Network Design Manual v7.7

Network Design Manual
Version: 7.7 Date of Issue: December 2006

Author: Tony Haggis Job Title: Lines & Cables Manager
Approver: Patrick Booth Job Title: Asset Manager
Revision Log
Version 7.7 Prepared by T Haggis Date December 2006
Notes: Re-branded to E.ON Central Networks.
Version 7.6 Prepared by T Haggis Date Sept 2005

Notes:
Section 1.3.2.2 Circuit Ratings for Industrial /Commercial Loads – 19 MVA and 32 MVA limits set
for standard 33/11kV transformers
Section 1.4.7.2 High Load Routes added
Section 1.2.5.3 kVA rating of residential cables adjusted to from 230v to 240v base – kVA rating
provided alongside amps.
1.2.6.7 Emergency Fire Fighting Supplies – new BS5588-5:2004 replaces BS5588-5:1991 and
relaxes the need for an independent HV fire fighting supply.
1.2.8.3 Outdoor meter boxes – minimum heigth of box changed from 750mm to 450mm to
align with Engineering Recommendation G81.
1.2.10.2 Transformer size calculation – 50 kVA transformers not to be used for residential
loads.
Section 1.3.1.3 Meter tails 2 per phase – now restrcietd to one per phase.
Section 1.3.1.3 kVA rating of industrial cables adjusted to from 230v to 240v base – all rating
incresed slightly. Typing error corrected for rating of 300 Wavecon - 256 kVA changed to 265
kVA and again adjustedto 275kVA to reflect 240v base.
Sections 1.3.1.7/8/9 - kVA bands adjusted to reflect changes to cable ratings above 200
becomes 210 kVA and 250 becomes 275 kVA.
1.3.1.11 Arrangement F. 11kV extension, 500 to 1000kVA substation & LV Air Circuit Breaker. –
where transformer is sited inside the customer’s boundary fence a Ring Mian Unit is now
used instead of a Switch Disconnector.
1.3.1.11 Arrangement F. 11kV extension, 500 to 1000kVA substation & LV Air Circuit Breaker. –
some of the detail removed and transferred to the “LV Air Circuit Breaker 500 to 1000 kVA
Substation Application Guide”
1.3.2.3/4/5/6/7/8 Standard HV Supply Arrangements H to L – Switchgear types amended to
include Lucy RMU & metering unit.
1.4.6.4 Overhead Line Rules – New ABSDs to be connected solid without live line taps.
Table 3.2.1.5 LV Cable data – Consac cables added
Tables 3.3.2.A to E - Autumn 11kV OHL ratings included plus additional sizes
Tables 3.3.1.4 A&B also 3.3.1.5A XLPE ratings recalculated using Crater V3.0 and 3 core XLPE
ratings added.
Table 3.3.1.5B – new table added for group of 2 XLPE Sustained & Cyclic ratings
3.3.4.1 11kV Distribution Substation Switchgear – tables updated to include Lucy Switchgear
and Central Networks contract references
Version 7.5 Tables 3.3.1.5 B,C,D,E,F now become Tables 3.3.1.5 C,D,E,F,G in version 7.6 to reflect
Version 7.7 Page 2 of 194

The offical current version of this Manual resides in the E.On UK Documentum database.
CAUTION – CD RoM & Printed copies may be out of date.
insertion of Table 3.3.1.5B

Table 3.3.1.5.D 11kV screened cable ratings – error in laid direct cyclic ratings corrected.
Section 3.4.3 ratings of transformers amemded to reflect the use of OFAF and OFAF CER
ratings for different load types.
Version 7.5 Prepared by T Haggis Date 23 June 2004

Notes: Typing errors corrrected –
Page 52 – 1500kVA changed to 1000KVA
Page 62 – 3.8MVA changed to 7.6MVA
Notes: Manual Issued with impimenttaion date of 1 July 2004 – File size reduced by altering jpg
pictures
Notes: Manual Issued with impimenttaion date of 1 July 2004
Notes: 1MW threshold for load connection referal raised from 200/500 kVA
Circulated for final comments prior to issue
Version 7.0 Prepared by T Haggis Date 28 May 2004
Notes: Circulated for final comments prior to issue
Version 6.8 Prepared by T Haggis Date 11 May 2004
Notes: Revisied to include Central Networks West
Font changed to Polo
Version 6.5 Prepared by T Haggis Date 2 April 2004
Notes: Penultimate draft sent for comment
Version 6.0 Prepared by T Haggis Date 1 March 2004

Notes: Final drafting after focus group work
Version 5.1 Prepared by T Haggis Date 5 Feb 2004
Notes: Circulated to Aquila prior to 10 Feb meeting
Version 4.8 Prepared by T Haggis Date 12 Jan 2004
Notes: Cirulated for comment
Version DEC 2002 Prepared by T Haggis Date Dec 2002
Notes: First Issue of interim document signposting user to ER G81

CONTENTS
CONTENTS .................................................................................................................................. 4
INTRODUCTION.......................................................................................................................... 11
1. NETWORK CONFIGURATION.................................................................................................12
1.1 Overview ............................................................................................................................12

1.1.1 400kV & 275kV Grid Substations ............................................................................................ 13
1.1.2 132kV Grid circuits .................................................................................................................. 13
1.1.3 132kV Substations................................................................................................................... 13
1.1.4 66kV & 33kV Primary circuits .................................................................................................. 14
1.1.5 Primary Substations ............................................................................................................... 14
1.1.6 11kV Secondary circuits........................................................................................................... 14
1.1.7 Secondary Substations ........................................................................................................... 15
1.1.8 Low Voltage (LV) Network ...................................................................................................... 15
1.2 LV Network ........................................................................................................................16

1.2.1 General Considerations at LV ................................................................................................. 16
1.2.2 LV Earthing ............................................................................................................................. 17
1.2.3 LV Protection .......................................................................................................................... 19
1.2.4 After Diversity Maximum Demands ....................................................................................... 22
1.2.4.1 Residential Loads ..................................................................................................................................................................22
1.2.4.2 Commercial and Industrial Loads...................................................................................................................................22
1.2.5 Residential Housing Developments ....................................................................................... 23
1.2.5.1 Connection Strategy for Residential Loads ................................................................................................................23
1.2.5.2 Voltage Regulation - Residential Housing ..................................................................................................................23
1.2.5.3 Mains Cables – Residential Housing .............................................................................................................................24
1.2.5.4 Service Cable Loading - Residential Housing.............................................................................................................25
1.2.5.5 Service Cable Voltage drop calculations - Residential Housing.........................................................................25
1.2.5.6 Service cable / cut-out maximum thermal loading.................................................................................................26
1.2.5.7 Servicing method ..................................................................................................................................................................28
1.2.5.8 Fusing - Residential Housing ............................................................................................................................................28
1.2.6 Supplies to Flats ..................................................................................................................... 29
1.2.6.1 Voltage Regulation - Flats ..................................................................................................................................................29
1.2.6.2 Service Cable Loading for Flats .......................................................................................................................................29
1.2.6.3 Service cable / cut-out maximum thermal loading - Flats ................................................................................... 31
1.2.6.4 Properties Converted into Flats ...................................................................................................................................... 31
1.2.6.5 Purpose Built Flats................................................................................................................................................................33
1.2.6.6 Fusing Feeders, Services & Cut-outs in Flats..............................................................................................................38
1.2.6.7 Emergency Fire Fighting Supplies – BS5588-5:2004 ...............................................................................................39
1.2.7 In-fill Developments ............................................................................................................... 40
1.2.8 Physical Position of Services .................................................................................................. 41

1.2.8.1 Residential Housing Underground Cable Service positions ................................................................................ 41
1.2.8.2 Residential Housing Overhead Line Cable Service positions.............................................................................. 41
1.2.8.3 Outdoor meter boxes ..........................................................................................................................................................42
1.2.8.4 Industrial and Commercial Service Positions ............................................................................................................46
1.2.8.5 Service Positions in Flats....................................................................................................................................................47
1.2.9 Street Lighting / Furniture Supplies ...................................................................................... 48
1.2.9.1 New developments...............................................................................................................................................................48
1.2.9.2 Existing networks..................................................................................................................................................................48
1.2.9.3 Complex / multiple connections.....................................................................................................................................48
1.2.10 Transformers Supplying LV Networks .................................................................................. 49
1.2.10.1 Type & Location.....................................................................................................................................................................49
1.2.10.2 Transformer size calculation...........................................................................................................................................50
1.3 Industrial and Commercial Supplies ................................................................................ 52

1.3.1 LV Metered Supplies............................................................................................................... 52
1.3.1.1 Connection Strategy for LV Industrial & Commercial Loads ................................................................................52
1.3.1.2 Voltage Regulation for LV Industrial & Commercial Loads ..................................................................................53
1.3.1.3 Cables for LV metered supplies.......................................................................................................................................53
1.3.1.4 Multi occupancy buildings.................................................................................................................................................54
1.3.1.5 Standard Service Arrangements .....................................................................................................................................55
1.3.1.6 Arrangement A. Direct Connection from Existing LV Network. .........................................................................57
1.3.1.7 Arrangement B. Direct Connection from modified LV network.........................................................................57
1.3.1.8 Arrangement C. Service direct from substation. ......................................................................................................58
1.3.1.9 Arrangement D. Second service direct from substation. ......................................................................................58
1.3.1.10 Arrangement E. 11kV extension to new substation. ............................................................................................59
1.3.1.11 Arrangement F. 11kV extension, 500 to 1000kVA substation & LV Air Circuit Breaker. ..........................60
Central Networks’ “LV Air Circuit Breaker 500 to 1000 kVA Substation Application Guide”............................60
Compact Unit Substation Option .............................................................................................................................................60
Separate Ring Main Unit & Transformer Option............................................................................................................... 61
Metering............................................................................................................................................................................................. 61
Length and rating of customer’s LV cables.......................................................................................................................... 61
Examples of cable sizes for customer owned LV tails.....................................................................................................62
1.3.2 HV Metered Supplies .............................................................................................................. 63
1.3.2.1 Connection Strategy for HV Industrial & Commercial Loads...............................................................................63
1.3.2.2 Circuit Ratings for Industrial /Commercial Loads....................................................................................................63
Ring Mains.........................................................................................................................................................................................63
Duplicate circuits.............................................................................................................................................................................64
1.3.2.3 Standard HV Supply Arrangements ..............................................................................................................................65
1.3.2.4 Arrangement H Single Transformer up to 3.8 MVA@11kV / 2.3 MVA@6.6kV..............................................67
1.3.2.5 Arrangement I Two or more Transformers totalling up to 7.6 MVA@11kV / 4.6 MVA@6.6kV ..............68
1.3.2.6 Arrangement J. Duplicate Ring Main Units totalling up to 7.6 MVA@11kV / 4.6 MVA@6.6kV..............69
1.3.2.7 Arrangement K. Customer switchboard up to 7.6 MVA@11kV / 4.6 MVA@6.6kV ......................................70
1.3.2.8 Arrangement L. Duplicate HV Supply .......................................................................................................................... 71
1.4 Secondary Network 11kV & 6.6kV ..................................................................................... 72

1.4.1 General Considerations – Secondary Network ....................................................................... 72
1.4.2 Connection Strategy ............................................................................................................... 72
1.4.3 11kV Earthing.......................................................................................................................... 73
1.4.3.1 11kV Earth Fault Current .....................................................................................................................................................73

1.4.3.2 Earthing of Plant ...................................................................................................................................................................73

1.4.4 11kV Protection....................................................................................................................... 75
1.4.4.1 Primary substation circuit breakers...............................................................................................................................75
1.4.4.2 Secondary substation protection ...................................................................................................................................76
1.4.4.3 HV Metered Customer Owned Substations ...............................................................................................................76
1.4.4.4 Padmount transformer protection ................................................................................................................................76
1.4.4.5 Pole mounted transformer protection.........................................................................................................................77
1.4.5 11kV Circuit Configuration & Loading ..................................................................................... 78
1.4.5.1 11kV Ring Mains .....................................................................................................................................................................78
1.4.5.2 Duplicate circuits ...................................................................................................................................................................78
1.4.5.3 Standard Equipment and Designs.................................................................................................................................79
1.4.5.4 Voltage Regulation ...............................................................................................................................................................79
1.4.5.5 11kV Cable Applications......................................................................................................................................................79
1.4.5.6 11kV Overhead Line Applications....................................................................................................................................82
1.4.6 11kV Connectivity Rules ......................................................................................................... 83
1.4.6.1 Derogations to 11kV Connectivity Rules and/or P2/5 .............................................................................................83
1.4.6.2 Ground Mounted Substation Rules ...............................................................................................................................84
1.4.6.3 Padmount Transformer Rules ..........................................................................................................................................86
1.4.6.4 Overhead Line Rules ............................................................................................................................................................88
1.4.6.5 LV Back-feeds and Mobile Generation.........................................................................................................................93
Ground Mounted Substations...................................................................................................................................................93
Padmount Transformers..............................................................................................................................................................94
Pole mounted transformers.......................................................................................................................................................94
1.4.7 Physical Siting of 11kV Substations, Cables & Lines ............................................................... 96
1.4.7.1 Location and Operational Access to Substations.....................................................................................................96
1.4.7.2 Routing of 11kv Overhead Lines ......................................................................................................................................97
High Load Routes............................................................................................................................................................................97
Un-insulated 11kV overhead Lines ...........................................................................................................................................97
Routing overhead lines across farmland ..............................................................................................................................98
Pole mounted transformers and switchgear. .....................................................................................................................98
Vehicular Access..............................................................................................................................................................................98
Hazard reduction.............................................................................................................................................................................99
1.4.7.3 Routing of 11kV Underground Cables.........................................................................................................................100
Preferred routes ............................................................................................................................................................................100
Routes to avoid ..............................................................................................................................................................................100
1.4.7.4 Earth Potential Rise............................................................................................................................................................ 101
1.4.7.5 Environmental Constraints ............................................................................................................................................. 101
Noise .................................................................................................................................................................................................. 101
Escape of Insulating Oil.............................................................................................................................................................. 101
Water and Gas................................................................................................................................................................................102
1.4.7.6 Substation Legal Requirements....................................................................................................................................102
1.4.8 11kV Network Automation ....................................................................................................103
Pole Mounted Auto Reclosers (PMAR) ................................................................................................................................103
Remote controlled switchgear................................................................................................................................................103
1.5 Primary Network 33kV ....................................................................................................105

1.5.1 33kV General Considerations.................................................................................................105
1.5.2 33kV Earthing ........................................................................................................................105
1.5.3 33kV Protection .....................................................................................................................105
1.5.4 33kV Circuit Configurations ...................................................................................................105

1.5.5 33kV Connectivity Rules........................................................................................................105

1.5.6 Physical Siting of 33kV Substations and Equipment.............................................................105
1.5.7 33kV Network Automation ....................................................................................................105
1.6 Grid Network 132kV.........................................................................................................105

1.6.1 General Considerations at 132kV ...........................................................................................105
1.6.2 132kV Earthing.......................................................................................................................105
1.6.3 132kV Protection....................................................................................................................106
1.6.4 132kV Circuit Configurations..................................................................................................106
1.6.5 132kV Connectivity Rules ......................................................................................................106
1.6.6 Physical Siting of 132kV Substations and Equipment ...........................................................106
2. NETWORK MODIFICATION PROCEDURE .............................................................................107
2.1 Introduction.....................................................................................................................107
2.2 Implications of Network Modifications ..........................................................................108

2.2.1 Cause and Effect Mechanisms ...............................................................................................108
2.2.2 Examples of Implications...................................................................................................... 110
3. STANDARD EQUIPMENT RATINGS AND DATA ................................................................... 112
3.1 General ............................................................................................................................ 112
3.2 Low Voltage Network Ratings and Data......................................................................... 112

3.2.1 LV Underground Cables ......................................................................................................... 112
3.2.1.1 Single Phase LV Service Cables ..................................................................................................................................... 112
3.2.1.2 Three Phase LV Cables ...................................................................................................................................................... 113
3.2.1.3 Transformer Tails (Distribution Substation)............................................................................................................. 114
3.2.1.4 Transformer Tails (Large LV Industrial Supplies) ................................................................................................... 115
3.2.1.5 LV Underground Cable Data Tables............................................................................................................................. 119
3.2.2 LV Overhead Lines .................................................................................................................120
3.2.2.1 Main and Service Lines.....................................................................................................................................................120
3.2.2.2 Pole Transformer Tails ......................................................................................................................................................120
3.2.2.3 LV Overhead Line Data Tables....................................................................................................................................... 121
3.2.3 LV Switchgear........................................................................................................................122
3.2.3.1 Service Cut-outs ...................................................................................................................................................................122
3.2.3.2 LV Distribution Cabinets ..................................................................................................................................................123
3.2.3.3 Underground Network Boxes........................................................................................................................................124
3.2.3.4 Pole Mounted LV Switchgear ........................................................................................................................................124
3.3 Secondary Network 11kV Ratings and Data ....................................................................124

3.3.1 11kV Cables ............................................................................................................................124
3.3.1.1 11 kV Network Cables.........................................................................................................................................................124
3.3.1.2 Transformer 11kV Tails ......................................................................................................................................................125
3.3.1.3 Cable Rating Criteria..........................................................................................................................................................126
Paper Insulated Cables...............................................................................................................................................................126
XLPE Insulated Cables ................................................................................................................................................................126
Duct type - important note. ......................................................................................................................................................126

5 Day Distribution Rating ..........................................................................................................................................................127
Cyclic Ratings..................................................................................................................................................................................128
Sustained Ratings.........................................................................................................................................................................128
Ratings used in GIS ......................................................................................................................................................................129
Caution - XLPE cable to Paper cable joints.........................................................................................................................129
Calculation of Loss Load Factor ..............................................................................................................................................129
3.3.1.4 11kV Cable 5 day Distribution Rating Tables............................................................................................................ 131
11kV XLPE Cables – Ungrouped - 5 Day Distribution Rating.......................................................................................132
11kV XLPE Cables – Group of 2 - 5 Day Distribution Rating.......................................................................................133
11kV XLPE Cable - Multiple Groups - 5 Day Distribution Rating.................................................................................134
11kV Belted Paper Cables – 5 Day Distribution Rating..................................................................................................135
Ungrouped - Metric sizes - Max 65OC conductor temp..............................................................................................135
11kV Belted Paper Cables – 5 Day Distribution Rating..................................................................................................136
Ungrouped - Imperial sizes - Max 65OC conductor temp ...........................................................................................136
11kV Screened Paper Cables – 5 Day Distribution Rating ............................................................................................137
Ungrouped - Metric sizes - Max 70OC conductor temp..............................................................................................137
11kV Screened Paper Cables – 5 Day Distribution Rating ............................................................................................138
Ungrouped - Imperial sizes - Max 70OC conductor temp ...........................................................................................138
3.3.1.5 11kV Cable Sustained & Cyclic Rating Tables ...........................................................................................................139
11kV XLPE Cables - Sustained & Cyclic Ratings .................................................................................................................140
Ungrouped - Max 90OC conductor temp..............................................................................................................................140
11kV XLPE Cables - Sustained & Cyclic Ratings ................................................................................................................. 141
Group of 2 - Max 90OC conductor temp ............................................................................................................................... 141
11kV Belted Paper Cables –Sustained & Cyclic Ratings.................................................................................................142
Ungrouped – Metric Sizes - Max 65OC conductor temp...............................................................................................142
11kV Belted Paper Cables - Cyclic & Sustained Rating.................................................................................................142
Ungrouped – Imperial Sizes - Max 65OC conductor temp...........................................................................................142
3.3.1.6 11kV Paper Cables - Grouping Correction Table......................................................................................................146
3.3.1.7 11kV Paper Cables - Loss Load Factor Correction Table.......................................................................................147
3.3.1.8 11kV Cable Electrical Data Tables..................................................................................................................................148
3.3.2 11kV Overhead Lines..............................................................................................................149
3.3.2.1 11kV Conductors...................................................................................................................................................................149
3.3.2.2 11kV Conductor Distribution Rating Tables..............................................................................................................149
3.3.2.3 11kV O/H Line Data Tables...............................................................................................................................................154
3.3.3 11 kV / LV Transformers ........................................................................................................155
3.3.3.1 Ground Mounted Transformers ....................................................................................................................................155
3.3.3.2 Padmount Transformers..................................................................................................................................................156
3.3.3.3 Pole Mounted Transformers ..........................................................................................................................................156
3.3.3.4 11kV/LV Transformer Data Tables ................................................................................................................................157
3.3.4 11kV Switchgear ....................................................................................................................158
3.3.4.1 11kV Distribution Substation Switchgear..................................................................................................................158
3.3.4.2 11kV Primary/Grid Substation Switchgear ...............................................................................................................165
3.3.4.3 Pole Mounted 11kV Switchgear ....................................................................................................................................165
3.4 Primary Network 33kV Rating and Data .........................................................................166

3.4.1 33kV Cables............................................................................................................................166
3.4.1.1 33 kV Network Cables ........................................................................................................................................................166
3.4.1.2 Transformer 33kV Tails......................................................................................................................................................166
3.4.1.3 33kV Cable Ratings.............................................................................................................................................................167
3.4.1.4 33kV Transformer Tail Rating Tables...........................................................................................................................167
Transformer 33kV XLPE Tails Sustained Ratings..............................................................................................................168

3.4.1.5 33kV Cable Data Tables.....................................................................................................................................................170
3.4.2 33 kV Overhead Lines ............................................................................................................170
3.4.2.1 33kV Conductors..................................................................................................................................................................170
3.4.3 33/11 kV Transformers........................................................................................................... 171
3.4.3.1 Standard Designs................................................................................................................................................................ 171
3.4.3.2 Identification of exceptional Conditions...................................................................................................................172
3.4.4 33 kV Switchgear...................................................................................................................172
3.5 Grid Network 132kV Ratings and Data............................................................................172

3.5.1 132 kV Cables .........................................................................................................................172
3.5.2 132 kV Overhead Lines...........................................................................................................172
3.5.3 132/33 kV & 132/11kV Transformers ......................................................................................172
3.5.4 132kV Switchgear ..................................................................................................................172
4. NETWORK VOLTAGE POLICY ..............................................................................................173
4.1 Voltage Limits .................................................................................................................173

4.1.1 Use of the Voltage Limits ......................................................................................................173
4.2 Voltage Control ...............................................................................................................174
4.3 Grid & Primary Substation 11kV Bus Bar Voltages..........................................................175
4.4 11kV Network Voltage Regulation ..................................................................................175

4.4.1 Standard Feeders ..................................................................................................................175
4.4.2 Long Feeders .........................................................................................................................175
4.4.3 HV Customers........................................................................................................................176
4.5 Secondary Transformer Voltage Regulation ...................................................................176
4.6 LV Network Voltage Regulation......................................................................................177
4.7 Line Drop Compensation.................................................................................................177
4.8 Voltage Regulators..........................................................................................................178

4.8.1 LV Regulators ........................................................................................................................178
4.8.2 Static Balancers.....................................................................................................................178
4.8.3 11kV Voltage Regulators........................................................................................................178
5. LV EARTH LOOP IMPEDANCE POLICY.................................................................................179
5.1 Introduction.....................................................................................................................179
5.2 Provision of Protective Multiple Earthing.......................................................................179
5.3 Specific Requirements .................................................................................................... 179

5.3.1 New LV networks...................................................................................................................179

5.3.2 Service alterations ................................................................................................................179
5.3.3 In-fill developments supplied from existing LV mains .........................................................180
5.3.4 Street lighting services .........................................................................................................180
5.3.5 Replacement or alterations to existing mains...................................................................... 181
5.3.6 Temporary Back-feeding ....................................................................................................... 181
5.4 Notes of Guidance to Loop Impedance Policy.................................................................182

5.4.1 Quality of supply:...................................................................................................................182
5.4.2 Protection of mains & services ..............................................................................................182
5.4.2.1 New developments.............................................................................................................................................................182
5.4.2.2 Existing networks ...............................................................................................................................................................183
5.4.2.3 Long Street Lighting cables ...........................................................................................................................................183
5.4.3 Protection of consumer’s equipment ....................................................................................184
5.4.4 Provision of PME terminal ....................................................................................................184
6. LOW VOLTAGE NETWORK DESIGN CALCULATIONS ............................................................187

6.1.1 Voltage Drop Calculation Methodology .................................................................................187
6.1.2 Mains LV volt drop calculation: .............................................................................................187
6.1.3 Service Voltage Drop Calculation...........................................................................................188
6.1.4 Loop impedance ....................................................................................................................188
7. DISTURBING LOADS & DISTRIBUTED GENERATION...........................................................189
8. ENVIRONMENTAL REQUIREMENTS....................................................................................189
8.1 Development at Sites that have Legal Environmental Protection..................................189
8.2 Escape of insulating oil ...................................................................................................189
8.3 Transformer loss evaluation criteria employed..............................................................190
9. NEW NETWORK ACCEPTANCE REQUIREMENTS................................................................. 191

9.1.1.1 Scope......................................................................................................................................................................................... 191
9.1.1.2 LV Networks and associated Substations ................................................................................................................. 191
10. APPLICABLE ENGINEERING STANDARDS.........................................................................193
11. GLOSSARY ........................................................................................................................194

INTRODUCTION
Central Networks is regulated by the Authority under primary legislation of the Electricity Act 1989 as
amended by the Utilities Act 2000 (the Act), the Competition Act 1998 and through the granting of an
Electricity Distribution Licence.
Central Networks has a duty under the Act to develop and maintain an efficient, co-ordinated and
economical system of electricity distribution and to facilitate competition in the supply and generation
of electricity.
All new additions and modifications to the distribution system must comply with the requirements of
this Manual which provides for an adequate standard of network performance and reliability
consistent with Central Networks’ obligations under the Act.
The requirements of this manual also apply to any assets to be adopted by Central Networks under
Competition in Electricity Connections.
This Manual relates to the areas of:

Central Networks East – formally East Midlands Electricity
Central Networks West –formally Aquila / GPU / Midlands Electricity.
Where technical differences exist between the two areas they are identified in the text. Otherwise this
Manual applies equally to both areas.
If any instances occur whereby the requirements of this Manual cannot be complied with or where it
is desired for a specific reason to depart from them, written permission must first be obtained from:
The Network Manager

Network Strategy & Regulation
Central Networks

1. Network Configuration
This section is provided for persons who may be unfamiliar with the technical aspects of the Central
Networks’ network. This may include: persons designing assets to be adopted by Central Networks,
Central Networks’ contractor’s staff, Central Networks staff new to network design work.
It describes how the Central Networks distribution system is configured so that they can identify where it
may differ from other Distribution Network Operator’s systems. e.g. The Central Networks’ distribution
system generally uses resistance earthing at Primary Substations. Other DNOs may have solid earthing
or arc suppression coil. Others may employ different transformer vector groups etc. The fundamental
aspects need to be understood before commencing any design work on Central Networks’ distribution
system.
1.1 Overview
Central Networks takes electricity from National Grid Transco substations at 132kV and distributes it to
end user customers via a network comprising 132kV, 66kV, 33kV 11kv 6.6kV and LV (400/230v) circuits.

Alternative circuits are provided where required to comply with the Energy Networks Association
Engineering Recommendation P2/5 minimum standards of supply security. Alternative supplies and
network switching points may be provided in excess of P2/5 requirements to meet the Regulator’s
customer service targets.
The following section describes salient details of the Central Networks distribution system to assist
with overall design considerations. Whilst the descriptions are typical of the majority of the Central
Networks distribution system some parts may differ in detail due to historical reasons or to address
specific engineering issues. Before extending or modify the distribution system the designer must
consider the specific part of the network involved and design accordingly. Particular attention must be
paid to the Network Modification Procedure section of this manual.
1.1.1 400kV & 275kV Grid Substations

400kV & 275kV Grid Substations (also know as Grid Supply Points or GSPs) are owned by National Grid
Transco and contain 400/132kV or 275/132kV step-down transformers feeding busbars from which
Central Networks 132kV circuits originate.
1.1.2 132kV Grid circuits

132kV Grid circuits normally comprise of overhead lines with earth wire on steel lattice towers.
Underground cables are used where overhead routes are inappropriate or unobtainable. Duplicate
circuits are normally arranged as a parallel pair to provide full alternative supplies in the event of one
circuit outage in compliance with P2/5. Some use is made of single circuit unearthed construction
wood pole lines.
Main protection provides fault clearance times under 200ms using distance or unit protection
schemes. Back-up protection is by IDMT relays.
1.1.3 132kV Substations

132kV Substations (also know as Bulk Supply Points or BSPs) are normally owned by Central Networks
and contain 132 kV switchgear, 132/33kV and/or 132/11kV transformers and 33kV and/or 11kV
switchgear from which 33kV and 11 kV circuits originate. Certain areas of Central Networks West (e.g.
Hereford & Worcester) employ 66kV as a primary voltage instead of 33kV. Some Bulk Supply Points
feed into 6.6kV city networks in which case the secondary voltages are 6.6kV instead of 11kV. Grid
transformers are normally installed in pairs to provide full alternative supplies in the event of one
circuit outage in compliance with Engineering Recommendation P2/5.
Transformers have unit protection, Buchholz relays and employ pilot wire inter-tripping to source. The
LV circuit breakers (33, 11, 6.6kV) have directional protection to prevent back-feeding 132kV earth
faults.
Transformers have on-load tap changers for voltage control. Typical vector groups are YNd11 or YNd1
for the 132/33kV transformers and YNyn0 for the 132/11Kv transformers. Earthing transformers of
vector group Zy1 or Zy11 provide substation LVAC supplies and either have sufficient impedance to
limit earth fault current or work in conjunction with a separate Neutral Earthing Resistor. Earth fault

currents are limited to a maximum of the full load current of the transformer. Lower values may apply
according to network earthing requirements.
1.1.4 66kV & 33kV Primary circuits

66kV & 33kV Primary circuits normally comprise of overhead lines on wood poles (unearthed
construction) or steel lattice towers (with earth wire). Underground cables are used where overhead
routes are inappropriate or unobtainable. Duplicate circuits are normally arranged as a parallel pair to
provide full alternative supplies in the event of one circuit outage. Older duplicate circuits have both
circuits located closely together on double circuit overhead line supports or in adjacent trenches
leaving them at risk from simultaneous damage. Modern designs remove this ‘linked risk’ by installing
single circuits as pairs but on separate routes wherever practicable.
Main protection provides fault clearance times under 200ms using high set over current relays. Some
circuits may have the protection slugged to a maximum of 500ms to accommodate downstream
protection stages such as Pole Mounted Auto Reclosers. There are some distance and unit protection
schemes on certain circuits. Back-up protection is by IDMT relays.
1.1.5 Primary Substations

Primary Substations are normally owned by Central Networks and contain 33 kV switchgear, 33/11kV
transformers and 11kV switchgear from which 11 kV circuits originate. Certain areas of Central
Networks West (e.g. Hereford & Worcester) employ 66kV as a primary voltage instead of 33kV. Some
Primary substations feed into 6.6kV city networks in which case the secondary voltages are 6.6kV
instead of 11kV. Primary transformers are normally installed in pairs to provide full alternative supplies
in the event of one circuit outage. However, some Primary Substations have single transformers and
rely on alternative supplies from the 11kV network but arranged to comply with Engineering
Recommendation P2/5.
Transformers have unit protection, Buchholz relays and employ pilot wire inter-tripping to source on
underground circuits and fault throwers on overhead circuits. Neutral Voltage Displacement detection
is provided on incoming overhead circuits. The LV circuit breakers (11, 6.6kV) have directional
protection to prevent back-feeding 33kV phase faults.
Transformers have on-load tap changers for voltage control. Vector group is Dyn11. Earth fault currents
are limited to a maximum of 1000 amps per transformer by neutral earthing resistors although lower
values may apply according to network earthing requirements.
1.1.6 11kV Secondary circuits

11kV Secondary circuits normally comprise of overhead lines on wood poles (unearthed construction)
in rural areas and underground cables in urban areas or where overhead routes are inappropriate or
unobtainable. Most circuits contain a mix of underground cable and overhead line. Circuits are
generally arranged as radial feeders terminating at normally open switches to form an open ring
main. Alternative supplies are applied either by manual switching or by remote operation of
automated switches by System Control. Switches are situated at distribution substations and on
overhead line poles to enable routine and emergency work to be carried out within the requirements
of Engineering Recommendation P2/5 and the Regulator’s customer service targets.
1.1.7 Secondary Substations

Secondary Substations deliver energy to Central Networks’ low voltage networks and to
industrial/commercial customer owned networks.
The vector group of 11kV/LV transformers is Dyn11 and off-load tap changers are provided for voltage
adjustment. Where customers take a supply at 11kV they own their 11kV/LV transformers and
sometime operate their own internal 11kV circuits. The majority of customers take supply from the
Low Voltage network supplied by 11kV/LV transformers owned by Central Networks.
Pole mounted transformers are either connected directly to ring main overhead lines or spur lines by
solid jumpers, live line taps, expulsion fuses, links or automatic sectionalising links as appropriate to
the local network configuration. Pole transformer locations have to be selected with consideration to
the risk of accidental contact with 11kV terminals taking into account the present and future use of the
land. Free-standing pole mounted transformers (cable connected rather than overhead supplied) are
no longer permitted in Central Networks East. Padmount transformers or Compact Unit Substations
are used where overhead line connections do not meet the requirements of the risk assessment
process. This policy will be applied in Central Networks West once Padmount transformers have been
introduced.
Ground mounted transformers have local switchgear incorporating transformer protection fuses or
circuit breakers. This switchgear is normally a Ring Main Unit that provides switching and earthing
facilities for the connected 11kV ring circuit cables. Some transformers are radial connected on tee-off
cables. These may be equipped with Ring Main Units or cable elbow connectors in the case of
padmount transformers.
Any radial connection of pole and ground mounted transformers is carefully regulated by Central
Networks to ensure compliance with the requirements of Engineering Recommendation P2/5 and the
Regulator's customer service targets. The provision of LV back-feeds and/or facilities for the
connection of mobile generators is an integral part of network design.
1.1.8 Low Voltage (LV) Network

Low Voltage (LV) Network comprises of three phase underground cables or overhead lines on wood
poles normally arranged as multi branched radial feeders. Some LV circuits interconnect with adjacent
transformers via underground link boxes or overhead fuse-gear to provide back-feeds during routine
and emergency work to ensure compliance with the requirements of Engineering Recommendation
P2/5 and Regulator’s customer service targets.
The majority of existing underground cables are paper insulated, lead covered, steel tape armoured
construction. Most cables are 4 core (3 phase + neutral) but areas of 5 core (3 phase + street light +
neutral), 2 core (1 phase + neutral) and 3 core (2 phase + neutral) also exist.
In some locations plain lead or steel wire armoured cables exist and some of the older cables may be
of concentric construction.
TN-S earthing is provided to many customers. TN-C-S earthing is also provided to customers where the
cable has been converted to Protective Multiple Earthing (PME).

A large proportion of existing overhead line is of bare wire construction. TT earthing is provided to
many customers. TN-C-S earthing is also provided to customers where the line has been converted to
Protective Multiple Earthing (PME).
New LV networks are constructed of underground cable. Three core XLPE insulated cable with
waveform combined neutral/earth (CNE) screen is employed to enable live jointing to take place
without interruption of the neutral/earth. TN-C-S earthing is available from waveform cables which are
all PME.
Replacement of, or extensions from, existing open wire overhead lines are by Aerial Bundled
Conductor (ABC). TN-C-S earthing is available from ABC lines which are all PME.
LV Circuits are protected by HRC fuses to BS88 part5. Central Networks’ policy is to fuse new LV mains
such that phase to neutral faults on mains and services are cleared within 100 seconds. Voltage drop
and loop impedance criteria are used to ensure the appropriate quality of supply and circuit protection
times are achieved. Refer to the Loop Impedance Policy section of this manual for more detail.
Services are breech jointed directly onto mains. Three phase and single phase services are used as
appropriate to the load.
All new domestic properties have outdoor meter boxes unless prohibited by local planning
regulations. Where the use of internal services is unavoidable they are terminated in a cut-out
immediately inside the building. New installations in blocks of flats have a three phase supply
installed at a central service / metering position and lateral connections to flats are provided and
owned by the owner/occupiers.
1.2 LV Network
1.2.1 General Considerations at LV
Supplies to groups of new customers shall normally be provided by underground cables.
Exceptionally, overhead lines may be used where the use of underground cable is not reasonably
practicable.
Mains cables shall normally laid along one side of the road. Road crossings shall be provided to service
properties on the opposite side. Double-sided mains may be used to accommodate large
concentrations of load or as a means of reinforcing existing developments. Cables shall be installed in
footpaths or service strips and positioned relative to other utility apparatus in accordance with
National Joint Utilities Group recommendations.
LV mains cables shall be laid in black rigid twinwall corrugated ducts to ENATS 12-24 with warning
tape. Short sections of LV mains cable may be laid direct only where ducts cannot be used such as
sharp bends or at jointing positions. Warning tape shall be used in these situations. Warning tape
may be omitted where trenchless excavation is employed.
All road crossings shall be ducted.

The Central Networks Cables, Cable Laying and Accessories Manual provides further detail.
LV circuits shall normally be arranged as multi-branched radial feeders. There shall be a maximum of
100 customer connected to each LV circuit. Un-used ways in the substation LV feeder pillar shall be
cabled and bottle ended outside the substation to enable future circuits to be added without making
the feeder pillar dead. The bottle ends shall not have PME earth electrodes installed as this may
compromise the separation of the HV and LV earth electrode systems.
Selected LV circuits shall interconnect with adjacent transformers via underground link boxes or
overhead fuse-gear to provide back-feeds during routine and emergency work on the 11kV system to
ensure compliance with the requirements of Engineering Recommendation P2/5 and the Regulator’s
customer service targets. Refer to sub-section 1.4.5.6 of this Manual for LV interconnection
requirements.
Services are breech jointed directly onto mains. Commercial and Industrial properties normally have
three phase services. Domestic properties have single phase services and these are distributed across
the phases of the mains cable as evenly as possible to balance the load. Electrically heated properties
may have three phase services according to local network requirements. Block of flats have a three
phase supply installed at a central service / metering position and lateral connections to flats are
provided and owned by the owner/occupiers.
The Standard Equipment Ratings and Data section of this manual defines the types and sizes of cable
and overhead lines that shall be used the designer.
Voltage drops are calculated as per section 6 of this Manual
1.2.2 LV Earthing
All new construction shall be to PME specification in accordance with the Central Networks Earthing
Manual - Section E6 Protective Multiple Earthing.
Existing networks should only be converted to PME where PME terminals are requested by customers
and if it is technically and economically practical to do so.
Regulation 8(2)b of the Electricity Safety Quality & Continuity (ESQC) Regulations 2002 states “the earth
electrodes are (to be) designed, installed and used in such a manner so as to prevent danger occurring in
any low voltage network as a result of any fault in the high voltage network.”
• Substation HV & LV earths shall be:

• segregated at sites where the Earth Potential Rise (EPR) is over 430 volts – Hot Sites
• combined at sites where the Earth Potential Rise (EPR) is below 430 volts – Cold Sites
according to the requirements of the Central Networks Earthing Manual.
• Where an LV circuit is to pass near HV equipment such as 11/33kV earthed poles or

132/275/400 kV towers the appropriate 430v or 650v EPR contour must be identified and
precautions taken against earthing the LV neutral within the zone either directly or via
street furniture.

A PME terminal is the preferred method of earthing for the majority of premises. However, it is not
suitable for every connection.
This table shows where a PME terminal may or may not be provided
Type of premises PME Comments

?
Domestic houses and YES Equipotential bonding must comply with the current edition
commercial / industrial of BS7671
premises in non-steel
framed buildings
Existing domestic houses NO PME can be provided if an equipotential mat has been
with downstairs bathrooms but installed in the concrete base and bonded in accordance
on concrete or non- with the current edition of BS7671. PME may also be
insulated floor provided if all water and waste pipes within the bathroom
are plastic.
Commercial / industrial YES YES if there is only one service into the building and
premises in steel framed but equipotential bonding complies with the current edition of
buildings BS7671
Multiple services – NO - may result in high EMF due to
neutral currents returning via the steel frame. See section
3.2.21 of section E6 of the Earthing Manual
Caravan, mobile home NO Prohibited by ESQC Regs
Permanently fixed ‘mobile’ YES Provided it is not mounted on wheels and does not have a
home but conductive surface – i.e. it is a prefabricated dwelling. See
section 3.2.21 of section E6 of the Earthing Manual
Railway electric traction NO See section 3.2.1 of section E6 of the Earthing Manual
systems
Construction Sites NO See section 3.2.2 of section E6 of the Earthing Manual
Temporary building not a YES See section 3.2.3 of section E6 of the Earthing Manual
construction site but
Farms and horticultural NO OK for the house but the remainder of the buildings need
premises but special consideration, especially dairies. See section 3.2.4 of
section E6 of the Earthing Manual
Sports Pavilions and NO Can be used provided the bonding arrangements are
Swimming Pools but specially designed. See section 3.2.5 of section E6 of the
Earthing Manual
Mines and Quarries NO OK for the offices but must be segregated from the
but production facilities. See section 3.2.7 of section E6 of the
Earthing Manual
Petrol Filling Stations NO OK for associated buildings like shops but must be
but segregated from the petrol filling areas. See section 3.2.8 of
section E6 of the Earthing Manual. Also Association for
Petroleum & Explosives Administration / Institute of
Petroleum Guide.
Continued PME ? Comments

Type of premises
Pubs and bars (tanker YES See section 3.2.9 of section E6 of the Earthing Manual
delivery supply)
High Rise Buildings YES See section 3.2.10 of section E6 of the Earthing Manual
Supplies provided from a YES See section 3.2.11 of section E6 of the Earthing Manual
separate position to the but
building
Outside water taps YES Not precluded by the current edition of BS7671. See
section 3.2.12 of section E6 of the Earthing Manual
LV Embedded Generators YES Refer to section 5.2 of EA Engineering Recommendation
G59/1
Street lighting and street YES See section 3.2.14 of section E6 of the Earthing Manual
furniture
Lightning protection YES See section 3.2.15 of section E6 of the Earthing Manual and
systems BS6651
Roadside public NO There are some limited exceptions. See section 3.2.16 of
telephones, bollards, but section E6 of the Earthing Manual
ticket machines etc.
Cathodic protection NO There are some limited exceptions. See section 3.2.17 of
installations but section E6 of the Earthing Manual
Small radio stations and NO OK for domestic or business radio stations. See section
cell phone stations but 3.2.18 of section E6 of the Earthing Manual
Cell phone base stations NO EA Engineering Recommendation G78 applies.
on 132/275/400 kV towers
Outside broadcast NO Previous approval has been withdrawn by the Dept of Trade
caravans & Industry.
1.2.3 LV Protection
LV Circuits are protected by High Rupturing Capacity (HRC) fuses to BS88 part 5. Central Networks’
policy is to fuse new LV circuits such that phase to neutral faults on mains and services are cleared
within 100 seconds after allowing a 15% voltage reduction for arc resistance. This limits the length of
mains and services according to the combined loop impedance of the transformer, main and service
cable and substation fuse size. Long street lighting circuits shall be protected by an additional cut-out
fuse normally installed in the first lamp on the run or a street lighting authority owned distribution
pillar.
The 100 second fault clearance times do not comply with the current edition of BS7671 “Requirements
for Electrical Installations”. Therefore service cables must be either terminated in outdoor meter boxes
or in cut-outs installed at the first reasonably practicable location within the building such that the
minimal length of service cable is within the building. Any Central Networks equipment inside the
building after the cut-out must comply with the current edition of BS7671. i.e. 5 second earth fault
clearance time.
Section 1.2.8 of this Manual specifies acceptable service locations for each building type.

