EDS+06-0017+Customer+Installation+Earthing+Design

Document Number: EDS 06-0017

Version: 3.1

Date: 22/08/2012

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ENGINEERING DESIGN STANDARD

EDS 06-0017

CUSTOMER INSTALLATION EARTHING DESIGN

Network(s): EPN, LPN, SPN

Summary: This standard provides guidance on the earthing of customer LV and HV installations.

Originator: Stephen Tucker Date: 22/08/2012

Approved By: Barry Hatton Approved Date: 23/04/2012

Review Date: 22/04/2015

This document forms part of the Companys Integrated Business System and its requirements are mandatory throughout UK Power Networks. Departure from these requirements may only be taken with the written approval of the Director of Asset Management. If you have any queries about this document please contact the originator of the current issue.

Document History (The document history notes below are intended as a guide only and may not cover all of the changes. If you wish to make use of this document it should be read in full.)

Version Date Details Originator

1.0 31/03/2008 Original Stephen Tucker/ Rob Weller

2.0 25/05/2011 Completely rewritten to provide a more consistent and practical approach and separated from LV network design

Stephen Tucker

2.1 03/08/2011 Reclassification of document from Earthing Design Manual Section 7

Stephen Tucker

3.0 01/02/2012 Street furniture, railway installations and multiple occupancy building sections updated

Stephen Tucker

3.1 22/08/2012 Minor updates to ensure consistency with other standards and new PME label (130) added to Section 5.5. Document reviewed for publishing on G81 website

Stephen Tucker

Document Number: EDS 06-0017 Customer Installation Earthing Design

Version: 3.1

Date: 22/08/2012

Contents

1 Introduction ................................................................................................................5 2 Scope ..........................................................................................................................5 3 References..................................................................................................................6 4 Definitions...................................................................................................................8 5 Customer LV Installations.......................................................................................12 5.1 Design Criteria .......................................................................................................... 12 5.2 Earth Terminal........................................................................................................... 12 5.3 Special Situations...................................................................................................... 13 5.4 Bonding Requirements.............................................................................................. 14 5.5 Warning Labels ......................................................................................................... 15 6 Earth Fault Loop Impedance...................................................................................17 7 Special Situations ....................................................................................................17 7.1 Construction and Demolition Sites ............................................................................ 17 7.1.1 Background ............................................................................................................... 18 7.1.2 Temporary Supplies .................................................................................................. 18 7.1.3 Transition to a Permanent Supply............................................................................. 20 7.2 Mines and Quarries................................................................................................... 21 7.3 Railway Installations.................................................................................................. 21 7.3.1 General ..................................................................................................................... 21 7.3.2 LV Supplies at Traction Supply Points ...................................................................... 21 7.3.3 LV Supplies for Sites with both AC and DC Traction Systems.................................. 22 7.3.4 LV Supplies Associated with AC Traction Systems at Locations other than

Traction Supply Points .............................................................................................. 22 7.3.5 LV Supplies Associated with DC Traction Systems .................................................. 23 7.3.6 Other Electrified Systems.......................................................................................... 24 7.3.7 Assessment Process................................................................................................. 24 7.4 Farms and Horticultural Premises............................................................................. 25 7.5 Swimming Pools........................................................................................................ 25 7.6 Sports Pavilions ........................................................................................................ 25 7.7 Street Lighting and Street Furniture .......................................................................... 26 7.7.1 Street Lighting and Road Signs (up to 500W)........................................................... 26 7.7.2 Street Furniture (not covered by 7.7.1) ..................................................................... 27 7.8 Multiple Occupancy Buildings ................................................................................... 30 7.8.1 Overview ................................................................................................................... 30 7.8.2 New Building Design Single Intake and Single Service ......................................... 31 7.8.3 New Building Design Multiple Intakes and One Service ........................................ 32

UK Power Networks 2012 All rights reserved 2 of 48


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7.8.4 Multiple Services to a Single Customer or Building................................................... 34 7.8.5 Existing Buildings ...................................................................................................... 35 7.8.6 End-of-main Earth Electrode..................................................................................... 35 7.9 Metal-clad Buildings .................................................................................................. 35 7.10 Conversion of Older Properties into Flats ................................................................. 35 7.11 Cathodic Protection Installations............................................................................... 35 7.12 Small Radio Stations Requiring a Functional Earth ............................................... 36 7.13 Mobile Phone Masts.................................................................................................. 36 7.14 Mobile Phone Base Stations and Masts Associated with Substations...................... 36 7.14.1 MPBS and Masts Adjacent to a COLD Secondary Substation ................................. 37 7.14.2 MPBS and Masts Adjacent to a HOT Secondary Substation/Pole-mounted Site ..... 37 7.14.3 MPBS and Masts within a Secondary Substation ..................................................... 38 7.14.4 Mobile Phone Base Stations Associated with Transmission Towers ........................ 38 7.15 Lightning Protection Systems.................................................................................... 38 7.16 LV Generators........................................................................................................... 38 7.17 HOT Sites.................................................................................................................. 39 7.18 Inset Networks .......................................................................................................... 39 8 Customer HV Installations ......................................................................................39 8.1 HV Supplies and Associated Substations ................................................................. 39 8.2 HV/LV Supplies to Same Customer or Building ........................................................ 40 Appendix A Legislation ....................................................................................................41 Appendix B Earthing Systems.........................................................................................42 B.1 IEC Standard for the Naming of Earthing Systems................................................... 42 B.2 BS 7671 Definitions................................................................................................... 42 B.3 TN-S (Terra-Neutral Separated) ............................................................................... 43 B.4 TN-C-S (Terra-Neutral-Combined-Separated) .......................................................... 43 B.5 TT (Terra-Terra) ........................................................................................................ 45 Appendix C Railways........................................................................................................46 C.1 Operators of AC Electrified Traction Systems in the UK........................................... 46 C.2 DC Electrified Traction Systems in the UK................................................................ 46 Appendix D Railway PME Application Form ..................................................................47 Appendix E Railway PME Assessment Form and Flowchart........................................47 Appendix F Neutral Current Diversion............................................................................48 F.1 Broken Neutral .......................................................................................................... 48 F.2 Unbalanced Loads .................................................................................................... 48