Cut-outs are fitted with fuses to BS 1361 (services to 100 amps) or BS88 Part5 (services to 630 amps).
The current edition of BS7671 “Requirements for Electrical Installations” allows the installation
designer to rely upon the cut-out fuse to protect his main switchboard / consumer unit. There is
therefore a maximum permissible loop impedance related to each size of cut-out fuse to ensure fault
clearance within 5 seconds.
Refer to Tables 1 to 4 in the LV Earth Loop Impedance Policy Section 5.3.4 of this Manual for fuse size /
loop impedance data.
Substation LV fuses must also grade with the HV protection of the local distribution transformer.
Central Networks standard transformer HV protection schemes comply with ENATS 12-6 and allow the
following maximum fuse sizes to be installed:
Refer to Table 1.2.3 LV Fuse sizes for further information.

Table 1.2.3 LV Fuse sizes

Transformer Type Nominal Max Normal fuse size
size impedance fuse
Residential non Residential
size electric heating electric heating &
amps Industrial /
Commercial
1 2 3 4 5 6
Notes 1& 2
1000 kVA Ground mounted 4.75% 630 315 400
Notes 1 & 2
750 / 800 kVA Ground mounted 4.75% 630 315 400
300 / 315 kVA Ground mounted 4.75% 315 315 315
50 kVA 1Ø ANSI Padmount 2.5% 315 315 315
Notes 3 & 4
315 kVA 3Ø Pole mounted 4.75% 400 315 400
Note 5
100 kVA 1Ø Pole mounted 2 wire 4.5% 400 315 400
All 100 amp Cut-out - - 80 100
Column 2 lists standard transformers to Electricity Association Technical Standard ENATS 35-1 except for
Padmounts which are to American National Standards Institute (ANSI) C57-12-25/26 standard.
Column 4 lists the maximum sizes allowable for HV/LV protection discrimination.
Column 5 lists the sizes to be installed on LV feeders supplying non-electric heated residential properties. The
maximum fuse size of 315 amps is chosen to enable the maximum LV feeder length to be utilised within voltage
drop limits and without unduly restricting the length for loop impedance criteria. Mains may be 95, 185 & 300
mm2
Column 6 lists the sizes to be installed on LV feeders supplying electric heated residential properties and
industrial / commercial properties. Mains may be 95, 185 & 300 mm2.
Note 1 – Some existing feeders with copper cables supply loads over 400 amps. On these circuits the fuse size
may be increased to 630 amps. However, L.V. distribution fuse-boards to ESI Standard 37-2, as supplied with
standard EME unit substations, have a standard 500A outgoing way with a 6-hour overload rating of 555A.
Therefore, the loading of a 630A fuse-link fitted in such a switchboard must be limited to these values
Note 2 – Industrial supply Arrangements C and D in Section 1.3.1.7 and 1.3.1.8 use a 630 amp fuse at the
substation in order to grade with the 400 amp cut-out fuses.
Note 3 - Where feeding onto an LV network two cables shall be used to split the load using 2 sets of 315 amp
fuses.
Note 4 - Where a single customer requires a supply up to 260 kVA then an LV cabinet shall be used with one set
of 400 amp fuses supplying a 300 mm2 cable. (Transformer output is limited to the 360 amp / 260 kVA rating of
a 300 mm2 cable in ducts.)
Note 5 - 400 amp fuses should only be used to supply a single cable from a 200 kVA P/T to utilise the full
overload rating of the transformer for a single customer. Where two cables are supplied then 2 sets of 315 amp
fuse shall be used.

1.2.4 After Diversity Maximum Demands

The following values shall be used as the basis for design. Note that the method of using
these values depends of the part of the network being designed i.e. mains, services or
transformer. Further sections of this manual define the method of application
1.2.4.1 Residential Loads
‘Non-electric’ and ‘No Central Heating’ values of After Diversity Maximum Demand (ADMD)
assume small or average houses and flats up to 4 bedrooms. For larger properties increase the
ADMD by 0.5kW per additional bedroom.
Heating Type ADMD kW

(LV Mains)
No central heating – gas available in the dwelling 2.0
No central heating – gas not available in the dwelling 3.0
Gas / Oil or Solid fuel central heating 2.0
Direct electric heating – ceiling / panel /convector 2.0 plus 50% of installed space
heating load (ignore water
heating)
Electric storage heating 100% of installed storage and

100% of water heating load
Electric storage heating plus direct heating 100% of installed storage and
50% of direct heating +100% of
water heating load
Note that the above values are “after diversity” demands and apply directly to the design of mains cables.
When designing individual services or sizing transformers, further formulae are applied to adjust the diversity
element as defined in the appropriate sections of this manual.
1.2.4.2 Commercial and Industrial Loads
The following table provides examples of typical commercial & industrial loads based on
Central Networks’ experience.
Customers tend to request a supply that is the arithmetical sum of the installed equipment
and this can give rise to an unrealistic maximum demand. Customer’s load estimates should
be benchmarked against this table.

Property Type Typical ADMD

Small shop Single ph 8 kW
Café/Restaurant Three ph 15 kW
Take away Three ph 20 kW
Church Three Ph supply lots with infra-red direct heating up to 50 kW
Farm Three Ph up to 100 kW
Farm with grain dryer Three Ph 200 kW depending on fan motor/heater
Beauty salon/ Tanning shop Three ph 10 kW
Hair dressers single ph 5 kW
Pubs Three ph 20 kW exceptionally up to 120 kW
Garages/workshops Three Ph 30 kW Compressor Motors and Welders
Village stores/ small supermarkets Three Ph 20 kW
Milking parlours Three Ph 50 kW (normally Single Ph motors)
Sewage pumps Three Ph 20 kW - 150Kw Compressor Motors?
Small hotels/guest houses Three Ph 20 kW
Small businesses Three ph 20 kW
Residential care/nursing homes Three Ph most have lifts and all-day heating can be 100 kW.
Note that the above values are “after diversity” demands and apply directly to the design of mains cables. When
designing individual services or sizing transformers, further formulae are applied to adjust the diversity element
as defined in the appropriate sections of this manual.
Other industrial and commercials loads need to be established in conjunction with

the customer’s electrical consultant or contractor.
1.2.5 Residential Housing Developments
1.2.5.1 Connection Strategy for Residential Loads
All proposed industrial / commercial connections shall comply with Section 1.4.2 Connections
Strategy.
1.2.5.2 Voltage Regulation - Residential Housing
• Section 6 of this Manual, Low Voltage Network Design Calculations, detailes the method to be
used to calculate voltage regulation on LV mains and services. The ADMDs shown in section
1.2.4.1 for residentail loads shall be used.
• Voltage regulation from LV busbars of the HV/LV transformer to any service cut-out shall not
exceed:
• 6% of 230 volts when supplied from “Standard 11kV Feeders”.
• 4% of 230 volts when supplied from “Long 11kV Feeders”.

A ‘Long 11kV Feeder’ is defined as extending beyond the 15km radius of a Bulk Supply Point or
Primary Substation. See Section 4 Network Voltage Policy.
This voltage regulation may be apportioned between the main and service cable provided that:
• The maximum service cable voltage drop is limited to 2.5%. This is to prevent excessive
voltage appearing on the neutral/earth of PME supplies.
• On new non-electrically and direct acting electrically heated properties the overall loop
resistance is no greater than 0.24 ohms (comprising the sum of transformer, main and
service cable go/return resistances). The 0.24 ohm limit applies to ‘Greenfield’ housing
developments only. Where extensions are made from existing LV networks this limit may be
relaxed in accordance with the LV Earth Loop Impedance Policy section of this Manual. Also
note that the resistance and not the impedance value is referred to here – this is in
recognition that large single phase switched loads on residential developments are normally
resistive e.g. electric showers up to 7.2 kW.
• On new electrical storage heated properties the total space and water heating load must
not cause a step voltage change greater than 3% when switched by the metering
time/tele switch.
Maximum Loop Resistance for simultaneously switched heating
devices
Nameplate kW rating @230v 6 7 8 9 10 11 12 13 14
Max Loop Resistance for 3%
0.24 0.21 0.18 0.16 0.14 0.13 0.12 0.11 0.10
step voltage change
1.2.5.3 Mains Cables – Residential Housing
Mains cable loading shall be based on the arithmetic sum of the After Diversity Maximum Demands
(ADMDs) of the properties connected to the cable.
The maximum continuous summer rating of the cable in ducts shall be used.
The following Combined Neutral Earth (CNE) cables shall be used;

Size Application Continuous summer rating in
ducts
95mm2 Wavecon Note 1 201 amps 145 kVA Note 3

Branches, cul de sacs etc.
185mm2 Wavecon Main feeders and interconnectors 292 amps 210 kVA Note 3
300mm2 Wavecon Electric heating sites. Other sites where current rating, 382 amps 275 kVA Note 3
loop impedance and/or voltage drop criteria cannot be
Note 2
achieved with 185mm2
Note 1 - Small developments may only require 95mm2 main feeders and branches.
Note 2 - the neutral of 300mm2 Wavecon is only 185mm2 and will have a reduced effect on loop
impedance reduction.
Note 3 – kVA rating based on 240v nominal running voltage – 4.167 amps per kVA

1.2.5.4 Service Cable Loading - Residential Housing
The service cable design load shall be calculated according to the type of heating to be used. The
formulae for “Non-electric heating” and “No central heating installed” make allowance for the future
installation up to 6kW of storage heating and electric water heating.
For houses over four bedrooms increase the ADMD by 0.5kW per additional bedroom.
The minimum design load for any service shall be 12kW
Service cable loading shall be based on the formulae:
Non-electric heating (i.e. gas, oil solid fuel)

Service design load = (2x ADMD x N) +8 kW
= 12kW single house
= 16kW if looped to second house
ADMD is normally 2kW
N = 1 for single house. 2 if loop service is installed.
No central heating installed and gas not available
Service design load = (2x ADMD x N) +8 kW
= 14kW
N = 1 loop service is not permitted
Direct acting electric heating (i.e. panel, ceiling heaters)
Service design load = (2x ADMD x N) +8 kW + 50% of electric space heating load
(ignore water heating)
N = 1 loop service is not permitted
Electric storage heating
Service design load = 4kW + 100% of installed electric storage and 100% of water
heating load
Mixed electric storage heating and direct heating
Service design load = 4kW + 100% of installed electric space and 100% of water heating
load + 50% of direct acting heating.
1.2.5.5 Service Cable Voltage drop calculations - Residential Housing
For voltage drop calculations the current per kW shall be based on a typical network running voltage
of 240v which equates to 4.167 amps per kW.

1.2.5.6 Service cable / cut-out maximum thermal loading
The nameplate kW ratings of electrical appliances are based on a nominal voltage of 230v. At higher
voltages the kW output of heating appliances rises above the nameplate rating as they draw more
current.
i.e.
The current taken by 1kW @ 230v is 1000 ÷230 = 4.35 amps (I=W ÷ V)
This equates to a resistance of 230 ÷ 4.35 = 52.9 ohms (R=V ÷ I)
At 253 volts the current rises to 253 ÷ 52.9 = 4.8 amps (I=V ÷ R)
This equates to 253 x 4.8 = 1.2 kW at 253v (W=V x I)
Therefore an appliance with a nameplate rating of 1kW will become a load of 1.2kW when supplied at
the upper statutory limit of 253v. Whilst this is not critical when sizing mains cables and transformers
it must be taken into account when establishing the maximum load capability of a service cable and
cut-out combination. Furthermore, a 100 amp cut-out installed in an outdoor meter box has a reduced
rating of 90amps. This must not be exceeded either on a single installation or through the connection
of a looped service.
The following table over the page shows the maximum loads allowed on services.

Maximum thermal loading on services

Service Size Application Cable Cut-out rating Maximum connected
continuous in meter box load ADMD or Heating
rating in Name Plate Rating
ducts
25mm2 Street lighting / 94 amps N/A N/A
Hybrid furniture service
35mm2 Single phase house 115 amps 80amps = 16 4.0 kW ADMD
Hybrid service 24kW kW Load = 2AN+8
80A cut-out fuse = 2x4x1+8 = 16kW
Non- electric heating
35mm2 Single phase house 115 amps 80amps = 16 2.5 kW ADMD
Hybrid services 24kW kW Load = 2AN+8
Looped 80A cut-out fuse (90amps = 18 = 2x2.5x2+8 = 18 kW
Service Non- electric heating kW main and
loop)
35mm2 Single phase house 115 amps 90 amps = 18 12kW of Space Heating
Hybrid service 24kW kW Load = 2AN+8 +50%
100A cut-out fuse heating
Direct Acting Electric = 2x2x1+8 + 12/2=
heating 18kW
35mm2 Single phase house 115 amps 90 amps =18 14kW of Space & Water
Hybrid service 24kW kW Heating
100A cut-out fuse Load =
Electric storage 4kW + 100% of storage
heating or +50% of direct = 18kW
Mixed storage /
direct
35mm2 3 phase house service 100 amps 90 amps = 54 50kW of Space & Water
Wavecon 100A cut-out fuses 62kW kW Heating
Electric storage Load =
heating or 4kW + 100% of storage
Mixed storage / +50% of direct = 54kW
direct =18+18+14 of heating
+4 of domestic load.
Load to be split:
RØ YØ BØ
House 1 14+4 18 18
House 2 18 14+4 18
House 3 8 18 14+4
House 4 14+4 18 18
Etc.
NOTE the load ratings assume 4.8 amps per kW i.e. appliance rated at nominal 230v running at
253v.
Looped services may be used on non electric heated houses subject to step voltage change and loop impedance
considerations being satisfied.
Looped services shall not used for houses with electric heating.
Looped services from 3 phase cut-outs are not permitted.
NOTE. Single phase cut-outs are designed, tested and approved for the additional heat generated by the looped
cable within the cut-out cable box cover. Three phase cut-outs have not been designed for this duty and the heat
generated by the additional 3 phase conductors with the confined space of the cable box may cause thermal
failure.

1.2.5.7 Servicing method
Two methods of servicing houses are acceptable:
i) Sites with footpaths or service strips and cables laid to NJUG 7 - Service joints – up to
four services per joint.
ii) PPG33 sites - Underground Service Distribution Boxes of an approved design may
be used where appropriate. All mains and services to be ducted where run in
roadways.
Planning Guidance Note PPG3 sites normally have few/no footpaths or service strips requiring mains
ands services to be laid in the roadway.
1.2.5.8 Fusing - Residential Housing
Non-electric heating
House service cut-outs shall be fused at 80 amps
Feeders supplying non-electric heating residential developments shall be fused at a maximum

of 315 amps to enable cables lengths to be maximised whilst providing protection within the
LV Earth Loop Impedance Policy.
Note – on non-electric heating developments loop impedance will normally be the limiting factor
on feeder length rather than volt drop or load current.
Electric heating
House service cut-outs shall be fused at 100 amps
185mm2 Wavecon feeders supplying electric heating sites shall be fused at 315 amps
300mm2 Wavecon feeders supplying electric heating sites shall be fused at 400 amps.
Note – on electric heating developments, voltage drop and/or load current will normally limit
feeding distances rather than loop impedance criteria.

1.2.6 Supplies to Flats
1.2.6.1 Voltage Regulation - Flats
Section 6 of this Manual, Low Voltage Network Design Calculations, detailes the method to be
used to calculate voltage regulation on LV mains and services.
Voltage regulation from LV bus-bars of the HV/LV transformer to any cut-out at the central
metering position shall not exceed:
Primary Substation. See Section 4 Network Voltage Policy 6% of 230 volts
provided that:
The overall loop resistance is no greater than 0.24 ohms at the service position at the head
of the most remote rising main (comprising the sum of transformer, main and service
cable go/return impedances).
• On electrical storage heated properties the total space and water heating load must not
cause a step voltage change greater than 3% when switched by the metering time/tele
switch.
Maximum Loop Resistance for simultaneously switched heating devices
Nameplate kW rating
6 7 8 9 10 11 12 13 14
@230v
Max Loop Resistance for

0.24 0.21 0.18 0.16 0.14 0.13 0.12 0.11 0.10
3% step voltage change
1.2.6.2 Service Cable Loading for Flats
The service cable design load shall be calculated according to the type of heating to be used. The
formulae for “Non-electric heating” and “No central heating installed” make allowance for the
future installation up to 6kW of storage heating and electric water heating.
The minimum design load for any service shall be 12kW

Non electric heating (i.e. gas, oil, solid fuel)

ADMD shall be 2 kW
Service cable design load = (ADMD x number of flats) +8 kW
e.g.
2 flats = 2kW x 2+ 8kW =12kW
26 flats = 2kW x 26 + 8kW =60kW
No central heating installed and gas not available

ADMD shall be 3 kW
Service cable design load = (ADMD x number of flats) +8 kW
e.g.
2 flats = (3kW x 2)+ 8kW =14kW
26 flats = (3kW x 26) + 8kW =86kW
Direct acting electric heating (i.e. panel, ceiling heaters)

ADMD shall be 2 kW + 50% of installed electric space heating load (ignore water heating)
Service cable design load = (ADMD x number of flats) +8 kW.
e.g.
2 flats with 8kW of panel heaters each = (2kW + 8kW/2) x 2) +8kW =20kW
26 flats with 10kW of panel heaters each = (2kW +10kW/2) x 26) + 8kW =190kW
Electric storage heating

ADMD shall be 100% of installed electric storage and water heating load
Service cable design load = (ADMD x number of flats) + 4kW
e.g.
2 flats with 8kW of storage & water heating = (8kW x 2) +4kW =20kW
26 flats with 10kW of storage & water heating

= (10kW x 26) +4kW =264kW
Mixed electric storage heating and direct heating

ADMD shall be 100% of installed electric space and water heating load plus 50% of direct
acting heating.
Service cable design load = (ADMD x number of flats) + 4kW
e.g.
2 flats with 6kW of storage & water heating plus 4kW of panel heaters
= (6kW + 4kW/2) x 2 +4kW =20kW
26 flats with 6kW of storage & water heating plus 4kW of panel heaters
= (6kW + 4kW/2) x 26 +4kW =212kW

1.2.6.3 Service cable / cut-out maximum thermal loading - Flats
Continuous summer cable ratings in ducts shall be used for services to flats;
The nameplate kW ratings of electrical appliances are based on a nominal voltage of 230v but
may operate at up to 253v. As explained in section 1.2.5.3, a 1kW appliance nameplate rating
equates to 1.2kW of load at 253v. The following service loadings are based on a current of 4.8
amps per kW:
35mm2 Hybrid – 90* Amps ≈ 18 kW

35mm2 Wavecon - 90* Amps ≈ 54 kW
95mm2 Wavecon - 201 Amps ≈ 125 kW
Note *
100 amp cut-out is limited to 90 amps in meter box or indoor cabinet
Maximum of 2 customers per phase on 35mm2 cable
These are the minimum size service cables to be used according to the service cable design
load. Sizes may have to be increased for voltage drop and loop impedance criteria depending
on the location of the substation and size of mains cable.
1.2.6.4 Properties Converted into Flats
Converting existing properties into flats presents a number of service design challenges. This
section of the Manual provides approved solutions for some commonly encountered
circumstances. Other situations (e.g. space over shops converted to flats) should be
approached using good engineering practice employing the spirit and intent of the approved
solutions to provide a safe installation.
Two storey buildings
Two outdoor service boxes installed at ground level each to supply one floor only. The
consumer’s switchgear must be within 3 metres of each service position. This requires that
the first floor consumer’s switchgear be directly above the meter box.
This method is only suitable where
• Space is available for meter boxes (surfaced or cavity)
• The building is not listed or in a conservation area where the Local Planning Authority
does not allow outdoor boxes.
Three or more floors
When the building has more than two floors or is covered by conditions listed above an
internal location can be used subject to:
• The internal location is a maximum 3 metres into the building from the outside front
or side wall of the property.
• It is situated on a ground floor within common area adjacent to front door.
• The service cable is protected in black PVC duct clearly marked “Electric Cable Duct”
and laid under a solid floor. Cables must not be routed under suspended floors
The meter position accommodation must:
• be for electrical equipment only

• be of adequate size for installation of Central Networks cut-out equipment.
• be of adequate size for installation of metering equipment
• be formed in brick or block work only NOTE 1. Plasterboard partitions are not acceptable
• have full width opening doors for access to equipment
• be fire resistant to comply with fire regulations.
• not contain other utility service equipment (e.g. water or gas)
• not be used as a storage room
• not be under stairs where headroom is less than 2 metres
NOTE 1 – The provision of brick or block may not be possible in timber framed buildings. The
wall behind the service/metering equipment must be fitted with a steel sheet (min 1mm thick)
bonded to the cut-out PME terminal. This is to protect persons drilling through the wall from
electric shock.
Cut-out arrangements shall be:

• 1 flat - all types of heating– single phase 100 amp cut-out. An additional bus bar
connected 100 amp cut-out may be used for the landlord’s supply if required. One
35mm2 hybrid cable.
• 2 flats - non-electric heating – single phase 100 amp cut-out plus loop to second cut-
out. An additional bus bar connected 100 amp cut-out may be used for the landlord’s
supply if required. One 35mm2 hybrid cable.
• 2 flats - electric heating – single phase 100 amp cut-out per flat. An additional bus bar
connected 100 amp cut-out may be used for the landlord’s supply if required. Two
35mm2 hybrid cables.

• 3 flats - all types of heating– three phase 100 amp cut-out - An additional bus bar
35mm2 Wavecon service cable.
• 4 to 10 flats (or 3 to 9 flats plus landlords supply) 10 way 100 amp cut-out - 95mm2
Wavecon service cable.
Note – it is forbidden to loop from 3 phase cut-outs as they have no certification for the
additional thermal duty.
1.2.6.5 Purpose Built Flats
Services shall be installed at group metering positions and the developer shall provide and
install lateral wiring to each flat complying with the current edition of BS7671 “Requirements
for Electrical Installations”. These laterals shall remain the property of the building landlord or
individual flat owners as appropriate.
The position and number of service positions shall depend on the distance from each flat such
that the lateral wiring can be installed within the design requirements of the current edition
of BS7671 “Requirements for Electrical Installations”.
In order of preference the number / position of services shall be:
1. A single position on the ground floor.

2. A single position on the ground floor plus a rising main to some or all floors.
3. Several positions on the ground floor plus a rising mains to some or all floors.
PME bonding to the building metalwork shall be made at the central metering position /
rising main only. This is to limit the circulation of neutral current around the building metalwork.
See Central Networks’ Earthing Manual Section E6 for more information
Where a landlord’s supply in excess of 70 kVA is required, e.g. for lifts, then a CT metered
Industrial / Commercial supply should be provided for this purpose.
Group meter positions must:
• be for electrical equipment only

• be of adequate size for installation of Central Networks’ cut-out equipment.
• be of adequate size for installation of metering equipment
• be formed in brick or block work only NOTE 1. Plasterboard partitions are not acceptable
• have full width opening doors for access to equipment
• be fire resistant to comply with fire regulations.
• not contain other utility service equipment (e.g. water or gas)
• not be used as a storage room
• not be under stairs where headroom is less than 2 metres
• not be in below ground level i.e. basements

NOTE 1 – The provision of brick or block may not be possible in timber framed buildings. The
wall behind the service/metering equipment must be fitted with a steel sheet (min 1mm thick)
bonded to the cut-out PME terminal. This is to protect persons drilling through the wall from
electric shock.
Cut-out arrangements shall be:
• 1 flat - all types of heating– single phase 100 amp cut-out. An additional bus bar
35mm2 hybrid cable.
• 2 flats - non-electric heating – single phase 100 amp cut-out plus loop to second cut-
out. An additional bus bar connected 100 amp cut-out may be used for the landlord’s
supply if required. One 35mm2 hybrid incoming cable plus one 35mm2 hybrid cable
loop.
• 2 flats - electric heating – single phase 100 amp cut-out per flat. An additional bus bar
connected 100 amp cut-out may be used for the landlord’s supply if required. Two
35mm2 hybrid cables.
• 3 flats - all types of heating– three phase 100 amp cut-out - An additional bus bar
35mm2 Wavecon service cable.
• 4 to 10 flats (or 3 to 9 flats plus landlords supply) 10 way 100 amp cut-out - 95mm2
Wavecon service cable.
• 10 to 24 flats (or 9 to 23 flats plus landlord’s supply) – 400 amp or 600 amp cut-out. 24
way 100amp multi-way cut-out mounted immediately above the cut-out or on the
appropriate floor of the rising main. 185mm2 or 300mm2 Wavecon cable dependant on
service cable design load.
• 25 to 30 flats (or 24 to 29 flats plus landlord’s supply) – 400 amp or 600 amp cut-out. 30
way 100amp multi-way cut-out mounted immediately above the cut-out or on the
appropriate floor of the rising main. 185mm2 or 300mm2 Wavecon cable dependant on
service cable design load.
• Above 30 flats – multiples of the above arrangements.
Notes
It is forbidden to loop from 3 phase cut-outs as they have no certification for the
additional thermal duty.
10 way 100 amp cut-outs fit cables up to 95 mm2
400 amp cut-outs fit cables up to 185mm2
600 amp cut-outs are required for 300mm2 cable

Rising Mains
• The incoming cable supplying a rising main shall be terminated in a cut-out

immediately inside the building. The rising main shall be taken from the top side of this cut-
out to the metering position floor. The rising mains shall not be taken directly from the
substation to the metering position floor. Note that this is to enable the rising main to be fused
in accordance with section 1.2.6.5 in order to comply with the current edition of BS7671
“Requirements for Electrical Installations”. Also it provides the facility for personnel not
authorised to enter substations to isolate the rinsing mains.
• The incoming cable shall be terminated in an insulated cut-out in-line with any metal
clad distribution board. It is forbidden to terminate a main cable emanating directly from the
substation into a metal clad multi-way distribution board. The provision of the insulated cut-out
provides the facility to make the metal clad distribution board dead prior to work. It also provides
a suitable termination for the Wavecon cable and a neutral connection for PME bonding.
• The developer is responsible for providing and installing suitable cable trays of 450mm
or 600mm width for the full height of the risers. Cable tray sections must be connected using
the manufacturer’s specified couplings and properly bonded and earthed. (Cables trays shall
be galvanised Admiralty Pattern or equivalent)
Number of cables per tray
Cable size Cable dia 450mm tray 600mm tray
35 mm2 Hybrid 20 mm 12 15
2
35 mm Wavecon 31 mm 8 10
2
95 mm Wavecon 35 mm 7 9
185 mm2 Wavecon 47 mm 5 7
300 mm2 Wavecon 56 mm 3 6
Cables shall be spaced one diameter apart to avoid de-rating.
• Nylon cable zip ties are to be used for fixing cables to trays. A cable cleat is to be
installed below each cut out for added support.
• Rising voids and cable trays must be positioned on each floor to open out into
common areas which provide 24-hour safe and unrestricted access to Central Networks
personnel. Each location should be provided with fully opening doors.
• Central Networks standard Wavecon and Hybrid cables shall be used for rising mains
unless local Fire and/or Building Regulations require additional measures such as low smoke
emission cables. It is responsibility of the developer to seek the advice of the Local Fire Officer.
However, the incoming service shall be standard Wavecon and Hybrid cable.

• The PME terminals of all cut-outs in a rising main shall be bonded together in
accordance with the Central Networks Earthing Manual Section E6 Protective Multiple
Earthing part 3.2.10.
Size of smallest cable Neutral size Size of Combined

in the Rising Main. Neutral/Earth Bonding
Cu
Conductor
(Colour green/yellow + blue
marker tape at each end)
35mm2 Wavecon 22 mm2 cu 25mm2 cu
• Where there are two or more rising mains in the same rising void their neutrals shall
be bonded together via their PME terminals with the appropriate sized cable which shall be
identified as a combine neutral earth conductor. i.e. green/yellow cable with blue marker tape
at each end. (see BS7671 clause 514-04-03)
• The PME terminal of a single cut-out at a group metering point shall be connected to
an earth rod outside the building via 16mm2 conductor (coloured green/yellow only).
• The developer’s electrical contractor is responsible for the installation of private

switchgear and sub mains from the common electrical riser to each dwelling.
• The landlord is responsible for providing access for occupiers to the equipment
installed within the common risers for maintenance or to check meter readings.
• The developer is responsible for ensuring that the design and installation of the rising
main void meets local fire regulations including any smoke or fire segregation provisions
between floors.

Private Laterals to flats
I Isolator
M
Meter
C/O
100A Cut-out with Links
Hybrid cable
24 /32 way
Distribution
Board 100 Amp Fuses
400 / 600 Amp
Cut-out with Links
Rising Main
400 / 600 Amp RISING MAIN VOID

Cut-out with Fuses RISING
Private MAIN
Laterals VOID
Service to flats
Private
Laterals Private
I I I I
to flats M M M M
I I I I
Laterals
M M M M
C/O C/O C/O C/O C/O C/O C/O C/O to flats
C/O C/O C/O C/O C/O C/O C/O C/O

M M M M M M M M I I I
Private I I I I I I I I
M M M
Laterals C/O C/O C/O
to flats
Private Private
Laterals Laterals
to flats I I I I I I I I
to flats Private
M M M M M M M M Laterals
C/O C/O C/O C/O C/O C/O C/O C/O to flats
M M M M M M M M I I I
I I I I I I I I
M M M
Private C/O C/O C/O
Laterals
to flats
Private WAVECON
Laterals CABLES
I I I I
to flats I I I I I I I I M M M M
M M M M M M M M
C/O C/O C/O C/O
C/O C/O C/O C/O C/O C/O C/O C/O PME
Bonding HYBRID
CABLES
M M M M M M M M E
I I I I I I I I
Private 10 way
Laterals
to flats PME
Earth
PME Bonding
2 OR MORE CUT-OUTS
BOND NEUTRALS TOGETHER PME EARTH ROD
EARTH ROD NOT REQUIRED ONLY REQUIRED FOR
SINGLE CUT-OUT
Typical rising main arrangements using a multi-way distribution board on each floor or 10 way Cur-out
on ground floor with Hybrid cables to upper floor(s).

1.2.6.6 Fusing Feeders, Services & Cut-outs in Flats
Substation feeders supplying flats shall be fused at:

• 315 amps – feeders up to 185mm2
• 400 amps – feeders of 300mm2 NB transformer must be 500kVA or greater
• 630 amps – feeders of 300mm2 and to grade with 400 amp fuse in cut-outs at high
load sites. NB transformer must be 800kVA or greater
Feeder length will be limited by the loop impedance required to clear the substation fuse in
100 seconds, step voltage change criteria and voltage drop.
The first cut-out within the building shall be fused to clear earth faults on any Central
Networks owned equipment inside the building within 5 seconds in accordance with the Earth
Loop Impedance Policy section of this Manual and the current edition of BS7671
“Requirements for Electrical Installations”.
This Table is a guide to fuse sizes and loop impedance criteria

Max loop
Max loop impedance at multi-
Substation impedance at Cut-out fuse
way cut-out at remote end of
Fuse first cut-out
any rising main (5 sec)
(100 sec)
Dominant loop impedance shown bold
315 Amps 0.24 ohms 100 Amps 0.38 ohms
Substation 3 or 10 way
MAIN Cut-out
100 amp fuses
MAIN Distribution
Board
100 amp fuses
MAIN Distribution
Board
LINKS 100 amp fuses
BUILDING

1.2.6.7 Emergency Fire Fighting Supplies – BS5588-5:2004
British Standard 5588-5:2004 “Fire Precautions in the design, construction and use of buildings – Part 5
Access and facilities for fire-fighting”
This standard requires that certain high rise buildings have dedicated lifts and stairs for use by fire
fighters to quickly reach floors affected by fire. These lift shafts and stair wells may also be force
ventilated to the clear smoke in a controlled manner. During the course of a fire the normal electricity
supply may fail either as a deliberate act to make internal services safe or by the effect of the fire
itself or water used in fire fighting.
Section 14 “Electrical Services” in BS5588-5 requires that the internal circuits supplying the lifts and
ventilation fans are specially protected against fire and water. Also a secondary supply must be
available which is independent of the normal supply. The customer’s switchgear is normally arranged
to run these circuits from the normal supply with auto changeover equipment to select the secondary
supply when required.
The secondary supply may be a standby generator or secondary supply from the local distribution
network. The previous version of this standard (BS 5588-5:1991) recommended the additional supply
be taken from a different 11kV (or 6.6kV) circuit to the normal supply. This has now been rescinded in
BS5588-5:2004.
The standard now provides for the secondary supply to be:
1. Standby generator capable of operating in fire conditions.
This option is totally independent of faults that could be caused to the Central Networks
system by the fire itself.
2. Where regular maintenance of a standby generator would not be expected the main and
secondary supplies may come from the same external substation via two separate intakes
and then by separate routes to the fire-fighting shaft.
Central Networks’ interpretation is that an external substation should be located such

that it would not be affected by the fire. A substation incorporated into the building
cannot be relied upon to maintain the secondary supply. Taking an LV supply from an
adjacent substation may not be sufficient if both substations are on the same HV feeder
as they would be shut down if the fire caused the HV feeder to trip. Central Networks will
not normally be able to provide and guarantee a secondary supply from an independent
HV feeder.
3. If protection against the occurrence of a fault on the high voltage system (unconnected with
the fire) is required by the occupier then a standby generator or an independent power
supply is required.
BS5588-5:2004 does not define what form an independent power supply should take.
Central Networks’ interpretation is that it would not be provided by Central Networks.

Central Networks cannot guarantee the availability of a secondary supply under all circumstances. It is
the customer’s responsibility to ensure compliance with BS5588-5:2004 and should seek the advice of
the local fire service and /or building control officer about the suitability of any proposed secondary
supply.
1.2.7 In-fill Developments

In-fill developments shall be designed using the same principles as Residential Housing
developments.
When calculating voltage drop, the ADMD of existing properties connected to the mains shall
normally be assumed to be 2kVA unless the existence of electric heating is suspected. Where
necessary the actual ADMD should be ascertained. e.g. by algorithms based on meter readings
or by taking the most recent winter maximum demand reading of the local transformer and
dividing it by the number of customers connected. This value must be multiplied by 1.33 to be
applicable the reduced diversity on individual LV mains. Where the result is greater than 2kW
this revised ADMD shall be used. If the local transformer does not have maximum demand
indictors the ADMD of a similar locality where the maximum demand is known may be used.
Some older networks with a low housing density (e.g. suburbs and villages) will have mains that
can accept additional load and remain within voltage limits but will exceed the loop impedance
criteria set for new housing.
Refer to the LV Earth Loop Impedance Policy section of this Manual for further guidance and
explanation.
The salient points of the LV Earth Loop Impedance Policy are summarised below together with
guidance on their application.
Where the substation fuse clearance time at the cut-out cannot be achieved then in the
following order of preference:
1. Consider if the substation feeder fuse can be reduced in size whilst ensuring that
the maximum feeder demand is within the proposed fuse rating.
2. Consider installing a 2 way link box or pole mounted fuses to sub-fuse the
network extension if the substation feeder fuse size cannot be reduced.
3. If 1 and 2 above are not reasonably practicable then ensure all service
terminations are installed in out-door meter boxes.
The step voltage change loop resistance criteria of 0.24 ohms for new developments is relaxed
for in-fill developments is relaxed from the cut-out position back to the point of common
coupling with other customers. Any further relaxation should be agreed with the customer and
the balance between cost and quality of supply made clear particularly where large electric
showers or storage heating loads could be installed.

Note that loop resistance is referred to here because the 3% step voltage criteria is based on a 7.2
kW shower load which is mainly resistive. Fault clearance criteria are calculated using impedance
values due to the power factor of the fault current. The resistance of an LV network is lower than
it’s impedance.
Where the loop impedance at the cut-out exceeds 0.35 ohms the designer of the premise’s
internal wiring shall be advised that this value exceeds the typical maximum values published in
Engineering Recommendation G23/1
However, the absolute maximum Loop impedance provided at new properties shall be 0.52
ohms and where necessary networks shall be reinforced to attain this value.
Central Networks may be prepared to fund some or all of the cost of any reinforcement to meet
loop impedance criteria.
1.2.8 Physical Position of Services
1.2.8.1 Residential Housing Underground Cable Service positions
The service cable shall be run from the main to the meter box in at straight line perpendicular to
the main wherever reasonably practicable. Service cables shall be installed in ducts and
provided with warning tape. (Refer to Cable, Cable Laying & Accessories Manual for details)
This requirement is to ensure that the service cable follows a predicable route where occupiers
would anticipate their presence during normal gardening and DIY activities.
1.2.8.2 Residential Housing Overhead Line Cable Service positions
Overhead services shall not normally be provided to new houses except where expressly
requested by the owner.
When renewing existing overhead services:
Central Networks West – copper concentric CNE service cable run as an aerial span and
continued clipped to the wall to the service box
Central Networks East - Aerial Bundled Conductor (ABC) shall be used for the overhead span
and anchored at high level on the building. The following methods of cabling from the aerial
span to the service position are acceptable:
1. Continue the ABC on the wall to the meter box or existing internal meter position.
Overhead Line Manuals Volume 1 Section 6 Drawings LV215 to LV219 details the
installation methods and defines the zone that needs additional protection.
2. Convert the ABC to Hybrid cable and run on the wall to the service position as above.
Because the Hybrid cable has an earthed metallic screen there is no requirement to
provide the additional protection shown in drawing LV215. However, a cable guard is
required where the cable is within 2.4m of the ground.

3. Entry to the meter box may be directly from the wall generally to drawing LV219 but
using cut and mitred cable guards instead of tubing.
4. Alternatively the cable may be taken down to ground level, ducted underground and
terminated into the meter box in the same manner as a totally underground service.
This method may be less visually intrusive than the above methods.
1.2.8.3 Outdoor meter boxes
New and re-serviced domestic housing service terminations shall be in outdoor meter boxes
mounted into an external wall on the front or side elevation of the property in front of any
fence or gates to allow unrestricted access to Central Networks staff from the public
environment.
Single phase service cables shall fixed to the surface of the wall and enter the meter box from
the underside and be capped with a cable guard or contained within in a pipe. Single phase
service cables shall not be run inside the wall cavity.
Three phase service cables shall enter the meter box via a pipe installed inside the cavity. NOTE
– The present design of large meter box does not allow surface entry. Installation via the wall
cavity will be discontinued once the design of three phase meter box has been modified to enable
external entry.
The meter tails to the consumer unit shall not exceed 3 metres in length.
The meter operator may install a 100 amp isolator in these meter tails within the meter box to
provide isolation facilities for the customer’s consumer unit.
The following positions are not permitted:
• at the rear of the property
• at the side of the property behind fences or gates
• within a coal, dustbin or refuse store, garage, porch.
• Under windows unless minimum installation height can be achieved.
The box shall be installed at a:
• maximum height of 1800mm from ground level to the top of the box – to enable access
without ladders or steps.
• minimum height of 450mm from ground level to the bottom of the box – to reduce the risk
of water entering the box as a result of flooding or fire fighting activities.
These requirements are necessary for the following reasons:
• To reduce risks to occupiers to the minium level
o Cables, fuses and meters generate heat during normal operation which must be
properly dissipated to ensure the equipment does not overheat and catch fire.
Outdoor meter boxes designed and manufactured to ENATS 12-03 have thermal
characteristics that enable the service termination equipment to be operated

safely. The thermal performance of indoor service positions may be compromised

by occupiers storing items close to the equipment resulting in overheating.
o Service cables are directly connected to high energy ditribution mains protected
by substation fuses up to 630 amp rating and designed to have a maximum fault
clearance time of 100 seconds. The protection of fixed wiring complying with
BS7671 inside buildings ensures fault clearance times of less than 5 seconds. By
placing the service cut-out fuse in a meter box outside the building Central
Networks is able to provide the same level of protection inside the building up to
the consumer unit as afforded by BS7671.
o An outdoor service position ensures that at the time of installation and during
subsequent building work and occupation the risk of tradesmen or occupiers
being injured by live high energy equipment is as low as is reasonably
practicable.
• To provide access to service equipment
o To enable regular meter reading by the meter operator to reduce the incidence of
estimated billing
o To reduce the hazards to meter operatives. Of all personnel involved in electricity
distribution, meter operatives have the highest injury rate. The injuries that occur
whilst inside private dwellings include slips trip & falls, dog attacks, assault by
occupiers. Removing the need to enter dwellings reduces these hazards.
o For testing of supplies to assist with effective network fault location
o For main fuse removal to prevent embedded generation (e.g. photo voltaic,
sterling engine combined heat & power gas boilers etc.) back-feeding the
network during dead working. This emerging technology is now available for new
build and retro fitting. The long term reliability of the loss of supply protection of
these units is not yet proven.
• Service cable fixed to surface of wall
o To prevent overheating of the cable due to cavity wall insulation
o To ensure that cable/satellite TV and telecoms installers do not drill through a
hidden service cable – risk of electrocution. Technology and product changes now
make this a frequent activity undertaken by tradesmen and DIY enthusiasts .
• Service not installed in coal, dustbin or refuse store, garage, porch.
o To prevent damage to service equipment and resulting danger from accidental
contact during normal domestic activities.
o To prevent damage and resulting danger from accidental contact by vehicles
and/or materials stored in garages
o To prevent danger to Fire Fighters from damaged service equipment where
dustbin / refuse stores are set alight accidentally or maliciously.

The following meter boxes to ENATS 12-03 are approved for use in Central Networks:
From left to right:
1. Small flush fitting box

2. Slim-line flush fitting box
3. Small surface mount box
4. Large flush fitting box (not shown)
1. Small flush fitting box.
Used for single phase services
This is designed to fit into the outer leaf of a

cavity wall in a space 7 bricks high by 1½
bricks wide. The overall outside dimensions
are approximately 600mm high by 410mm
wide. This box will accommodate a single
phase cut-out, time-switch, two rate meter
and isolator.
Knock-outs are provided for service cable

entry via the cavity or the wall surface. Cavity
entry is not permitted by Central Networks.
2. Slim-line flush fitting box.

cavity wall in a space 9 bricks high by 1 brick
wide. The overall outside dimensions are
approximately 830 mm high by 275 mm wide.
This box will only accommodate a single phase
cut-out and a single meter with built in tele-
switch and isolator.
When opting for slim-line boxes the developer

must ensure that the intended meter operator
can provide this type of meter.
Knock-outs are provided for service cable entry by the

cavity or the wall surface. Cavity entry is not permitted
by Central Networks.

3. Small surface mounted box.
Similar to item 1 but designed to fit on the

surface of walls without a cavity. The overall
outside dimensions are approximately 600
mm high by 400mm wide. The box projects
about 235mm from the surface of the wall.
This box will accommodate a single phase cut-
out, time-switch, two rate meter and isolator.
Knock-outs are provided for service cable entry via the

wall surface only.
No photo available 4. Large flush fitting box.

Similar to small flush fitting box
Used for three phase services

cavity wall in a space 9 bricks high by 2 bricks
wide. The overall outside dimensions are 790
mm high by 565 mm wide. This box will
accommodate a three phase cut-out, time-
switch, two rate meter and isolator.
Knock-outs are provided for service cable entry via the

cavity only. For the time being cavity entry is permitted
by Central Networks until such time as the standard
design is altered to allow wall surface entry.
Indoor meter positions

Where the developer can demonstrate that the Local Planning Authority will not permit an
outdoor meter box by virtue of the building being listed or in a conservation area a service
position inside the house may be proposed. Central Networks’ acceptance of any proposal will
be subject to the following conditions:
1. The design of the indoor service position shall ensure that at the time of
installation and during subsequent building work and occupation the risk of
tradesmen or occupiers being injured by live high energy equipment is as low as is
reasonably practicable.
2. The service cable shall be routed inside the building by the shortest and most
direct route possible and shall be ducted. The internal end of the duct shall be
sealed immediately after cable installation to prevent the ingress of natural gas.
3. The service equipment shall be installed on a brick or block-work wall. Where
reasonably practicable this will be an external wall.
4. In timber framed buildings a suitable brick or block-work wall may not be
available. In these cases a steel sheet (min 1mm thick) shall be fixed behind the
service cable, cut-out and meter which shall be earthed. This it to protect persons
drilling through the wall from electric shock.
5. The developer shall provide accommodation for the cable, cut-out and meter in a
meter cabinet extending from floor level. The free air space inside the cabinet

should equal or exceed that of an out-door meter box and ventilation shall be
provided to enable the heat generated by the service cable, cut-out fuse and
meter to be safely dispated. Local fire regulations may require heat run tests to be
carried out on the proposed design.
6. The service cable above the floor must not be obscured by panelling of any type or
routed behind the backs of any cupboards or fitments. The cable may be covered
with a cable guard or capping which is easily identified as cable protection. There
have been fatal accidents where persons have drilled through panelling into live
service cables.
7. The standard fibre glass outdoor meter box shall not be used indoors as it does
not comply with the appropriate British Standards for fire resistance and fume
emissions. The developers shall provide a meter box that complies with fire
regulations and any local by-laws.
The following service positions are not permitted

• Inside a coal store, dust bin or refuse store, cellar, lavatory, kitchen or bathroom.
• Over doorways.
• On partition wall made of plasterboard, drywall or similar material.
• Under stairs where headroom is less than 2m
• Any location where it is not possible to comply with the current edition of BS7671
Existing properties
Where existing underground and overhead service positions are altered at the request of the
customer the new service shall be subject to the same provisions as for new developments.
Where existing underground and overhead services are replaced at Central Networks
instigation, e.g. due to poor condition, it will not normally be reasonably practicable to alter
the service position to fully comply with the provisions for new developments. Existing meter
positions may be retained unless a more favourable position can be established at no
additional cost or disturbance
1.2.8.4 Industrial and Commercial Service Positions
Service up to 100 Amps
Industrial and commercial service terminations up to 100 Amps per phase shall normally be in
an outdoor meter box mounted into an external wall on the front or side elevation of the
property in front of any fence or gates where practicable. Where the installation of a meter box

is not feasible, e.g. shop frontage, then the service may be installed internally as per the
provisions in section 1.2.8.1
The present design of large meter box does not facilitate cable entry from the outside surface of
the wall. Until such time as the large box is re-designed to facilitate surface entry it is
permissible to install 3 phase service cables via the wall cavity.
The meter tails to the consumer unit shall not exceed 3 metres in length.
The meter box shall not be accommodated within a coal, dustbin or refuse store, garage or
porch.
Services over 100 Amps
The developer shall provide accommodation for the cable, cut-out and meter in a dedicated
meter room / cupboard. The service cable above floor must not be obscured by panelling of any
type or routed behind the backs of any cupboards or fitments.
The design of the indoor service position shall ensure that at the time of installation and during
subsequent building work and occupation the risk of tradesmen or occupiers coming into
accidental contact with live equipment is as low as reasonably practical.
The service cable shall be routed inside the building by the shortest route and most direct route
possible and shall be ducted. The duct shall be terminated the service position either in a slow
elbow bend or a pulling pit. The arrangement shall enable a cable of 56mm diameter with a
620mm minimum bending radius to be installed (i.e. up to 300mm2 Wavecon). The internal end
of the duct shall be sealed immediately after cable installation.
The standard fibre glass outdoor meter box shall not be used indoors as it does not comply
with the appropriate British Standards for fire resistance and fume emissions. The developers
shall provide a meter box that complies with fire regulations and any local by-laws.
The following service positions are not permitted

• Inside a coal store, dust bin or refuse store, cellar, lavatory, kitchen, bathroom.
• Over doorways.
• On partition wall made of plasterboard, drywall or similar material.
• Under stairs where headroom is less than 2m
• Any location where it is not possible to comply with the current edition of BS7671
1.2.8.5 Service Positions in Flats
Guidance on service positions and rising mains is provided in the Supplies to Flats section.

1.2.9 Street Lighting / Furniture Supplies

Street lighting columns and other street furniture shall be serviced using :
Central Networks West - 16mm2 or 25mm2 copper concentric cable
Central Networks East - 25mm2 Hybrid cable.
The earth loop impedance criteria specified in Section 5 LV Earth Loop Impedance Policy, of this
Manual shall be observed to ensure that phase to neutral/earth faults are cleared effectively. This is to
prevent PME earthed lamp columns and cabinets remaining alive for prolonged periods. Section 5.3.4
Street Lighting Services provides information on fuse sizes and loop impedance criteria.
1.2.9.1 New developments
Street lighting cables up to 20m long may be directly jointed onto the main without
the need for a loop impedance calculation. The normal LV design procedures will
automatically ensure that loop impedance criteria are met.
Street lighting cables over 20m long and those looped to other lamps / furniture must
be subject to a loop impedance study and sub fusing shall be provided where
necessary.
1.2.9.2 Existing networks
New street lighting cables up to 5m long may be directly jointed onto existing mains
without the need for loop impedance calculation.
Street lighting cables over 5m long and those looped to other lamps / furniture must
be subject to a loop impedance study and sub fusing shall be provided where
necessary.
1.2.9.3 Complex / multiple connections
Road junctions with traffic lights, carriageway lighting etc. A single service shall be
provided to a lighting authority owned distribution pillar to enable them to carry out
their own distribution to the street furniture. Multiple service connections from the LV
network to separate items of equipment, which themselves may be interconnected by
light authority cables, become complex to design and control. There is scope for
accidental parallels to be made between DNO mains and substations which could result
in danger.
Runs of more than 2 looped street lighting columns (e.g. carriageway lighting) shall
not be jointed directly to LV mains. A single service shall be provided to a lighting
authority owned distribution pillar to enable them to carry out their own distribution
to the street furniture

1.2.10 Transformers Supplying LV Networks
1.2.10.1 Type & Location
Substations and Padmounts shall be positioned in accordance with Section 1.4.6 Physical Siting
of 11kV Substations and Equipment.
Note that Padmounts cannot be used in Central Networks West at present
LV networks on residential development shall be supplied from:
A Ground Mounted Substation(s) provided specifically for the development and sited
near the load centre to facilitate practical and economic LV network layout. The
developer shall provide a substation site measuring 4m x 4m. (see section 1.4.6.4
Substation Legal Requirements).
o The substation shall normally be housed in a Glass Reinforced Plastic (GRP)

housing with approved explosion venting provisions.
o The developer may elect to accommodate the substation in a brick built

substation to match the surrounding properties. A tiled roof is not permitted on
these buildings because they cannot be readily removed for equipment change-
outs and they have no controlled explosion venting characteristics. The Central
Networks Plant Specification Manual specifies the approved types of roofs and
doors that may be used. Roof options include GRP hipped roofs designed to
match various types of tiles.
o Substations shall not be built into or onto residential properties or garages.

Substations shall be free standing to prevent the transmission of noise/vibration
into adjacent properties and to avoid collateral damage in the event of
explosion.
All substation housings/buildings must have suitable explosion venting built into the
design – normally in the form of a lightweight tethered roof.
An LV network extension from an existing Ground Mounted Substation, Padmount or

Pole Transformer of sufficient capacity and where voltage drop and earth loop
impedance criteria can be met. This option applies mainly to small infill developments.
An LV network extension from a new Ground Mounted Substation, or where no other

option exists, a Padmount or Pole Transformer located off the site and used to reinforce
the local LV network. Where a 4m x 4m site cannot be obtained locally for a Ground
Mounted Substation a smaller site may be used to accommodate a Padmount
Transformer. This option applies mainly to small infill developments.
A Pole Mounted Transformer installed on an 11kV overhead line. Where the 11kV
overhead line is extended towards the development the pole transformer site shall be
selected in accordance with a risk assessment procedure taking into account potential
danger to the public and the present and projected use of the land. If a safe location
cannot be established then an 11kV underground cable and a Ground Mounted

Substation or Padmount shall be used.
Free-standing Pole Transformers shall not be used either within or at the periphery of residential
developments. Central Networks consider that the presence of exposed high voltage terminals
close to houses presents an unacceptable risk to members of the public going about normal DIY,
hobby and play activities at or near home through inadvertent contact or deliberate interference.
The hazard is eliminated by the use of ground mounted non-exposed conductor equipment.
The Ring Main Units of Ground Mounted Substations shall be loop connected into the 11kV
network and LV back-feeds provided in accordance with Section 1.4.5 11kV Connectivity Rules.
Padmount and pole transformers shall be tee connected into the 11kV network and LV back-
feeds provided in accordance with Section 1.4.5 11kV Connectivity Rules. Where the LV back-
feeding provision of the 11kV Connectivity Rules cannot be met then a Ring Main Unit equipped
Ground Mounted Substation shall be used.
1.2.10.2 Transformer size calculation
The initial calculated load shall not exceed the transformer name-plate rating. Any overload
capability shall be used to accommodate future load growth.
Standard transformer sizes for use on residential developments

Ground Mounted Padmount Pole Mounted
200 kVA 100 kVA 100 kVA
315 kVA 200 kVA 200 kVA
500 kVA
800 kVA NOTE 1
1000 kVA NOTE 1
NOTE 1 – 800 kVA and 1000 kVA for electric heating developments only or where motor starting current (e.g. for
lifts or pumps) dictates a larger size.
NOTE 2 - 50 kVA transformers shall not be used to supply residential loads. The combination of unbalanced load
and the high impedance of these units results in levels of negative phase sequence voltage which adversely
effects electric motors.
The maximum size of transformer supplying a non-electric heated residential development shall
not exceed 500kVA. This is to enable LV back-feeding to be feasible in accordance with the
criteria in Section 1.4.5 11kV Connectivity Rules.
Electrically heated developments will have relatively short LV feeders due to the high loading
making back feeding feasible at light load periods.
On residential developments the total load on the transformer shall be calculated according to
the following formula:
Load = N x A x Ft x F2
Where F2=1+12/AN A = ADMD in KW N = No of houses Ft = 70%
This formula recognises that large groups of customers and higher ADMDs will have greater load
diversity than small groups at low ADMDs.

Calculated values of Ft x F2 are shown below

Values of Ft x F2 for ADMDs of:
No of
houses
N 2 kW 3 kW 4 kW 6 kW 10 kW 15 kW 20 kW
1 4.90 3.50 2.80 2.10 1.54 1.26 1.12
2 2.80 2.10 1.75 1.40 1.12 0.98 0.91
3 2.10 1.63 1.40 1.17 0.98 0.89 0.84
4 1.75 1.40 1.23 1.05 0.91 0.84 0.81
5 1.54 1.26 1.12 0.98 0.87 0.81 0.78
6 1.40 1.17 1.05 0.93 0.84 0.79 0.77
7 1.30 1.10 1.00 0.90 0.82 0.78 0.76
8 1.23 1.05 0.96 0.88 0.81 0.77 0.75
9 1.17 1.01 0.93 0.86 0.79 0.76 0.75
10 1.12 0.98 0.91 0.84 0.78 0.76 0.74
15 0.98 0.89 0.84 0.79 0.76 0.74 0.73
20 0.91 0.84 0.81 0.77 0.74 0.73 0.72
25 0.87 0.81 0.78 0.76 0.73 0.72 0.72
30 0.84 0.79 0.77 0.75 0.73 0.72 0.71
40 0.81 0.77 0.75 0.74 0.72 0.71 0.71
50 0.78 0.76 0.74 0.73 0.72 0.71 0.71
60 0.77 0.75 0.74 0.72 0.71 0.71 0.71
70 0.76 0.74 0.73 0.72 0.71 0.71 0.71
100 0.74 0.73 0.72 0.71 0.71 0.71 0.70
150 0.73 0.72 0.71 0.71 0.71 0.70 0.70
200 0.72 0.71 0.71 0.71 0.70 0.70 0.70
250 0.72 0.71 0.71 0.71 0.70 0.70 0.70
300 0.71 0.71 0.71 0.70 0.70 0.70 0.70
Examples
1. 4 houses at 2 kW ADMD Trans load = 4 x 2kW x 1.75 = 14 kW

For residential properties assume that the power factor is nearly unity, therefore kW ≈
kVA. The transformer size should be selected at the next standard nameplate rating up
from the calculated load using the Ft x F2 correction factor.
Note that in example 2 the simple use of the number of houses and ADMDs would
result in 70 x 6KW = 420 KW and this would result in over-planting the substation
with a 500kVA transformer when a 315 kVA would suffice.
Likewise example 3 would have resulted in the use of a 800kVA transformer instead
of 500 kVA.

1.3 Industrial and Commercial Supplies

1.3.1 LV Metered Supplies
LV metered supplies are to be used for:
• Loads up to 1000kVA where

• the customer has not specifically requested an HV metered supply
• It is electrically feasible for the customer to distribute the load at LV within the site.
Only one point of LV supply will be provided to each customer within a building or site.
Where two or more customers occupy the same building refer to Section 1.3.1.3 Multi occupancy
buildings.
Note that if two or more services were to be provided there is a danger that neutral current from the
electrically remote one service(s) may return to the substation via the service cable electrically nearest
the substation. This is due to the interconnection of the service neutrals by the PME equipotential
bonding and other conductive routes such as metallic pipe-work. The passage of neutral current may
cause high electro magnetic fields (emf) which may cause electronic equipment to malfunction. More
importantly, the PME bonding and metallic pipes etc. are not designed to be load carrying and may
represent a fire risk.
This does not prelude the use of two incoming cables supplied from the same substation to a single
metering room as per Section 1.3.1.8 Arrangement D. “Second service direct from substation”. In this
case both services have the same size neutral and are of a similar length. The cut-outs are positioned
within the same room and the neutrals bonded together with a large conductor capable of carrying
full load current. (185mm2 and 300mm2 Wavecon both have 116mm2 copper neutrals and the bonding
conductor is 120mm2)
Loads unsuitable for LV metered supplies
• All loads above 1000kVA.

• Loads where the customer specifically requests an HV metered supply.
• Loads where the customer’s site is extensive and voltage considerations make distribution at LV
impractical.
• Disturbing loads where the Point of Common Coupling with other customers needs to be at HV.
1.3.1.1 Connection Strategy for LV Industrial & Commercial Loads
Strategy.

1.3.1.2 Voltage Regulation for LV Industrial & Commercial Loads
Voltage regulation from LV busbars of the HV/LV transformer to any service cut-out shall not
exceed:
Provided that
The overall power factor of the loads is better than 0.95 which will cause no more than 4%
voltage drop on the LV bus-bars local secondary substation transformer at full load.
If the power is known to be worse than 0.95 or the transformer is to be run into overload
the additional transformer voltage drop shall be deducted from the maximum allowable
LV network voltage regulation. See Section 4.2.3 of this Manual.
and
The voltage drop on the mains and service cables are calculated according to the
methodology described in the Low Voltage Network Design Calculations Section 6 of this
Manual.
1.3.1.3 Cables for LV metered supplies

The service cable and meter tail size shall be selected according to the Authorised Supply
Capacity.
Cable / cut-out maximum thermal loading
The nameplate kVA ratings of electrical machines such as motors, air conditioning units etc
are based on a nominal voltage of 400v. Unlike heating appliances the current does not
necessarily increase as the supply voltage increases. Generally at higher voltages the machine
draws less current to maintain the same output. The kVA ratings of cables shall be based on a
nominal running voltage of 415/240v.
For industrial and commercial loads the cable / cut-out loading is based on:
1000 VA ÷ √3 x 415 v = 1.39 amps per kVA
Continuous cable ratings in ducts shall be used for industrial services;
35mm2 Wavecon - 100* Amps ≈ 70 kVA
95mm2 Wavecon - 201 Amps ≈ 140 kVA
185mm2 Wavecon – 292 Amps ≈ 210 kVA
300mm2 Wavecon - 382 Amps ≈ 275 kVA

Note *
100 amp cut-out is limited to 90 amps (62 kVA) if the service terminates in an outdoor meter box or indoor cabinet without a free
flow of air.
These are the minimum size cables to be used according to the Authorised Supply Capacity.
Sizes may have to be increased for voltage drop and loop impedance criteria depending on
the location of the substation and size of mains cable.
Examples of Meter Tail Sizes

Service Cable Service Cut-out Suggested Number and Size of Meter Suggested Min Size of
Size Cable Fuse Tails per phase Minimum Equipotenti
Wavecon cable Current Size Size of al Bonding
Rating
Main Conductor
PVC XLPE Earthing
BS7671 Table 4D1A BS7671 Table 4E1A Conductor
Method Method From cut-out

Method Method
1 1 to i.e. water,
Aluminium phase 3 in 3 in
Amps Amps Clipped Clipped customer’s gas, oil,
(Copper neutral) trunking trunking
direct direct main earth frame
mm2 cu mm2 cu
mm2 cu mm2 cu bar
35 mm2 (22 mm2) 100/90 100 1 x 35 1 x 35 1 x 35 1 x 35 16 mm2 10 mm2
95 mm2 (60 mm2) 201 200 1 x 70 1 x 95 1 x 50 1 x 70 35 mm2 16 mm2
185 mm2 (116 mm2) 292 315 1 x 150 1 x 240 1 x 95 1 x 150 70 mm2 35 mm2
300 mm2 (116 mm2) 382 400 1 x 185 1 x 400 1 x 150 1 x 240 95 mm2 50 mm2
NOTES
Two tails per phase are not permitted as they will not pass through metering CTs
The Customer’s electrical contractor should refer to the current edition of BS7671 ‘Requirements for
Electrical Installations’ to confirm that these sizes are adequate for the proposed method installation.
1.3.1.4 Multi occupancy buildings

Where two or more customers occupy separate units within the same building there are two options
available:
1. Group metering position. Preferred option. A single metering position is installed using a
single or duplicate service cable (see arrangements A to D below). The developer should
provide customer owned cabling from the group metering position to each to each unit. This
option has two advantages:
a. Lower cost of mains and services
b. All earthing and bonding is made at a common point
c. A PME terminal is available for each unit for TNCS earthing.
If two or more service cables are required they must be positioned in the same room and have
the same sized neutral conductors. The neutrals of the cut-outs shall be bonded together with
120mm2 copper conductor coloured green/yellow with blue tape marker at each end to denote
that it is a current carrying neutral/earth bond.
2. Individual services to each unit. A distribution main is provided outside the building and
individual services are connected to each unit. This option may be considered in large
buildings where it may be impractical to provide long customer owned cabling from a central
metering point.
This option has the following disadvantages:

a. Higher cost of mains and services
b. Earthing and bonding is made at numerous points. Where TNCS earthing (PME) is
used much of the neutral current will return via the building’s metalwork and the
service cable electrically closest to the substation. This will result in high electrical
fields which may affect electrical equipment. More importantly, it may also overheat
the PME bonding conductors and the neutral conductors of the service cable
electrically closed to the substation. This effect is especially pronounced in metal
framed buildings.
c. To avoid this phenomenon PME shall not be provided to any unit. All units should
employ TT earthing. The current edition of BS7671 permits a building’s metal frame to
be used as the main earth electrode. See Central Networks’ Earthing Manual Section
E6 for more information.
d. Where current transformer (CT) metering is used the metal meter cabinet shall be
bonded to the neutral of the cut-out. Any trunking between the metering cabinet and
the customer’s equipment shall be plastic or other insulating material.
Where multiple LV cables are used they shall originate from the same substation. Providing LV
supplies from two or more substations to the same building is not permitted on safety
grounds.
1.3.1.5 Standard Service Arrangements
The next pages contain drawings of the following standard LV supply arrangements:
Arrangement A. Direct connection from existing LV Network.

Arrangement B. Direct connection from modified LV network.
Arrangement C. Service direct from substation
Arrangement D. Second service direct from substation
Arrangement E. 11kV extension to new substation.
Arrangement F. 11kV extension, 500 to 1000kVA substation & LV Air Circuit Breaker.
These are designed to accommodate the majority of supply requirements. Standard items of plant are
used as ‘building blocks’ to ensure a straight forward approach to construction, operation and
maintenance on the Central Networks system.
Any departure from these standards will be resisted unless exceptional local circumstances or supply
requirements make a bespoke solution necessary. The Networks Manager must approve any proposed
bespoke solution before it is installed.
With all these arrangements, reference should also be made to:

• Section 6 of this Manual – Low Voltage Network Design Calculations
• Section 5 of this manual – LV Earth Loop Impedance Policy
• Earthing Manual – PME requirements
KEY TO LV CIRCUIT SYMBOLS
11kV Ring Main Unit - 2 x 630A ring switches 1 x circuit breaker 200 amp or 630 amp
all with integral earth switches. Earth fault indicator on one ring switch
11kV/LV Transformer
M Metering unit - Voltage reference and current transformers
Low Voltage distribution cabinet - incoming isolating links, outgoing 500 amp fuse ways
LV Air Low Voltage distribution cabinet - incoming isolating links, one outgoing 500 amp fuse way
Circuit Air Circuit Breaker up to 2500 amp and meter unit
Breaker
M
New LV cable
Existing LV cable
Boundary of customer's property

1.3.1.6 Arrangement A. Direct Connection from Existing LV Network.

May be used for loads up to
and including 210 kVA, provided
that:
11kV Ring
• Cable ratings are not
Customer's exceeded.
11kV switchgear • LV fuse ratings are not
exceeded
11kV/LV • Voltage drop and earth
Distribution M
Substation Metering loop impedance limits are
not exceeded
LV Cut-out Max 315 amp Fuse
Service cable
up to 185 CNE If these conditions cannot be
LV Distribution main
met, then arrangement B, C or
E may be used as appropriate.
One or more outgoing
cables to LV Network
1.3.1.7 Arrangement B. Direct Connection from modified LV network.

To be used for loads up to and
including 210 kVA, where LV
network modifications are
11kV Ring
required in order to meet the
Customer's limits specified in Arrangement
11kV switchgear
A.
11kV/LV
Distribution M If these conditions cannot be
Metering
Substation met with this arrangement, then
Cut-out Max 315 amp Fuse arrangement C or E may be used
LV
Service cable as appropriate.
up to 185 CNE
Reinforced section
of LV Distribution main

1.3.1.8 Arrangement C. Service direct from substation.
11kV Ring To be used for:

• Loads up to 210 kVA where
Customer's arrangements A or B cannot be
11kV switchgear used, or
• Loads between 210 kVA and 275
New or existing kVA.
11kV/LV
Distribution M
Substation Metering If voltage drop and / or earth loop
impedance limits prevent this
LV Cut-out arrangement being used, then
Arrangement E should be used.
185 or 300 CNE

Do not connect other
customers to this cable
1000 or 800 kVA transformer – 630 amp S/S fuse – 300 CNE cable – 400 amp cut-out fuse – 275 kVA load
500 kVA transformer – 400 amp S/S fuse – 300 CNE cable – 400 amp cut-out fuse – 275 kVA load
1.3.1.9 Arrangement D. Second service direct from substation.
11kV Ring To be used to up-rate an existing

Customer's Customer's 275 kVA supply (Arrangement C) to
existing new 550 kVA. The customer must install
11kV
switchgear switchgear a second LV switchboard to divide
the loads into two separate blocks.
New or existing
Each block must not exceed 275
11kV/LV
Distribution M M kVA.
Metering
Substation
Cut-out Maximum cable length is 200m at
LV 120mm full load.
N/E Bond
Existing cable Both cables must come from the
same substation and terminate in
Additional 185 or 300 CNE
One or more outgoing the same switch-room. The neutral
Do not connect other
cables to LV Network customers to these cables must be bonded with 120mm2
conductor.
Refer to Arrangement F if new
Only for up-rating existing supplies.
substation is required
Use Arrangement F for new supply applications over 275 kVA.
1000 or 800 kVA transformer – 630 amp S/S fuse – 300 CNE cable – 400 amp cut-out fuse – 2 x 275 kVA load
500 kVA transformer – 400 amp S/S fuse – 300 CNE cable – 400 amp cut-out fuse – 2 x 250 kVA load*
315 kVA transformer – 315 amp S/S fuse – 185 CNE cable – 200 amp cut-out fuse – 2 x 140 kVA load*
* provided capacity is available on the substation

1.3.1.10 Arrangement E. 11kV extension to new substation.

A new substation may be required for
11kV Ring arrangements A to D.
The substation shall be installed in a
GRP housing or brick substation on or
Loop outside the customer’s property
connection boundary with direct access from the
public environment.
The new substation shall always be
looped into the existing HV system in
accordance with Section 1.4.5 11kV
11kV Connectivity Rules.
LV as per Arrangements C or D

1.3.1.11 Arrangement F. 11kV extension, 500 to 1000kVA substation & LV Air Circuit Breaker.
To be used for:
11kV Ring
11kV Loads from 275 to 1000
Ring Main Unit kVA unless the
sited on or outside customer specifically
RN2c-T1/21 boundary fence
11kV/LV requires an HV supply.
or VRN2A
Compact Unit
Substation sited
on or outside Boundary fence Choice of Compact
boundary fence Unit Substation or
separate Ring Main
One outgoing Unit and Transformer
cable to LV Air depending on load
LV Network Circuit
if required centre.
Breaker
M RN2c-T1/21
or VRN2A
or SN6-S1/21 TRANSFORMER A.C.B.
Customer's RATING RATING
LV Tails
11kV/LV Transformer
500kVA 800A
Customer's sited at customers load
switchgear centre
800kVA 1250A
LV Air
Circuit
Do Breaker
not M
use
1000kVA 1600A
Customer's
Customer's switchgear
LV Tails
Central Networks’ “LV Air Circuit Breaker 500 to 1000 kVA Substation Application Guide”
This guidance document provides further technical detail ands information and should be made
availble to the customer’s electrical contractor.
Compact Unit Substation Option
The Compact Unit Substation must be located in a GRP housing or brick substation on the customer’s
property boundary with direct access from the public environment.
The Central Networks/Customer boundary will be the outlet terminals of the LV cabinet. The customer
will provide and own the cable tails between Central Networks’ LV cabinet and the customer’s switch
board. It is unlikely that the customer will be in possession of a licence to lay cables in the public
highway so the substation will need to be sited on, or immediately adjacent to, land in the ownership
of the customer.
One additional LV fuse-way is provided which can be used to supply the local network if required.
Separate Ring Main Unit & Transformer Option
Where the distance from the property boundary to the customer’s load centre makes LV cabling
impractical or uneconomic the transformer and LV Air Circuit Breaker cabinet may be located at the
customer’s load centre. In this case an 11kV Ring Main Unit must be placed at the customer’s property
boundary to enable Central Networks to perform network operations and fault restoration on the ring
main without the need to enter the customer’s property.
The transformer must be equipped with switchgear to isolate and earth the incoming cable and the
transformer in order to enable operation under Central Networks’ Safety Rules. This may be achieved
using either:
RN2c-T1/21 or VRN2A Ring Main Unit – in this case the size of the protection CTs and
TLFs of the boundary RMU and the Transformer RMU must be the same.
SN6-S1/21 Switch Disconnector
Note that due to purchase volumes the Ring Main Unit is a lower cost than the Switch
Disconnector. All Central Networks’ projects shall use the Ring Main Unit.
Refer to Table 3.3.4.1 of the Network Design Manual for a complete list of Central Networks approved 11
kV switchgear.
The Central Networks/Customer boundary will be the outlet terminals of the LV cabinet. The customer
will provide and own the cable tails between Central Networks’ LV cabinet and the customer’s switch
board.
As there will not be direct access from the public environment to the transformer and LV cabinet the
additional LV fuse-way shall not be used to supply other customers.
Metering
The customer shall provide suitable accommodation for the meters.

The maximum length of cable from the metering CTs and VTs to the meter must be limited to 15m of
12 core 2.5mm2 cable so that the maximum burden on the CTs is not exceeded. The metering cubicle
should have dimensions a minimum of 1m wide x 1.5m high by 400mm deep to accommodate the
meters and provide sufficient space for safe working.
(NOTE in practice the metering panel will need to no more than 12 m from the LV metering unit to allow
the cable to be run down into the cable trench and up the wall to the metering panel.)
Length and rating of customer’s LV cables

The customer’s electrical contractor must design the LV cables to comply with the current edition of
BS7671 “Requirements for Electrical Installations”, including the following criteria:
1. An LV earth fault at any point up to the customer’s first circuit breaker or fuse will trip the
Central Networks owned Air Circuit Breaker within 5 seconds as required by the current
edition of BS7671. The worst case scenario is for an earth fault at the substation end of an
LV cable.
2. The customer’s electrical designer should be provided with the protection settings applied
to the Central Networks owned Air Circuit Breaker. An extract from the Central Networks
Protection Manual is included below.
3. Where the Central Networks ACB is to provide both overload and short circuit protection
the LV tails should be sized to carry overloads up to the setting of the Central Networks Air
Circuit Breaker and not just the transformer name plate rated load or Authorised Supply
Capacity (ASC). Overload settings are shown in column 2 of tables 1.3.1.10 A & 1.3.1.10 B
below.
4. Alternatively the Central Networks ACB can be employed as short circuit protection only
and the customer’s main incoming circuit breaker or fuses may be employed as overload
protection. In this case smaller LV tails may be permissible. (refer to BS 7671 clause 473-01-
02). The customer’s electrical contractor should ascertain that the characteristics of the
Central Networks ACB provide effective short circuit protection for the size of LV tails
selected.
5. Voltage drop on the customer’s cable must comply with the current edition of BS7671
“Requirements for Electrical Installations” at transformer name plate rating.
6. Central Networks is not an enforcing or advisory body for BS 7671. Where questions of the
adequacy of the customer’s installation need to be resolved the electrical contractor should
seek advice from the trade body providing his accreditation. e.g. Electrical Contractors
Association (ECA), National Inspection Council for Electrical Installation Contracting
(NICEIC) etc.
Examples of cable sizes for customer owned LV tails
The Central Networks “LV Air Circuit Breaker 500 to 1000 kVA Substation Application Guide” contains
tables cable ratings for typical installation scenarios. Central Networks project managers and Service
Providers should refer to this document to assess the fitness of the customers LV tails prior to
energisation.

1.3.2 HV Metered Supplies
HV metered supplies are to be used for:
• All loads above 1000kVA

• Loads below 1000kVA, where the customer specifically requires an HV metered supply.
• Loads where the customer’s site is extensive and voltage distribution at LV impractical.
• Disturbing loads where the Point of Common Coupling with other customers needs to be at HV.
Otherwise, use an LV metered arrangement.
1.3.2.1 Connection Strategy for HV Industrial & Commercial Loads
Strategy.
1.3.2.2 Circuit Ratings for Industrial /Commercial Loads
Ring Mains
All new 11 kV cables shall be laid in black rigid twinwall corrugated ducts to ENATS 12-24 with warning
tape. Short sections of 11kV cable may be laid direct only where ducts cannot be used such as sharp
bends or at jointing positions. Warning tape shall be used in these situations.
Where trenchless or narrow trenching cable laying techniques are used the cable shall be ducted but
the warning tape shall be omitted.
Cable ratings on ring mains shall be based on the Winter 5 Day Distribution Rating – 50% or 75%
Utilisation as appropriate to the type of circuit. See section 3.3.1.4.
New ring main cables shall be XLPE Triplex. The ratings shown below are obtained from Table 3.3.1.4B
and assume two cables leading to a ring main substation, laid in the same trench with the ducts
touching. i.e. a group of 2 cables not thermally independent.
185 mm2 XLPE Al Triplex in ducts – 366 amps – normal ring mains
300 mm2 XLPE Al Triplex in ducts – 485 amps – large capacity ring mains
300 mm2 XLPE Cu Triplex in ducts – 625 amps – exceptional applications only
Tables 3.3.1.4D and 3.3.1.4E contain 5 Day Distribution Ratings – 50% Utilisation for existing 11kV belted
Paper Insulated Cables - metric & imperial sizes.

The above ducted ratings of XLPE Triplex cable match the ratings of belted PICAS cable laid direct.
However, care must be taken when connecting a ducted XLPE cable to a paper insulated cables. The XLPE
cable ratings are obtained at a conductor temperature to 90OC. Paper cables are rated at 65OC. There is a
danger of transferring excessive heat from the XLPE cable to the paper cable via the transition joint. To
overcome this, ducted XLPE cable must be laid direct for a length of at least 1 metre after leaving a duct
to allow the conductor temperature to drop to 65OC. (Thermodynamic calculations have demonstrated
that a 1 metre length is sufficient reduce the core temperature by 25OC.)
The 5 Day Distribution ratings assume a mix of domestic, industrial and commercial loads. If the ring
consists of exclusively industrial / commercial load then sustained ratings may have to be used.
Groups of mixed residential, industrial and commercial loads up to 3.8 MW at 11kV may be connected
into 11kV ring mains provided there is sufficient circuit capacity to provide a full switched alternative
supply on each half of the ring. The total of the loads connected to both sides of the normal open
point must not exceed the circuit’s 5 Day Distribution Rating. Refer to Section 1.4.4 1kV Circuit
Configuration & Loading for further guidance.
Note – Whilst the normal open point should be located to split the load approximately 50/50 over the two
halves of the ring it may sometimes be necessary to apply a 60/40 or even 30/70 split depending on
circumstances. In this case 75% utilisation ratings must be applied to the cables.
Duplicate circuits
Industrial and commercial loads that cannot be accommodated on ring mains shall have a pair of
dedicated 11kV cables direct from a Primary or BSP Substation.
All new 11 kV cables shall be laid in black rigid twinwall corrugated ducts to ENATS 12-24 with warning
tape. Short sections of 11kV cable may be laid direct only where ducts cannot be used such as sharp
bends or at jointing positions. Warning tape shall be used in these situations.
Cable ratings on duplicate circuits shall be based on the Summer Sustained Rating unless the
proposed load cycle is known with confidence. See section 3.3.1.5. and Table 3.3.1.5A
Directional or unit protection may be installed to provide a firm supply for a single circuit outage if the
customer requires this level of security.
Loads with an authorised supply capacity over 12MW must have a firm supply to comply with
Engineering Recommendation P2/5.
Loads over 12 MW may require three or more firm circuits at 11kV or a 33kV supply as decided by
Networks Manager.

Authorised Supply Capacity Circuit sizes

Based on sustained summer ducted ratings
11kV 6.6 kV
5.9 MW 3.5 MW 185mm2 al XPLE triplex two circuits

O/H line two circuits
150mm2 ACSR Central Networks East
100mm2 AAAC Central Networks West
7.7 MW 4.6 MW 300mm2 al XLPE triplex two circuits

150mm2 ACSR Central Networks East
9.8 MW 5.9 MW 300mm2 copper XLPE triplex two circuits

300mm2 HDA Central Networks East
12MW 7.3 MW 400mm2 cu XLPE triplex two circuits

300mm2 HDA Central Networks East
33kV
19 MW 2 x 12/24 MVA 33/11kV Transformers (20OC AFAF rating of 19 MVA)

150mm2 cu XLPE double circuit
150mm2 ACSR O/H line two circuits
32 MW 2 x 24/40 MVA 33/11kV Transformers (20OC AFAF rating of 32 MVA)

400mm2 cu XLPE double circuit
300mm2 HDA O/H line two circuits
Over 32MVA Special Design
1.3.2.3 Standard HV Supply Arrangements
• The following standards shall be used for all new connections. Modifications to existing
supplies shall wherever reasonably practicable comply with these standards.
• Only one point of HV supply will be provided to each customer within a building or site. Note
this does not prelude the use of two or more incoming cables supplied into the same or adjacent
switchboards in the substation.
• All substations shall be loop connected to provide switched alternative 11kV supplies in
accordance with Section 1.4.5 “11kV Connectivity Rules”

• All substations shall be placed on or outside the boundary of the customer’s property
boundary so that Central Networks operational staff have direct access from the public
environment at all times. Where necessary the customer’s transformers and switchgear may
be sited within the property at the load centre and connected to the Central Networks
switchgear by cable. The standard arrangements cater for this option. This requirement has
become necessary due to the general increase in security arrangements at customer’s sites.
Significant supply restoration delays are being caused when Central Networks operational staff
are unable to gain access or are refused access to substations inside customer’s sites.
The next pages contain drawings of the following standard HV supply arrangements:
Arrangement H. Single Transformer up to 3.8 MVA@11kV / 2.3 MVA@6.6kV

Arrangement I. Two or more Transformers totalling up to 7.6 MVA@11kV / 4.6 MVA@6.6kV
Arrangement J. Duplicate Ring Main Units totalling up to 7.6 MVA@11kV / 4.6 MVA@6.6kV
Arrangement K Customer switchboard up to 7.6 MVA@11kV / 4.6 MVA@6.6kV
Arrangement L. Duplicate firm HV Supply up to 12 MVA@11kV / 7.2 MVA@6.6kV
These are designed to accommodate most of supply requirements. Standard items of plant are used
as ‘building blocks’ to ensure a straight forward approach to construction, operation and maintenance
of the Central Networks system.
Any departure from these standards will be resisted by Central Networks unless exceptional local
circumstances or supply requirements make a bespoke solution necessary. The approval of the
Network Manger shall be obtained before any proposed bespoke solution is implemented.
With all these arrangements, reference should also be made to:
• Central Networks’ Protection Manual Section 4.0 “Recommendations for the Protection of
Customer-owned HV networks”.
• Central Networks’ Earthing Manual
• Network Design Manual Table 3.3.4.1 Central Networks approved 11kV switchgear.
KEY TO HV CIRCUIT SYMBOLS

11kV Ring Switch or Isolator with circuit earthing switch
11kV Circuit breaker with circuit earthing switch
Directional Protection
Bus bar
11kV Ring Main Unit - 2 x 630A ring switches 1 x circuit breaker 200 amp or 630 amp
all with integral earth switches. Earth fault indicator on one ring switch
11kV/LV Transformer
M Metering unit - Voltage transformer and current transformers

Trip Emergency trip switch for customer's use
Boundary of customer's property
Limit of equipment ownership

1.3.2.4 Arrangement H Single Transformer up to 3.8 MVA@11kV / 2.3 MVA@6.6kV
11kV Ring 11kV Ring
11kV 11kV
Ring Main Unit Ring Main Unit
& Metering Unit RN2c-T1/21 & Metering Unit
sited on or outside or VRN2A sited on or outside
boundary fence boundary fence
M M
Customer's
11kV Cable e.g. Boundary
Transformer & Trip SN6-S1/21 fence
LV switchgear
Trip
Customers
11kV/LV Transformer
sited at customers load
centre
• Non-extensible Ring Main Unit with HV metering unit.
• RMU
• Merlin Gerin RN2c-T1/21 with TLFuse protection - up to 1.0 MVA@11kv / 0.5 MVA@6.6kV
• Merlin Gerin - RN2c-T2/21 with VIP 300 protection – up to 3.8 MVA@11kv / 2.3 MVA@6.6kV
• Lucy Sabre VRN2A with TLF protection - up to 1.0 MVA@11kv / 0.5 MVA@6.6kV
• Lucy Sabre VRN2A with Micom protection - up to 3.8 MVA@11kv / 2.3 MVA@6.6kV
• Metering unit
• Merlin Gerin MU2-M2/16 – 100/50/5 up to 1.9MVA@11kv / 1.1 MVA@6.6kV
• Merlin Gerin MU2-M3/16 – 200/100/5 up to 3.8MVA @11kv / 2.3 MVA@6.6kV
• Lucy AIMU – 100/50/5 up to 1.9MVA@11kv / 1.1 MVA@6.6kV
• Lucy AIMU – 200/100/5 up to 3.8MVA@11kv / 2.3 MVA@6.6kV
• Central Networks RMU to be located in GRP housing or brick substation on or outside the customer’s
• Customer’s transformer may be sited adjacent to the substation or located remotely at the load
centre.
• Central Networks recommend that the customer’s transformer be equipped with a Switch Disconnector
or Ring Main Unit (equivalent to SN2-S2/21, RN2c-T1/21 or VRN2A) where the HV cable cannot be
traced visibly back to the Central Networks RMU to comply with Distribution Safetey Rules.
• RMU circuit breaker rated at 200 amps load current with 3.15kA earth switch. (minimum rating)
• TLF protection on circuit breaker for transformers up to and including 1000kVA rating, relay protection
for transformers above 1000kVA rating.
• Provision must be made for remote emergency tripping of circuit breaker for use by the customer.

1.3.2.5 Arrangement I Two or more Transformers totalling up to 7.6 MVA@11kV / 4.6 MVA@6.6kV
11kV Ring 11kV Ring
Trip
RN6c-T1/21 11kV RN6c-T1/21
Boundary
M Customer's
M fence
Equipment
Cable Cable
Boundary
e.g. Trip
fence
SE6-S1/21
Customer's
11kV Network Customer's 11kV
Equipment
Customer's
11kV Network

• RMU
• Merlin Gerin RN6c-T1/21 with VIP 300 protection
• Metering unit
• Merlin Gerin MU2-N1/16 – 400/200/5 up to 7.6MVA@11kv / 4.6 MVA@6.6kV
• Relay protection on Central Networks circuit breaker to grade with customer’s 11kV protection.
NOTE that the RN6c is necessary even where the load is below 1.9MVA. This is because the 200/1 CTs
of the RN2c restrict the setting range of the VIP 300 relay such that it cannot be set to grade with the
customer’s downstream protection. The 800/400/1 CTs of the RN6c provide sufficient setting range.
• Central Networks RMU to be located in GRP housing or brick substation on or outside the customer’s
• Customer’s 11kV switchgear and transformer may be sited adjacent to the substation or located
remotely at the load centre.
• Central Networks recommend that the customer’s switchboard be equipped with an incoming isolator
(equivalent to Merlin Gerin SN6-S1/21) where the HV cable cannot be traced visibly back to the
Central Networks RMU to comply with Distribution Safetey Rules.
• RMU circuit breaker rated at 630 amps load current and the earth switch must be fully rated.

1.3.2.6 Arrangement J. Duplicate Ring Main Units totalling up to 7.6 MVA@11kV / 4.6 MVA@6.6kV
11kV Ring 11kV Ring
Duplicate 11kV
Ring Main Units
& Metering Units RN2c-T2/21
sited on or outside or VRN2A RN6c-T1/21
boundary fence
M M Boundary M Trip M
fence
Customer's
11kV Cable
Transformer & Trip Customer's
LV switchgear 11kV Network
• This arrangement is applicable only where the customer requires a full switched alternative supply and
total load will not exceed 7.6 MVA
• The Central Networks normal open point will NOT be placed on either of the interconnecting switches
between the Ring Main Units to prevent the customer accidentally paralleling the Central Networks system
via his own network.
• The consumer must maintain an open point on his ring main except by specific agreement with Central
Networks. (e.g. the consumer’s network may be designed to run as a closed ring employing unit
protection.)
• Exceptionally, where the customer requires an enhanced level of security via auto changeover on his
switchgear the open point may be placed between the two RMUs. However, this must have specific
approval from the Network Manager. Before accepting this arrangement consideration must be given to
system losses, regulation, security of other customer’s supplies and the desirability of limiting the choices
of future network developments. In this case interlocking will be required to prevent a system parallel
being made via the customer’s network.
• RMU
• Merlin Gerin - RN2c-T2/21 with VIP 300 protection – up to 3.8 MVA - two transformers – Back-
feed only possible via LV
• Lucy Sabre VRN2A with Micom protection - up to 3.8 MVA - two transformers – Back-feed only
possible via LV
• RN6c-T1/21– with VIP 300 protection up to 7.6 MVA - Back-feed possible via customer’s 11kV
network – NB RMU earth switch must be fully rated.
• Metering unit
• Merlin Gerin MU2-M2/16 – 100/50/5 up to 1.9 MVA@11kv / 1.1 MVA@6.6kV
• Merlin Gerin MU2-M3/16 – 200/100/5 up to 3.8 MVA@11kv / 2.3 MVA@6.6kV
• Merlin Gerin MU2–N1/16 – 400/200/5 up to 7.6 MVA@11kv / 4.6 MVA@6.6kV
• Lucy AIMU – 100/50/5 up to 1.9 MVA@11kv / 1.1 MVA@6.6kV
• Central Networks RMUs to be located in GRP housing or brick substation on or outside the customer’s
• Customer’s 11kV transformers and/or substations may be sited adjacent to the substation or located
remotely at the load centre.
• Customer’s 11kV transformers should be equipped with an isolator where the HV cable cannot be traced
visibly back to the controlling circuit breaker.
• Relay protection on Central Networks circuit breakers to grade with customer’s 11kV protection.

1.3.2.7 Arrangement K. Customer switchboard up to 7.6 MVA@11kV / 4.6 MVA@6.6kV
11kV Ring 11kV Ring
Trip
RN6c-T1/21 11kV RN6c-T1/21
Boundary
M Customer's
M fence
Equipment
Cable Cable
Boundary
e.g. Trip
fence
SE6-S1/21
Customer's
11kV Network Customer's 11kV
Equipment
Customer's
11kV Network
• Depending on the load this arrangement may be connected into a ring main or have dedicated circuits from
a Primary Substation.
• This arrangement does not provide the same level of supply security as can be obtained from
Arrangements J & L. However, motor actuators and radio control may be fitted to provide restoration by
remote control or auto change-over where the network can accommodate it.
• Central Networks extensible equipment:
• RMU - RN6c-T2/21– with VIP 300 protection
• Metering unit MU2-M2 – 100/50/5A up to 1.9 MVA@11kv / 1.1 MVA@6.6kV
• Metering unit MU2-M3 – 200/100/5A up to 3.8 MVA@11kV / 2.3 MVA@6.6kV
• Metering unit MU2-M4– 400/200/5A up to 7.6 MVA@ 11kV / 4.6 MVA@6.6kV
• Central Networks equipment to be located in a brick substation on or outside the customer’s property
boundary with direct access from the public environment.
• Customer’s 11kV switchgear may be sited adjacent to the Central Networks substation or located remotely
at the load centre.
• Central Networks recommend that the customer’s switchboard be equipped with an incoming isolator
(equivalent to Merlin Gerin SE6-S1/21) where the HV cable cannot be traced visibly back to the Central
Networks RMU to comply with Distribution Safetey Rules.
• Relay protection on Central Networks bus section circuit breaker to grade with customer’s 11kV protection.

1.3.2.8 Arrangement L. Duplicate HV Supply
Central Networks Primary

or BSP Substation
11kV 11kV
DNO Circuits DNO Circuits DNO Circuits DNO Circuits
A A A A
B 11kV B
M M M M
C C Boundary C C
Trip fence
Cable Cable
Customer's Customer's Trip
bus bars & bus bars &
Circuits 11kV
Circuits
ENA Assessed DNO & Customer's Optional
Switchgear accomodated on boundary bus section
Non ENA Assessed Customer's Switchgear
OR
Customer's Switchgear located inside boundary
• This arrangement is applicable where the customer requires an Authorised Supply Capacity up to 12 MVA at
11kV
• The maximum load is based on a single circuit outage at summer maximum continuous cable rating. Where
the customer’s load profile is known then it may be possible to apply higher cable ratings and an increased
Authorised Supply Capacity.
• Maximum summer sustained load using 400mm2 copper 11kV or 33kV XLPE cables
• 33kV – 32 MVA
• 11kV – 12 MVA
• 6.6kV – 7.3 MVA
• Customer to have emergency tripping facility on the metering circuit breakers ‘C’.
• The incoming circuit breakers ‘A’ may be fitted with directional or unit protection to provide supply security
during a fault on one of the incoming cables.
• A pilot cable may be provided to take alarms / indication information to the Primary S/S from the customers
substation to Central Networks’ Control room where considered necessary.
• If the customer does not require a high level of supply security and the ASC is below 12MW the incoming
switches ‘A’ and bus section ‘B’ may be isolators. In this case a fault on one circuit will result in both circuits
tripping at the Primary Substation. Supply will be restored by manual switching in less than 3 hrs as
permitted under Engineering Recommendation P2/5 for loads under 12 MW.
• The Central Networks substation is to be located on or outside the customer’s property boundary and have
direct access from the public environment.
• Note - Metering on the incoming circuits is no longer permitted by Central Networks. During abnormal
running conditions other customer’s load current may circulate through bus section ‘B’ resulting in metering
abnormalities.

1.4 Secondary Network 11kV & 6.6kV

1.4.1 General Considerations – Secondary Network
The standard Secondary distribution voltage will be 11kV and all new circuits must be capable of 11kV
operation. All work undertaken within established 6.6kV systems shall employ dual ratio 11/6.6kV
transformers, 11kV cables and 11kV switchgear to facilitate eventual up-rating to 11kV. All references to
11kV in this manual shall equally apply to 6.6kV networks.
Expansion of established 6.6kV systems beyond their existing geographical areas shall only be
permitted where there are exceptional circumstances preventing connections being made to an
established 11kV network.
The eventual up-rating of Central Networks 6.6kV networks to 11kV will only be carried out when this
can be financially or technically justified by Central Networks Asset Development.
11kV networks are supplied from Primary substations which step down the voltage from 66kV or 33kV
to 11kV or 6.6kV. Certain Bulk Supply Point substations transform directly from 132kV to 11kV or 6.6kV.
All references to Primary substations this section of the manual shall include Bulk Supply Point
substations that supply 11kV and 6.6kv networks.
In order to comply with license conditions and to meet supply restoration targets set by the industry
regulator all 11kV distribution circuits and plant shall be configured to exceed the minimum security
standards of Engineering Recommendation P2/5. Therefore all new connections to the 11kV network
shall comply with Section 1.4.5 - 11kV Connectivity Rules.
Network extensions and new connections and shall be carried out using new materials. Refurbished
equipment shall only be used to augment, repair or replace existing facilities subject to approval by
the Asset Standards Manager.
1.4.2 Connection Strategy
All proposed connections:

above 1000kVA
that take the overall load on a ring main above 100% of the circuit rating
shall be referred to the Network Manager for an assessment of the strategic effect on the network.
If the proposal is not approved, the Network Manager shall provide an alternative connection strategy
and/or standard supply arrangement acceptable to Central Networks.
Designers should be aware that connections below 1000kVA will have an effect upon the network. In
particular the following load thresholds should act as a trigger for an evaluation of the 11kV network
by the designer:
loads above 500 kVA on “Standard 11kV Feeders”
loads above 200 kVA on “Long 11kV Feeders”
Disturbing loads such as welders, motors pumps etc.
Where necessary the designer should obtain guidance from the Network Manger.
The following shall be considered:

• Network loading
• Supply security
• Overall network operability in line with 11kV and 33kV Connectivity Rules
• The ‘fit’ with any existing connection strategy for the locality.
1.4.3 11kV Earthing
1.4.3.1 11kV Earth Fault Current
The value of earth fault current on 11kV systems is restricted to a maximum of 3,000 amps by the use
of a Neutral Earth Resistors (NER) at 33/11kV and 132/11kV substations. At certain substations the
earth fault current is limited to lower values to control Earth Potential Rise. The screens of 11kV cables
used on the network must have a one second short circuit rating of at least 3,000 amps.
There are two Arc Suppression Coil trails taking place in Central Networks West. Earth faults are not
held indefinitely, after a time delay a permanent earth fault is diverted to the NERs to operate the
normal earth fault protection.
1.4.3.2 Earthing of Plant
The Central Networks Earthing Manual specifies the detailed requirements for earthing distribution
plant. The main requirements for new installations are summarised below:
Pole mounted switchgear
o Manually operated 11kV Air Break Switch Disconnectors. Non-automated remote

control ABSDs shall have high level rod operated mechanisms. The equipment
steelwork shall not be earthed or bonded to the pole top crossarm. Live local
operation is permissible.
o Very occasionally it is necessary to install or retain a manually operated ABSD on an

earthed pole. Live operation is not permissible with earthing installed to ENATS 42-
24:Issue 1:1992. Individual earthed ABSDs may be specifically approved for live
operation following a bespoke design of the earthing system
o Existing Pole Mounted 11kV Automated Reclosers. Where these devices have control
boxes mounted at ground level they shall be earthed. Live local operation is not
permissible with earthing installed to ENATS 42-24:Issue 1:1992.
o New Pole Mounted 11kV Automated Reclosers shall be installed with the control boxes
at high level with the earth lead insulated and ducted to an electrode positioned a

safe distance from the pole. Local live operation is permissible where rod operation
facilities exist.
o New Pole Mounted Sectionalisers and Air Break Switch Disconnectors fitted with
remote control shall be installed with the control boxes at high level. The equipment
steelwork shall not be earthed or bonded to the pole top crossarm. Local Live
operation is permissible where rod operation facilities exist.
o The low voltage supply for the control circuits of automated equipment shall be
derived from a dedicated transformer on the same pole. The transformer LV neutral
shall be directly earthed to the equipment’s steelwork. This transformer shall not be
used to supply customers. Likewise, transformers supplying customer shall not be
used to supply the control box.
o Expulsion Fuses/Links, Automatic Sectionalising Links – Where installed on earthed

poles (e.g. transformers and cable terminations) their steelwork shall be bonded to
the pole earth. Where installed on un-earthed poles their steelwork shall not be
bonded to the pole top cross-arm or earthed.
Distribution substations shall have an HV earth electrode system around the perimeter to
provide an equipotential zone around the equipment. The local earth electrode system shall
be less than 20 ohms.
Pole mounted transformers, auto reclosers, cable terminations and other plant fitted with
surge arresters shall have an HV earth electrode system of less than 10 ohms and shall include
a deep driven electrode at the base of the pole acting as a lightning earth.
An Earth Potential Rise (EPR) above 430 volts is classified as a ‘Hot’ site. An EPR below 430
volts is classified as a ‘Cold’ site.
Transformers at ‘Hot’ sites shall have their HV and LV earths segregated. At ‘Cold’ sites a
combined HV and LV earth may be used.
At ‘Hot’ sites the LV earth electrode shall be segregated by installing it outside the 430 volt
EPR contour and connecting it to the transformer star point via insulated cable. This
segregation distance depends on the EPR, soil resistivity and physical area of the earth
electrode system. This can vary from 5m to 25m for ground mounted substations with a 20
ohm earth. Pole transformers employ a physically larger earth electrode system in order to
obtain 10 ohms and the segregation distance can vary from 4m to 43m or more. See Section
E5 of the Earthing Manual for typical values.
A site supplied from a 132/11kV or 33/11kV substation can be assumed to be ‘Cold’ if:
o the normal and alternative 11kV feed is via an entirely underground cable route. This
will route the majority earth fault current back to source via the cable screens rather
than the HV earth electrode.
OR

o the site is in an urban / suburban network that can be defined as a ‘Global Earth’. In
practice a network consisting of mainly lead covered cables with an area over 10km2
may be considered to be a ‘Global Earth’.
Sites that are not part of a ‘Global Earth’ and have any overhead line in either the normal or
alternative 11kV feed shall be considered to be ‘Hot’.
At ‘Hot’ consumer substations it can be difficult to obtain segregation between the HV and LV
earths due to the proximity of the consumer’s transformer to the LV switchboard. An earthing
specialist shall be commissioned to design each installation.
Supplies to mobile phone base stations accommodated on 132kV, 275kV & 400kV towers shall
be provided in accordance with EA Engineering Recommendation G78. This is essential to
prevent dangerous potentials being transferred onto the distribution network during earth
faults on the tower. Refere to Central Networks’ Application Guide “Supplies to Mobile
Phone Base Stations mounted on EHV Towers (G78 Installations)”
1.4.4 11kV Protection

The Central Networks Protection Manual provides detailed information on standard 11kV protection
schemes. The main requirements are summarised below.
1.4.4.1 Primary substation circuit breakers
There are five standard protection schemes:
1. Transformer circuit breaker – overhead incoming network.
a. Neutral Voltage Displacement

b. IDMT overcurrent and earth fault
c. Directional overcurrent and earth fault
d. Inter-trip send to fault thrower
e. Standby earth fault
f. Restricted earth fault
g. Auto Reclose
2. Transformer circuit breaker – underground incoming network.
a. IDMT overcurrent and earth fault

b. Directional overcurrent and earth fault
c. Inter-trip send to source substation via pilot wire.
d. Standby earth fault
e. Restricted earth fault

3. Bus Section circuit breaker

4. Feeder circuit breaker – overhead network.

b. Instantaneous overcurrent and earth fault slugged to 500ms
c. Sensitive earth fault.
d. Auto Reclose
5. Feeder circuit breaker – underground network.
1.4.4.2 Secondary substation protection
There are two standard protection schemes:
1. Time Limit Fuse with CT release

a. Time graded overcurrent
b. Instantaneous earth fault.
c. Transformers up to 1000 kVA at 11kV and 500 kVA 6.6kV
2. Merlin Gerin VIP 300 self powered relay
a. IDMT earth fault and overcurrent
b. Transformers and loads up to 3,800 kVA at 11kV and 2,280 kVA 6.6kV
1.4.4.3 HV Metered Customer Owned Substations
The protection scheme will depend upon the size of the load and the customer’s network
configuration. The standard schemes outline in 1.3.3.1 and 1.4.3.2 above shall be used as a basis of
design. See section 1.3.2 ‘HV Metered Supplies’ supplies for typical network configurations to HV
customers.
The customer should be provided with Section 4.0 of the Central Networks Protection Manual
“Guidance for protection of customer owned HV networks”. This outlines the issues that the customer
should address when designing his system.
1.4.4.4 Padmount transformer protection
A combination of a back-up (partial range) current limiting fuse in series with an under oil expulsions
fuse. The combination is ‘melt matched’ to provide full range protection.
1. Back-up Current Limiting Fuse

a. Not field replaceable
b. Covers current range 600 amps to 50,000 amps
c. Sized to detect faults up to the HV winding only

2. Bay-o-Net under oil expulsion fuse

a. Field replaceable
b. Covers current full load to 2,500 amps
c. Sized to clear LV bus bar zone fault in compliance with ENATS 12-06
1.4.4.5 Pole mounted transformer protection
Individual pole transformers rely upon 11kV circuit protection to detect faults and operate circuit
breakers.
The extent of supply interruption may or may not be limited by the inclusion of expulsion fuses or
automatic sectionalising links on spurs which operate after a sequence of circuit breaker operations.
Central Networks East - Expulsion fuses are standardised at 50 amp slow blowing and are designed
not to melt during instantaneous trips of the source circuit breaker to avoid operation on transient
faults. These are intended operate after an auto reclose operation whilst the source protection is set
to IDMT.
Central Networks West - Expulsion fuses are sized according to the transformer size.
Automatic Sectionalising Links (ASL) count the number of fault current pulses whilst a multi shot auto
reclose circuit breaker operates. On detecting 2 pulses the device registers a permanent fault. During
the next dead time of the auto reclose cycle a chemical actuator de-latches the ASL which drops open
under gravity. A further close operation of the auto reclose cycle restores supplies to the remaining
system. ASLs should not be used on circuits without multi shot auto reclose with at least 3 shots to
lock out. All PMARs are multi shot. Modern primary substations circuit breakers can be set to multi
shot. Most existing Primary substations have protection that is limited to 2 shots.
All new pole transformers are factory fitted with lightning arresters on the HV and LV side.
15 kV MCOV arresters are fitted between the HV bushings and the transformer tank to protect
the HV windings
A single Transient Voltage Clamper is fitted between the LV neutral bushing and the
transformer tank to protect the LV winding which is earthed remotely from the transformer
steelwork earth.

1.4.5 11kV Circuit Configuration & Loading
1.4.5.1 11kV Ring Mains
11kV networks shall be developed to form open ring mains across either side of the bus
sections at primary substations or as interconnectors between two or more primaries to
facilitate the provision of alternative supplies or load transfer and management operations.
There may be further interconnections between rings depending on the local network
configuration. There should not normally be more than 4 ends (i.e. points of HV isolation) to a
section of cable or overhead line.
The normal open point should be located to split the load approximately 50/50 over the two
halves of the ring. However, it may sometimes be necessary to apply asymmetrical loading to
the ring (e.g. 60/40 split) depending on circumstances. No circuit shall be loaded beyond 75%
utilisation without the specific approval of the Network Manger. Where one half of the ring is
loaded above 50% then cables ratings based on 75% utilisation need to be used.
Ring mains shall not be loaded beyond a total of 100% circuit rating without the specific
approval of the Network Manger.
Circuit configurations that rely on multiple switching operations in order to facilitate back-
feeding will only be permitted with the specific approval of the Network Manager.
Existing circuits are limited to a maximum current of 400 amps by the rating of substation
switchgear. Often the rating is lower depending on the size of the cables and/or any current
transformers in the circuit. Where necessary, new ring mains can be designed to operate up to
600 amps with the appropriate sized cables and provided all substations have 630 amp
switchgear.
Whilst the open ring or interconnector is the preferred arrangement, closed rings and parallel
feeders with appropriate protection may be used where approved by Network Manager.
1.4.5.2 Duplicate circuits
Industrial and commercial loads that cannot be accommodated on ring mains shall have a pair
of dedicated 11kV cables direct from a Primary or BSP Substation.
Cable ratings on duplicate circuits shall be based on the Summer Sustained Ducted Rating
unless the proposed load cycle is known with confidence. See section 3.3.1.5. and Table 3.3.1.5A
Directional or unit protection may be installed to provide a firm supply for a single circuit
outage if the customer requires this level of security.
Loads with an authorised supply capacity over 1 MW must have a firm supply to comply with
Loads over 12 MW may require three or more firm circuits at 11kV or a 33kV supply as decided
by the Network Manager.
1.4.5.3 Standard Equipment and Designs
Standard designs using the equipment contained in Section 3 of the Manual shall normally be
used for all network modifications or extensions.
Where standard designs and/or equipment are not suitable for a specific application the Asset
Standards Manager may approve the use of a specifically engineered solution with reference
to the following documents;
• Central Networks “Local Management Instruction for the Selection and Approval of
equipment to be used on the Distribution Network”
• Central Networks “Local Management Instruction for Modification of the Distribution
Network”
• Section 2 of this Manual, Network Modification Procedure.
1.4.5.4 Voltage Regulation
Voltage regulation on 11kV & 6.6kV feeders shall be in accordance with Section 4.2.2 of this
manual. A power factor of 0.97 should be normally assumed for the calculation. Supplies to
large industrial loads may need special consideration if a less favourable power factor is
suspected.
Circuit configurations that rely on multiple switching operations in order to maintain voltage
during back-feeding will only be permitted with the specific approval of the Network Manager.
When comparing different network design options the circuit copper losses shall form part of
the decision making process.
See Tables 3.3.1.5 and 3.3.2.3 for Cable and Overhead Line Regulation and Losses.
1.4.5.5 11kV Cable Applications

Underground cables
All new 11 kV cables shall be laid in black rigid twinwall corrugated ducts to ENATS12-24 with
warning tape. Short sections of 11kV cable may be laid direct only where ducts cannot be used
such as sharp bends or at jointing positions. Warning tape shall be used in these situations.
Warning tape may be omitted where cables are installed in ducts by trenchless cable laying
techniques.
Cable ratings on ring mains shall be based on the Winter 5 Day Distribution Rating – 50%
Utilisation. Where the ring is asymmetrically loaded ( e.g. 65% - 35%) the 75% utilisation rating
shall be used.
New ring main cables shall be XLPE Triplex. The ratings shown in Table 3.3.1.4B shall be used
which assume two cables leading to a ring main substation, laid in the same trench with the
ducts touching. i.e. a group of 2 cables not thermally independent.
The 5 Day Distribution ratings assume a mix of domestic, industrial and commercial loads. If the
ring consists of exclusively industrial / commercial load then sustained ratings may have to be
used.
Where tee-off connections are permitted by the 11kV Connectivity Rules 185mm2 Al XLPE
Triplex shall be used in urban and suburban areas to cope with system fault level. In rural
areas 70mm2 Al XLPE Triplex may be used where the fault level is below 126MVA.
The first sections of a ring main from a Primary or BSP Substation shall be a minimum of 300
mm2 XLPE Al Triplex for sufficient distance to for to reduce the load by 650kVA (34 amps
@11kV). The remaining parts of the ring main shall be 185 mm2 XLPE Al Triplex (rated at 366
amps) This ensures an overall ring main rating of 400 amps.
Where the Network Manager specifies a 600 amp ring main this will be specially designed.
Cable ratings on duplicate circuits supplying industrial /commercial loads shall be based on
the Summer Sustained Rating unless the proposed load cycle is known with confidence. See
section 3.3.1.5. and Table 3.3.1.5A
Tables 3.3.1.4D and 3.3.1.4E contain 5 Day Distribution Ratings – 50% Utilisation for existing
11kV belted Paper Insulated Cables - metric & imperial sizes.
The above ducted ratings of XLPE Triplex cable match the ratings of PICAS belted cable laid
direct. However, care must be taken when connecting a ducted XLPE cable to a paper insulated
cables. The XLPE cable ratings are obtained at a conductor temperature to 90OC. Paper belted
cables are rated at 65OC. There is a danger of transferring excessive heat from the XLPE cable to
the paper cable via the transition joint. To overcome this, ducted XLPE cable must be laid direct
for a length of at least 1 metre after leaving a duct to allow the conductor temperature to drop to
65OC. (Thermodynamic calculations have demonstrated that a 1 metre length is sufficient reduce
the core temperature by 25OC.)

Table 1.4.4.1 11kV Cable Applications
Size / Type Application
70 mm2 Al XLPE Triplex • Central Networks EAST only

• Fault level must be below 6.6kA (126 MVA)
25mm2 wire screen
• Earth fault level must be below 3.2kA for 1 second.
• Tee-off circuits on rural networks up to 52 amps (1 MVA) P2/5
security standard
95 mm2 Al XLPE Triplex • Central Networks WEST only

• Fault level must be below 8.9A (170 MVA)
35mm2 wire screen
• Tee-off circuits on rural networks up to 52 amps (1 MVA) P2/5
security standard
185 mm2 Al XLPE Triplex • Fault level must be below 17.4kA (332 MVA)
35mm2 wire screen
• Tee-off circuits on all urban networks and where the fault level
exceeds rating of 70 mm2 or 95 mm2Triplex
• Ring Main circuits up to 368 amps
• Industrial/Commercial loads up to a firm capacity of 309 amps (5.9
MVA @ 11kV 3.5 MVA @ 6.6kV)
300 mm2 Al XLPE Triplex • Fault level must be below 28.2kA (538 MVA)
35mm2 wire screen
• Ring Main circuits up to 489 amps.
MVA @ 11kV 4.6 MVA @ 6.6kV)
• High capacity inter-connectors between Primary/Grid Substations
• First 1 km from Primary/BSP Substations
• Situations where 185 mm2 would result in excessive voltage drop.
300 mm2 Cu XLPE Triplex • Earth fault level must be below 4.5kA for 1 second.
• Ring Main circuits up to 629 amps.
35mm2 wire screen
• Multiple runs of cables exiting Primary substations –to offset de-rating
MVA @ 11kV 5.9 MVA @ 6.6kV)
400 mm2 Cu XLPE Triplex • Earth fault level must be below 4.5kA for 1 second.
• Multiple runs of cables exiting Primary substations –to offset de-rating
35mm2 wire screen
• Industrial/Commercial loads up to a firm capacity of 600 amps (12
MVA @ 11kV 7 MVA @ 6.6kV)

1.4.5.6 11kV Overhead Line Applications
The ratings of lines built before 1971 are based on 50oC conductor temperature. See tables
3.3.2.2C,D&E in section 3.3.2.2. These lines can be identified by the conductors’ imperial sizes.
Lines build after 1971 use metric size conductors and have ratings based on 75oC (Central
Networks West) or 60oC (Central Networks East) conductor temperature.
Central Networks West

Overhead line ratings on ring mains shall be based on the Winter Distribution ratings at 75oC
conductor temperature. See table 3.3.2.2B in section 3.3.2.2.
Overhead line ratings on duplicate circuits supplying industrial /commercial loads shall be
based on the Summer Distribution ratings at 75oC conductor temperature. See table 3.3.2.2B in
section 3.3.2.2.
Table 1.4.4.2 11kV Overhead Line Applications – Central Networks West

50 mm2 AAAC (Hazel) • Fault level must be below 4.5kA (86 MVA) *
• Ring Main circuits
• Tee-off circuits up to 52 Amps (1 MVA) P2/5 security standard
* Fault level may be up to 10kA (190 MVA) if protected with NEMA
standard 11kV expulsion fuses fitted with arc shortening rods.
100 mm2 AAAC (Oak) • Fault level must be below 8.9kA (170 MVA).
• Interconnectors between Primary Substations
• Ring Main circuits
• Tee-off circuits where the fault level exceeds the rating of 50 mm2
AAAC which cannot be fused.
200 mm2 AAAC (Poplar) • Interconnectors between Primary Substations
Central Networks East

Overhead line ratings on ring mains shall be based on the Winter Distribution ratings at 60oC
conductor temperature. See table 3.3.2.2A in section 3.3.2.2.
Overhead line ratings on duplicate circuits supplying industrial /commercial loads shall be
based on the Summer Distribution ratings at 60oC conductor temperature. See table 3.3.2.2A in
section 3.3.2.2.
Table 1.4.4.3 11kV Overhead Line Applications - Central Networks East


50 mm2 ACSR (Rabbit) • Fault level must be below 4.5kA (86 MVA) *
• Tee-off circuits up to 52 Amps (1 MVA) P2/5 security standard
• Sub ring circuits (e.g. spurs interconnected to improve local
security)
* Fault level may be up to 10kA (190 MVA) if protected with NEMA
standard 11kV expulsion fuses fitted with arc shortening rods.
100 mm2 ACSR (Dog) • Fault level must be below 8.9kA (170 MVA).
• Tee-off circuits where the fault level exceeds the rating of 50 mm2
ACSR which cannot be fused.
• Minor diversions of existing 100mm2 ACSR lines.
150 mm2 ACSR (Dingo) • Fault level must be below 13.5kA (260 MVA).
• Ring Main circuits up to 400 amps (7.6MVA @ 11kV)
• Interconnectors between Primary/Grid Substations
• Industrial/Commercial loads up to a firm capacity of 7.4 MVA – two
lines at 50% of continuous summer rating with 300 mm2 Al XLPE cable
sections.
300 mm2 HDA (Butterfly) • Fault level must be below 23.4kA (450 MVA).
• Normally used for 33kV circuits
• May be used at 11kV for Industrial/Commercial loads up to a firm
capacity of 12.0 MVA – two lines at 50% of continuous summer
rating – 400 mm2 Cu XLPE cable sections.
1.4.6 11kV Connectivity Rules
Under Condition 9 of our Electricity Distribution Licence, Central Networks has a duty to comply with
the Distribution Code of England and Wales. Engineering Recommendation P2/5 under this Code sets
out the minimum levels of supply security for distribution networks.
P2/5 addresses levels of supply security for distribution networks classified in ranges of group demand
only and not individual customers. The application of P2/5 to loads below 1MW results in an open
ended restoration time i.e. repair time. A network designed to this minimum requirement would result
in customer service levels, (customer minutes lost and time to supply restoration) well below
Regulatory targets and customer expectations.
These 11kV Connectivity Rules specify the methods of connecting substations and transformers to the
11kV network to enable:
the 11kV network to be broken up into sections by switching to enable faults to be localised.
faulty sections to be isolated and the remaining healthy network sections to be restored to
service.
supplies to customers within faulty sections of 11kV network to be restored by LV back-feeding

or mobile generation.
sections of the network to be isolated for routine work with minimal disruption to customers.
1.4.6.1 Derogations to 11kV Connectivity Rules and/or P2/5
These rules shall not apply to installations designed or installed before 1st July 2004.
Existing networks shall be brought up to these standards during refurbishment or replacement as

specified by the Network Manager.
Derogation of these rules at a customer’s request, for example to reduce to the cost of a new
connection, shall not be permitted unless the intended occupiers of the premises to be supplied are
aware of, and accept in writing, the reduced supply security and it’s implications. Central Networks
will not accept requests for derogations from developers or their agents as they are generally not in a
position to fully comprehend the requirements of the future occupiers. Network extensions that fail to
meet these requirements shall not be connected to Central Networks’ network.
In practice this means:

Residential housing and Commercial / Industrial estates – at the time the new connection is
designed it is unlikely that all or any of the intended occupiers will be known. As it is not
possible to seek their approval of a proposed derogation then the 11kV Connectivity Rules
shall be applied in full.
Large individual Industrial / Commercial premises – it may be possible to identify the
intended occupier e.g. the client. Written confirmation shall be obtained from that part of
the client’s business responsible for the future operation of the facility and not just those
involved in constructing it. Otherwise the 11kV Connectivity Rules shall be applied in full.
Derogations to Engineering Recommendation P2/5 will not be agreed by Central Networks
without a referral to the industry regulator for a determination.
Where a customer is prepared to pay for a level of supply security exceeding the Central Networks
11kV Connectivity Rules and/or P2/5 this may be provided subject to the solution being technically
feasible, does not prejudice the flexible operation and/or security of the network and is approved by
the Network Manager.
1.4.6.2 Ground Mounted Substation Rules
All new substations shall be equipped with 11kV ring main switches.
An earth fault indicator shall be provided on one of the ring switches.
Where connected to an underground or overhead line system the substation shall be ring
connected if within 200m of the ring main or spur line. All loads over 1000kVA shall be ring
connected regardless of the distance from the ring main.
Tee connected substations shall not be connected to the first cable section from a Primary or
BSP substation. Interconnected tee connections may be acceptable provided the
interconnection is not to another first section of the same ring main. Note – the first cable
section is defined as the cable to the first ring connected substation or overhead line terminal
pole.
Substations shall not be connected in such a manner that part of the network is sterilised
during maintenance work. e.g. a single substation tee connected across two sections of the
same ring main would require part or all of the ring to be shut down for work on the substation.

Primary or BSP Examples of ring main

S/S 11kV
sterilisation.
A single substation requires

both side of the ring main to
OK if two or more be isolated for maintenance
substations are work on that substation.
connected.
A second substation ensures

Single substations
can sterilise that isolation is available
the ring main without interrupting both side
of the ring main
simultaneously.
Ring connection may be achieved by:
1. Laying two cables in common or separate trenches and looping into an

existing ring main.
2. Laying one cable from an existing main or substation and extending via the
new substation to link up with another substation or ring main. It may
sometimes be necessary to await future developments to complete the ring
or interconnector resulting in spurs outside the requirements of the 11kV
Connectivity Rules and/or P2/5. Such arrangements should not be
undertaken without a connection strategy, approved by the Network
Manager, which provides for completion of the ring within a reasonable
period. Each case must be judged on merit but in general the aim should
be provision within 3 years.
3. Where the ring main is an overhead line it shall be terminated in both
directions and the substation cables connected into the ring. If terminal
poles are impractical e.g. for wayleave reasons, then two adjacent section
poles shall be created leaving a dead span between with the substation
cables connected into the ring at each section pole. Under no circumstances
shall two cables be connected either side of a single section pole.
4. Where an overhead line spur is the only reasonably practicable connection

point the substation shall be ring connected as per ‘3’ in order to provide
sectionalising facilities to aid fault localisation and to limit the need for
mobile generation. Where the spur is underground cable the substation shall
be ring connected into the spur. Future interconnection at the remote end of
the spur will enable a full switched alternative supplies to be available
without further work to install a second circuit into the substation.

1.4.6.3 Padmount Transformer Rules
Padmount transformers shall not be connected to the first cable section from a Primary or
BSP substation. Note – the first cable section is defined as the cable to the first ring connected
substation or overhead line terminal pole.
Padmount transformers up to 200 kVA may be tee connected to ring mains or spurs under the
following circumstances:
1. Where a full LV back feed is available from a source(s) that can remain on supply
whilst the section that the padmount is connected to is isolated. Note that this back-
feed will need to come from each of the substations either side of this section so at
least one will be available during maintenance work. Maximum of one padmount
between switching points.
2. Without LV back-feeds on low customer density rural underground ring mains or spurs
subject to the following requirements being met:
Maximum of 50 customers and 2 padmounts between switching points
and
An aggregate transformer nameplate capacity of 400kva with a
maximum individual 200kva transformer in any group of up to 2
padmounts
and
Customer base is predominately domestic, farming or light
industrial/commercial i.e. no individual customers over 140kVA
(200amps).
and
There is sufficient space near each padmount to safely park a mobile
generator and fuel tank during supply interruptions. (e.g. 100kV
generator – 4m x 2.5m plus 2m x 2m for fuel tank)
or
The customers have their own contingencies for supply interruption. e.g.
pumping stations, mobile phone base stations etc.
Individual loads over 140kVA shall be supplied from suitably rated compact unit
substation connected into the ring main.
Where the proposed number of padmounts will exceed 2 then a compact unit
substation instead of a padmount or the section broken up with a free standing ring
main unit.
The following drawings show typical circuit layouts using these rules

Up to 2 Padmounts
Low density rural G G
Primary or BSP underground
S/S 11kV More than 2 -
split the section
with a 200kVA ring
A/R main unit or S/S
RC RC RC
G
M M
Domestic / HV Metered supply
Light Comerical
1/3 LV back-feed NOP
High security HV
Metered supply
Change-over on
customer's side
M M NOTE SINGLE
C/O RMU NOT OK
across ring
Insustrial /
NOP
G
Heavy Comerical
No LV back-feed
Use Mobile Generator
High Rise Building

Domestic /
Light Comerical
1/3 LV back-feed
M HV Metered
supply A/R
Tee-off OK up to 3.5MW RC
Two LV Backfeeds
NOP
LV Distribution Pillar LV Link Box G Provision to connect generator M HV Metering
Fault Flow Indicator

Underground Cable Expulsion
Link NOP Normal Open Point
Overhead Line Fuse
Automatic Live Line Taps RC Remote Control
Sectionalising link
Ring Switch Solid connection A/R Auto Reclose
Air Break Switch
Disconnector
Circuit Breaker CENTRAL NETWORKS
Pole Mounted
Sectionaliser Ground Mounted Substation and Padmount
Padmount
11kV Circuit Configurations
Transformer
Pole Mounted DRAWN BY TH Jan 04 Scale N.T.S. Class-
Transformer Automated Recloser CHECKED BY

1.4.5.2 & 3
APPROVED BY

1.4.6.4 Overhead Line Rules
Few circuits consist wholly of overhead line. The Ground Mounted Substation and Padmount
Transformer Rules apply where appropriate in mixed underground/overhead circuits.
Switching devices
The following overhead line switching devises shall be used where defined in these rules:
Air Break Switch Disconnectors (ABSD) – three phase ganged, manual independent
spring assisted high level mechanism, 230MVA fault close, 630 amp load break.
NEMA Universal Fuse Mounts – single phase un-ganged, 27kV insulators, maximum
connected transformer capacity of 500kVA, maximum fault level 190MVA and fitted
with either:
o Expulsion fuse – 50 amp slow blowing with arc shortening rod.
o Automatic Sectionalising Link (ASL) – 50 amp pickup, 2 pulse
o Solid link – rated at 8kA for 1 second fault current.
Live Line Taps – 400 amp rating, used with 95mm2 aluminium or 70mm2 copper flexible
jumper and connected to the main line via bail clamps.
Jumpers comprising of the same material as the overhead line or coverd conductor
which can be removed to provide an isolation point either by hot glove/stick working
or under Permit to Work. Sizes shall be equal to or greater than the overhad line size
or 50mm2 al / 32mm2 cu for transformer connections.
Remote controlled equipment, such as Pole Mounted Auto Reclosers, Motor Operated
ASBDs, Gas Insulated Switches, are installed at strategic positions on the network as
decided by the Network Manger. The provision of such equipment is not covered by
these rules. Refer to Section 1.4.7 11kV Network Automation.
Application of switching devices.
1. Air Break Switch Disconnectors shall be installed:
To divide ring mains into sections if any of the criteria below are exceeded:
A maximum of 1000 kVA of transformer capacity
A maximum of 500 customers
A maximum of 5 km of ring main
Additional connections that cause any of the above criteria to be exceeded shall
require an additional ABSD installing.
On the next pole to a Pole Mounted Auto Recloser or Gas Insulated Switch for
use as a point of isolation that complies with the Distribution Safety Rules.

2. Air Break Switch Disconnectors shall be installed on spurs:

Of with between 500kVA and 1000kVA of connected transformer capacity.
Additional connections that take the capacity over 1000kVA shall require the
spur to be converted to a ring main.
On spurs with ferroresonance problems. i.e. a mixture of underground cables
and transformers that cannot be isolated independently. e.g. when adding a
cable section to a spur not all existing transformers will have local solid links.
Rather than install links on a number of transformer the installation of an ABSD
may be more economical.
On spurs over 5 spans where the fault level is over 190MVA thus preventing
the use of ASLs or fuses
3. Solid links in NEMA universal fuse mounts shall be installed:
Only where the fault level is below 190 MVA.
Only where the connected transformer capacity is less than 500 kVA
On all pole transformers and on all cable spurs to provide isolation facilities
during fault localisation. NB the present use of high speed protection all but
eliminates visible damage to transformers and surge arrestors making it difficult
to identify faulted plant. Local isolation facilities enable restoration staff carry
out further fault localisation within sections of line.
On cables or lines in ferroresonant spurs to enable transformers and cables to
be independently isolated. In some circumstances a small number of links
may be a more economical option than installing an ABSD.
4. Automatic Sectionalising Links in NEMA universal fuse mounts shall be installed:
On all spur lines at the first pole away from the tee-off or other accessible
location before the first transformer where:
• The spur is downstream of a Pole Mounted Auto Recloser or the
Primary Substation circuit breaker fitted with multi-shot auto
reclosing with at least 3 shots.
5. Expulsion Fuses in NEMA universal fuse mounts shall be installed:

On all spur lines at the first pole away from the tee-off or other accessible
location before the first transformer and where the spur is upstream of the
first Pole Mounted Auto Recloser on the circuit and the Primary Substation
circuit breaker is not fitted with multi-shot auto reclosing with at least 3 shots.
6. Live Line Taps shall be installed:
To connect all Pole Mounted Auto Reclosers, Sectionalisers and Remote

Controlled Switches to lines to enable routine maintenance to be carried out.
New ABSDs, Fuses, Links, ASLs, Spurs or Transformers (with links) may initially
be connected by live line taps where hot glove/stick connection of jumpers is
not reasonably practicable or does not meet live line risk assessment criteria.
They should be replaced by jumpers during the next line outage.
7. Jmpers shall be installed:
In all equipment not specified above including new ABSDs, Fuses, Links, ASLs,
Cable Terminations & Section Poles.
Ferroresonance
Ferroresonance occurs when transformers and cables are switched together by single phase
devices. The capacitance of the cable is effectively placed in series with the non-linear inductance
of the transformer winding. When one or two phases are made dead the circuit resonates and
creates high over-voltages which can damage equipment. In practice the surge arresters fitted to
cables and transformers clip the voltage to about 25kV but then quickly overheat and become
short circuit for several hours before cooling down and recovering. Re-energising the circuit before
the arresters have recovered results in an earth fault.
When the transformers have more than 5% of full load connected to the LV terminals
ferroresonance does not occur so the operation of fuses or ASLs on a loaded ferroresonant spur is
not a problem.
However, planned work is often done with the LV loads disconnected or being run on mobile
generation. Therefore facilities have to be provided to enable transformers and cables to be
switched separately.
The following drawings show typical overhead circuit layouts using these rules

Primary or BSP
S/S 11kV
2 shot A/R A/R Multi shot (3 or more) Interconnector

RC RC to another
Primary S/S
A/R
Fault Level, RC
above 190 MVA
NO FUSE, LINKS
or ASLs
190 MVA
A/R
Fault Level, 190 MVA RC
below 190 MVA
ASL OK if
Maximum of
source A/R
2 PMARs
is multi shot
in series
See Drawing 1.4.5.4.b for

detailed arrangement for
spur lines A/R
No ASLs RC
upstream of
first PMAR
Interconnector A/R
to another RC
Primary S/S NOP
FFIs, PMARs, RC
NOP Remote control ABSDs
& Sectionalisers to meet
strategic supply restoration
targets
RC
A/R
P/T on main line
connected via links RC
Underground Cable Surge Arrestor

Link Fault Flow Indicator
Overhead Line Expulsion
Fuse Live Line Taps
NOP Normal Open Point
RC Remote Control
Ring Switch Automatic Solid connection
Sectionalising link A/R Auto Reclose
Air Break Switch
Circuit Breaker Disconnector
Central Networks
Pole Mounted General configuration of
Padmount 11kV Overhead Network
Sectionaliser
Transformer

APPROVED BY 1.4.5.4.a

Fault Level above 190 MVA Fault Level

NO FUSE, LINKS or ASLs BELOW 190 MVA
P/T on main line
P/T s on spur line

Upstream of PMAR
1000 kVA Max 500 kVA Max
P/T s on spur line

Downstream of PMAR
or multi shot source
circuit breaker 1000 kVA Max 500 kVA Max 500 kVA Max
Too many existing P/T

Examples of Ferroresonant spurs to fit individual links
Fit links in line and on
P/Ts upstream of cable
to enable seperate
isloation of cable and
transformers
Padmount to be
connected to complex
spur. Install ABSD as well
as links on cable to new
padmount.
Underground Cable Surge Arrestor

Link Fault Flow Indicator
Overhead Line Expulsion
Fuse Live Line Taps
NOP Normal Open Point
RC Remote Control
Ring Switch Automatic Solid connection
Sectionalising link A/R Auto Reclose
Air Break Switch Central Networks
Circuit Breaker Disconnector
Pole Mounted 11kV Overhead Spur Configuration
Padmount Overhead Network
Sectionaliser
Transformer

APPROVED BY 1.4.5.4.b

1.4.6.5 LV Back-feeds and Mobile Generation
Ground Mounted Substations
Where technically possible sufficient LV back-feeds shall be provided to ground mounted

substations ensuring that during back-feeding:
The maximum regulation on the LV network being back-feed does not exceed
12% of 230v when supplied from “Standard 11kV Feeders”.
10% of 230 volts when supplied from “Long 11kV Feeders
assuming 33% of the design ADMD. i.e. routine work planned for times of light load. The
ring main connection allows for full HV restoration for 11kV cable faults.
and
The maximum loop impedance on the LV network does not exceed
o 0.52 ohms on non-electric heating networks
o 0.38 ohms on electric heating networks
o 0.19 ohms on commercial / industrial networks
These values will ensure that cut-out fuses will operate with 5 seconds as required under
the current edition of BS7671.
The remaining risk of LV cable faults clearing in over 100 seconds is deemed to be
acceptable as the network will be under close technical supervision during the course of
the work.
Where the LV back-feed cannot be designed to meet these criteria then there must be
sufficient space and access for a mobile generator to be parked and connected. Note that
generators over 100 kVA are delivered and parked on the trailer of an articulated lorry.
Typical 200kVA generator used for supply restoration. Larger sizes have a separate fuel tank.

Padmount Transformers
Padmount Transformers on low density rural underground networks need not have LV back-
feeds where provision is technically impossible. Mobile generator access must be available to
restore supplies during cable faults and for planned work where justified.
Where technically possible sufficient LV back-feeds shall be provided to padmount

transformers ensuring that during back-feeding:
assuming 100% of the design ADMD. Because the padmount is tee connected the LV back
feed could be required at times of maximum load due to an 11kV cable fault.
and
the work.
If necessary both LV cables should have separate back-feeds to avoid feeding through the
bus-bars.
If the above criteria cannot be met then a 200 kVA ring main substation shall be installed.
Pole mounted transformers
Pole Mounted Transformers need not have LV back-feeds where provision is technically
impossible. Mobile generator access must be available to restore supplies during transformer
faults and for planed work where justified.
Where technically possible sufficient LV back-feeds shall be provided to padmount
transformers ensuring that during back-feeding:

assuming 33% of the design ADMD if the spur is totally overhead or 100% if there is
underground cable in the spur. Most transformer faults occur during the summer in
lightning storms. Cable faults may occur at times of maximum load.
and
o 0.19 ohms on commercial / industrial networks
the work.
Where there is more than one LV cable connected to the Pole Transformer each will
require a back-feed as Pole Transformers are not equipped with transformer LV isolators.
If the above criteria cannot be met then mobile generator access must be available to
restore supplies during transformer or cable faults and for planned work where justified.

1.4.7 Physical Siting of 11kV Substations, Cables & Lines

All references to substations in this section shall also include switching stations and padmount
transformers.
1.4.7.1 Location and Operational Access to Substations
Substations shall be sited at ground level and have 24 hour access available from the public
environment by using the Central Networks standard operational key.
• Compact Unit Substations up to 1000 kVA and ring main + metering units shall normally be
housed in the standard Central Networks Glass Reinforced Plastic (GRP) housing with
approved explosion relief features. A plot of land 4m x 4m is required.
• Developers may opt to provide a free standing brick build housing to Central Networks’
specification employing a GRP roof with approved explosion relief features. A plot of land 4m
x 4m is required. Substations shall not be attached to houses or garages.
• Substations shall not normally be situated inside customer’s premises or built into the wall of
a building without the permission of the Networks Manger. Where this is unavoidable the
allocated space must provide certain clearances around the equipment for operational access,
maintenance and internal arc relief. For compact unit substations and ring main units these
clearances are;
o Equipment side/rear to wall – 750mm - maintenance & internal arc relief
o Front – 1000mm or less if towards fully opening doors – operational requirement
o Min ceiling height of 2.1m but at least 500mm clearance above the switchgear – to
comply with the internal arc relief requirement s of ENATS 41-36
• Substations shall not be situated on roofs or in basements.
• Substations shall not normally be installed inside residential buildings. See section 1.4.7.5.
• Substations supplying LV networks without LV back-feeds shall have suitable space available
within 25 metres to safely park and connect mobile generation of the required capacity.
In high density urban situations these rules may be relaxed at the discretion of Network Manager.
However, developers are expected to make provision for substations at an early stage of the design
work.
The Construction Design & Management Regulations require the designer to consider the entire life cycle of an
installation including construction, use, maintenance and eventual demolition. Substations inside customer’s
property cannot be immediately disconnected from the network upon notification that the supply is longer required
as network alterations have to be planned and executed to joint out the substation. There have been a number of

instances where third parties have been injured whilst interfering with high voltage equipment located inside the
boundary of a vacated property. The Health and Safety Executive have been critical of having live high voltage
equipment inside vacated premises. Placing the Central Networks 11kV switchgear on or outside the boundary fence
enables Central Networks to make all equipment inside the property dead by switching upon notification that the
supply is no longer in use.
1.4.7.2 Routing of 11kv Overhead Lines
Overhead lines represent a potential hazard to members of the public and workers from accidental
contact. Overhead line routes shall be planned bearing in the mind the type of use, or foreseeable use,
to which the land is or may be put.
Central Networks follows the requirements of ENATS 43-8 ‘Overhead Line Clearances’. Road clearances
are applied to agricultural land in recognition of the size of agricultural equipment now in common
use. See Section 3 of the Overhead Line Manual Volume 1 for further information.
High Load Routes

Certain roads are designated as High Load Routes by the Department of Transport. These require a
ground clearance of 7.1m in order to accommodate vehicles up to 6.1m high. See ENATS 43-08 section
6.1.
When planning a line to cross a road the designer shall establish whether or not it is a designated
high road route and specify the clearance appropriately.
Customer & Network Operations maintains the list of the high load routes and line crossings within
Central Networks.
Un-insulated 11kV overhead Lines
Un-insulated overhead lines shall not be routed across land used for:
Public recreation including;

Playing fields
Car parks
Caravan / camping sites
Lakes, rivers and canals
Gardens of residential dwellings.
Woods / forests
Any sites identified as being used for recreation with or without the owner’s permission.
Work activities other than farming including;

Factory yards, loading bays etc.

Commercial vehicle parking areas
Hopyards, orchards, garden centres
Commercial wood/forests
Any sites identified as being a unacceptable hazard to workers
Routing overhead lines across farmland

A risk assessment must be carried out and any unacceptable locations must be avoided. Examples
include:
Locations where regular loading /unloading activities take place
Fields where portable irrigation pipes are regularly used
Any locations identified as being a potential hazard to farm workers
Pole mounted transformers and switchgear.

Live jumpers may be as low as 4.3m from ground level – such equipment must be placed
where accidental contact is judged to be unlikely.
Free standing (cable supplied) pole transformers shall not normally be used as locations
unsuitable for overhead line are unlikely to be suitable for low level exposed live terminals.
Overhead line Wayleave terminations – these are often prompted by a change use of the land.
Pole transformers should not be retained as free standing transformers unless it is clear that
the new use of the land does not place persons at an unacceptable risk.
Vehicular Access
The Working at Height Regulations require that a safe method access is provided to tall structures.
The Health & Safety Executive’s preferred hierarchy of access is

1. Mobile elevated work platform (MEWP)
2. Ladders
3. Free climbing with permanent attachment for fall arrest equipment
4. Free climbing by first person to install temporary fall arrest equipment.
Options 2 to 4 are only acceptable where vehicular access for a MEWP is not possible on existing
overhead lines constructed before the Working at Height Regulations came into force.
All new overhead lines must be constructed along routes accessible to a four wheel drive MEWP.

Hazard reduction
Where un-insulated overhead lines represent an unacceptable risk the following alternatives are
available in order of preference;
1. Underground cable
3. Covered Conductor.
4. Large cross section bare conductors – risk assessment
1. Underground cable
a. This option eliminates the risk of accidental contact with exposed live equipment.
b. May not be economically viable or practicable in certain situations such as river /
canal crossings.
c. May not be an environmentally acceptable solution through woodland where
damage to tree roots may occur during cable trenching.
d. Not suitable in commercial forests where excavations during harvesting and re-
planting may represent an unacceptable risk of cable damage.
2. Covered Conductor.
e.g. BLX, SAX, PAS, Amokraft.
a. This is NOT fully insulated. Covered conductor itself is not designed to protect persons
from accidental contact. Contact with trees can be maintained for a few hours or
days depending on the insulation type.
b. Every 2nd or 3rd pole top has exposed live connections for Arc Protection Devices
(arcing horns known as APDs). These prevent the line being burnt down by direct
lightning strikes. These may be omitted within wooded areas where direct strikes will
occur to trees rather than the line.
c. May be used to cross rivers, dykes, canals and small lakes. This is primarily to reduce
fault incidence caused by water-fowl strikes. Covered conductor may reduce, but not
eliminate, the hazard of electrocution resulting from accidental contact by fishing
rods / boat masts. APDs must not be installed on the poles either side of the crossing
or on access routes to the water where contact could be made with the exposed live
APDs. Warning signs must be displayed and fishing/boating activities must be
restricted in proximity to covered conductor lines to exactly the same standards as
bare wire overhead lines.
d. Must not be used alongside rivers, canals dykes, canals, lakes and other leisure areas
as the primary method of protecting the public.
e. Available up to 185mm2 i.e. equivalent to 150mm2 O/H line.
3. Large cross section conductors

a. Covered conductor systems are not available for conductors over 185mm2.
b. New heavy duty lines, such as 200 mm2 or 300mm2, should be routed to avoid, or be
under-grounded in, areas of high risk where reasonably practicable.
c. Where river, canal, dykes crossing are unavoidable then a risk assessment should be
made taking into account the frequency and type of activities at risk. The crossing
should preferably be at right angles but no more 30O to minimise the length exposure
of the un-insulated line to a minimum. Taller poles shall be used where these can
mitigate the risk. The Overhead Line Manual Volume 1 Section 3 “Overhead Line
Clearances” Table 6 lists clearances across certain named waterways in the Central
Networks East area as agreed with the British Waterways Board.
1.4.7.3 Routing of 11kV Underground Cables
Underground cables are vulnerable to damage by persons excavating and may then be a source of
danger due to the explosive release of energy during failure. However, cables need to be excavated
and worked on from time to time for routine repairs, networks extensions, replacement etc. Cables
shall be laid in black rigid twinwall corrugated ducts to ENATS 12-24 and identified with warning tape
at the depths specified in the Central Networks Cables, Cable Laying and Accessories Manual, Section
3, “Cable Laying Technical Requirements”.
Preferred routes
Underground cable routes shall be planned bearing in the mind the type of use, or foreseeable use, to
which the land is or may be put.
The preferred options for cable routes are along the footpaths and across roads of public highway.
Routes should not be planned along carriageway unless other alternative are not reasonably
practicable. Where possible joints should be located off the carriageway by diverting the duct line into
suitable locations.
Where a route is across private land is unavoidable then a Permanent Easement must be obtained.
Routes to avoid
Areas of land and natural features have legal protection due to their environmental character or
sensitivity. Such designations include Sites of Special Scientific Interest (SSSI), National Nature
Reserves, Special Areas of Conservation and Scheduled Ancient Monuments. See Section 8.
“Environmental Requirements” for more information.
Special precautions are needed when digging near trees to avoid damage to tree roots. Hand digging
may make a shorter route through trees more expensive than a longer route to avoid them.

Routes through commercial forests should be avoided. Mechanical equipment used to harvest and re-
plant trees exposed the cable to accidental contact. The lack of permanent features to act as tie-in
points makes meaningful recording of the route and future use of plans difficult.
Routes across agricultural land should avoid locations where excavation may be predicted e.g. gate
posts. When crossing ditches the laying depth should be increased to keep the cable well below ditch
clearing excavations. It should also be ascertained if the landowner carries out sub-soil ploughing to
un-compact the soil. Where necessary an increased laying depth should be agreed.
1.4.7.4 Earth Potential Rise
At hot sites there may be issues relating to the proximity of Central Networks earthing to third part
equipment, buildings, fencing etc. which could be affected by transfer potentials during earth faults
on the Central Networks 11kV network.
Also, Central Networks 11kV & LV equipment may be affected by transferred potentials from the earths
of higher voltage installations in the proximity. (e.g. 33kV, 132kV, 275kV, 400kV circuits and railway
25kV supplies).
The Earthing Manual shall be consulted and where reasonably practicable the substation shall be
sited in a position where a standard earthing design will be safe. Where necessary an earthing
consultant shall be engaged to design a specific earthing system for that location.
1.4.7.5 Environmental Constraints
Noise
Substations shall not normally be installed inside residential buildings. Low frequency vibrations from
transformers will be transmitted through the building foundations and become apparent to residents
at periods of low ambient noise. Where it is unavoidable, typically in high density urban areas, Central
Networks may permit a substation to be installed inside provided the transformer foundations are
acoustically isolated from the building foundation.
Escape of Insulating Oil
All proposed substations shall be individually assessed to ascertain the environmental risk, if any,
associated with its immediate surrounding environment. The key point being to identify any potential
for pollution of controlled waters in the event of an oil release and thus determine the need for
control measures.

Control measures include:
• Siting the substation in a less sensitive location – preferred option.

• Using a bunded foundation to contain escaping oil
• Use of an environmentally benign alternative to mineral oil approved by Central Networks.
Refer to Section 8 Environmental Requirements for further information.

.
Water and Gas
The Electricity Safety, Quality, and Continuity Regulations 2002 Regulation 3(4) require generators and
distributors to take precautions to prevent, so far as is reasonably practicable, danger due to the influx
of water, or any noxious or explosive liquid or gas, into any enclosed space, arising from the
installation or operation of their equipment.
Environments affected by this regulation include customers’ premises, e.g. basements and stairwells,
and generators’ and distributors’ own premises, e.g. substations and cable basements.
Examples of substances that may cause danger or disruption to the public supply of electricity include:
• water due to burst mains, flash floods or fire fighting activities
• methane leaking from gas pipes in the ground
• leakage of SF6 from switchgear displacing air
New substations shall not be placed below ground level.

All cables ducts into substations shall be sealed.
Equipment designed specifically for submersible operation may be used to replace existing
underground substations only.
1.4.7.6 Substation Legal Requirements
Please refer to the Central Networks Wayleaves and Property Policy and Procedures Manual - Freehold
/ Leasehold Acquisitions section. Note that this Manual is confidential to Central Networks.
Information from this manual may be released to third parties where relevant to the work being
undertaken.

1.4.8 11kV Network Automation

The term Network Automation includes equipment that either:
1. Autonomously carries out a sequence of pre-planned switching operations in the event of a

supply failure to isolate faulty network and re-connect healthy sections.
and/or
2. May be operated by remote control from the distribution network control centre.
The use of autonomous automation is generally confined to auto changer schemes on 33kV and/or
11kV transformer circuit breakers at Primary Substations supplied from incompatible sources than
cannot be permanently paralleled. Certain high voltage customers also have auto changeover scheme
installed.
Remote control of network plant is employed on the 33kV & 11kV network to reduce customer
interruptions and restoration times.
The Networks Manager will specify which locations shall be provided with remote control in order
meet strategic customer service level targets.
Pole Mounted Auto Reclosers (PMAR)
These are installed in circuits with a maximum of two in series on each half of a ring main. Fault
discrimination is achieved by definite time grading. The Primary substation circuit breaker
instantaneous relay is slugged to 400 ms, the first PMAR to 200ms and the second to 50ms. The
normal open point is fitted with either a PMAR or other remote controlled device. See the Central
Networks Protection manual for more details.
Whilst initial fault interruption is carried out automatically by a PMAR tripping, supplies to healthy
sections between PMARs are restored by the distribution control engineer using remote control and
local operational staff.
PMARs cannot be used as a Point of Isolation under the Distribution Safety Rules. An ABSD must be
situated on an adjacent pole to provide isolation.
Remote controlled switchgear
Certain makes and types of switchgear can have motorised actuators and communications equipment
fitted which enables remote control via radio link.
Current options are:

• Power Isolators Rapier Air break Switch Disconnector (ABSD) fitted with double acting
spring mechanism and motor drive control box. The connection rod from the drive motor is
fitted with a mechanical de-coupling devise that enables the Rapier ABSD to be used as a
Point of Isolation under the Distribution Safety Rules.
• Merlin Gerin / NuLec RL27 overhead line gas insulated switch with motor drive control box.
The RL27 cannot be used as a Point of Isolation under the Distribution Safety Rules. An ABSD
must be situated on an adjacent pole to provide isolation.
• Merlin Gerin Ringmaster range of 11kV switchgear may have motor control actuators fitted
and are pre-wired to accept the control box. The motor actuators are easily removable to
enable the switchgear be used as a Point of Isolation under the Distribution Safety Rules.
• Certain makes of other switchgear, such as the Long & Crawford T3FG3 and T4GF3 may be
retrospectively fitted with motor control actuators. There are several makes of motor
actuator available. They must be capable of being mechanically disconnected to enable the
switchgear be used as a Point of Isolation under the Distribution Safety Rules.

1.5 Primary Network 33kV

The 33kV section is under preparation and will be included in due course.
1.5.1 33kV General Considerations
1.5.2 33kV Earthing
1.5.4 33kV Circuit Configurations
1.5.6 Physical Siting of 33kV Substations and Equipment
1.5.7 33kV Network Automation
1.6 Grid Network 132kV

The 132kV section is under preparation and will be included in due course.
1.6.1 General Considerations at 132kV
1.6.2 132kV Earthing

1.6.4 132kV Circuit Configurations
1.6.6 Physical Siting of 132kV Substations and Equipment

2. Network Modification Procedure
2.1 Introduction
This section of the Network Design Manual is aligned to comply with the requirements of the
Powergen Engineering Minimum Standard 017 Plant Modification Procedure. It is implemented in
Central Networks by the Local Management Instruction for Modification of the Distribution Network.
In an electricity distribution network the installation, replacement or disconnection of transformers,

cables, overhead lines and switchgear etc. can have safety and operational implications.
This procedure applies to actions that:
• Reduce or increase network impedance

• Reduce or increase earth electrode resistance
• Reduce or increase protection clearance times
• Change equipment technology
• Change the application of approved equipment from approved standard
arrangements/applications.
• Any other actions that are identified as possible threats to the continued safe and reliable
operation of Distribution Network.
This procedure does not apply to:
• Actions that are identified by a suitably experienced and qualified professional engineer as having
no implications to the continued safe and reliable operation of Distribution Network.
• Actions that were taken before the first publication date of this Manual.
Principles
• For the continued safe and reliable operation of the network all modifications and extensions
must be designed and implemented in a manner that that takes into account their effect on the
existing network.
• Where effects on the Distribution Network’s safety case are identified they are brought to the
attention of a suitably experienced and qualified Professional Engineer for resolution.
• All Central Networks business streams, contractors and consultants making network modifications
shall ensure that they have access to suitably qualified Professional Engineers, experienced in
distribution system design to provide first line technical support to Project Managers, Contract
Managers and Instructed Persons and to liaise with the Assets Manager’s technical specialists.

• Where routine design work is delegated to ‘Instructed Persons’ then their design procedures /
tools must be capable of identifying and highlighting issues that need to be referred to a
Professional Engineer.
• Pre-approved standard designs, equipment and applications are to be used wherever possible on
the distribution network. Any deviation from these standards must be pre-authorised by the Asset
Standards Manager.
The following wording is taken from the Powergen Minimum Engineering Standard 017:
“For the continued safe operation of plant and equipment, all modifications must be designed and
implemented in a manner that is consistent with the original design intent. This has long been
recognised in the nuclear side of the power industry, where formal procedures exist to ensure that
proposed modifications are checked to ensure that they do not breach the safety case.
The danger that can result from inadequate attention to plant modification practices have been
dramatically demonstrated by disasters in many countries around the world. For example, the incident on
the Flixborough chemical plant, UK, in 1974, killed 28 people and injured a further 36. The disaster was
formally attributed to “the introduction into a well designed and constructed plant of a modification
which destroyed its integrity”.
It is therefore essential that adequate formal procedures are drawn up and implemented within the
Company, to ensure that modifications are designed, constructed, tested and maintained to a standard
that is the same as, or better than, the plant being modified.”
2.2 Implications of Network Modifications

2.2.1 Cause and Effect Mechanisms
Cause and effect mechanisms include:
Cause How Effect Down stream Implications

1 Install additional or larger Increased fault level Over-stress plant, cables and
Reduced network plant or circuits. lines.
impedance General protection grading
issues
Paralleling of plant or Increased earth fault
circuits. current Increased Earth Potential Rise
Reinforcement of circuits. Reduced LV loop Possible rise above 16kA max

impedance PSCC
2 Remove plant or circuits. Reduced fault level Increased Protection times
Increased network
impedance Install smaller plant or Increased earth fault
circuits. Reduced earth fault clearance times or operation
current of SEF instead of IDMT
Un-paralleling of plant or
circuits. Increased S/S LV fuse
clearance times
Increased LV loop Cut-out fuse clearance times
impedance may no longer comply with
the current edition of BS7671
Reduced quality of supply –
more flicker

Continued
Cause How Effect Down stream Implications
3 Disconnection of lead Reduced earth fault Increased earth fault
Increased earth covered cables / current clearance times.
electrode resistance replacement by plastic Operation of SEF instead of
sheath cables IDMT
Disconnection of earth Increased Earth Potential Safe voltage / time criteria

electrode e.g. disposal of Rise may be exceeded.
part of a substation site.
S/S or equipment may
Insert O/H line into U/G become ‘hot’
circuit - no metallic earth
path to source.
Replace lead covered LV Loss of separate earth Non PME customers may be
cables with CNE cable conductor affected
4 Remove plant or circuits. Increased duration of Over-stress plant, cables and
Increased protection overcurrent lines.
clearance times Install smaller plant or
circuits. Increased duration of Safe voltage / time criteria for
earth fault current touch and step potentials may
Un-paralleling of plant or be exceeded.
circuits.
General protection grading
Insert additional protection issues
stages – slug upstream
protection
5 Lower protection settings Reduced duration of fault Failure to grade with
Decreased protection current downstream protection –
clearance times Insert additional protection extended interruption.
stages – fast or
instantaneous protection Minimal damage to plant
making fault identification
difficult
6 Replace old pole transformer Lightning impulses Must have 10 ohm HV deep
Change of equipment for new one fitted with HV & transferred to LV neutral. earth and 20 ohm segregated
Technology or the LV surge arresters. LV earth to keep with safe
application of levels .
equipment
As above but LV cable is HV / LV earth segregation
lead covered – HV earth was will be compromised when Earthing must be modified to
remotely earthed. the deep earth is installed provide HV LV segregation.
Oil filled switchgear in Risk of leaked gas pooling

basement S/S replaced by – suffocation risk Control measures required or
SF6 gear re-site above ground
Small earth system – high
132kV Gas Insulated EPR Special earthing design
Switchgear (GIS) installed. High frequency earthing needed.
issues
Other new technologies Previous design Thorough design and risk

assumptions may not be assessment required.
valid

2.2.2 Examples of Implications

Examples include:
a) Installation of new 132/33kV, 132/25kV, 132/11kV or 33/11kV substation:

i) Increased 3 phase fault level on the network.
ii) Over-stress 33kV and 11kV switchgear at existing substations
iii) Protection schemes & settings may need altering
iv) Increased earth fault current
v) Increased Earth Potential Rise (EPR) – Primary may become ‘Hot’ or ‘hotter’ –
increase in 650v and 430v contours – effect on telephones at S/S and local
houses.
vi) Transfer of increased EPR onto 11kV cable screens – distribution S/S may
become ‘hot’.
vii) The EPR rise at secondary substations may require that HV and LV earths to be
segregated.
viii) Raised fault level on 11kV circuits– Over-stress Secondary S/S switchgear
and/or exceed Air Break Switch Disconnector fault close capabilities. Small size
11kV cables may become over-stressed.
b) Increase transformer size at existing 132/33kV, 132/25kV, 132/11kV or 33/11kV substation:

i) In addition affecting other parts of the network as in 1 above, plant at
substation may be effected by increased 3 phase fault level
ii) Protection may be effected
iii) Increased earth fault level may increase the substation’s EPR
iv) Transformer tails, switchgear and CTs may be overloaded
c) Disconnect lead covered cables or replace with plastic sheathed cable.

i) Increase in overall earth resistance which and may cause the EPR to rise at
substations.
ii) The EPR rise at secondary substations may require that HV and LV earths to be
segregated.
iii) Lead covered LV cables replaced or reinforced with CNE cables will have PME
implications.
iv) Replacement of cable with a cable of smaller cross sectional area will increase
the LV network loop impedance. LV fuse clearance criteria must be checked.
d) Replace part of an underground circuit with overhead line.

i) On HV cables the metal sheath may have formed an important route for earth
fault current. EPR may be raised at substations.
ii) The EPR rise at secondary substations may require that HV and LV earths to be
segregated.
iii) Lead covered LV cables replaced or reinforced with ABC O/H line will have PME
implications.

iv) Replacement of cable with an overhead line of smaller cross sectional area will
increase the LV network loop impedance. LV fuse clearance criteria must be
checked.
e) Replace a bare wire overhead line with ABC.

i) The mechanical duty on the poles and stays will change – any undersized
components must be replaced with the correct size.
ii) The ABC may have a higher loop impedance than some of the larger copper
conductors on the network. This will increase the LV network loop impedance.
LV fuse clearance criteria must be checked.
.
f) Replace switchgear, transformer, cables or LV pillar at an existing site.
i) Operator’s standing position may have altered in relation to the earthing
system. Safe touch, step and transferred potentials cannot be assumed.
Earthing system must be re-evaluated and brought up to modern standards
ii) Replacement of lead covered cables with plastic sheathed cables will raise the
EPR.
iii) The EPR rise at secondary substations may require that HV and LV earths to be
segregated
iv) Lead covered LV cables replaced with CNE cables will have PME implications.
v) Replacement of existing pole transformers with new units fitted with HV & LV
lightning protection will require earthing to be brought up to modern
specification to control the magnitude of the lightning impulse transferred to
the LV network. Existing lead covered LV cable may have to be replaced with
plastic sheath cable to get HV / LV earth separation.
vi) Replacement of distribution transformers with a lower KVA rating will increase
the LV network loop impedance. LV fuse clearance criteria must be checked.

3. Standard Equipment Ratings and Data
3.1 General
This section contains the standard equipment types, ratings, circuit sizes and data that shall be used
for network design and construction. This equipment shall be applied within its capabilities and
limitations as part of standard network designs.
Where standard designs and/or equipment are not suitable for a specific application it will be
necessary to engineer a bespoke solution with reference to the following documents:
• Central Networks “Local Management Instruction for the Selection and Approval of Equipment to
be used on the Distribution Network”
• Central Networks “Local Management Instruction for Modification of the Distribution Network”
• Section 2, Network Modification Procedure of this Manual.
The data tables in this section include both standard and legacy equipment to facilitate design
calculations involving the extension and/or modification of the existing network. Standard equipment
is denoted by a tick in the column “Central Networks New Build Size”.
Detailed equipment specifications are contained in:

• Central Networks “Plant Specification Manual”
• Central Networks “Cables, Cable Laying and Accessories Manual”
• Central Networks “Overhead Line Manual Volumes1 & 2 ”
3.2 Low Voltage Network Ratings and Data
3.2.1 LV Underground Cables
3.2.1.1 Single Phase LV Service Cables
Hybrid CNE Cable: Solid aluminium core, singles phase, XLPE insulated, concentric bare copper
neutral/earth wires forming a combined neutral/earth conductor (CNE), 600/1000 volt, PVC sheathed
overall, to specification BS 7870-3.1:1996 and ENATS 09-7 Table 2 (XLPE variant).
As detailed in Central Networks Cables Cable Laying & Accessories Manual Section 2.1

Service Cables - Central Networks East
Size / Type Use
25 mm2 Hybrid Public Lighting - PME
Single phase services up to 90 amps - PME

35 mm2 Hybrid - single and looped domestic houses
- single light commercial properties

Hybrid Concentric Neutral HCN Cable: Solid aluminium core, singles phase, XLPE insulated, concentric
bare copper neutral/earth wires forming a combined neutral/earth conductor (CNE), 600/1000 volt, PVC
sheathed overall.
Copper Concentric Neutral CCN Cable: Stranded copper core, singles phase, XLPE insulated, concentric
bare copper neutral/earth wires forming a combined neutral/earth conductor (CNE), 600/1000 volt, PVC
sheathed overall.
Copper Split Concentric CNE Cable: Stranded copper core, singles phase, XLPE insulated, concentric
bare copper earth wires, insulated neutral wires forming a Separate neutral/earth conductor (SNE),
600/1000 volt, PVC sheathed overall.
Service Cables - Central Networks West
Size / Type Use
10 mm2 Al HCN Street lighting PME
25 mm2 Al HCN House services PME
35 mm2 Al HCN House services PME
25 mm2 Cu CCN House services PME
35 mm2 Cu CCN House services PME
16 mm2 Cu Split Concentric Street lighting Non- PME
25 mm2 Cu Split Concentric House services Non- PME
35 mm2 Cu Split Concentric House services Non- PME
3.2.1.2 Three Phase LV Cables
Wavecon CNE Cable: Three phase, three core, solid aluminium conductor, XLPE insulated, single rubber
bedding, copper wire waveform concentric forming a combined neutral/earth conductor (CNE), rated
voltage 0.6/1 kV, PVC sheathed overall, to specification BS 7870-3.40:2001 (Implementation of HD 603)

NB Prior to year 2000 the type of CNE cable used in Central Networks East had aluminium waveform
concentric neutral/earth conductors. This type of cable is known as Alpex. The electrical parameters of
Wavecon and Alpex are identical.

Hybrid Concentric Neutral HCN Cable: Three phase, three core, solid aluminium conductor, XLPE
insulated, single rubber bedding, copper wire helical concentric forming a combined neutral/earth
conductor (CNE), rated voltage 0.6/1 kV, PVC sheathed overall
Wavecon CNE Cable: Three phase, three core, solid aluminium conductor, XLPE insulated, single rubber
bedding, copper wire waveform concentric forming a combined neutral/earth conductor (CNE), rated
voltage 0.6/1 kV, PVC sheathed overall, to specification BS 7870-3.40:2001 (Implementation of HD 603)
Table 1.3.1.2
LV Cables - Central Networks East & West Continuous Rating (Summer) AMPS
Size / Type Use Laid Direct Ducted
35 mm2 Wavecon / HCN 3 phase services 132 106
95 mm2 Wavecon 3 phase services & LV mains 245 201
The above ratings assume:

Maximum conductor temperature 90OC
Ground temp 15OC
Soil Thermal Resistivity 1.2 OC m/W
1 cable laid in the trench.
Laid direct
125 mm duct
Laid 450mm deep to top of cable / duct
3.2.1.3 Transformer Tails (Distribution Substation)
Single core, solid aluminium sectoral conductor, XLPE insulated, aluminium wire armoured, 600/1000
volt, PVC sheathed overall, to specification BS5467:1997.

Transformer Tails (Distribution Substation
Size / Type Use
4 x 600 mm2 315 kVA or 500 kVA Transformers to LV pillar.

1 per phase + 1 neutral
7 x 600 mm2 800 kVA or 1000 kVA Transformers to LV pillar.

2 per phase + 1 neutral
These cables are for use in distribution substations only and do not have to comply with the current
edition of BS7671 “Requirements for Electrical Installations”. They shall not be used on customer
owned installations.
3.2.1.4 Transformer Tails (Large LV Industrial Supplies)
The LV tails from the substation LV Air Circuit Breaker to the customer’s switchgear are owned by the
customer and must comply with the current edition of BS7671 ‘Requirements for Electrical
Installations’
Section 1.3.1.11 Arrangement F. 11kV extension, 500 to 1000kVA substation & LV Air Circuit Breaker
suggests combinations of cables and transformer sizes based on these tables.
The following tables have been calculated from the current edition of BS7671 for cables laid in covered
trenches. The customer’s electrical contractor should refer to the current edition of BS7671 to confirm
that these sizes are adequate for the proposed method of installation.
Note: the tables, methods and columns refer to the tables in the current edition of BS7671
Table 3.2.1.4.A
Single Core Copper 70OC thermoplastic (PVC) non-magnetic armour Table 4D3A
of BS7671 ARMOURS BONDED AT BOTH ENDS
Table Table 4B3 correction Current rating per Total Capacity
4D3A factor for cables in core Amps Amps
Size
Rating covered trenches
Amps (methods 18 & 19)
Method 12 1 core 2 cores 3 cores 1 2 3 1 2 3
Horizontal per per per core cores cores core cores cores
flat spaced phase phase phase per per per per per per
column 10 phase phase phase phase phase phase
185 mm2 490 0.77 0.7 0.65 377 343 319 377 686 957
240 mm2 566 0.76 0.69 0.63 430 391 357 430 782 1071
300 mm2 616 0.74 0.68 0.62 456 419 382 456 838 1146
400 mm2 674 0.73 0.66 0.6 492 445 404 492 890 1212
500 mm2 721 0.72 0.64 0.58 519 461 418 519 922 1254
630 mm2 771 0.71 0.63 0.57 547 486 439 547 972 1317
800 mm2 824 0.7* 0.62* 0.56* 577 511 461 577 1022 1383
1000 mm2 872 0.69* 0.61* 0.55* 602 532 480 602 1064 1440
• Values for cables over 630 mm2 extrapolated from table 4B3 of the current edition of
BS7671

Table 3.2.1.4.B
Single Core Copper 90OC thermosetting (XLPE) non-magnetic armour Table 4E3A
of BS7671 ARMOURS BONDED AT BOTH ENDS
Table Table 4B3 correction Current rating per Total Capacity
4E3A factor for cables in core Amps Amps
Size
Rating covered trenches
(methods 18 & 19)
Amps
185 mm2 618 0.77 0.7 0.65 476 433 402 476 866 1206
240 mm2 715 0.76 0.69 0.63 543 493 450 543 986 1350
300 mm2 810 0.74 0.68 0.62 599 551 502 599 1102 1506
400 mm2 848 0.73 0.66 0.6 619 560 509 619 1120 1527
500 mm2 923 0.72 0.64 0.58 665 591 535 665 1182 1605
630 mm2 992 0.71 0.63 0.57 704 625 565 704 1250 1695
800 mm2 1042 0.7* 0.62* 0.56* 729 646 584 729 1292 1752
1000 mm2 1110 0.69* 0.61* 0.55* 766 677 611 766 1354 1833
* Values for cables over 630 mm2 extrapolated from table 4B3 of the current edition of
BS7671
Table 3.2.1.4.C
Single Core Copper 70OC Thermoplastic (PVC) Table 4D1A of BS7671
UNARMOURED (or ARMOURED and EARTHED at ONE END ONLY)
Table Table 4B3 correction Current rating Total Capacity
4D1A factor for cables in per core Amps
Size
Rating covered trenches Amps
(methods 18 & 19)
Amps
185 mm2 521 0.77 0.7 0.65 401 365 339 401 730 1017
240 mm2 615 0.76 0.69 0.63 467 424 387 467 848 1161
300 mm2 709 0.74 0.68 0.62 525 482 440 525 964 1320
400 mm2 852 0.73 0.66 0.6 622 562 511 622 1124 1533
500 mm2 982 0.72 0.64 0.58 707 628 570 707 1256 1710
630 mm2 1138 0.71 0.63 0.57 808 717 649 808 1434 1947
800 mm2 1265 0.7* 0.62* 0.56* 886 784 708 886 1568 2124
1000 mm2 1420 0.69* 0.61* 0.55* 980 866 781 980 1732 2343
• Values for cables over 630 mm2 extrapolated from table 4B3 of the current edition of
BS7671

Table 3.2.1.4.D
Single Core Copper 90OC Thermosetting (XLPE) Table 4E1A of BS7671
UNARMOURED (or ARMOURED and EARTHED at ONE END ONLY)
4E1A factor for cables in per core Amps
Size
(methods 18 & 19)
Amps
185 mm2 651 0.77 0.7 0.65 501 456 423 501 912 1269
240 mm2 769 0.76 0.69 0.63 584 531 484 584 1062 1452
300 mm2 886 0.74 0.68 0.62 656 602 549 656 1204 1647
400 mm2 1065 0.73 0.66 0.6 777 703 639 777 1406 1917
500 mm2 1228 0.72 0.64 0.58 884 786 712 884 1572 2136
630 mm2 1423 0.71 0.63 0.57 1010 896 811 1010 1792 2433
800 mm2 1581 0.7* 0.62* 0.56* 1107 980 885 1107 1960 2655
1000 mm2 1775 0.69* 0.61* 0.55* 1225 1083 976 1225 2166 2928
* Values for cables over 630 mm2 extrapolated from table 4B3 of the current edition of
BS7671
Table 3.2.1.4.E
Four Core Copper 70OC Thermoplastic (PVC) Table 4D4A of BS7671
STEEL WIRE ARMOURED and EARTHED at BOTH ENDS
4D4A factor for cables in per core Amps
Size
(methods 18 & 19)
Amps
Method 13 2 cables 3 cables 4 cables 2 3 4 2 3 4
Horizontal in in in cables cables cables cables cables cables
flat spaced parallel parallel parallel
column 5
column 4 column 5 column 7
2
120 mm 290 0.75 0.69 0.68 218 200 197 435 600 789
2
150 mm 332 0.74 0.67 0.67 246 222 222 491 667 890
2
185 mm 378 0.73 0.65 0.65 276 246 246 552 737 983
2
240 mm 445 0.71 0.63 0.63 316 280 280 632 841 1121
2
300 mm 510 0.69 0.62 0.62 352 316 316 704 949 1265

400 mm2 590 0.67 0.59 0.6 395 348 354 791 1044 1416
Table 3.2.1.4.F
Four Core Copper 90OC Thermosetting (XLPE) Table 4E4A of BS7671
STEEL WIRE ARMOURED and EARTHED at BOTH ENDS
4E4A factor for cables in per core Amps
Size
(methods 18 & 19)
Amps
Method 13 2 cables 3 cables 4 cables 2 3 4 2 3 4
Horizontal in in in cables cables cables cables cables cables
flat spaced parallel parallel parallel
column 5
column 4 column 5 column 7
2
120 mm 353 0.75 0.69 0.68 265 244 240 530 731 960
2
150 mm 406 0.74 0.67 0.67 300 272 272 601 816 1088
185 mm2 463 0.73 0.65 0.65 338 301 301 676 903 1204
240 mm2 546 0.71 0.63 0.63 388 344 344 775 1032 1376
300 mm2 628 0.69 0.62 0.62 433 389 389 867 1168 1557
400 mm2 728 0.67 0.59 0.6 488 430 437 976 1289 1747

3.2.1.5 LV Underground Cable Data Tables

Application New Resistance Reactance Continuous Rating
Size build (Summer)
size
Phase Neutral R Phase Neutral Laid Ducted
mm2 R W /km jX jX Direct
W /km W /km W /km AMPS AMPS
4 mm2cu Street Lighting 4.52 4.8 0.054 53 44

25 mm2 Hybrid Street Lighting 1.180 1.240 0.043 115 94
35 mm2Hybrid 1 Ø service 0.851 0.900 0.041 140 115
35 mm2 HCN 3 Ø service 0.939 0.939 0.076 0.015 132 106
35 mm2 Wavecon 3 Ø service 0.939 0.939 0.076 0.015 132 106
95 mm2 Wavecon Main/Service 0.320 0.320 0.075 0.016 245 201
600 mm2XLPE Transformer 0.0515 0.2 0.088 0.088 See Cables Cable Laying &
Tails Accessories Manual section 2.1.3
single core
35 mm2 Alpex 3 Ø service 0.939 0.939 0.076 0.015 132 106

70 mm2Alpex Main/Service 0.443 0.443 0.076 0.015 196 159
Consac – Paper Insulated Aluminium Sheathed based on Alpex

70 mm2 Consac Main/Service 0.433 0.433 0.061 0.015 185 150
PILC - Paper Insulated Lead Covered

4/35 mm2al Main/Service 0.868 0.868 0.075 0.075 125 105
0.1 in2 al Main/Service 0.456 0.456 0.073 0.073 185 150

2
0.0225 in cu Main/Service 1.258 1.258 0.086 0.086 100 83
0.04 in2 cu Main/Service 0.703 0.703 0.079 0.079 140 115
Resistance values are Maximum Conductor temperature:- Depth of lay 0.45m
DC at 20OC - Wavecon & Alpex 90OC (XLPE) Ground temperature 15 OC (Summer)
- Hybrid 700C (PVC) Soil thermal resistivity 1.2 OC m/w (Summer)
- PILC and Consac 800C

3.2.2 LV Overhead Lines
3.2.2.1 Main and Service Lines
Aerial Bundled Conductor (ABC) to ENATS 43-12,13 &14

As detailed in Central Networks Overhead Line Manual Section 6
Size / Type Use
2 x 35 mm2 ABC 1 phase services up to 100 Amps
4 x 35 mm2 ABC 3 phase services 100 Amps
4 x 95 mm2 ABC 3 phase services up to 135kVA

LV mains up to 228 Amps
3.2.2.2 Pole Transformer Tails
Aerial Bundled Conductor (ABC) to ENATS 43-12,13 &14

As detailed in Central Networks Overhead Line Manual Section 6
Stranded copper core, red PVC insulated, brown PVC sheathed to Basec ref 61817 and BS 6004
Size / Type Use

2 x 95 mm2 ABC (cores 2 & 3 1 phase Pole Transformers up to 50 kVA
left disconnected)
4 x 95 mm2 ABC or 3 phase Pole Transformers up to 100 kVA
2
4 x 70 mm Cu PVC/PVC
4 x 120 mm2 Cu PVC/PVC 3 phase Pole Transformers 200 kVA
7 x 120 mm2 Cu PVC/PVC 3 phase Pole Transformers 315 kVA – 2 way LV fuse cabinet

3.2.2.3 LV Overhead Line Data Tables

Size Application New build Resistance Reactance Continuous Rating
size
Phase Neutral Phase Neutral
R R jX jX AMPS AMPS
W /km W /km W /km W /km
Aerial Bundled Conductor Pole Top Under-

eaves
2/25 ABC 1 Ø service 1.200 1.200 0.086 0.086 119 109
2/35 ABC 1 Ø service 0.868 0.868 0.086 0.086 144 133
4/35 ABC 3 Ø service 0.868 0.868 0.086 0.086 120 114
4/50 ABC 0.641 0.641 0.083 0.083 144 140
4/70 ABC 0.443 0.443 0.080 0.080 180 -
4/95 ABC Main 0.320 0.320 0.077 0.077 228 191
4/120 ABC 0.253 0.253 0.075 0.075 300 270
Cu PVC/PVC Pole Transformer Tails Cleated to pole *

* Table 4D1A BS 7671 column 11
70 mm2 Trans tails 0.268 0.268 - - 254 -
120 mm2 Trans tails 0.153 0.153 - - 382 -
185 mm2 Trans tails - - 480 -
240 mm2 Trans tails - - 569 -
Open wire – metric sizes Winter Summer

50 HDA 4w 0.542 0.542 0.297 0.297 185 154
100 HDA 4w 0.270 0.270 0.276 0.276 288 240
Open wire – imperial sizes

0.025 cu 4w = 16mm2 1.083 1.083 0.347 0.347 122 102
0.04 cu 4w = 25mm2 0.681 0.681 0.305 0.305 162 135
0.05 cu 4w = 32mm2 0.541 0.541 0.297 0.297 190 159
2
0.06 cu 4w = 38mm 0.439 0.439 0.293 0.293 220 184
0.1 cu 4w = 64mm2 0.272 0.272 0.289 0.289 283 236
0.15 cu 4w = 96mm2 0.183 0.183 0.278 0.278 363 303
Open wire – SWG sizes

No 6 cu = 0.03 in2 or 18mm2 0.975 0.975 0.337 0.337 129 103
2 2
No 5 cu = 0.035 in or 22mm 0.800 0.800 0.319 0.319 148 118
No 4 cu = 0.04 in2 or 27mm2 0.640 0.640 0.303 0.303 167 133
No 3 cu = 0.05 in2 or 32mm2 0.540 0.540 0.297 0.297 185 148
No 2 cu = 0.06 in2 or 38mm2 0.439 0.439 0.293 0.293 209 167
No 1 cu = 0.071 in2 or 45mm2 0.390 0.390 0.292 0.292 233 186
1/0 cu = 0.082 in2 or 53mm2 0.360 0.360 0.291 0.291 258 206
2 2
2/0 cu = 0.095 in or 61mm 0.286 0.286 0.290 0.290 283 226
3/0 cu = 0.109 in2 or 70mm2 0.250 0.250 0.288 0.288 309 247
Resistance values are DC at 20 OC ABC ratings are based on ERA Open wire ratings are based on;
Report 90/0386 July 1990 Maximum conductor temperature 50
O
ABC ratings are based on ERA Report Maximum conductor temperature C
90/0386 July 1990 75 OC Ambient temperature 20 OC Summer,
Maximum conductor temperature 75 OC Ambient temperature 25 OC 5 OC Winter
Ambient temperature 25 OC Wind speed 0.5m/s
Wind speed 0.5m/s Solar radiation 1kW/m2 Data shown in italics for Open Wire
Solar radiation 1kW/m2 SWG sizes has been extrapolated
from Open Wire Imperial Size data.

3.2.3 LV Switchgear
3.2.3.1 Service Cut-outs
LV service cut-outs shall be as detailed in the Central Networks ant Specification Manual Section 3G.
Street lighting cut-outs shall be to BS 7654.
Single and three phase cut-outs (up to 100A) shall be to BS 7657.
Three phase cut-outs shall accept BS 88 part 5 fuses
Size / Type Use
1 phase 25 amp to BS Street lamp services plus loop to other street lamps
7654:1997 (A1)
1 phase 100 amp to BS 7657 1 phase services up to 100 amps plus loop to second domestic property
(B2A)
3 phase 100 amp to BS 7657 3 phase services 100 amps (loop to second cut-out forbidden)
(B3)
3 phase 400 amp (D1) 3 phase services 400 amps for aluminium CNE cables up to 185mm2 (loop
to second cut-out forbidden)
3 phase 600 amp (D2) 3 phase services 600 amps for aluminium CNE cables up to 300mm2 (loop
to second cut-out forbidden)

3.2.3.2 LV Distribution Cabinets
LV fuse cabinets / pillars and metering/protection cubicles free-standing or transformer mounted shall
be in accordance with ENATS 37-2 Issue 3 and as detailed in the Central Networks Plant Specification
Manual Section 3G.
Size / Type Use
Figures in brackets refer to Plant Specification Manual
600 Amp 1phase Fuse carrier (E1)

300 Amp 1 phase Fuse carrier (E2) Single phase 50 kVA padmount transformer
400 Amp 2 way (3.6) incl: Three phase 100 kVA & 200 kVA padmount
1 x 400 Amp transformer links transformer
2 x 400 Amp outgoing feeder ways
1 x 400 Amp generator connection
500 Amp 2 way (3.1) incl: 200 kVA transformer
1 x 500 Amp transformer links • LV Network supplying mixed loads
2 x 500 Amp outgoing feeder ways • Single direct service for industrial /
3 x incoming CTs & MDIs commercial load up to 225 kVA
4 x 630 Amp outgoing feeder ways • Single direct service for industrial /
3 x incoming CTs & MDIs commercial load up to 300 kVA
4 x 630 Amp outgoing feeder ways • Single or double direct services for
3 x incoming CTs & MDIs industrial / commercial loads up to 600
kVA
1600 Amp 5way(3.5) incl: 800 or 1000 kVA transformer
5 x 630 Amp outgoing feeder ways • Single or double direct services for
3 x incoming CTs & MDIs industrial / commercial loads up to 600
kVA
800 Amp ACB cabinet (6.4) incl:
1 x 800 Amp transformer links 500 KVA transformer
1 x 800 Amp Air Circuit Breaker 250 to 500 kVA LV metered supply to single
1 x 630 Amp outgoing feeder ways customer from plus feed out onto local LV
3 x incoming 800/5A CTs & MDIs network
1 x Micrologic Trip Unit
1 x 1250 Amp Air Circuit Breaker
1 x 500 Amp outgoing feeder ways 501 to 800 kVA LV metered supply to single
3 x incoming 1200/5A CTs & MDIs customer plus feed out onto local LV network
1 x 630 Amp outgoing feeder ways 801 to 1000 kVA LV metered supply to single
3 x incoming 1600/5A CTs & MDIs customer plus feed out onto local LV network
1 x 630 Amp outgoing feeder ways *DISCONTINUED*
3 x incoming 2500/5A CTs & MDIs
1 x Micrologic Trip Unit 1001 to 1500 kVA LV metered supply to single
customer plus feed out onto local LV network

3.2.3.3 Underground Network Boxes
See Plant Specification Manual

Inter-connectors between substations for back-feeding purposes.
LV 2 Way Underground Link Box.
Sub fusing point for network extensions where earth loop impedance is
high.
Inter-connectors between substations for back-feeding purposes.
LV 4 Way Underground Link Box.
Where economical use over multiple 2-way link boxes can be justified.
3.2.3.4 Pole Mounted LV Switchgear
Single pole LV fuse units for mounting on wood poles shall be to BS7656 and shall accept BS 88 part 5
fuses with 88mm centres as detailed in the Central Networks Plant Specification manual Section 3G.
Three pole ABC Network switches and ABC Service Box shall be as detailed in the Central Networks
Overhead Line Manual Vol 1 Section 6
Size / Type Use
Figures in brackets refer to Plant Specification

and O/H Line Manuals
1ph 400 amp to BS7656 and suitable for BS88 • Controlling LV circuits supplied from pole transformers
fuse links with 82mm centres.
• Open points on lines rated above 245 Amps
Item (F1) in Plant Specification Manual
250 Amp ABC Network Switch with size NH00 • Reconnecting ABC circuits on-load after temporary
245 amp links (Item LV220 in Overhead Line disconnection
Manual Vol 1)
• Open points on ABC lines.
ABC Service Box (Item LV221 in Overhead Line • Connecting from 4 to 12 services at a single pole
Manual Vol 1)
• Wall box to transitions from 35 CNE to ABC or Hybrid
under-eaves services.
3.3 Secondary Network 11kV Ratings and Data

3.3.1 11kV Cables
3.3.1.1 11 kV Network Cables
11kV Triplex: Solid circular aluminium conductor (SAC), Cross Linked Polyethylene (XLPE) insulation,
copper wire screen, red MDPE over-sheath, three single cores laid up in trefoil formation to
specification BS7870 Part 4.10:1999 & IEC 502 as detailed in Central Networks Cables Cable Laying &
Accessories Manual Section 2.2
The standard sizes to be used in Central Networks are:

• 70 mm2 aluminium XLPE Triplex , 25mm2 wire screen (East)

• 95 mm2 aluminium XLPE Triplex , 35mm2 wire screen (West)
• 185 mm2 aluminium XLPE Triplex , 35mm2 wire screen
• 300 mm2 aluminium XLPE Triplex , 35mm2 wire screen
The following sizes are used for special applications :
• 300 mm2 copper XLPE Triplex , 35mm2 wire screen
• 400 mm2 copper XLPE Triplex , 35mm2 wire screen
For applications refer to Section 1.4.4.1
3.3.1.2 Transformer 11kV Tails
Single core, circular stranded copper conductor, extruded conductor screen, XLPE insulation, helical
copper wire screen (35mm2), red MDPE oversheath 6350/11000v cable to BS 7870-4.10:1999 as detailed
in Central Networks Cables Cable Laying & Accessories Manual Section 2.2.
The standard sizes to be used in Central Networks are:
• 400 mm2 copper XLPE single core , 35mm2 wire screen
• 630 mm2 copper XLPE single core , 35mm2 wire screen

Transformer CER Winter CER Summer Tails per phase Current per core
Nameplate Rating MVA Amps MVA Amps Winter Summer
4/8 MVA 11kV 8 420 6.5 341 1 x 400 mm2 420 341
6/12 MVA 11kV 12 628 9.5 498 1 x 400 mm2 628 498
12/24 MVA 11kV 24 1257 19 996 2 x 400 mm2 628 498
20/40 MVA 11kV 40 2096 32 1676 3 x 630 mm2 698 558
6/12 MVA 6.6kV 12 1044 9.5 826 2 x 400 mm2 522 413
12/24 MVA 6.6kV 24 2088 19 1653 3 x 630 mm2 696 551
20/40 MVA 6.6kV 40 3580 32 2784 Special design requires 3500 amp s/gear
These combinations of cable are based on the Sustained Ratings of the cables installed to the
configurations shown in Table 3.3.1.5 G employing single point screen bonding.
The winter and summer ratings of the cables match or exceed the respective winter and summer
Certified Emergency Ratings of the transformers. Other configurations may not provide the full
transformer rating – consult the Assets Manager if site conditions prevent these configurations from
being used.

3.3.1.3 Cable Rating Criteria
Current ratings of 11kV cables are determined from Engineering Recommendation P17 ‘Current
Ratings Guide for Distribution Cables’
Three types of rating are employed in Central Networks:
5 Day Distribution Rating (5 day cyclic rating)
Cyclic (Continuous) rating
Sustained (Continuous) rating
Paper Insulated Cables
The ratings for paper insulated cables are based on the tables contained in Engineering
Recommendation P17 parts 1 & 2.
Central Networks East has traditionally used belted cables for which P17 prescribes a maximum
conductor temperature of 65OC.
Central Networks West has traditionally used screened cables for which P17 prescribes a maximum
conductor temperature of 70OC.
XLPE Insulated Cables
The ratings for XLPE cables have been calculated using the ‘CRATER’ spreadsheet developed by EA
Technology Ltd. Version 1.0 Feb 2004. This tool uses Engineering Recommendation P17 Part 3
calculation criteria but some results differ slightly from the P17 tables. Explanations are provided in
the ‘CRATER’ user manual.
P17 prescribes a maximum conductor temperature of 90OC for XLPE cables. This value is applied to
Central Networks East & West.
‘CRATER’ is capable of calculating a wider range of installation circumstances than can be contained in
tables. e.g. various laid direct and duct configurations, multiple circuits, spacing, soil types etc.
Some common multiple cable configurations have been included in the tables. However, where large
multiple runs are being designed, e.g. congested exits from Primary Substations, the Equipment
Specialists in Asset Standards will be able to provide guidance using ‘CRATER’.
Duct type - important note.
The ratings that ‘CRATER’ calculates for cables installed in corrugated polyethylene duct (e.g.
Ridgiduct) are significantly lower than for smooth wall ducts. EATL STP Project SO389 Report No 5617
identified that trapped air in the closed sections of the corrugations insulates 65% of the effective
duct length. The report calculated that the overall thermal resistivity of corrugated polyethylene duct
varies from 4.3 to 13.2 OCm/W depending on whether the outside corrugations are filled with soil or air.
‘CRATER’ applies an average value of 8.7 OCm/W when ‘Ridgiduct’ is selected.
Appendix D of Report 5617 details the method of calculating the thermal resistivity of Ridgiduct. The
figure of 4.3 OCm/W was calculated assuming soils with a resistivity of 1.2 OCm/W (as used in summer
calculations). Re-calculating using the winter soil thermal resistivity of 0.9 OCm/W lowers the overall
thermal resistivity of 3.7 OCm/W. This compares with 1.2 OCm/W for earthenware ducts, 2.5 OCm/W for
polyethylene ducts and 5.0 OCm/W for PVC ducts. For the purpose of Central Networks cables ratings it
is assumed that the outer corrugations will fill with fine soil or silt after laying and a thermal resistivity
of less than 5.0 OCm/W will be attained in practice. Therefore the ducted cable ratings in this Manual
have been calculated in ‘CRATER’ by selecting PVC duct 126.3mm I/D 134.6mm O/D to represent
Ridgiduct. For this assumption to remain valid it is essential that Ridgiduct are bedded in material that
contains elements small enough to fill the corrugations. The Central Networks Cable, Cable Laying an
Accessories Manual provides guidance on backfill materials.
5 Day Distribution Rating
This is a cyclic rating that assumes the cable forms part of an open ring on a Primary Substation or an
interconnector between Primaries feeding average domestic, small industrial and commercial loads in
average environmental conditions.
The cable is normally carrying only 50% of the load prior to being required to supply the second half of
the ring in an emergency, such as a cable fault.
The rating can also be based on the cable carrying 75% of the load prior to an emergency. This has
been employed in certain urban networks where 4 circuits have been arranged such that 3 healthy
circuits can each pick up 25% of the load of a faulted circuit by multiple switching operations. New
circuits or existing circuits presently operating at 50% utilisation shall not be run at 75% utilisation
without approval of the Network Manager.
The 5 Day Distribution Rating tables for XLPE cables have 50% and 75% utilisation ratings. Tables are
provided for
• Un-grouped cables – single cable without the possibility of being heated by an adjacent cable
on the same ring main.
• Group of 2 cables – this assumes two cables of a ring main in the same trench connected to a
distribution substation. During emergency feeding both cables run to their 5 Day Distribution
rating to supply the rest of the ring.
• Multiple groups of cable - this assumes the entire group normally runs to 50% utilisation.
During emergency feeding one of the group runs to its 5 Day Distribution rating.
Distribution Ratings depend on the load being cyclic in nature so that heating during periods of high
load is followed by periods of cooling during low load. For calculation purposes the shape of the load
curve is express as a value called Load Loss Factor. The Distribution Ratings in P17 are based on ‘Load

Curve G’ (Loss Load Factor =5.061) which is shown later in this section together with the method of
calculation the Loss Load Factor for actual load curves.
Cyclic Ratings
These are similar to the 5 day Distribution ratings but assume that the loading condition is maintained
indefinitely and so take into account long term heating of the soil. The Cyclic ratings published are
based on Engineering Recommendation P17 – ‘Load Curve G’ (Loss Load Factor = 5.061). They are not
suitable for industrial and large commercial loads unless adjusted to reflect the actual Loss Load
Factor. Refer to the Assets Manager for further information.
Cyclic ratings are applicable to ring mains supplying average domestic, small industrial and
commercial loads in average environmental conditions where anticipated repair times are in excess of
5 days.
Sustained Ratings
Cables supplying only industrial and large commercial loads, where loads close to maximum demand
are sustained for long periods, do not benefit form periodic cooling during light load periods.
Sustained Ratings, corrected for ambient ground temperature, are therefore appropriate for these
loads. Summer ratings will normally be used unless there is firm evidence of seasonal load variation.
Note that commercial summer loads are often equal to or higher than winter loads due to air
conditioning.
Sustained Ratings are used for the secondary tails of 132/11kV & 33/11kV transformers which can be
called upon to carry their Certified Emergency Rating for periods greater than 5 days.
As transformer tails are often used in combinations of 2 or 3 per phase for great care has to be used
during installation to ensure that the correct grouping/spacing is used to ensure that the Certified
Emergency Rating of the transformer is achievable and not limited by the 11kV cables.
Section 3.3.1.2 11kV Transformer Tails – states the combinations and sizes to be used for each
transformer size. These are based on the ratings of the cables installed to the configurations shown in
Table 3.3.1.5F employing single point screen bonding.
The Loss Load Factor for Sustained Ratings is 1.0 and the Utilisation Factor is 100%.

Ratings used in GIS
The 11kV cable ratings used in the Central Networks Graphical Information Systems (GIS) system
diagrams are based on:
Central Networks East:
Cyclic Rating, Group of 2, Winter, Table 3.3.1.5 B
5 day Distribution Rating 50% utilisation, Group of 2 Winter Table 3.3.1.4 B
Sustained Rating, Ungrouped, Summer, Table 3.3.1.5 A
Both laid direct and ducted ratings used.
The Central Networks East 11kV System Diagram displays cable rating in the following format:
334 (368) = Cyclic rating (5 day distribution rating)
[309 (309)] = Sustained Summer rating – Note the square brackets denote a contrained cable rating.
Caution - XLPE cable to Paper cable joints
Caution should be exercised when using XLPE cable in existing paper insulated networks. The rating of
paper insulated cable is based on a conductor temperature of 70OCfor screen cables and 65OC for
belted cables. For XLPE it is based on 90OC.
For example: On the face of it 185 mm2 XPLE in a duct has the same rating to a 185 mm2 belted PICAS
laid direct.
However, at a joint between the ducted XLPE cable and the lid direct PICAS cable 90OC conductor
temperature of the XLPE cable core would transfer through the joint and overheat the PICAS cable,
possibly resulting in joint failure.
To avoid this the XLPE cable must be laid direct for a distance of one metre before jointing to a paper
insulated cable to allow the core temperature to drop to 65OC.
Calculation of Loss Load Factor
An approximate value of Loss Load Factor can be calculated by the following method:
Draw an the actual load curve under consideration making maximum demand equal to 1 and
other levels in proportion.
Approximate this to a series of rectangles with height representing an average proportion of

the maximum demand over a period and width representing the length of that period.
Example Load Curve

1
0.9
0.8
0.7
Load pu
0.6
0.5
0.4
0.3
0.2
0.1
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Time
The Loss Load Factor
= Sum of the square of the per unit loads x durations divided by 24 hours
= (0.52 x(2+2+3) + 0.42 x(4+2) + 0.752 x(2+2) + 12 x(5) + 0.852 x(2)) ÷ 24
= (0.25x7 + 0.16x6 + 0.5625x4 + 1x5 + 0.7225x2) ÷ 24
= (1.75 + 0.96 + 2.25 + 5 + 1.445) ÷ 24 = 11.405 ÷ 24
= 0.475
Load Curve G gives a Loss Load Factor of 0.5061 and this is assumed in the 5 Day Distribution rating in this
Manual.

3.3.1.4 11kV Cable 5 day Distribution Rating Tables
5 day Distribution limited time current ratings for 11kv cables in open rings across a primary or
interconnector between primaries.
• Values for normal domestic/small commercial loads and average environmental conditions
• Ambient ground temperature 10°C Winter, 15°C Summer.
• Soil thermal resistivity 0.9 °C m/W Winter, 1.2 °C m/W Summer.
• Soil Thermal Diffusivity of 5 x 10-7 x (g/0.9) m2/s where g is the soil thermal resistivity in OCm/W
• Loss Load Factor 0.5 Load Curve G
• Maximum conductor temperature of
• 65OC for belted paper insulation (Central Networks East)
• 70OC for screened paper insulation (Central Networks West)
• 90OC for XLPE insulation. (Central Networks East & West)
• Depth of 600mm to the top of the cable or duct
• No account taken of soil drying out. (Not applicable to Distribution ratings)
• Where 'Off Peak' loads predominate a correction factor of 0.97 should be applied.
• The 5 day Distribution limited time rating can be applied for the duration of an emergency lasting
not more than 5 days assuming cable carrying only 50% of load before the emergency. (or 75% in
the table for 75% utilisation)
• Duct sections less than 15m long can be assumed to be laid direct.
The following 5 Day Distribution Ratings tables are provided:
Table 3.3.1.4 A 11kV XLPE Cables - Ungrouped - 5 Day Distribution Rating - 50% & 75% Utilisation
Table 3.3.1.4 B 11kV XLPE Cables – Group of 2 - 5 Day Distribution Rating - 50% & 75% Utilisation
Table 3.3.1.4 C 11kV XLPE Cable - Multiple Groups - 5 Day Distribution Rating - 50% & 75% Utilisation
Table 3.3.1.4 D 11kV Belted Paper Cables – 5 Day Distribution Rating - 50% & 75% Utilisation Ungrouped -
Metric sizes
Table 3.3.1.4 E 11kV Belted Paper Cables – 5 Day Distribution Rating - 50% & 75% Utilisation Ungrouped -
Imperial sizes
Table 3.3.1.4 F 11kV Screened Paper Cables – 5 Day Distribution Rating - 50% & 75% Utilisation
Ungrouped - Metric sizes
Table 3.3.1.4 G 11kV Screened Paper Cables – 5 Day Distribution Rating - 50% & 75% Utilisation
Ungrouped - Imperial sizes
Table 3.3.1.4 H De-rating factors for Grouped Paper Cables

Table 3.3.1.4 A Central Networks East & West

11kV XLPE Cables – Ungrouped - 5 Day Distribution Rating
CABLE WINTER SUMMER

Amps Amps
Laid Direct Ducted Laid Direct Ducted
Utilisation 50% 75% 50% 75% 50% 75% 50% 75%
70 mm2 Al Triplex 292 284 216 213 260 253 208 205
95 mm2 Al Triplex 346 337 257 253 309 300 247 243
2
150 mm Al Triplex 445 433 330 325 396 385 317 312
185 mm2 Al Triplex 507 494 376 370 451 439 360 355
2
240 mm Al Triplex 592 576 439 432 525 511 420 413
300 mm2 Al Triplex 672 654 499 492 595 579 476 469
2
300 mm Cu Triplex 866 842 642 632 766 745 612 602
400 mm2 Cu Triplex 989 962 818 796 873 850 766 746
95 mm2 Al 3-Core 310 305 239 236 283 278 231 228
185 mm2 Al 3-Core 457 450 353 348 342 409 339 335
2
300 mm Al 3-Core 613 603 473 467 556 547 454 448
300 mm2 Cu 3-Core 792 780 616 608 718 707 590 583
The above ratings have been calculated on ‘Crater V3.0 Oct 2004’
Assumptions:
Max 90OC conductor temp
Winter - soil temp 10OC , Thermal resistivity 0.9 m OC/W

Summer - soil temp 15OC , Thermal resistivity 1.2 m OC/W
Screens bonded at both ends
1 cable laid in the trench.
Laid direct
125 mm duct up to 300mm2
3 x 125 mm duct for 400mm2

Table 3.3.1.4 B Central Networks East & West

11kV XLPE Cables – Group of 2 - 5 Day Distribution Rating
CABLE WINTER SUMMER

Amps Amps
Utilisation 50% 75% 50% 75% 50% 75% 50% 75%
2
70 mm Al Triplex 283 267 211 203 250 238 202 193
95 mm2 Al Triplex 335 316 251 241 296 281 239 228
2
150 mm Al Triplex 430 404 323 308 379 358 307 292
185 mm2 Al Triplex 489 459 368 350 430 406 349 331
240 mm2 Al Triplex 570 534 430 407 500 472 407 384
2
300 mm Al Triplex 646 605 489 462 566 534 462 435
300 mm2 Cu Triplex 832 779 629 595 728 686 593 559
2
400 mm Cu Triplex 949 887 796 750 828 780 739 696
95 mm2 Al 3-Core 300 286 235 228 271 260 225 218
185 mm2 Al 3-Core 442 418 346 333 399 379 331 317
2
300 mm Al 3-Core 593 558 464 445 533 504 442 422
300 mm2 Cu 3-Core 767 721 604 580 689 651 574 549
The above ratings have been calculated on ‘Crater V3.0 Oct 2004’
Assumptions:

Screens bonded at both ends
2 cables laid in the trench.
Laid direct
220
25
125 mm duct up to 300mm2
3 x 125 mm duct for 400mm2
25
Table 3.3.1.4 C Central Networks East & West

11kV XLPE Cable - Multiple Groups - 5 Day Distribution Rating

Winter amps Summer amps
Formation Utilisation 50% 75% 50% 75%
Size Type
185 mm2 Al Triplex 443 352 383 309
300 mm2 Al Triplex 582 457 500 401
6 circuits
300 mm2 Cu Triplex 749 589 643 515
400 mm2 Cu Triplex 852 667 730 582
185 mm2 Al Triplex 342 284 323 263
300 mm2 Al Triplex 454 369 427 341
300 mm2 Cu Triplex 584 475 549 438
6 circuits
400 mm2 Cu Triplex 736 582 671 531
185 mm2 Al Triplex 461 379 393 328
300 mm2 Al Triplex 605 493 514 426
4 circuits
300 mm2 Cu Triplex 767 626 661 548
400 mm2 Cu Triplex 873 709 751 620
185 mm2 Al Triplex 350 304 331 283
300 mm2 Al Triplex 465 398 438 370
4 circuits
300 mm2 Cu Triplex 598 512 562 475
400 mm2 Cu Triplex 745 612 683 561
185 mm2 Al Triplex 434 328 374 288
300 mm2 Al Triplex 569 425 489 372
6 circuits
300 mm2 Cu Triplex 733 548 628 479
400 mm2 Cu Triplex 833 619 712 540
185 mm2 Al Triplex 340 271 319 250
300 mm2 Al Triplex 451 352 421 324
6 circuits
300 mm2 Cu Triplex 580 453 541 417
400 mm2 Cu Triplex 721 543 655 494
185 mm2 Al Triplex 411 328 352 249
300 mm2 Al Triplex 538 368 459 321
9 circuits
300 mm2 Cu Triplex 692 474 590 413
400 mm2 Cu Triplex 786 535 668 466
185 mm2 Al Triplex 326 241 304 221
300 mm2 Al Triplex 433 311 401 285
9 circuits
300 mm2 Cu Triplex 557 400 515 366
400 mm2 Cu Triplex 686 472 621 428
185 mm2 Al Triplex 396 256 338 224
300 mm2 Al Triplex 517 330 440 288
300 mm2 Cu Triplex 665 425 565 370
12 circuits
400 mm2 Cu Triplex 755 479 640 416
185 mm2 Al Triplex 322 224 292 201
300 mm2 Al Triplex 426 289 387 259
300 mm2 Cu Triplex 540 366 497 332
12 circuits 400 mm2 Cu Triplex 660 420 595 378
Laid Direct - 150mm spacing between cables
125 mm ducts touching
For other combinations contact the Assets Manager

Table 3.3.1.4 D Central Networks East

11kV Belted Paper Cables – 5 Day Distribution Rating
Ungrouped - Metric sizes - Max 65OC conductor temp
CABLE WINTER SUMMER
Amps Amps
Utilisation 50% 75% 50% 75% 50% 75% 50% 75%
2
95 mm Al PICAS 245 239 190 185 220 214 178 174
185 mm2 AL PICAS 370 360 290 282 329 320 272 265
2
240 mm Cu PICAS 545* 531 436* 425 485* 473 409* 399
300 mm2 Al PICAS 490 478 395 385 436 425 371 362
95 mm2 Al SWA 260 253 200 195 234 228 188 183
2
185 mm Al SWA 390 380 300 292 347 338 282 275
300 mm2 Al SWA 525 512 410 400 467 455 385 375
Assumptions:


Table 3.3.1.4 E Central Networks East

11kV Belted Paper Cables – 5 Day Distribution Rating
Ungrouped - Imperial sizes - Max 65OC conductor temp
CABLE WINTER SUMMER
Amps Amps
Utilisation 50% 75% 50% 75% 50% 75% 50% 75%
0.06 in2 Al SWA 145 141 115 112 132 129 109 106
2
0.1 in Al SWA 195 190 155 151 175 171 145 141
0.15 in2 Al SWA 250 244 195 190 225 219 183 178
0.2 in2 Al SWA 300 293 235 229 270 263 221 215
0.25 in2 Al SWA 345 336 270 263 311 303 254 248
2
0.3 in Al SWA 390 380 310 302 347 338 290 283
0.4 in2 Al SWA 470 458 370 361 418 408 348 339
2
0.5 in Al SWA 530 517 420 410 471 459 395 385
0.0225 in2 Cu SWA 102 100 90 88 93 91 85 83

0.04 in2 Cu SWA 145 141 115 112 132 129 109 106
0.06 in2 Cu SWA 185 180 150 146 168 164 142 139
0.10 in2 Cu SWA 255 249 205 200 230 224 192 187
2
0.15 in Cu SWA 320 312 255 249 288 281 240 234
2
0.2 in Cu SWA 385 375 305 297 347 338 285 278
0.25 in2 Cu SWA 440 429 350 341 396 386 329 321
0.3 in2 Cu SWA 500 488 400 390 445 434 375 366
0.4 in2 Cu SWA 595 580 475 463 530 517 498 486
2
0.5 in Cu SWA 670 653 535 522 596 581 560 546
Assumptions:


Table 3.3.1.4 F Central Networks West

11kV Screened Paper Cables – 5 Day Distribution Rating
Ungrouped - Metric sizes - Max 70OC conductor temp
CABLE WINTER SUMMER
Amps Amps
Utilisation 50% 75% 50% 75% 50% 75% 50% 75%
95 mm2 Al PICAS 270 263 205 200 240 234 193 187
2
185 mm AL PICAS 400 390 310 302 356 347 291 284
2
300 mm Al PICAS 530 517 420 410 472 460 395 385
185 mm2 Cu PICAS 540 527 410 400 481 469 385 375
2
300 mm Cu PICAS 719 701 555 541 640 624 521 508
95 mm2 Al SWA 286 279 215 210 257 251 202 197
185 mm2 Al SWA 420 410 320 312 374 365 301 294
300 mm2 Al SWA 567 553 435 424 505 492 409 399
Assumptions:


Table 3.3.1.4 G Central Networks West

11kV Screened Paper Cables – 5 Day Distribution Rating
Ungrouped - Imperial sizes - Max 70OC conductor temp
CABLE WINTER SUMMER
Amps Amps
Utilisation 50% 75% 50% 75% 50% 75% 50% 75%
2
0.06 in Al SWA 160 156 125 122 146 142 118 115
2
0.1 in Al SWA 220 215 170 166 198 193 160 156
2
0.15 in Al SWA 275 268 215 210 248 242 202 197
2
0.2 in Al SWA 330 322 255 249 297 290 240 234
2
0.25 in Al SWA 375 366 295 288 338 330 277 270
2
0.3 in Al SWA 425 414 335 327 383 373 315 307
2
0.4 in Al SWA 510 497 400 390 454 443 376 367
2
0.5 in Al SWA 570 556 450 439 507 494 423 412
0.0225 in2 Cu SWA 116 113 101 98 106 103 96 94

0.04 in2 Cu SWA 165 161 130 127 150 146 143 139
0.06 in2 Cu SWA 210 205 165 161 191 186 182 177
2
0.10 in Cu SWA 285 278 220 215 257 251 242 236
2
0.15 in Cu SWA 355 346 280 273 320 312 301 293
0.2 in2 Cu SWA 420 410 330 322 378 369 355 346
0.25 in2 Cu SWA 480 468 375 366 432 421 406 396
2
0.3 in Cu SWA 545 531 430 419 491 479 462 450
2
0.4 in Cu SWA 645 629 510 497 547 533 514 501
0.5 in2 Cu SWA 720 702 570 556 641 625 603 589
Assumptions:


3.3.1.5 11kV Cable Sustained & Cyclic Rating Tables
• Values are for average environmental conditions

• Ambient ground temperature 10°C Winter 15°C Summer.
• Soil thermal resistivity 0.9°C m/w Winter, 1.2°C m/w Summer
• Loss Load Factor 0.5 Load Curve G ( applicable to Cyclic ratings only)
• Maximum conductor temperature of
• 65OC for belted paper insulation (Central Networks East)
• 70OC for screened paper insulation (Central Networks West)
• 90OC for XLPE insulation. (Central Networks East & West)
• For other conditions including cables in air or clipped to wall use Engineering Recommendation
P17 tables with all appropriate correction factors. The EA Technology Ltd ‘CRATER’ spreadsheet is
available to carry out these calculations for XLPE cables.
The following Sustained & Cyclic Ratings tables are provided:
Table 3.3.1.5 A 11kV XLPE Cables Sustained & Cyclic Rating – Ungrouped
Table 3.3.1.5 B 11kV XLPE Cables Sustained & Cyclic Rating – Group of 2
Table 3.3.1.5 C 11kV Belted Paper Cables - Sustained Rating & Cyclic Rating – Ungrouped – Metric Sizes
Table 3.3.1.5 D 11kV Belted Paper Cables - Sustained Rating & Cyclic Rating – Ungrouped – Imperial Sizes
Table 3.3.1.5 E 11kV Screened Paper Cables - Sustained Rating & Cyclic Rating – Ungrouped – Metric Sizes
Table 3.3.1.5 F 11kV Screened Paper Cables - Sustained Rating & Cyclic Rating – Ungrouped – Imperial Sizes
Table 3.3.1.5 G Transformer 11kV XLPE Tails Sustained Ratings

Table 3.3.1.5 A Central Networks East & West

11kV XLPE Cables - Sustained & Cyclic Ratings
Ungrouped - Max 90OC conductor temp
CABLE WINTER SUMMER

Utilisation Sustained Cyclic Sustained Cyclic Sustained Cyclic Sustained Cyclic
70 mm 2
Al Triplex 246 276 196 212 214 243 181 199
95 mm 2
Al Triplex 292 327 233 252 253 288 214 237
150 mm 2
Al Triplex 371 420 297 323 321 368 273 303
185 mm 2
Al Triplex 421 477 337 367 363 419 309 344
240 mm 2
Al Triplex 487 555 391 427 420 486 357 400
300 mm 2
Al Triplex 550 630 442 485 474 551 403 453
300 mm 2
Cu Triplex 705 810 567 623 606 707 516 580
400 mm 2
Cu Singles* 798 922 674 764 686 804 601 699
2 x 185 mm2 Al Triplex** 336 385 + 293 317 + 287 330 + 264 294 +
+ 336 385 + 293 317 + 287 330 + 264 294
2 x 300 mm2 Al Triplex ** 433 502 + 382 418 + 370 428 + 342 382 +
+ 433 502 + 382 418 + 370 428 + 342 382
95 mm2 Al 3-Core 268 298 219 234 236 268 203 221
185 mm2 Al 3-Core 389 437 319 344 342 393 294 324
300 mm2 Al 3-Core 514 584 424 460 451 523 389 431
300 mm2 Cu 3-Core 662 755 550 598 580 675 504 560
The above ratings have been calculated on ‘Crater V3’ and assume:
Circuits should take separate routes if possible
The cables are NOT grouped – they may be laid in the same trench but not fully loaded
simultaneously – e.g. parallel feeders to customer substation sharing the load 50% / 50% or
100% / 0% during single circuit outage.
50% OR 100%
50% 0%
Laid direct – 120mm between the cable centres

125mm ducts 25mm apart 25
Screens bonded at both ends 220
*400 mm2 Singles – laid direct in trefoil,

- ducted in 3 separate ducts 220
25
25
220
** 2 x 185/300 Al Triplex per circuit 220 25
600mm cover to top cables
800mm cover to lower cables Circuit 1 Circuit 2 Circuit 1 Circuit 2

Table 3.3.1.5 B Central Networks East & West

11kV XLPE Cables - Sustained & Cyclic Ratings
Group of 2 - Max 90OC conductor temp
CABLE WINTER SUMMER

70 mm 2
Al Triplex 212 245 176 196 183 215 160 182
95 mm 2
Al Triplex 250 290 208 231 215 253 185 211
150 mm 2
Al Triplex 315 368 264 295 270 321 234 267
185 mm 2
Al Triplex 355 416 298 334 305 363 264 302
240 mm 2
Al Triplex 409 482 345 388 351 419 304 350
300 mm2 Al Triplex 460 544 388 438 394 472 342 395
300 mm 2
Cu Triplex 589 699 498 563 504 607 437 507
400 mm2 Cu Singles* 664 793 553 675 567 687 486 608
2 x 185 mm 2
Al Triplex** 302 + 364 + 237 + 285 + 258 + 316 + 209+ 256 +
302 364 237 285 258 316 209 256
2 x 300 mm2 Al Triplex ** 390 + 475 + 306 + 369 + 333 + 411 + 268 + 331 +
390 475 306 369 333 411 268 331
95 mm2 Al 3-Core 233 268 192 212 201 235 175 197
2
185 mm Al 3-Core 333 388 277 307 287 340 252 285
300 mm2 Al 3-Core 434 510 365 407 374 448 330 376
300 mm2 Cu 3-Core 553 651 474 530 482 579 427 488
The above ratings have been calculated on ‘Crater V3’ and assume:
Theses ratings are for cables installed on a ring main. Whilst for most of the ring they run
seperatly they do run togther where they loop in and out of the ring man substation in the same
trench. During abnormal feeding they are both subject to the full ring main load.
Open
#Laid direct – 150mm between the cables

125mm ducts – touching 25
Screens bonded at both ends 220
*400 mm2 Singles – laid direct in trefoil,

220
- ducted in 3 separate ducts
25
25
220
** 2 x 185/300 Al Triplex per circuit 220 25
600mm cover to top cables
800mm cover to lower cables Circuit 1 Circuit 2 Circuit 1 Circuit 2

Table 3.3.1.5 C Central Networks East

11kV Belted Paper Cables –Sustained & Cyclic Ratings
Ungrouped – Metric Sizes - Max 65OC conductor temp
CABLE WINTER SUMMER

95 mm 2
Al PICAS 205 230 170 181 185 207 160 169
185 mm 2
AL PICAS 300 340 250 273 270 303 230 256
240 mm2 Cu PICAS 441* 501* 375* 406* 393* 446* 352* 380*
300 mm2 Al PICAS 390 446 335 363 355 397 305 341
95 mm2 Al SWA 220 244 180 190 195 220 170 179
185 mm 2
Al SWA 320 359 260 282 285 319 240 265
300 mm2 Al SWA 420 478 350 377 380 425 315 354
* Mathematically derived from aluminium values (no manufacturer’s data)
Table 3.3.1.5 D Central Networks East

11kV Belted Paper Cables - Cyclic & Sustained Rating
Ungrouped – Imperial Sizes - Max 65OC conductor temp
CABLE WINTER SUMMER
2
0.06 in Al SWA 128 138 105 110 116 125 99 105
0.10 in 2
Al SWA 168 183 140 147 151 165 131 138
0.15 in2 Al SWA 213 235 174 185 191 212 163 174
0.20 in 2 Al SWA 252 279 207 221 227 251 194 208
0.25 in 2 Al SWA 286 321 235 254 258 289 221 239
0.3 in2 Al SWA 320 359 270 288 285 319 252 270
0.4 in2 Al SWA 381 432 315 344 339 385 296 324
0.5 in2
Al SWA 424 482 353 386 377 429 332 363
0.0225 in2 Cu SWA 93 99 85 89 85 90 80 84

0.04 in 2
Cu SWA 129 138 106 112 117 125 100 106
0.06 in 2
Cu SWA 163 176 137 144 148 160 129 136
0.10 in 2
Cu SWA 219 240 185 144 198 216 173 182
0.15 in 2
Cu SWA 272 301 227 242 245 271 214 228
0.2 in2
Cu SWA 323 358 268 287 291 323 251 268
0.25 in 2 Cu SWA 365 409 305 329 329 368 286 309
0.3 in2 Cu SWA 410 460 348 372 365 409 326 349
0.4 in2 Cu SWA 482 547 404 442 429 488 423 463
0.5 in2 Cu SWA 536 610 449 492 477 542 470 515

Table 3.3.1.5 E Central Networks West

11kV Screened Paper Cables –Sustained & Cyclic Ratings
Ungrouped – Metric Sizes - Max 70OC conductor temp
CABLE WINTER SUMMER
2
95 mm Al PICAS 224 251 182 195 199 223 172 183
2
185 mm AL PICAS 324 368 267 288 288 328 250 271
2
300 mm Al PICAS 419 482 353 386 373 430 332 363
185 mm2 Cu PICAS 437 497 353 381 390 443 331 358
300 mm2 Cu PICAS 568 654 466 511 506 582 438 479
95 mm2 Al SWA 237 266 191 204 213 239 180 192
185 mm2 Al SWA 340 386 275 298 303 344 259 280
300 mm2 Al SWA 448 516 365 400 399 460 344 376
Table 3.3.1.5 F Central Networks West

11kV Screened Paper Cables - Cyclic & Sustained Rating
Ungrouped – Imperial Sizes - Max 70OC conductor temp
CABLE WINTER SUMMER
2
0.06 in Al SWA 138 150 113 119 126 137 106 112
2
0.10 in Al SWA 187 207 150 162 168 186 141 152
0.15 in2 Al SWA 231 256 187 202 208 231 176 190
0.20 in2 Al SWA
274 307 222 237 247 276 209 223
0.25 in2 Al SWA
308 345 254 274 277 311 238 258
0.3 in2 Al SWA
344 391 285 312 310 352 268 293
0.4 in2 Al SWA
408 464 336 368 363 413 316 346
0.5 in2 Al SWA 450 519 374 410 401 461 351 385
2
0.0225 in Cu SWA 104 113 93 99 95 103 88 94
2
0.04 in Cu SWA 144 157 117 125 131 143 129 137
0.06 in2 Cu SWA 181 197 149 157 164 180 164 173
0.10 in2 Cu SWA 242 268 194 209 218 242 213 230
0.15 in2 Cu SWA 298 330 244 263 269 298 262 283
0.2 in2 Cu SWA 349 391 287 307 314 352 309 330
0.25 in 2 Cu SWA
394 442 323 349 354 397 349 378
0.3 in2 Cu SWA
441 501 366 400 398 452 393 430
0.4 in2 Cu SWA
516 587 428 469 438 498 432 473
0.5 in2 Cu SWA
569 655 473 519 506 583 500 549

Table 3.3.1.5 G
Transformer 11kV XLPE Tails Sustained Ratings
Formation Size Type Winter amps Summer amps
A In trefoil – 1 core per 400 mm2 Cu Singles 821 798 706 686
phase R
YB 630 mm2 Cu Singles 1046 1006 897 861
B Laid singly – 1 core per 400 mm2 Cu Singles 885 785 759 671
phase – 2D spacing
between cores
630 mm2 Cu Singles 1155 968 988 824
RYB
C Laid singly – 1 core per 400 mm2 Cu Singles 778 655 701 586
phase - ducts touching
RYB 630 mm2 Cu Singles 1010 808 906 718
D In trefoil – 2 cores per 400 mm2 Cu Singles 700 681 598 581
phase - 300mm spacing
R R
630 mm2 Cu Singles 884 850 754 724
YB YB
E Laid singly – 2 cores per
400 mm2 Cu Singles 659 563 579 494
phase - ducts touching
R R
YB YB
630 mm2 Cu Singles 848 694 743 606
F In trefoil – 3 cores per 400 mm2 Cu Singles 620 603 528 513
phase - 300mm spacing
R R R 630 mm2 Cu Singles 779 749 662 636
YB YB YB
G Laid singly – 3 cores per

phase - ducts touching 400 mm2 Cu Singles 579 495 505 431
R R R
YB YB YB 630 mm2 Cu Singles 743 608 645 527
Cable screens bonded to earth at one end only – NORMAL ARRANGMENT
Cable screens bonded to earth at both ends – DO NOT USE
The current ratings are given for each core. For the total capacity of multiple cores per phase multiply the
rating by the number of cores.
Transformer CER Winter CER Summer Tails per phase Current per core
Nameplate Rating MVA Amps MVA Amps Winter Summer
6/12 MVA 11kV 12 628 9.5 498 1 x 400 mm2 628 498
12/24 MVA 11kV 24 1257 19 996 2 x 400 mm2 628 498
20/40 MVA 11kV 40 2096 32 1676 3 x 630 mm2 698 558
6/12 MVA 6.6kV 12 1044 9.5 826 2 x 400 mm2 522 413
12/24 MVA 6.6kV 24 2088 19 1653 3 x 630 mm2 696 551
20/40 MVA 6.6kV 40 3580 32 2784 Special design

EATL ‘CRATER’ Spreadsheet settings used for Transformer Tail Ratings

Common settings
Cable Characteristics
Cable type Copper wire screen BS 7870-4.10 (11kV)
Conductor Copper stranded
Conductor size mm2 400 & 630
Insulation type XLPE
Sheath type Polyethylene
Colour of Sheath N/A
Cu Wire Screen area mm2 35 mm2
Bonding arrangement Single point & Solid, No Transposition as required
Operating Conditions
Environment In soil
Drying out of soil No Account taken
Conductor temp 90 OC
Soil Temp 10 OC Winter 15 OC Summer
Soil Thermal Resistivity 0.9 mOC/W Winter 1.2 mOC/W Winter
Soil depth 600 mm
Loss Load Factor 1.0 (set to 1.0 so that Dist/Cyclic/Sust rating are equal)
Utilisation % 100% (set to 100% so that Dist/Cyclic/Sust rating are equal)
Limited Time days N/A
Cable Grouping See below
Soil Thermal diffusivity 5.0E-07x(0.9/g)^0.8
Soil depth measured to Top surface
Arrangement
A B C D E F G
Cable grouping No No No Yes Yes Yes Yes
Cable & Duct configuration
Single cores flat 2D Spacing between Y
centres
Single core in trefoil Y Y Y
Three ducts flat touching Y
Three ducts flat 2D spacing
Three ducts trefoil Y Y
One duct 3 singles in trefoil
Duct Characteristics
Duct type Ridgiduct
Duct size 125mm ID 148mm OD
Grouping
Grouping – trefoils flat N/A N/A N/A Y Y Y
Grouping -trefoils tiered N/A N/A N/A Y
Spacing mm N/A N/A N/A 300 225 300 225
No of Trefoils 2 2 3 3
The Distribution rating as displayed on
the ‘Grouped’ result screen is equal to
the Sustained rating because Loss
Load Factor is 1.0 & Utilisation is 100%

3.3.1.6 11kV Paper Cables - Grouping Correction Table
Where multiple cables are installed in close proximity (i.e. grouped) the heat dissipation is reduced
thus de-rating the cables. The following table may be used to calculate the degree of de-rating of
grouped legacy cables.
Table 3.3.1.6A
De-rating factors for Grouped Paper Cables
5 Day Distribution Rating, Cyclic and Sustained
Laid Direct Ducted
S
S
S
Soil Thermal Spacing
Resistivity of cables Number of Cables in Group Number of Cables in Group
O
Cm/W mm 2 3 4 5 6 2 3 4 5 6
0.9 Winter 150 0.97 0.93 0.91 0.89 0.88 0.98 0.96 0.95 0.94 0.93
300 0.98 0.95 0.94 0.92 0.92 0.99 0.97 0.96 0.95 0.95
450 0.98 0.96 0.95 0.94 0.94 0.99 0.98 0.97 0.97 0.96
600 0.98 0.97 0.96 0.96 0.95 0.99 0.98 0.98 0.97 0.97
1.2 Summer 150 0.96 0.92 0.90 0.88 0.86 0.98 0.95 0.94 0.92 0.91
300 0.97 0.94 0.93 1.91 0.90 0.98 0.96 0.95 0.94 0.94
450 0.98 0.96 0.95 0.93 0.93 0.99 0.97 0.96 0.96 0.95
600 0.98 0.96 0.96 0.95 0.94 0.99 0.98 0.97 0.97 0.96
Formation of Cables Formation of Ducts

S 3 4 6 9 12 3 4 6 9 12
Soil Thermal Spacing
Resistivity of cables
S
O
Cm/W mm
0.9 Winter 150 0.93 0.9 0.85 0.78 0.73 0.96 0.94 0.91 0.87 0.83
300 0.94 0.92 0.88 0.82 0.77 0.97 0.95 0.93 0.89 0.86
450 0.95 0.93 0.89 0.84 0.79 0.97 0.96 0.94 0.9 0.87
600 0.95 0.94 0.9 0.85 0.8 0.97 0.96 0.94 0.91 0.88
1.2 Summer 150 0.92 0.89 0.83 0.76 0.7 0.95 0.93 0.89 0.84 0.8
300 0.94 0.91 0.86 0.8 0.74 0.96 0.94 0.91 0.86 0.82
450 0.94 0.92 0.88 0.82 0.77 0.96 0.95 0.92 0.88 0.84
600 0.94 0.93 0.89 0.83 0.78 0.96 0.95 0.93 0.89 0.85
Based on Engineering Recommendation P17 Table 10
For other values of Soil Resistivity please refer to P17

3.3.1.7 11kV Paper Cables - Loss Load Factor Correction Table
The 5 Day Distribution ratings in Section3.3.1.4 and the Cyclic ratings in Section 3.3.1.5 are based on
‘Load Curve G’ which has a Loss Load factor of 0.5 representing a typical mixed
industrial/commercial/residential distribution feeder load.
Where the load mix is predominantly industrial/commercial the Loss Load Factor is often significantly
higher thus de-rating the cables to a value closer to the Sustained ratings.
The following factors can be used to assess the capability of legacy cables to safely accommodate
industrial/commercial loads.
Table 3.3.1.6B
De-rating factors for Variation in Loss Load Factors
Paper Cables - 5 Day Distribution & Cyclic Ratings
Soil Resistivity Loss Load Factor over Laid Direct Ducted

24 hr Period
O
Cm/W Belted Screened Belted Screened
0.9 Winter 0.45 – 0.5 1.0 1.0 1.0 1.0
0.5 – 0.55 0.98 0.98 0.99 0.99
0.55 – 0.6 0.97 0.97 0.98 0.98
0.6 - 0.65 0.96 0.95 0.97 0.97
0.65 – 0.7 0.94 0.94 0.96 0.96
0.7 – 0.75 0.93 0.93 0.96 0.95
0.75 – 0.8 0.92 0.91 0.95 0.95
0.8 – 0.85 0.91 0.90 0.94 0.94
0.85 – 0.9 0.90 0.89 0.93 0.93
0.9 – 0.95 0.89 0.88 0.93 0.92
0.95 – 1.0 0.88 0.87 0.92 0.91
1.2 Summer 0.45 – 0.5 1.0 1.0 1.0 1.0

0.5 – 0.55 0.98 0.98 0.99 0.99
0.55 – 0.6 0.96 0.96 0.98 0.98
0.6 – 0.65 0.95 0.95 0.96 0.96
0.65 – 0.7 0.93 0.93 0.95 0.95
0.7 – 0.75 0.92 0.92 0.94 0.94
0.75 – 0.8 0.91 0.91 0.93 0.93
0.8 – 0.85 0.90 0.89 0.92 0.92
0.85 – 0.9 0.88 0.88 0.92 0.91
0.9 – 0.95 0.87 0.86 0.91 0.90
0.95 – 1.0 0.86 0.85 0.90 0.89
Based on Engineering Recommendation P17 Table 13
For lower values of Loss Load Factor please refer to P17

3.3.1.8 11kV Cable Electrical Data Tables
CABLE 1 sec Resistance Reactance % Volt Losses

S/C drop at 1MVA
Note 1
rating
Size Type Core Screen R % jX % % per km KW per
kA kA W /km (100MVA W /km (100MVA per MVA km
DC @ base) base)
20OC
70 mm2 Al Triplex 6.6 3.2 0.443 36.61 0.125 10.33 0.356 3.445
2
95 mm Al Triplex 8.9 4.5 0.320 26.45 0.119 9.835 0.258 2.488
150 mm2 Al Triplex 14.1 4.5 0.206 17.02 0.112 9.256 0.167 1.602
2
185 mm Al Triplex 17.4 4.5 0.164 13.55 0.108 8.926 0.133 1.275
240 mm2 Al Triplex 22.5 4.5 0.125 10.33 0.104 8.595 0.102 0.972
2
300 mm Al Triplex 28.2 4.5 0.100 8.26 0.101 8.347 0.083 0.778
300 mm2 Cu Triplex 42.9 4.5 0.060 4.96 0.0961 7.94 0.052 0.467
400 mm2 Cu singles 57.2 4.5 0.047 3.88 0.095 7.851 0.042 0.365
630 mm2 Cu singles 90.1 4.5 0.0283 2.34 0.089 7.355 0.029 0.220
300 mm2 Al PICAS 23.4 23.4 0.100 8.26 0.077 6.35 0.082 0.778
2
240 mm Cu PICAS 28.6 4.5 0.0754 6.23 0.078 6.45 0.062 0.586
185 mm2 Al PICAS 14.4 14.4 0.164 13.55 0.080 6.61 0.132 1.275
2
95 mm Al PICAS 7.4 7.4 0.320 26.44 0.087 7.19 0.257 2.488
300 mm2 Al SWA 23.4 0.100 8.26 0.077 6.35 0.082 0.778
185 mm2 Al SWA 14.4 0.164 13.55 0.080 6.61 0.132 1.275
95 mm2 Al SWA 7.4 0.320 26.44 0.087 7.19 0.257 2.488
0.5 in2 Al SWA 23.4 0.0923 7.63 0.0742 6.13 0.075 0.718
0.3 in2 Al SWA 14.4 0.152 12.56 0.0778 6.43 0.123 1.182
0.15 in2 Al SWA 7.4 0.312 25.78 0.0839 6.93 0.251 2.426
0.10 in2 Al SWA 5.0 0.456 37.68 0.0897 7.41 0.366 3.546
2
0.3 in Cu SWA 23.4 0.0920 7.60 0.0778 6.43 0.075 0.715
2
0.2 in Cu SWA 14.4 0.142 11.73 0.082 6.78 0.115 1.104
0.15 in2 Cu SWA 11.5 0.188 15.54 0.0839 6.93 0.152 1.462
2
0.10 in Cu SWA 7.4 0.276 22.81 0.0897 7.41 0.222 2.146
0.06 in2 Cu SWA 4.5 0.463 38.26 0.0962 7.95 0.372 3.600
2
0.04 in Cu SWA 2.9 0.703 58.10 0.102 8.43 0.564 5.466
0.0225 in2 Cu SWA 1.8 1.258 103.96 0.114 9.42 1.009 9.782
Derived from:-
%Volt Drop = (√ ((0.97R%)2 + (0.24X%)2) ) / 100
assuming 0.97 p.f. load, Voltage = 11kV

2
Losses =I x R (where I = 50.91 Amps for 1MW at 11.0 kV and 0.97 pf)
Note 1
Losses at any other load cab be calculated from :
Loss = Loss at 1MVA x (Actual Load in MVA)2
1 sec S/S ratings for imperial cables shown in italics have been extrapolated from drawing DPM 423 in the EMEB
Distribution Planning Manual

3.3.2 11kV Overhead Lines
3.3.2.1 11kV Conductors
Normal applications: 11kV unearthed bare wire construction shall be to ENATS 43-40 as detailed in
Central Networks Overhead Manual Volume 1.
Special applications e.g. wooded areas, protection of water-fowl, recreational areas: 11kV unearthed
covered conductor construction shall generally be to ENATS 43-40 and ENATS 43-119,120,121. There is no
Central Networks standard for covered conductor lines – refer to the Technology & Standards Manager
for guidance.
Steelwork and fittings shall generally be to ENATS 43-95 and as detailed in Central Networks’ Overhead
Manual Volume 2.
The standard sizes to be used in are:
• 50 mm2 AAAC ‘Hazel’ (All Aluminium Alloy Conductor)

• 100 mm2 AAAC ‘Oak’ (All Aluminium Alloy Conductor)
• 200 mm2 AAAC ‘Poplar’ (All Aluminium Alloy Conductor)
• 50 mm2 ACSR ‘Rabbit’ (Aluminium Core Steel Reinforced)

• 100 mm2 ACSR ‘Dog’ (Aluminium Core Steel Reinforced)
• 150 mm2 ACSR ‘Dingo’ (Aluminium Core Steel Reinforced)
• 300 mm2 HDA ‘Butterfly’ (Hard Drawn Aluminium)
For applications refer to Section 1.4.4.2
3.3.2.2 11kV Conductor Distribution Rating Tables

Continuous current ratings are based on Engineering Recommendation P27 and calculated using the
EA Technology Ltd Overhead Line Calculator Spreadsheet. (NB. Cyclic ratings are not applicable to
overhead lines).
Maximum conductor temperature 50 OC pre line built before 1971
Maximum conductor temperature 60 OC pre line built after 1971 in Central Networks East
Maximum conductor temperature 75 OC pre line built after 1971 in Central Networks West
Exceedance (excursion time) of 0.001%

Wind speed 0.5m/s
Solar radiation nil
Ambient temperature:
20 OC Summer (May June, July, August)
9 OC Spring /Autumn (March, April / Sept, Oct, Nov)
2 OC Winter (Dec, Jan, Feb)

Table 3.3.2.2A New Lines and Lines Built After 1971

60 OC Conductor Temperature (Central Networks East standard)
(New build standard shown bold) Summer Autumn Winter
Spring
Size Type Code Stranding Amps Amps Amps
Bare Conductors
40 mm2 ACSR Ferret 6/1/3.0 mm 156 175 186
50 mm2 ACSR Rabbit 6/1/3.35 mm 181 203 215
100 mm2 ACSR Dog 6/4.72 & 7/1.57 mm 286 318 338
150 mm2 ACSR Dingo 18/1/3.35 mm 386 432 458
2
150 mm ACSR Wolf 30/7/2.59 396 443 470
175 mm2 ACSR Lynx 30/2/2.79 437 489 518
50 mm2 AAAC Hazel 7/3.30 mm 188 221 224

2
100 mm AAAC Oak 7/4.65 mm 297 333 353
150 mm2 AAAC Ash 19/3.48 mm 393 440 467
2
200 mm AAAC Poplar 37/2.87 mm 474 531 562
300 mm2 AAAC Upas 37/3.53 mm 626 700 742
50 mm2 HDA Ant 7/3.10 mm 186 209 222

100 mm2 HDA Wasp 7/4.39 mm 295 331 351
300 mm2 HDA Butterfly 19/4.65 mm 622 695 737
Covered Conductors
50 mm2 XLPE CC HazelCC AL3 7/3.30 mm 201 225 239
2
50 mm BLX / PAS 50CC AL2 7/compacted 168 188 200
120 mm2 BLX / PAS 120CC AL2 19/compacted 300 336 356
185 mm2 BLX / PAS 185CC AL2 34min/compacted 394 441 467
2
185 mm BLX / PAS 185CC AL3 34min/compacted 411 460 488
Engineering Recommendation P27 calculations carried out on EA Technology Ltd Spreadsheet:

Bare conductors – OHRAT2 (version 5th May 2004) ANATS 43-122 Appemdix B1
Covered Conductors - OHRATv3.01 (version 7th July 2004)

Table 3.3.2.2B New Lines and Lines Built After 1971

O
75 C Conductor Temperature (Central Networks West standard)
(New build standard shown bold) Summer Autumn Winter
Spring
Bare Conductors
40 mm2 ACSR Ferret 6/1/3.0 mm 181 197 206
50 mm2 ACSR Rabbit 6/1/3.35 mm 209 228 238
100 mm2 ACSR Dog 6/4.72 & 7/1.57 mm 329 359 375
2
150 mm ACSR Dingo 18/1/3.35 mm 447 485 507
150 mm2 ACSR Wolf 30/7/2.59 459 498 521
175 mm2 ACSR Lynx 30/2/2.79 507 550 575
50 mm2 AAAC Hazel 7/3.30 mm 219 238 249

100 mm2 AAAC Oak 7/4.65 mm 345 375 392
150 mm2 AAAC Ash 19/3.48 mm 457 496 518
200 mm2 AAAC Poplar 37/2.87 mm 551 598 626
300 mm2 AAAC Upas 37/3.53 mm 729 790 826
50 mm2 HDA Ant 7/3.10 mm 216 235 246

2
100 mm HDA Wasp 7/4.39 mm 343 372 390
300 mm2 HDA Butterfly 19/4.65 mm 723 785 820
Covered Conductors
50 mm2 XLPE CC HazelCC AL3 7/3.30 mm 233 253 265
2
2
Engineering Recommendation P27 calculations carried out on EA Technology Ltd Spreadsheet:

Bare conductors – OHRAT2 (version 5th May 2004) ANATS 43-122 Appemdix B1
Covered Conductors - OHRATv3.01 (version 7th July 2004)

Table 3.3.2.2C Lines Built Before 1971 50 OC Conductor Temperature
Aluminium Summer Autumn Winter

Spring
2
0.02 in ACSR Squirrel 6/1/.083 in 86 100 109
2
0.025 in ACSR Gopher 6/1/.093 in 100 116 126
0.03 in2 ACSR Weasel 6/1/.102 in 113 131 141
0.04 in2 ACSR Ferret 6/1/.118 in 136 159 171
0.05 in2 ACSR Rabbit 6/1/.132 in 158 184 198
2
0.06 in ACSR Mink 6/1/.144 in 178 205 223
2
0.075 in ACSR Raccoon 6/1/.161 in 207 239 257
0.1 in2 ACSR Dog 6/.186 & 7/.062 in 252 289 310
0.15 in2 ACSR Dingo 18/1/.132 in 337 391 421
0.15 in2 ACSR Wolf 30/7/.102 in 346 401 431
0.175 in2 ACSR Lynx 30/2/2.79 in 382 442 476
0.025 in2 AAAC Almond 7/.092 in 105 122 132

0.03 in2 AAAC Cedar 7/.100 in 117 136 146
2
0.04 in AAAC Fir 7/.116 in 142 165 178
2
0.05 in AAAC Hazel 7/.130 in 165 191 206
0.06 in2 AAAC Pine 7/.142 in 186 215 232
0.075 in2 AAAC Willow 7/.160 in 215 250 269
0.1 in2 AAAC Oak 7/.183 in 259 301 324
2
0.15 in AAAC Ash 7/.137 in 343 397 428
0.175 in2 AAAC Elm 19/.148 in 380 440 474
0.2 in2 AAAC Poplar 37/.113 in 414 479 515
0.3 in2 AAAC Upas 37/.139 in 545 631 679
0.05 in2 HDA Ant 7/0.122 in 163 189 204

0.1 in2 HDA Wasp 7/0.173 in 258 299 322
0.3 in2 HDA Butterfly 19/0.183 in 542 628 675
0.4 in2 HDA Centipede 37/0.149 in 852 924 966
Engineering Recommendation P27 calculations carried out on EA Technology Ltd Spreadsheet OHRAT2 (version
5th May 2004

Table 3.3.2.2D Lines Built Before 1971 50 OC Conductor Temperature
Copper (Imperial sizes) Summer Autumn Winter

Spring
Size Type Metric eq Stranding Amps Amps Amps
0.017 in 2 Cad cu 13 mm2 3/0.93” 3/2.36mm 78 90 97
0.0225 in2 Cad cu 17 mm2 7/.069” 7/1.75mm 89 104 112
0.025 in2 Cad cu 19 mm2 7/.073” 7/1.85mm 96 111 120
2
0.025 in Cad cu 19 mm2 3/.112” 3/2.84mm 99 115 124
2 2
0.04 in Cad cu 31 mm 7/.093” 7/2.36mm 132 153 165
0.05 in2 Cad cu 38 mm2 3/.158” 3/4.01mm 156 181 195
2 2
0.05 in Cad cu 38 mm 7/.103” 7/2.62mm 151 175 189
0.075 in2 Cad cu 60 mm2 7/.127” 7/3.23mm 197 229 247
2 2
0.1 in Cad cu 80 mm 7/.146” 7/3.71mm 237 275 296
2 2
0.15 in Cad cu 113 mm 7/.179” 7/4.55mm 312 362 390
0.15 in2 Cad cu 110 mm2 19/.109” 19/2.77mm 314 364 392
2 2
0.175 in Cad cu 110 mm 19/.118” 19/3.00mm 347 402 433
0.022 in2 Cu 15 mm2 7/.064” 7/1.63mm 89 103 111

2 2
0.025 in Cu 16 mm 3/.104” 3/2.64mm 98 114 123
2 2
0.04 in Cu 27 mm 3/.131” 3/3.37mm 135 157 169
0.05 in2 Cu 33 mm2 3/.147” 3/3.73mm 154 179 193
2 2
0.075 in Cu 49 mm 3/.180” 3/4.57mm 202 234 252
0.075 in2 Cu 48 mm2 7/.116” 7/2.95mm 192 223 241
2 2
0.1 in Cu 65 mm 7/.136” 7/3.45mm 236 274 295
0.15 in2 Cu 110 mm2 19/.109” 19/2.77mm 342 397 427
2 2
0.2 in Cu 130 mm 19/.116” 19/2.95mm 364 422 454
Engineering Recommendation P27 calculations carried out on EA Technology Ltd Spreadsheet OHRAT2 (version
5th May 2004
Table 3.3.2.2E Lines Built Before 1971 50 OC Conductor Temperature
Solid Copper (SWG sizes) Summer Autumn Winter

Spring
Size Type Metric eq Diameter Amps Amps Amps
2
No 6 cu SWG 18.6 mm 4.88 mm 103 120 129
2
No 5 cu SWG 22.8 mm 5.38 mm 118 137 148
No 4 cu SWG 27.3 mm2 5.89 mm 133 155 167
2
No 3 cu SWG 32.2 mm 6.4 mm 148 171 185
2
No 2 cu SWG 38.6 mm 7.01 mm 167 194 209
No 1 cu SWG 45.6 mm2 7.62 mm 186 216 233
2
1/0 cu SWG 53.2 mm 8.23 mm 206 239 258
2/0 cu SWG 61.4 mm2 8.84 mm 226 263 283
2
3/0 cu SWG 70.1 mm 9.45 mm 247 287 309
SWG values calculated on EATL spreadsheet OHRAT” (5th May 2004 version) – approx only – based on
equivalent cross section 7 strand conductor as there is no option for solid conductor.

3.3.2.3 11kV O/H Line Data Tables
Conductor 1 sec S/C Resistance Reactance % Volt Losses

rating drop at 1MVA
Note 1
Size Type kA MVA R % jX % per km per km

W /km (100M W /km (100M per MVA per MVA
VA VA
Metric
40 mm 2
ACSR Ferret 3.6 69 0.677 55.95 0.408 33.72 0.549 5.264
50 mm 2
ACSR Rabbit 4.4 86 0.543 44.88 0.395 32.64 0.442 4.222
50 mm2 AAAC Hazel 0.550 45.45 0.351 29.01 0.446 4.277
100 mm 2
ACSR Dog 8.9 170 0.273 22.56 0.378 31.24 0.231 2.123
100 mm2 AAAC Oak 0.277 22.89 0.351 29.01 0.233 2.154
150 mm 2
ACSR Dingo 13.5 260 0.182 15.04 0.335 27.69 0.160 1.415
150 mm 2
ACSR Wolf 0.183 15.12 0.335* 27.69 0.161 1.423
2
150 mm AAAC Ash
175 mm2 ACSR Lynx
200 mm2 AAAC Poplar 0.139 11.49 0.325 26.86 0.129 1.081
300 mm 2
HDA Butterfly 23.4 450 0.0892 7.37 0.316 26.12 0.095 0.694
300 mm 2
HDA Upas 0.0916 7.57 0.316* 26.12 0.097 0.712
50 mm2 HazelCC AL3 0.55 0.351*
50 mm2 BLX 50CC AL2 0.72 0.351*
120 mm2 BLX 120CC AL2 0.288 0.343*
2
185 mm BLX 185CC AL2 0.188
50 mm2 BLX 50CC AL3 0.61 0.351*
120 mm2 BLX 120CC AL3 0.272 0.343*
2
185 mm BLX 185CC AL3 0.176
* extrapolated values
Imperial
0.0225 in2 Cad cu 1.350* 111.6 0.38* 31.41 1.085 10.50
2
0.025 in Cu eq 1.7 32 1.100 90.90 0.375 30.99 0.956 9.090
0.04 in2 Cu eq 3.6 69 0.691 57.10 0.360 29.75 0.625 5.710
2
0.05 in Cu eq 4.4 86 0.549 45.37 0.353 29.17 0.510 4.537
0.075 in2 Cu eq 6.8 130 0.378 31.24 0.343 28.36 0.371 3.124
2
0.1 in Cu eq 8.9 170 0.276 22.81 0.333 27.52 0.287 2.281
0.15 in2 Cu eq 13.5 260 0.184 15.21 0.320 26.44 0.211 1.521
SWG
No 6 cu = 18.6 mm2 0.975 80.58 0.375 30.99 0.785 7.581
2
No 5 cu = 22.8 mm 0.800 66.12 0.360 29.75 0.645 6.220
No 4 cu = 27.3 mm2 0.640 52.89 0.360 29.75 0.518 4.976
No 3 cu = 32.2 mm2 0.540 44.63 0.353 29.17 0.439 4.199
No 2 cu = 38.6 mm2 0.439 36.28 0.343 28.36 0.358 3.413
No 1 cu = 45.6 mm2 0.390 32.23 0.343 28.36 0.320 3.032
1/0 cu = 53.2 mm2 0.360 29.75 0.343 28.36 0.296 2.799
2/0 cu = 61.4 mm2 0.286 23.64 0.333 27.52 0.239 2.224
2
3/0 cu = 70.1 mm 0.250 20.66 0.320 26.44 0.210 1.944
The values for SWG conductors are approximations only.

Derived from:-
%Volt Drop = (√ ((0.97R%)2 + (0.24X%)2) ) / 100
assuming 0.97 p.f. load, Voltage = 11kV
2
Losses =I x R (where I = 50.91 Amps for 1MW at 11.0 kV and 0.97 pf)
Note 1
Losses at any other load cab be calculated from :
Loss = Loss at 1MVA x (Actual Load in MVA)2
Data in italics is extrapolated from adjacent values.
3.3.3 11 kV / LV Transformers
3.3.3.1 Ground Mounted Transformers
Ground mounted transformers shall generally be in accordance with ENATS 35-1 and as detailed in the
Central Networks’ Plant Specification Manual Section 3F. Free-standing and as part of Compact Unit
Substation (CUS)
Free-standing and as part of Compact Unit Substation (CUS)
Size / Type Use
200 kVA CUS with • LV Network supplying mixed loads

11kV RMU • Single direct service for industrial / commercial load up to 225
500A 2 way LV Pillar kVA

11kV RMU & • Single direct service for industrial / commercial load up to 300
800A 4 way LV Pillar kVA

11kV RMU & • Single or double direct services for industrial / commercial loads
1600A 4 way LV Pillar up to 600 kVA

11kV RMU & • Single or double direct services for industrial / commercial loads
1600A 4 way LV Pillar up to 600 kVA
500 kVA CUS with • 301 to 500 kVA LV metered supply to single customer from plus feed
11kV RMU & out onto local LV network
LV 1250A ACB cabinet • Substation located next to public highway
11kV RMU & out onto local LV network
1000 kVA CUS with • 801 to 1000 kVA LV metered supply to single customer from plus
11kV RMU & feed out onto local LV network
11kV RMU & feed out onto local LV network
11kV feeder switch & out onto local LV network
LV 1250A ACB cabinet • Substation located remotely from RMU

11kV feeder switch & out onto local LV network

11kV feeder switch & feed out onto local LV network
11kV feeder switch & feed out onto local LV network
3.3.3.2 Padmount Transformers
Padmount transformers shall generally be in accordance with ANSI C57-12-25/26 and as detailed in the Central
Networks Plant Specification Manual Section 3F.
Size / Type Use
50 kVA single phase padmount • LV Network supplying mixed loads
with one feeder way • Single phase loads up to 50kVA
100 kVA three phase padmount • LV Network supplying mixed loads

• Single direct service for industrial / commercial load up to 110
with two feeder ways kVA
200 kVA three phase padmount • LV Network supplying mixed loads
• Single direct service for industrial / commercial load up to 225
with two feeder ways kVA
3.3.3.3 Pole Mounted Transformers
Pole mounted transformers shall generally be in accordance with ENATS 35-1 and as detailed in the Central
Networks Plant Specification Manual Section 3F.
Size / Type Use
25 kVA single phase 2 wire • Single direct service for domestic / farming load up to 25 kVA
• LV Network supplying mixed loads

50 kVA single phase 2 wire
• Single phase loads up to 50 kVA
100 kVA single phase 3 wire
• 480/240v split phase loads up to 100 kVA
Single direct service for domestic load or pumping station up to 50 kVA
50 kVA three phase • Must not be used for unbalanced loads as negatve phase sequence
voltage may damage motors.
100 kVA three phase
• Single direct service for industrial / commercial load up to 100 kVA
200 kVA three phase
315 kVA three phase

3.3.3.4 11kV/LV Transformer Data Tables

Transformers
Size / Type Pole or New build R jX ohms Max Max Cu Max Cyclic
Ground size ohms Fe Losses loading
Mounted Losses W
W
100 kVA ANSI Padmount GM 0.0271 0.0401 200 1500 150%
200 kVA ANSI Padmount GM 0.0074 0.0258 370 1600 150%
25 kVA ENATS PM 0.2080 0.2660 130%
50 kVA ENATS PM 0.0876 0.1440 130%
100 kVA ENATS PM 0.0371 0.0810 175 1700 130%
200 kVA ENATS PM 0.0158 0.0406 275 3000 130%
200 kVA ENATS GM 0.0158 0.0406 275 3000 130%
300 kVA ENATS GM 0.0095 0.0277 130%
315 kVA ENATS GM 0.0090 0.0268 425 5100 130%
315 kVA ENATS PM 0.0090 0.0268 425 5100 130%
500 kVA ENATS GM 0.0051 0.0171 600 7000 130%
750 kVA ENATS GM 0.0031 0.0115 130%
800 kVA ENATS GM 0.0029 0.0107 1500 10000 130%
1000 kVA ENATS GM 0.0022 0.0086 1800 13300 130%
1500 kVA ENATS GM 0.0013 0.0067 130%
50 kVA 1Ø ANSI Padmount GM 0.0182 0.0206 170 750 150%
16 kVA 1Ø ENATS PM 0.1074 0.139 130%
25 kVA 1Ø ENATS PM 0.0612 0.0944 130%
50 kVA 1Ø ENATS PM 0.0266 0.0496 110 800 130%
100 kVA 2Ø ENATS PM 0.0165 0.0255 130%
Note –impedances are referred to the LV side of the transformer

3.3.4 11kV Switchgear
3.3.4.1 11kV Distribution Substation Switchgear
11kV Distribution Switchgear shall generally be in accordance with ENATS 41-36 with more specific
technical design information, as detailed in the Central Networks Plant Specification Manual.
This secondary switchgear includes RMUs (Transformer mounted and free-standing), Metering units,
Multi-panel boards for consumer s/s including extensible circuit breakers, switch disconnectors etc.
Table 3.3.4.1 1 of 7
Central Networks Schneider Lucy
Application
Contract Reference designation designation
Merlin Gerin Lucy Sabre • Ring feed substations with free standing
RMU2-CC/TLF Ringmaster equipment.
• Switchgear replacement.
Free standing ring main RN2c-T1/21 VRN2A • Transformer protection up to
equipment with 200A • 1000 kVA@11kV
circuit breaker with • 500 kVA 6.6kV
cable box. • c/w Overcurrent and Earth Fault
Protection using CT operated trip coils with
TLF Protection provision for time limit fuses (TLF)
Non extensible
630 amp switch Tee-off CB must not be used to feed circuits with a
disconnectors direct 11kV back-feed – earth switch is limited to
Fault flow indicator & 3.15kA fault level.
CTs on left ring switch
RMU2-DC/TLF Merlin Gerin Lucy Sabre • Consumer Substations.

Ringmaster • Transformer protection up to
Free standing ring main • 1000 kVA@11kV
equipment with 200A RN2c-T1/21 VRN2A + • 500 kVA 6.6kV
circuit breaker complete + AIMU • c/w Overcurrent and Earth Fault
with directly mounted MU2-M2/16 Protection using CT operated trip coils with
11kV metering unit. provision for time limit fuses (TLF)
TLF Protection
Tee-off CB must not be used to feed circuits with a
Non extensible
direct 11kV back-feed – earth switch is limited to
630 amp switch
3.15kA fault level.
disconnectors
Fault flow indicator &
Specify :
VT Voltage – 11kV or 6.6kV
CT Ratio – 100/50/5

Table 3.3.4.1 2 of 7
Merlin Gerin Lucy Sabre • Consumer Substations.
• 3.8 MVA@11kV
RN2c-T2/21 VRN2A + • 2.3 MVA @ 6.6kV
+ AIMU • c/w Overcurrent and Earth Fault
RMU2-DC/REL MU2-M3/16 Protection using Self Powered relay
Free standing ring main
equipment with 200A Tee-off CB must not be used to feed circuits with a
circuit breaker complete direct 11kV back-feed – earth switch is limited to
with with directly 3.15kA fault level.
mounted 11kV metering
unit. Specify :
VT Voltage – 11kV or 6.6kV
Self Powered relay. CT Ration – 200/100/5
Non extensible
630 amp switch
disconnectors
RMU2- Merlin Gerin Lucy Sabre • Automated Ring feed substations with
Ringmaster free standing equipment.
CC/AUTO/TLF/EFC • Switchgear repalcement.
T/1ACT RN2C-T1/21 VRN2A-FS • Transformer protection up to
T200E-4MI- • 1000 kVA@11kV
GPU • 500 kVA 6.6kV
equipment with 200A
• c/w Overcurrent and Earth Fault
circuit breaker with
Protection using CT operated trip coils with
cable box.
provision for time limit fuses (TLF)
1 actuator. For Tee-off CB must not be used to feed circuits with a

automation use (c/w direct 11kV back-feed – earth switch is limited to
RTU) 3.15kA fault level.
TLF Protection
Non extensible
630 amp switch
disconnectors
RMU2-TC/TLF Merlin Gerin • Compact Unit Substations

Transformer mounted VRN2A - TC • 1000 kVA@11kV
ring main equipment RN2c-T1/21 • 500 kVA 6.6kV
with 200A circuit Lucy Sabre • c/w Overcurrent and Earth Fault
breaker with cable box. Protection using CT operated trip coils with
provision for time limit fuses (TLF)
TLF Protection
Non extensible Tee-off CB must not be used to feed circuits with a
630 amp switch direct 11kV back-feed – earth switch is limited to
disconnectors 3.15kA fault level.

Table 3.3.4.1 3 of 7
RMU6- Merlin Gerin Lucy Sabre • Consumer Substations.
DC/REL/EFCT VRN6A • 3.8 MVA@11kV
Free standing ring main RN6c- +AIMU • 2.3 MVA @ 6.6kV
equipment with 630A T1/21+ • c/w Overcurrent and Earth Fault
circuit breaker complete MU2-N1/16 Protection using Self Powered relay
with with directly
mounted 11kV metering Tee-off CB may be used to feed circuits with a
unit. direct 11kV back-feed – earth switch is fully rated.
Self Powered relay. Specify :

Non extensible VT Voltage – 11kV or 6.6kV
630 amp switch CT Ration – 400/200/5
disconnectors
RMU6- Merlin Gerin Lucy Sabre • Network Automation

Ringmaster • Installed with the 630 amp CB in the ring
CC/AUTO/REL/EFC VRN6A-FS main as free standing equipment.
T/2ACT RN6c-T1/21 • Switchgear repalcement.
T200E-4MI- • Not used to protect local transformer
GPU
equipment with 630A
circuit breaker with Tee-off CB is used to feed circuits with a direct
cable box. 11kV back-feed – earth switch is fully rated.
2 actuators.
For automation use (c/w
RTU)
Non-self powered relay
Non extensible
630 amp switch
disconnectors
Merlin Gerin • Radial connected transformers

NOT ON Ringmaster • CB is suitable for addition of remote control
CONTRACT actuator.
CN2-T6 N/A • Transformer protection up to
• 1000 kVA@11kV
Transformer mounted
• 500 kVA 6.6kV
200 amp circuit breaker
• Overcurrent and Earth Fault Protection using
with cable box.
CT operated trip coils with provision for time
TLF Protection limit fuses (TLF)
Non extensible • CTs 100/50/5
Fully rated earth switch
on incoming cable.
3.15kA rated earth
switch towards
transformer.

Table 3.3.4.1 4 of 7
Merlin Gerin • Radial connected transformers
NOT ON Ringmaster • CB is suitable for addition of remote control
CONTRACT actuator.
CN2-T1/21 N/A • Transformer protection up to
• 3.8 MVA@11kV
Transformer mounted
• 2.3 MVA @ 6.6kV
200 amp circuit breaker
• c/w Overcurrent and Earth Fault Protection
with cable box.
using Self Powered relay
Self powered relay • CTs 200/100/5
Non extensible
on incoming cable.
3.15kA rated earth
switch towards
transformer.
Merlin Gerin • Radial connected transformers sited remote

NOT ON Ringmaster from controlling CB or S/Fuse
CONTRACT • Switch Disconnector is suitable for addition of
SN6-S1/21 N/A remote control actuator
• No Protection
Transformer mounted
630 Switch Disconnector
with cable box.
Non extensible
on incoming cable.
3.15kA rated earth
switch towards
transformer.
ECB Merlin Gerin • Un-metered Feeder up to 12MVA

Extensible free standing Ringmaster • Consumer substations
circuit breaker 630A N/A • CB is suitable for addition of remote control
cable connected. CE6-T8/21 actuator.
• c/w self powered IDMT Overcurrent and Earth
Fault Protection using VIP 300 relay.
• Protection CT’s – 800/1A, Class X & Shunt
Trip Coil 20v DC-250v AC
• Ammeter 0 -600 amp
CB may be used to feed circuits with a direct 11kV
back-feed – earth switch is fully rated

Table 3.3.4.1 5 of 7
• Consumer substations
NOT ON Merlin Gerin
• Metered consumer’s feeders up to 12 MVA
Ringmaster
CONTRACT • CB is not suitable for addition of remote
CE6-T5/21
N/A control actuator.
Extensible free standing (CE6-T6/21
• c/w self powered IDMT Overcurrent and Earth
circuit breaker 630A 6.6kV)
Fault Protection using VIP 300 relay.
cable connected. • Protection CT’s – 800/1A, Class X & Shunt
Metering CTs & VT Trip Coil 20v DC-250v AC
• Ammeter 0 -600 amp
CB may be used to feed circuits with a direct 11kV
back-feed – earth switch is fully rated
ESW Merlin Gerin

Extensible free standing Ringmaster • Ring feed substations with 3 or more feeder
switch 630A cable circuits (with Circuit Breaker CE2-T8/21 or
N/A CE6-T5/21)
connected. SE6-S2/21
• Consumer substations
• Switching stations
• Switch disconnectors is suitable for addition of
remote control actuator.
• No Protection
Merlin Gerin • Consumer substations employing extensible

NOT ON Ringmaster switchgear with 630Amp SE6-S2/21 Switch
CONTRACT Disconnectors
N/A • CB is not suitable for addition of remote
CE6-B3/21
Bus Section Circuit control actuator.
Breaker • Right hand Bus-bar Earth switch
Extensible free standing • c/w self powered IDMT Overcurrent and Earth
630A Fault Protection using VIP 300 relay.
Metering CTs & VT • Protection CT’s – 800/1A, Class X & Shunt
Trip Coil 20v DC-250v AC
• Metering 400/200/5 CTs

Table 3.3.4.1 6 of 7
NOT ON Ringmaster switchgear with 630Amp SE6-S2/21 Switch
CONTRACT N/A Disconnectors.
SE6-B1/21
Bus Section Switch • Right hand Bus-bar Earth switch
Extensible free standing • No Protection
630A
• No Metering

NOT ON Ringmaster switchgear
CONTRACT N/A
SE6-E1/21 • Used to earth left hand bus-bars in
Bus-bar Earthing Switch conjunction with
Extensible free standing
630A • Bus section CB CE6-B3/21
• Bus section switch SE6-B1/21
Merlin Gerin Lucy Sabre Specify :

Ringmaster VT Voltage – 11kV or 6.6kV
AIMU CT Ratio
MU2-M2/16 200/100/5
11kV 100 ?50/5
MU2-M5/16
FMU2 6.6kV
200A free standing
metering unit with
11000/110
or 6600/110 VT
Cable connected both
sides
Merlin Gerin Specify :

Ringmaster VT Voltage – 11kV or 6.6kV
Lucy Sabre CT Ratio
MU2-M2/16 200/100/5
MU2 11kV AIMU 100 ?50/5
200A free standing MU2-M5/16
metering unit with 6.6kV
11000/110
or 6600/110 VT
Direct coupled to RMU
Cable connected towards
consumer

Table 3.3.4.1 7 of 7
Merlin Gerin Specify :
Ringmaster Lucy Sabre VT Voltage – 11kV or 6.6kV
CT Ration – 600/400/5
MU6 MU6-N1/16 AIMU
630A free standing 11kV
metering unit with
11000/110
Direct couplet to RMU

Cable connected towards
consumer

3.3.4.2 11kV Primary/Grid Substation Switchgear
11kV Distribution Switchgear shall generally be in accordance with ENATS 41-36, with more specific
technical design information, as detailed in the Central Networks Plant Specification Manual.
This Primary switchgear includes indoor type multi panel switchboards etc, all available on a bulk
purchase ‘call-off’ contract from the supplier.
The standard switchgear panel options available are all listed in the Central Networks drawings ref
02602.001 to 02602.008.
Size / Type Use

1250Amp or 2000Amp • Transformer Incomer c/w protection relay functions :-
LV IDMT Overcurrent, Directional Overcurrent,
Circuit Breaker Transformer Restricted Earth Fault, Standby Earth Fault Stage 1 & 2, NVD, Transformer
Incomer Panel (T1/1 to T3) & Tapchanger Buchholz Trip & WTI Trip, Telecontrol, Auto-reclose
1250Amp or 2000Amp • Bus-section c/w protection relay functions:-
IDMT Overcurrent, IDMT Earth Fault, Auto Trip Alarm,
Circuit Breaker Bus-Section Panel Trip Circuit Supervision, Telecontrol
(B1/1 to B4)
630Amp Circuit Breaker • Underground Feeder c/w protection relay functions:-

Underground Feeder Panel (F1/1 Trip Circuit Supervision, Telecontrol
to F7)
630Amp Circuit Breaker • Overhead Feeder c/w protection relay functions:-

Overhead Feeder Panel Trip Circuit Supervision, Auto-reclose, Telecontrol
(F3 & F5)
3.3.4.3 Pole Mounted 11kV Switchgear
Section under preparation

3.4 Primary Network 33kV Rating and Data

3.4.1 33kV Cables
3.4.1.1 33 kV Network Cables

Single Core 33kV X.L.P.E. Insulated, Copper Wire Screened Solid Cables, Stranded Copper
Conductors, Generally to BS 7870 4-10:1999, as detailed in Central Networks Cables Cable
Laying & Accessories Manual Section 2.3
Size / Type Use
150 mm2 Cu • Fault level must be below 21.4kA (706 MVA) for 1 second.
• Circuit supplying 24 MVA 33/11kV transformer
400 mm2 Cu • Fault level must be below 57.2kA (1887 MVA) for 1 second.
• Circuit supplying 40 MVA 33/11kV transformer
• May be used for 24 MVA circuit where 150 mm2 would result
in excessive voltage drop.
3.4.1.2 Transformer 33kV Tails
Single Core 33kV X.L.P.E. Insulated, Copper Wire Screened Solid Cables, Stranded Copper Conductors,
Generally to BS 7870 4-10:1999, as detailed in Central Networks Cables Cable Laying & Accessories
Manual Section 2.3
Size / Type Use
• For use with 4/8MVA, 6/12MVA & 12/24MVA transformers.

150 mm2 Cu XLPE singles
• HV tails of 33/11kV transformers
1 per phase
• LV tails of 132/33kV transformers
• For use with 20/40MVA transformers.

400 mm2 Cu XLPE singles
• HV tails of 33/11kV transformers
1 per phase
• LV tails of 132/33kV transformers
2
500 mm Cu XLPE singles • For use with 30/60/78 MVA transformers
2 per phase • LV tails of 132/33kV transformers
2
500 mm Cu XLPE singles • For use with 45/90/117 MVA transformers.
3 per phase • LV tails of 132/33kV transformers

3.4.1.3 33kV Cable Ratings

• Ambient ground temperature 10°C Winter, 15°C Summer.
• Soil thermal resistivity 0.9°C m/w Winter, 1.2°C m/w Summer.
• Loss Load Factor 0.5 - Load Curve G
• The Distribution 5 day limited time rating are not applicable to 33kV circuits as 33kV transformer repairs
normally take more than 5 days to complete.
• Cables in trefoil and solid bonded.
NOTE 1
3.4.1.3 A Transformer Feeders - 50% utilisation – ungrouped
CABLE WINTER SUMMER

Size / Type Sustained Cyclic Sustained Cyclic Sustained Cyclic Sustained Cyclic
XLPE Cu
150mm2 477 540 433 483 416 479 391 446
singles
XLPE Cu
400mm2 796 917 693 785 690 807 619 718
singles
XLPE Cu
500mm2 900 1041 776 883 778 916 691 806
singles
NOTE 1 – Assumes that only one circuit is running at full load and the other is dead.
NOTE 2
3.4.1.3 B Interconnectors - 100% utilisation – group of 2 spaced 300mm apart
CABLE WINTER SUMMER

Size / Type Sustained Cyclic Sustained Cyclic Sustained Cyclic Sustained Cyclic
XLPE Cu
150mm2 426 482 387 432 368 424 344 392
singles
XLPE Cu
400mm2 675 805 610 691 602 704 537 623
singles
XLPE Cu
500mm2 681 910 681 775 676 796 597 697
singles
NOTE 2 – Assumes that both cables are simultaneously running at full load e.g. to provide support during a
132kV outage.
If the two cables are thermally independent – e.g. on separate routes then use Table 3.4.1.3A Ratings
For cable separation other than 300mm consult the Assets Manager.
3.4.1.4 33kV Transformer Tail Rating Tables
Transformer tails are based on Sustained ratings on the basis that:
Transformer repairs will normally take more than 5 days top complete
Load cycles may be less favourable than Engineering Recommendation P17 ‘Load
Curve G’.
Assumptions
• Values for average environmental conditions
• Ambient ground temperature 10°C Winter 15°C Summer.
• Soil thermal resistivity 0.9°C m/w Winter, 1.2°C m/w Summer

• For other conditions including cables in air or clipped to wall use Engineering Recommendation P17 tables
with all appropriate correction factors.
Table 3.4.1.4. A
Transformer 33kV XLPE Tails Sustained Ratings
Formation Size Type Winter amps Summer amps
A In trefoil – 1 core per phase 150mm2 Cu Singles 482 477 421 416
R
YB 400 mm2 Cu Singles 816 796 707 690
2
500mm Cu Singles 926 900 802 778
B Laid singly – 1 core per phase – 150mm2 Cu Singles 511 489 445 425
2D spacing between cores
400 mm2 Cu Singles 876 787 759 678
RYB 500mm2 Cu Singles 1003 879 867 755

C Laid singly – 1 core per phase - 150mm2 Cu Singles 473 442 429 399
ducts touching
2
400 mm Cu Singles 806 699 724 623
RYB
500mm2 Cu Singles 921 779 825 692
D In trefoil – 2 cores per phase - 150mm2 Cu Singles 418 414 362 359
300mm spacing
2
R R 400 mm Cu Singles 696 679 599 584
YB YB 500mm 2
Cu Singles 787 764 676 656
E Laid singly – 2 cores per phase
150mm2 Cu Singles 390 368 346 326
- ducts touching
R R 400 mm2 Cu Singles 651 579 574 508

YB YB
500mm2 Cu Singles 740 645 650 565
F In trefoil – 3 cores per phase - 150mm2 Cu Singles 374 371 323 320
300mm spacing
400 mm2 Cu Singles 616 602 529 516
R R R
YB YB YB 500mm2 Cu Singles 695 675 596 578
G Laid singly – 3 cores per phase 150mm2 Cu Singles 346 327 304 286
- ducts touching
400 mm2 Cu Singles 573 509 500 443
R R R
YB YB YB 500mm2 Cu Singles 649 566 566 492
Cable screens bonded to earth at one end only – NORMAL ARRANGMENT
Cable screens bonded to earth at both ends – DO NOT USE
The current ratings are given for each core. For the total capacity of multiple cores per phase multiply the
rating by the number of cores.

Transformer Nameplate CER Winter CER Summer Tails per Current per core
Rating phase
MVA Amps MVA Amps Winter Summer
6/12 MVA 33kV 12 210 9.5 166 1 x 150mm2 210 166

2
12/24 MVA 33kV 24 420 19 332 1 x 150mm 420 332
20/40 MVA 33kV 40 700 32 560 1 x 400 mm2 700 560
30/60/78 MVA 33kV 78 1365 60 1050 2 x 500 mm2 683 525
45/90/117 MVA 33kV 117 2047 90 1575 3 x 500 mm2 682 525
EATL ‘CRATER’ Spreadsheet settings used for Transformer Tail Ratings

Common settings
Cable Characteristics
Cable type Copper wire screen BS 7870-4.10 (11kV)
Conductor Copper stranded
Conductor size mm2 150, 400 & 500
Insulation type XLPE
Sheath type Polyethylene
Colour of Sheath N/A
Cu Wire Screen area mm2 35 mm2
Bonding arrangement Single point & Solid, No Transposition as required
Operating Conditions
Environment In soil
Drying out of soil No Account taken
Conductor temp 90 OC
Soil Temp 10 OC Winter 15 OC Summer
Soil Thermal Resistivity 0.9 mOC/W Winter 1.2 mOC/W Winter
Soil depth 600 mm
Loss Load Factor 1.0 (set to 1.0 so that Dist/Cyclic/Sust rating are equal)
Utilisation % 100% (set to 100% so that Dist/Cyclic/Sust rating are equal)
Limited Time days N/A
Cable Grouping See below
Soil Thermal diffusivity 5.0E-07x(0.9/g)^0.8
Soil depth measured to Top surface
Arrangement
A B C D E F G
Cable grouping No No No Yes Yes Yes Yes
Cable & Duct configuration
Single core in trefoil Y Y Y
Single cores flat 2D Spacing between Y
centres
Three ducts flat touching Y
Three ducts flat 2D spacing
Three ducts in trefoil Y Y
One duct 3 singles in trefoil
Duct Characteristics
Duct type Ridgiduct
Duct size 125mm ID 148mm OD
Grouping
Grouping – trefoils flat N/A N/A N/A Y Y Y
Grouping -trefoils tiered N/A N/A N/A Y
Spacing mm N/A N/A N/A 300 225 300 225
No of Trefoils 2 2 3 3
The Distribution rating as displayed on
the ‘Grouped’ result screen is equal to
the Sustained rating because Loss
Load Factor is 1.0 & Utilisation is 100%

3.4.1.5 33kV Cable Data Tables
Under preparation
CABLE 1 sec S/C Resistance Reactance % Volt Losses
rating drop
Size Type Core Screen R % jX % per km per per km per
kA kA W /km W /km MVA MVA
3.4.2 33 kV Overhead Lines
3.4.2.1 33kV Conductors
Normal applications: 33kV unearthed bare wire construction shall generally be to ENATS 43-40 as
detailed in Central Networks Overhead Manual Volume 1.
Special applications for wooded areas, protection of water-fowl, recreational areas: 33kV unearthed
covered conductor construction shall generally be to ENATS 43-40 and ENATS 43-119,120,121. There is no
standard Central Networks standard for covered conductor lines – refer to the Assets Manager for
guidance.
Steelwork and fittings generally to ENATS 43-95 as detailed in Central Networks Overhead Manual
Volume 2.
Size / Type Use

150 mm2 ACSR (Dingo) • Fault level must be below 13.5kA (780 MVA).
Central Networks East • Circuit supplying 24 MVA 33/11kV transformer
2
300 mm HDA (Butterfly) • Fault level must be below 23.4kA (450 MVA).
Central Networks East • Circuit supplying 40 MVA 33/11kV transformer
• May be used for 24 MVA circuit where 150 mm2 would result in
excessive voltage drop.
100 mm2 AAAC (Oak) • 22 MW Circuit

200 mm2 AAAC (Poplar) • 36 MW Circuit


3.4.3 33/11 kV Transformers

33/11kV & 33/11-6.6kV Dual Ratio Primary Transformers shall generally be in accordance with ENATS 35-
2 with more specific technical design information, as detailed in the Central Networks’ Plant
Specification Manual.
• Primary Substations
4/6/8MVA 13% Impedance • HV Network supplying mixed loads up to 8 MVA
@ 8MVA • Industrial / commercial load up to maximum 6.2 MVA
6/9/12MVA 12% Impedance • HV Network supplying mixed loads up to 12MVA
@ 12MVA • Industrial / commercial load up to maximum 9.6 MVA,
12/19/24MVA 24% • HV Network supplying mixed loads up to 24 MVA
Impedance @ 24MVA • Industrial / commercial load up to maximum 19 MVA,
20/32/40MVA 28% • HV Network supplying mixed loads up to 40 MVA
Impedance @ 40MVA • Industrial / commercial load up to maximum 32 MVA,
The above ratings relate to the cooling designations of ONAN/OFAF/OFAF CER where :-
i) ONAN (Oil Natural Air Natural) is the BS rating with natural oil circulation cooling only, i.e. without fans and
pumps and at an ambient temperature of 20 deg C.
ii) OFAF (Oil Forced Air Forced)is the BS rating with forced (pumps) and air forced (fans) cooling, again at an
ambient temperature of 20 deg C
iii) OFAF CER (Oil Forced Air Forced Certified Emergency rating) is the Continuous Emergency Rating, again with
the fans and pumps in operation but at a lower ambient temperature of 5 deg C
Mixed loads - the OFAF rating at 5OC is applicable
Industrial / Commercial loads - the OFAF CER rating at 20OC is applicable
3.4.3.1 Standard Designs
The standard transformer designs are of the compact design incorporating the following features:-
Integral Coolers (Tank mounted radiators) including fans and pumps.
Integral 11kV Neutral Earthing Resistor (Tank mounted NER)
Integral Cooler & Voltage Control Cubicle.
Separable Elbow type cable terminations for both the 33 & 11kV.
Standard Central Networks equipment is intended for new-build applications with no exceptional
conditions prevailing.
The designer must confirm that the standard plant will be suitable for the actual application. Where
exceptional conditions are identified assistance must be obtained from the Central Networks Asset
Standards Manager.

3.4.3.2 Identification of exceptional Conditions
Examples of the exceptional conditions that would need ‘non standard’ designs and special ordering
include:-
Overhead type terminations, new shopping centre and underground/basement type applications,
noise enclosures, installation in existing sites with separate cooler type plinths, non-standard
impedance and/or NER ratings for local network peculiarities.
3.4.4 33 kV Switchgear
Under preparation
3.5 Grid Network 132kV Ratings and Data

3.5.1 132 kV Cables
Under preparation
3.5.2 132 kV Overhead Lines
Under preparation
3.5.3 132/33 kV & 132/11kV Transformers
Under preparation
3.5.4 132kV Switchgear
Under preparation

4. Network Voltage Policy
4.1 Voltage Limits

The network shall be designed to keep voltage levels within the statutory limits of:
Customer connected at Low Voltage – 230 volts +10% - 6%
Customers connected at High Voltage (6.6, 11 & 33kV ) declared voltage +/– 6%
Customers connected at Extra High Voltage (132kV ) declared voltage +/– 6%
4.1.1 Use of the Voltage Limits

The 11kV and LV network shall be designed to make use of the full extent of the statutory limits taking
into account normal and abnormal feeding arrangements.
To enable rational design of the network by independent parties the voltage limits are sub-divided
and allocated to discrete elements of the network.
The allotment voltage drop to the HV/LV transformer and LV network will depend on the 11kV feeder
length, loading and power factor. 11kV feeders extending further than 15km from the BSP/Primary
substation should be classified as ‘long’ and the amount of voltage drop apportioned to the LV
network may need to be reduced in line with the diagram below.
NOTE that the statistical likelihood of maximum voltage drop occurring simultaneously in each element of the
network is remote. The overlap between network elements represents the performance of the overall network in
reality.
Typical apportionment of Voltage Drop down the Network
+10% 2% BSP / Primary Transformer
253 V tap-changer bandwidth
Residential
LONG FEEDERS STANDARD FEEDERS
over 15km from under 15km from Industrial
GSP/ Primary GSP/ Primary
Residential Commercial
4% 11kV (6.6kV) system
Industrial
Commercial NOTE1 3%
6% 11kV system 2% HV/LV transformer NOTE 2
NOTE1
3% 2% HV/LV transformer 6% LV Main and Service
NOTE 2
5%
3% 4% LV Main and Service

230 V
6% Contingency for 11kV or LV back-feeding

(not simultaneous)
- 6%
216 V
NOTE 1 - 2 % in distribution transformer assumes power factor of unity and 100% loading.
NOTE 2 - 3 % in distribution transformer assumes power factor of 0.95 and 100% loading.

4.2 Voltage Control

The on-load tap changers on 132/11kV, 66/11kV, & 33/11kV transformers at Grid and Primary substations
are the final stage of automatic voltage control on the network. The bus-bar voltage levels must not be
allowed to exceed values that will cause customers to receive voltage above the statutory maximum.
The worst case scenario is an LV customer connected adjacent to the first secondary substation on an
11kV (or 6.6kV) feeder at the time of minimum load.
The voltage on the LV bus-bars of secondary transformers depends on:
The voltage of the input voltage at that part of the 11kV network.
The nominal ratio of the secondary transformer
The setting of the off-load tap changer of the secondary transformer.
Voltage drop in the secondary transformer due to load
The nominal ratio of a standard secondary transformer is 11kV to 433v (250v). In effect this gives a 4%
boost above 415v (240v) which used to be the nominal LV network voltage ( pre 1998 the statutory
limits were 240v +/- 6%).
Transformers at Secondary substations are fitted with off-load tap changers which have the following
ranges:
Tap number
Transformer type
1 2 3 4 5
11kV/LV single ratio +5% +2.5% 0% -2.5% -5%
6.6kV/LV single ratio +5% +2.5% 0% -2.5% -5%
11/6.6kV/LV dual ratio

set to 11kV +5% +2.5% 0% -2.5% -5%
set to 6.6kV +8% +4% 0% -4% -8%
Single phase pole transformers +5% 0% -5%
Note
The + & - % values refer to the number of turns on the HV winding. Therefore a +5% setting reduces
the LV voltage by 5% and a -5% setting increases the LV voltage for a given 11kv input voltage.
The majority of off-load tap changers of secondary transformers are set to tap 3. However, there are
variations throughout Central Networks for historical reasons or to meet local network needs. e.g.
• The 11kV network in Leicester City centre is run on tap 2 to allow a running level of 11.4kV to
maximise load transfer capacity.
• May parts of Lincolnshire run on tap 4 to allow for voltage drop on long feeders.

4.3 Grid & Primary Substation 11kV Bus Bar Voltages

Designers of new Primary and Grid substations shall establish what the local running voltages and
distribution taps settings are and ensure that the 11kV or 6.6kV bus-bar voltage remains within a 2%
bandwidth peaking at the following voltages:
Tap setting of distribution Maximum Primary/Grid S/S bus bar voltage

transformers
11kV system 6.6 kV system
Tap 5 10.60 kV 6.15 kV * (-8.0% tap)
Tap 4 10.85 kV 6.4 kV * (-4.0% tap)
Tap 3 11.13 kV 6.68 kV
Tap 2 11.40 kV 6.85 kV * (+2.5% tap)
Tap 1 11.69 kV 7.01 kV * (+5.0% tap)
Note
*Most 6.6kV systems will have a mixture of dual ratio and single ratio transformers. The 6.6kV bus bar voltage
has been selected to keep the transformers on the most unfavourable tap ratio inside the upper statutory limit.
4.4 11kV Network Voltage Regulation

4.4.1 Standard Feeders
Designers of 11kV networks shall ensure that under normal feeding arrangements the 11kV
voltage drop shall not exceed 4% at the normal open point.
The 11kV network must be designed to ensure that during abnormal feeding the overall 11kV
voltage drop at any part of the system does not exceed 10%. (i.e. back-feeding may use some
or all of the 6% contingency element).
4.4.2 Long Feeders

Long rural feeders may be designed to have a voltage drop that does not exceed 6% at the
normal open point. The voltage drop allocated to the LV network must be reduced by 2%.
This applies to the entire feeder as LV networks close to the source will be affected by large
HV voltage drop during back feeding.
The 11kV network must be designed to ensure that during abnormal feeding the overall 11kV
voltage drop at any part of the system does not exceed 12%. This may be achieved by ensuring
that appropriate load transfers can be made to reduce the load on associated parts of the
network affected by or providing the back feed.
A ‘Long Feeder’ is defined as extending beyond the 15km radius of a Bulk Supply Point or
Primary Substation.

Some feeders constructed of large cross-section conductors may be capable of maintaining

the 11kV voltage to within “Standard Feeder” limits. Where calculations show this is feasible a
feeder extending beyond the 15km radius may be defined as “Standard Feeder”.
4.4.3 HV Customers
High voltage metered customers must be maintained within nominal voltage +/- 6%.
An 11kV feeder supplying only HV customers may be designed to 6% voltage drop.
Where LV customers are supplied from the same 11kV circuit the apportionment of voltage
drop to the LV networks must be reduced according to the ‘Long Feeder’ criteria.
4.5 Secondary Transformer Voltage Regulation

The voltage drop through a secondary transformer depends on the load and the power factor of the
load.
% voltage drop at Power Factor
Size kVA Unity 0.99 0.98 0.95 0.9 0.8
1000 1.4 2.7 3.3 4.3 5.4 5.7
800 1.3 2.5 3.0 3.9 4.9 6.1
500 1.5 2.5 2.9 3.7 4.5 5.5
315 1.7 2.6 2.9 3.6 4.3 5.1
200 1.6 2.4 2.7 3.3 3.9 4.7
100 1.8 2.6 2.8 3.4 3.9 4.6
200 padmount 0.9 1.4 1.6 2.0 2.4 3.0
100 padmount 1.6 1.9 2.1 2.4 2.6 2.9
Calculations based on the transformer data in section 3.3.3.4
The values in the above table are at transformer name plate rating. Where the transformer is running
into it’s permissible overload rating the voltage drop values should be increased by the percentage
overload.
A power factor of Unity may be assumed for domestic and light commercial loads and 1.5% allocated
to the transformer voltage drop. Full use can me made of the 6% allocated to LV cable and service volt
drop.
Industrial and heavy commercial loads will normally have a less favourable power factor often around
0.9 to 0.95. Allocate 4% to the transformer and reduce the allocation to mains and services to 5%.
Note 1 Cable loading is often the limiting factor on LV feeder length rather than voltage drop on
industrial / commercial networks.
Note 2 Poor power factor may be encountered where electricity retail company’s prices do not charge for
maximum demand. This can be the cause voltage complaints on some industrial / commercial networks.
It may be possible to redress the problem by raising the off-load tap on the secondary transformer but

care should be taken to ensure that volts do not exceed the statutory maximum at light load periods. It
may be necessary to apply Line Drop Compensation at the Primary substation to buck the voltage at light
loads.
4.6 LV Network Voltage Regulation

Designers of LV networks shall ensure that under normal feeding arrangements the combined LV
main and service voltage drop shall not exceed:
Domestic housing / light commercial (up to 140kVA)
o 6% Standard 11kV Feeders

o 4% Long 11kV Feeders
Industrial / heavy commercial
o 5% Standard 11kV Feeders

o 3% Long 11kV Feeder
o Networks with poor power factor or transformer running on overload, the values for
Standard or Long 11kV Feeders less the increase in transformer regulation over 4%.
Abnormal feeding (i.e. back-feeding) may use some or all of the 6% contingency bandwidth given that
planned work should normally be time for periods of light load.
See Section 6 of this Manual for the method to be used for voltage drop calculation.
4.7 Line Drop Compensation

Line drop compensation is applied to some Primary and Grid Substations within East Midlands
Electricity, but the use of this is dependent on the geographical location of the particular
substation and the nature of the circuits it feeds.
During the design of line drop compensations voltage control schemes, system volt drop
calculations for the 11kV outgoing feeders have to be conducted and the line drop compensation
settings applied accordingly. Distribution substations supplied from the Primary or Grid
Substation will then have their taps set according to their distance (circuit length) from the
source, which is usually done in a “zoned” manner.
CAUTION. The presence of embedded generation will effect the operation of Line Drop
Compensation rendering it inappropriate for some 11kV networks.

4.8 Voltage Regulators

4.8.1 LV Regulators
Historically, some voltage complaints due to long LV feeders have been resolved by installing
shaded pole voltage regulators controlled by Astatic relays. Invariably the loop impedance of such
networks is well outside the limits specified in the current Loop Impedance Policy. No new LV
voltage regulators shall be installed on the Central Networks network.
4.8.2 Static Balancers

Interconnected Star Balancing Transformers, commonly known as Static Balancers improve
voltage regulation by redistributing some of the neutral current across the phases. They have
proved to be particularly useful on long LV feeders serving small numbers of customers by
improving the load balance. Again, the loop impedance of such networks is often well outside the
limits specified in the current Loop Impedance Policy. No new LV static balancers shall be
installed as a permanent voltage complaint remedy. However, refurbished units may be used as
an interim measure whilst permanent network reinforcement is organised.
4.8.3 11kV Voltage Regulators

11kV voltage regulators are not yet a standard plant item on the Central Networks network.
However, their use may be developed in future to resolve issues causes by embedded generation
or to deal with voltage drop on long feeder where Line Drop Compensation is not appropriate.
The voltage regulators presently available are single phase units designed for pole mounting.
Only two single phase units, connected in open delta, are required to regulate a three phase line.
No 11kV voltage regulators are to be installed on the network without the involvement of
Technology & Standard section of Central Networks Asset Development.

5. LV Earth Loop Impedance Policy
5.1 Introduction
Low voltage networks must be designed to provide acceptable quality of supply and safe fault
clearance times. These requirements are fundamentally influenced by the loop impedance of the low
voltage network comprising of the combined impedance of the transformer, LV main and service
cable.
5.2 Provision of Protective Multiple Earthing

There is no maximum limit of loop impedance for a PME supply defined in either the current edition of
BS7671 “Requirements for Electrical Installations” or Electricity Association Engineering
Recommendation P23/1.
PME terminals may be provided at any value of loop impedance.
5.3 Specific Requirements

5.3.1 New LV networks
New LV networks shall be designed to a maximum loop impedance of 0.24 ohms at the most remote
cut-out. (Limiting factor – quality of supply - step voltage change)
Where the LV feeder fuse is 400 amp or 630 amps, the maximum loop impedance shall be further
limited to 0.19 or 0.12 ohms respectively. See Table 1. (Limiting factor – fault clearance time)
5.3.2 Service alterations

Where the loop impedance at the cut-out will exceed the values in Table 1, the service position shall
not be permitted to be inside the building.
Where the loop impedance at the cut-out does not exceed 0.38 ohms, a 100 amp cut-out fuse may be
used. See Table 3
The allowable loop impedance may be increased to 0.52 ohms provided that a 80 amp cut-out fuse is
used. See Table 3
Where the loop impedance exceeds 0.52 ohms, the LV network must be reinforced to reduce it to this
value.

Where the loop impedance exceeds 0.35 ohms, the customer must be advised that the loop
impedance exceeds the typical maximum value shown in Engineering Recommendation P23/1 as it
may affect the design of the building’s electrical installation.
5.3.3 In-fill developments supplied from existing LV mains

In-fill developments supplied from existing LV mains shall be designed to a maximum loop resistance
of 0.24 ohms at the service breech joints.
Where the loop resistance above exceeds 0.24 ohms, the LV network must be reinforced to reduce it to
this value.
Where the loop impedance at the cut-out will exceed the values in Table 1, the service position shall
not be permitted to be inside the building.
Where the loop impedance exceeds the criteria for substation fuse operation then consideration
should be given to changing the fuse to a smaller size before resorting to system reinforcement
subject to network loading.
5.3.4 Street lighting services

On new developments, runs over 20m of 25mm2 hybrid must have a loop impedance calculation
carried out to confirm that the substation fuse will clear correctly and is within the limits shown in
Table 1. In some circumstances it may be necessary to extend the mains cable towards the street lamp
to reduce the service cable length.
Where street lamp cables over 5 metres are connected to existing mains the loop impedance
calculation must be carried out to confirm that the substation fuse will clear correctly and is within the
limits shown in Table 1. Again, a mains cable extension may be required.
Long runs of service cable supplying a number of lamps must be sub-fused in the first lamp on the run
to protect the remainder of the service cable. The size of the sub-fuse should correlate to the loop
impedance at the last lamp on the run. See Table 2.
Where the loop impedance exceeds 0.35 ohms the street lighting authority must be advised that the
loop impedance exceeds the typical maximum value shown on Engineering Recommendation P23/1.
The maximum cut-out fuse size should be selected from Table 4 to protect the internal wiring of each
lamp.

Table 1 Table 2
LV Fuse Max Loop Impedance Street Lamp Max Loop
(to BS 88 Part 5) Fuses to BS 88 Impedance
(100 sec) note1 (5 sec) note4
part1&2 (100 sec) note2
S/S Feeder Cut out

630 Amps 0.12 ohms 0.054 ohms 25 Amps 3.27
6 Amps 19.32
Table 3 Table 4
House Cut-out Fuses Max Loop Street Lamp Max Loop
to BS 1361 Impedance Fuses to BS 88 Impedance
(5 sec) note3 part1&2 (5 sec) note3
6 Amps 14.1 ohms
note1
based on Central Networks Protection & Control Manual
note2
based on maximum curves in BS 7654
note3
based on table 41D in the current edition of BS7671
note4
based on Cooper Bussmann BS Fuse Links Catalogue Jan 2003
• The 100 sec values relate to the maximum allowable fuse clearance times for Distribution Network
Operator owned underground cables.
• The 5 sec values relate to the maximum allowable fuse clearance times for wiring inside buildings
according to the current edition of BS7671 ”Requirements for Electrical Installations (IEE Wiring
Regulations 16th Edition.)”
5.3.5 Replacement or alterations to existing mains

Any work to replace or alter existing mains shall not result in an increase in the loop impedance values
at the remote ends of the network.
5.3.6 Temporary Back-feeding

The loop impedance limits may be exceeded during LV back-feeding or temporary generation.

5.4 Notes of Guidance to Loop Impedance Policy
East Midlands Electricity’s Loop Impedance Policy has been formulated from the following
considerations:
5.4.1 Quality of supply:
The connected loads will act upon the network impedance to produce cumulative voltage drop
along the cables. The magnitude of the loads, the balance across the phases and the
impedance of the network all affect the voltage drop.
Disturbing loads will cause sudden changes in voltage, which may be noticed by customers
even though the voltage level does not go outside statutory limits. The effects of disturbing
loads are primarily influenced by loop impedance. Electricity Association Engineering
Recommendation P28 recommends that the maximum step voltage change caused by a
switched single phase load of 7.2kW (i.e. an electric shower) should not exceed 3%. This
equates to a maximum loop resistance of 0.24 ohms. P28 requires that the 3% limit applies at
the point of common coupling with other customers (i.e. the service breech joint or at the first
cut-out of a looped service).
New housing developments should normally be designed with the 7.2kW/3% step voltage
limit applied at every cut-out in order to pre-empt complaints from large numbers of
customers affected by their own shower installations. Exceptionally, the limit may be applied
at the service joint where longer mains are unavoidable such as small in-fill housing
developments.
Some parts of existing networks may exhibit higher step voltage changes as they were
installed according to the engineering standards applicable at the time.
5.4.2 Protection of mains & services
5.4.2.1 New developments

Feeder Protection
Central Networks policy is to fuse new LV mains such that phase to neutral faults on mains
and services are cleared within 100 seconds after allowing a 15% voltage reduction for arc
resistance.
House service cut-outs

Non-electric heating – 80 amps
Electric Heating 100 amp
Industrial / Commercial 100, 200, 315, 400, 630 amp as appropriate to the load.
For Central Network’s standard fuse range, the corresponding loop impedances are:

Table 1 Table 2
LV Feeder Fuse Max Loop Street Lamp Max Loop
(to BS 88 Part 5) Impedance Fuses to BS 88 Impedance
630 amps 0.12 ohms 25 amps 3.27

400 amps 0.19 ohms 20 amps 3.86
315 amps 0.27 ohms 16 amps 6.07
200 amps 0.45 ohms 10 amps 9.66
6 amps 19.32
note1
based on Central Networks CoP17 Protection & Control
note2
based on maximum curves in BS 7654
5.4.2.2 Existing networks
Many existing LV networks have loop impedances in excess of these values in Table 1 as they
were installed according to the engineering standards applicable at the time.
Where cables or overhead lines are replaced or altered it is not necessary to bring the feeder
into compliance with the loop impedance value for a new network. However, the loop
impedance must not be made worse as this may result in a deterioration on quality of supply
(flicker and/or volt-drop) or increased operation time of protective devices. Particular care
should be taken when replacing the larger sizes of copper O/H line with 95 ABC.
Where service alterations are carried out or small in-fill developments have to be supplied it
may not be reasonably practicable to achieve these values of loop impedance at the cut-out. In
these situations the hazard from an un-cleared service cable or cut-out fault is controlled by
ensuring that the service cable does not enter the building (i.e. outdoor meter box and cable
fixed externally to the wall.)
The design of hybrid service cable results in most faults self-clearing by burning back the
aluminium phase core down inside the XLPE insulation. Furthermore, XLPE does not burn as
easily as PVC and any fumes produced are external to the dwelling. The local neutral/earth
potential rise is controlled in a safe manner by the equipotential bonding of the building’s
wiring according to the current edition of BS7671 “Requirements for Electrical Installations”.
5.4.2.3 Long Street Lighting cables

Short lengths of street lighting cable have similar loop impedances to house services.
On new developments, runs over 20m of 25mm2 hybrid require a loop impedance calculation
to ensure that the substation fuse will clear correctly.
Where street lamp connections are made to existing mains the loop impedance must be
ascertained for runs in excess of 5m of 25mm2 hybrid.
Long runs of cable supplying a number of lamps may introduce loop impedances higher than
can be protected by the substation fuse.
This can result in:
Open circuit faults presenting a danger to staff at lamp columns

Short circuits burning out entire cable runs

The lamp columns becoming and remaining alive in the street presenting a danger to
passers-by and staff.
In these cases a sub-fuse must be introduced into the first lamp on the run to protect the
remainder of the circuit. Then size of the sub-fuse should correlate to the loop impedance at
the last lamp on the run according to Table 2 above.
5.4.3 Protection of consumer’s equipment
The current edition of BS7671 “Requirements for Electrical Installations” requires that circuits
be protected such that earth faults are cleared within:
5 seconds on stationary equipment
0.4 seconds on circuit supplying socket outlets.
With knowledge of the loop impedance at the supply terminals the installation designer can
select appropriate fuse/MCB ratings to protect the sub-circuits.
The designer may also rely upon the Distribution Network Operator’s cut-out fuse to protect
the bus-bars of the installation’s consumer unit.
The maximum loop impedance that will ensure a 5 second fault clearance at the consumer
unit is determined by the house service cut-out fuse size as shown in Table 3.
Similarly street lamp cut-out fuses provide protection to the street lighting authority’s
protective devices. Maximum loop impedances at street lamps are shown in Table 4.
Table 3 Table 4
House Cut-out Fuses Max Loop Street Lamp Max Loop
to BS 1361 Impedance Fuses to BS 88 Impedance
100 amps 0.38 ohms 25 amps 2.40 ohms
6 amps 14.1 ohms
note3
based on table 41D in the current edition of BS7671
Newly designed networks should not normally produce loop impedances in excess of the
above values except on long street lighting runs. In this case the street lighting designer must
co-ordinate the cut-out fuse rating with the prevailing loop impedance.
This requirement is applicable to new connections whether or not a PME earth is made
available.
Installations with services installed prior to the 15th Edition of the IEE Wiring Regulation 1981
need not comply with these cut-out fuse clearance times.
5.4.4 Provision of PME terminal
There is no maximum limit of loop impedance for a PME supply defined in either the current
edition of BS7671 “Requirements for Electrical Installations” or Electricity Association
The values of loop impedance in Electricity Association Engineering Recommendation P23/1

are indicative values for the majority of installations whether they use PME or other forms of
earthing. They are not maximum values permitted for provision of PME.
Engineering Recommendation P23/1 ‘Customer’s Earth Fault Protection for Compliance with
the IEE Wiring Regulations for Electrical Installations
This document provides guidance to electrical installation designers who need to use the loop
impedance to ensure that sub circuit fuses clear within the times specified in the current
edition of BS7671 “Requirements for Electrical Installations”. (i.e. 0.4 seconds for portable
equipment or 5 seconds for stationary equipment). This requirement first appeared in the 15th
Edition of the IEE Wiring Regulation issued in 1981.
To meet this requirement, the installation designer needs to know the loop impedance at the
cut-out of the property resulting from the length and size of Distribution Network Operator’s
LV network.
For a building with an existing service this value can be readily obtained by carrying out a loop
impedance test. The value obtained must then be added to loop impedance of the internal
wiring to determine whether the proposed fuse clearance times are satisfactory.
However, at the design stage of a new building there is no existing cut-out to test but the
installation designer still needs to establish a loop impedance figure to base his design on. As
he cannot carry out a loop impedance measurement at this stage he should normally ask the
Supply Authority to provide the loop impedance of the proposed LV network at each property
to be connected.
In order to prevent Distribution Network Operators being inundated with this type of enquiry,
the Electricity Association produced Engineering Recommendation P23/1.
P23/1 provides a typical maximum value of LV network loop impedance (e.g. 0.35 ohm for
supplies up to 100amp) with the caveat:
“Higher values could apply to consumers supplied from small capacity pole transformers
and/or long lengths of low voltage overhead lines.”
These values and the caveats on their use are reproduced in East Midlands Electricity’s 'Notes
of Guidance to Electrical Contractors on Earthing and the Characteristics of Supply'.

Table 5
Type of supply Typical Max Loop
Impedance
Single or three phase supply up to 100 amps 0.8 ohms
Cable sheath earth terminal
Single phase supply up to 100 amps 0.35 ohms
PME or PNB earth
Three phase supply up to 200 amps 0.35 ohms
PME or PNB earth
Three phase supply 200 to 300 amps 0.2 ohms
PME or PNB earth
Three phase supply 300 to 400 amps 0.15 ohms
PME or PNB earth
Building installation designers will normally use the above figures to design circuit protection.
Central Networks’ standard LV network design procedure will not normally result in the values
being exceeded on Greenfield sites.
However, in-fill developments and service alterations may result in higher values. In these
cases the building installation designer must be advised of the actual values.

6. Low Voltage Network Design Calculations

6.1.1 Voltage Drop Calculation Methodology
i) Central Networks LV network design is based on the principles of ACE Report No 13:1966.
ii) LV networks for adoption may be designed to other standards (e.g. ACE Report No 49:1981
using Debut). However Central Networks will judge the design results against ACE 13 criteria.
Central Networks’ application of the ACE 13 design principals is explained below.
6.1.2 Mains LV volt drop calculation:

i) First the voltage drop is calculated assuming that the customers are uniformly distributed
along each section and equally distributed across the three phases. This is the Balanced
Voltage Drop. In order to account for variations in customer’s utilisation patterns the Balanced
Voltage Drop is multiplied by Correction Factors F1 (Unbalance Factor) and F2 (Diversity
Factor).
ii) The Correction Factors produce high multipliers for low customer numbers and/or low
ADMDs. As the number of customers and/or ADMDs increases the Correction Factors decrease
to represent the aggregation of individual utilisation patterns.
iii) The voltage drop on each section is calculated by considering the voltage drop due to
customers connected to the section (distributed customers) separately from the voltage drop
due to customers supplied through the section (terminal customers). The two volt drops are
then summated. Note that the Diversity Factor F2 differs between the distributed and terminal
calculations. The distributed calculation considers the sum of the distributed and terminal
customers affecting the section whilst the terminal calculation considers only the number of
terminal customers.
Volt drop due to distributed customers in section:
Vd = Rp*L/2 * F1*F2d*Nd*(a/3)*4.166
Volt drop due to terminal customers supplied through section

Vt = Rp*L* F1*F2t*Nt*(a/3)*4.166
Section voltage drop is then Vs = Vd + Vt

The cumulative voltage drop along the feeder is the sum of each section voltage drop.
Key to formulae
N = Number of customers supplied by service (e.g. 2 = loop service)
Nd = Number of customers distributed along the section of main
Nt = Number of customers terminally supplied through the section of main
Rp = phase resistance

Rn = neutral resistance
a = ADMD in kW
U = Unbalance = 1/sqrN
Unbalance Correction Factor F1 = 1+4.14U
Diversity Correction Factor F2d = 1+12/a.(Nd+Nt)
Diversity Correction Factor F2t = 1+12/a.Nt
Combined Correction Factor = F1 * F2d or F1 * F2t
4.166 = amps per kW based on 240v nominal running voltage
6.1.3 Service Voltage Drop Calculation
i) This is calculated from the single phase voltage drop due to the maximum load expected on
the service. This load is estimated to be 2aN+8 kW.
ii) Service Voltdrop = (2aN+8)*4.166 * (Rp + Rn)
6.1.4 Loop impedance

Maximum earth loop resistance to the most electrically remote service cut-out shall not
exceed the values detailed in the Loop Impedance Policy Section of this Manual.

7. Disturbing Loads & Distributed Generation

Under preparation
8. Environmental Requirements
8.1 Development at Sites that have Legal Environmental Protection

Certain areas of land and natural features have legal protection due to their environmental character
or sensitivity. Such designations include Sites of Special Scientific Interest (SSSI), National Nature
Reserves, Special Areas of Conservation and Scheduled Ancient Monuments.
It is an offence to access or carry out work on these sites without the express consent of the
appropriate regulatory body. Any development proposed by the company must therefore be carried
out so as to avoid any potential impact on these sites or their immediate vicinity, wherever
practicable. If work cannot be planned so as to avoid these sites, consultation must be carried out
with the relevant regulatory body and an appropriate consent obtained to allow agreed actions to be
carried out at the site. When carrying out work that has a potential impact on SSSIs, any additional
actions that could enhance the site should also be agreed with the regulatory body.
In Central Networks East the information relating to these sites is held on the company’s Geographic
Information System (GIS). All sites display a warning of their protection and an “environmental
sensitivity” layer contains additional information that provides details about the site and appropriate
contact details.
Further information regarding the legal protection of these sites and the required process of
consultation, is contained in the company’s Wayleaves and Property Policy and Procedures Manual.”
8.2 Escape of insulating oil

The main environmental risk posed by the release of oil from equipment is the pollution of water
resources such as rivers, lakes and underground aquifers. The Water Resources Act 1991 prohibits
causing or knowingly permitting the discharge or entry of polluting matters, such as oil, into
controlled waters.
The sections of this Manual dealing with the siting of Secondary, Primary and 1232kV substations
specify the requirements for each type of installation.

8.3 Transformer loss evaluation criteria employed
i) The transformer data tables in section 3.3.3.4 of this manual shows the maximum losses of
transformers purchased by Central Networks together with the maximum loading permissible
on a domestic load cycle.
ii) When establishing the permissible maxim cyclic loading for each size and type of
transformer Central Networks considers the environmental impact of losses. As a result the
maximum cyclic loading of low loss transformers may be higher than for standard loss
transformers whilst still incurring similar daily kWh losses.
iii) Accordingly, should the Applicant wish to offer a higher loss transformer than Central
Networks’ standard unit then the Applicant must demonstrate that the loading applied to the
transformer does not result in excessive losses. In practice this means that a high loss
transformer must be run to a lower loading than a standard loss unit.
iv) Generally, daily transformer losses should not exceed 1.5% of the total daily energy
supplied during periods of maximum demand.
Based on a model domestic load cycle of:

7hrs @ 0.3 pu
7hrs @ 0.4 pu
2hrs @ 0.5 pu
3hrs @ 0.6 pu
3hrs @ 0.8 pu
2hrs @ 1.0 pu

9. New Network Acceptance Requirements
9.1.1.1 Scope
All new network extensions and modifications must be tested in accordance with the requirements of
this Manual. This includes:
• Network installed by Central Networks direct labour and Contractors directly employed by Central
Networks and their sub contractors
• Networks to be adopted by Central Networks under Ofgem’s Competition in Connections process.
9.1.1.2 LV Networks and associated Substations

Test Requirements:
Tests must be carried out either before or immediately after energising the network as required by the
Testing Schedule. Documented results for all tests shall be submitted to Central Networks within two
working days of energisation except for those tests indicated below as being required before
energising the network .
Central Networks will not energise any network extension unless the Applicant’s contractor is
available on site ready to commence the electrical testing required immediately after energising. This
is to ensure that any unsafe electrical conditions are expediently identified and made safe. Central
Networks inspectors may witness a sample of these tests.

Testing Schedule
Equipment Test required Action When
Each service Insulation resistance Over 50MΩ Prior to jointing
PME & Phase identification Mark phase colour as connected in Immediately after
service joint and fit PME label jointing
Cut-out sealing Insert the fuse and carrier into the

cut-out and seal with plastic tick Immediately after
seals jointing the service.
Each service Polarity check Check according to Electricity Immediately after
Association Engineering energising the network
Recommendation G14
Immediately after
Phase rotation (3ph) Check rotation and mark ABC or ACB energising the network
at cut-out.
Immediately after
energising the network
Earth loop impedance Less than 0.19 or 0.24 ohms
depending on S/S fuse size Immediately after testing
the service.
Cut-out sealing Insert the fuse and carrier into the

cut-out and seal with plastic tick
seals
Each new section Insulation resistance Over 50 MΩ Prior to jointing

of main between phases and phase
to neutral/earth.
Continuity Continuity of the mains will be Immediately after

proved by carrying out earth loop energising the network
impedance tests on each service.
(see above)
Each P.M.E. Earth resistance Tests not required. N/A
electrode
Substation Soil resistivity Use to establish earthing design All these results will be
earthing required two working
Hot Site: HV electrode resistance days BEFORE Central
LV electrode resistance Networkswill energise
HV/LV separation distance the substation
Cold Site: Combined HV/LV electrode resistance
LV fuse cabinet Insulation resistance Over 50 MΩ Prior to energising to

network
HV/LV Insulation resistances HV- Manufactures test sheets acceptable These results will be
transformer LV winding/earth required two working
Off site oil moisture content days BEFORE Central
oil electric breakdown Networks will energise
strength the substation
measured losses
statement of pcb content
HV/LV Pressure test See HV Cable Tests Prior to energising to

transformer network
On site
Phasing checks Check phase rotation correct Immediately after
Immediately after
Tap commissioned on Central Networks to advise on energising the network
required setting for each locality
Immediately after
No load output voltage at
commissioning

Testing Schedule continued

Equipment Test required Action When
HV Switchgear Pressure test See HV Cable Tests All these results will be
On site required two working
Protection test, secondary Manufactures test acceptable days BEFORE Central
injection of CB relay Networks will energise
the substation
Dummy HV fuse trip tester
for switch/fuses Test on site
Functional test of interlocks

and operation Check all operations
Busbar resistance if work

includes connection or ‘Ducter’ tests
extension of busbars.
Gas pressure if gas filled
Check gauge fitted to switchgear

HV Cables Pressure test
Cables & switchgear only 30 kV dc centre point earthed AB-C, All these results will be
on site including transformer switch open A-BC 15 minutes each required two working
compact unit days BEFORE Central
substation / Cables, switchgear & Networks will energise
padmount jointed transformer the substation
into circuit transformer switch closed 18 kV dc ABC – Earth 15 minutes
Continuity
Check all phases have equal

resistance
Completed Network phasing Confirm that HV & LV busbars are Immediately after
Substation National Standard (i.e. A=Red, energising the network
B=Yellow, C=Blue)
Where the local LV network is not

national standard it may be
necessary to cross or roll LV cable
connections onto the LV board. Liaise
with Central Networks if non National
Standard phasing has to be used.
Check phasing with LV

interconnection (if provided) Immediately after
LV phasing energising the network
Check HV ring phasing is correct Immediately after
before making system parallel. (in energising the network
HV Ring conjunction with Central Networks
System Control)
Insulation tests of LV equipment may be carried out using 500 or 1000 volt instruments
10. Applicable Engineering Standards

Under preparation

11. Glossary
Under preparation


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Network Design Manual

Version: 7.7 Date of Issue: December 2006

Version 7.6 Prepared by T Haggis Date Sept 2005

Version 7.7 Page 2 of 194

insertion of Table 3.3.1.5B

Version 7.5 Prepared by T Haggis Date 23 June 2004

Version 6.0 Prepared by T Haggis Date 1 March 2004

Version 7.7 Page 3 of 194

1.1 Overview ............................................................................................................................12

1.2 LV Network ........................................................................................................................16

1.2.8 Physical Position of Services .................................................................................................. 41

1.3 Industrial and Commercial Supplies ................................................................................ 52

1.4 Secondary Network 11kV & 6.6kV ..................................................................................... 72

Version 7.7 Page 5 of 194

1.4.3.2 Earthing of Plant ...................................................................................................................................................................73

1.5 Primary Network 33kV ....................................................................................................105

Version 7.7 Page 6 of 194

1.5.5 33kV Connectivity Rules........................................................................................................105

1.6 Grid Network 132kV.........................................................................................................105

2. NETWORK MODIFICATION PROCEDURE .............................................................................107

2.2 Implications of Network Modifications ..........................................................................108

3. STANDARD EQUIPMENT RATINGS AND DATA ................................................................... 112

3.1 General ............................................................................................................................ 112

3.2 Low Voltage Network Ratings and Data......................................................................... 112

3.3 Secondary Network 11kV Ratings and Data ....................................................................124

Duct type - important note. ......................................................................................................................................................126

3.4 Primary Network 33kV Rating and Data .........................................................................166

Transformer 33kV XLPE Tails Sustained Ratings..............................................................................................................168

3.5 Grid Network 132kV Ratings and Data............................................................................172

4. NETWORK VOLTAGE POLICY ..............................................................................................173

4.1 Voltage Limits .................................................................................................................173

4.2 Voltage Control ...............................................................................................................174

4.3 Grid & Primary Substation 11kV Bus Bar Voltages..........................................................175

4.4 11kV Network Voltage Regulation ..................................................................................175

4.5 Secondary Transformer Voltage Regulation ...................................................................176

4.6 LV Network Voltage Regulation......................................................................................177

4.7 Line Drop Compensation.................................................................................................177

4.8 Voltage Regulators..........................................................................................................178

5. LV EARTH LOOP IMPEDANCE POLICY.................................................................................179

5.2 Provision of Protective Multiple Earthing.......................................................................179

5.3 Specific Requirements .................................................................................................... 179

Version 7.7 Page 9 of 194

5.3.1 New LV networks...................................................................................................................179

5.4 Notes of Guidance to Loop Impedance Policy.................................................................182

6. LOW VOLTAGE NETWORK DESIGN CALCULATIONS ............................................................187

7. DISTURBING LOADS & DISTRIBUTED GENERATION...........................................................189

8.1 Development at Sites that have Legal Environmental Protection..................................189

8.2 Escape of insulating oil ...................................................................................................189

8.3 Transformer loss evaluation criteria employed..............................................................190

9. NEW NETWORK ACCEPTANCE REQUIREMENTS................................................................. 191

10. APPLICABLE ENGINEERING STANDARDS.........................................................................193

11. GLOSSARY ........................................................................................................................194

Version 7.7 Page 10 of 194

This Manual relates to the areas of:

The Network Manager

Version 7.7 Page 11 of 194

Version 7.7 Page 12 of 194

1.1.1 400kV & 275kV Grid Substations

1.1.2 132kV Grid circuits

1.1.3 132kV Substations

Version 7.7 Page 13 of 194

1.1.4 66kV & 33kV Primary circuits

1.1.5 Primary Substations