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Figures

Figure 1 Temporary Building Supply with a TT Earthing System .......................................18 Figure 2 TN-S Earth from a Dedicated Transformer...........................................................19 Figure 3 TN-S Earth from an Isolating Transformer............................................................20 Figure 4 Direct Bonding for Equipment in Contact with the Traction Return Circuit ...........23 Figure 5 Indirect Bonding for Equipment not in Contact with the Traction Return Circuit. ..23 Figure 6 Examples of Street Lighting Earth Electrode Positions ........................................26 Figure 7 Metallic Pillar with a Grading Earth Electrode.......................................................29 Figure 8 Metallic Pillar Earthing at Network Rail Sites........................................................29 Figure 9 Multiple Occupancy Building Examples................................................................30 Figure 10 Single Intake Supply Arrangement New PME Service from Secondary

Substation ............................................................................................................31 Figure 11 Single Intake Supply Arrangement - New PME Service from PME Main ...........32 Figure 12 Multiple Intake Supply Arrangements .................................................................33 Figure 13 Multiple Services to a Single Customer or Building ............................................34 Figure 14 COLD Substation with Adjacent MPBS or Mast .................................................37 Figure 15 HOT Substation with Adjacent MPBS or Mast....................................................37 Figure 16 Substation with MPBS or Mast within the Boundary Fence................................38 Figure 17 TN-S Earthing System ........................................................................................43 Figure 18 PME Earthing System.........................................................................................44 Figure 19 PNB Earthing System .........................................................................................44 Figure 20 TT Earthing System ............................................................................................45 Figure 21 The Effect of a Broken Neutral when Multiple Services from a CNE Network

are provided to a Steel-framed Building ...............................................................48

Tables

Table 1 Special Situations Use of PME Earth Terminal...................................................13 Table 2 General Bonding Requirements.............................................................................14 Table 3 Warning Labels ......................................................................................................15 Table 4 Electric Vehicle Charging Point Earthing Options..................................................27 Table 5 PME Maximum Load and Earth Electrode Resistance Values for Class I Street

Furniture ...............................................................................................................28



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1 Introduction This standard (previously Section 7 of the Earthing Design Manual) provides guidance on earthing of customer LV and HV installations. It brings together a common approach to system design and operation and has been written to provide guidance to designers, meter operators, developers and contractors. This standard supersedes all previous EPN, LPN and SPN specific guidance on customer installation earthing including:

LPN E14-26 A guide to the connection of customers' electrical installations to protective multiple earthed networks.

SPN PME Guide A guide to the application of Protective Multiple Earthing to customers' electrical installations.

LV Earthing Design Manual version 1 and 2. Earthing Design Manual Section 6B Construction Site Earthing Requirements. Earthing Design Manual Section 7 Customer Installation Earthing Design. This standard includes content from the original LV Earthing Design Manual and the Construction Site Earthing Requirements document. It aims to build on these original documents while providing a more consistent and practical approach. This document is based on best practice from the latest draft of ENA ER G12/4 and also reflects the requirements of BS 7671:2008 incorporating Amendment No 1: 2011 (IET Wiring Regulations Seventeenth Edition).

Note: It is the Installer's responsibility to ensure that the earthing system is safe and complies with the relevant regulations.

Significant changes from existing practice or previous documents:

The statutory requirements from the electricity, supply quality and continuity (ESQC) regulations 2002 are included.

The guidance from the latest draft of ENA G12/4 is included. The requirements from BS 7671:2008 (2011) are included where applicable. References to relevant planning standards are included. The criteria for where PME can and cannot be used have been clarified and, where

appropriate, alternative earthing systems suggested. The supply arrangements for construction sites, railways (including London

Underground) have been revised and clarified. The supply arrangements for multiple occupancy buildings have been revised in line with

ENA ER G87. Bonding the earthing system to a customers lightning earthing system is now

recommended. The guidance on street furniture and metallic pillars has been simplified and updated to

include electric vehicle charging points. Customer HV and combined HV/LV supplies to same customer or building are included. The range of warning labels has been updated.

Note for customers: For further information on any aspect of a project please contact the nominated UK Power Networks' Connections Project Designer for the project.

2 Scope This standard applies to the earthing used in customer installations.



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3 References EDS 02-0030 LV supplies to Mobile Phone Base Stations Mounted on 132, 275 and 400kV Towers (internal document only)

EDS 06-0001 Earthing Standard

EDS 06-0004 Earth Fault Loop Impedance Requirements (internal document only)

EDS 06-0014 Secondary Substation Earthing Design

EDS 06-0015 Pole-mounted Equipment Earthing Design

EDS 06-0016 LV Network Earthing Design

EDS 06-0017D Railway PME Application Form

EDS 06-0017E Railway PME Assessment Form (internal use only)

EDS 07-0009 Signs and Labels for Operational Sites

EDS 07-0102 Secondary Substation Civil Design Standards

EDS 08-0106 New Distributed Generation Connections

EDS 08-0113 Inset Networks

EDS 08-0118 Supplies to Multi Occupied Buildings (future issue)

EDS 08-0121 Supplies to HOT Sites and National Grid (future issue)

EDS 08-0129 Underground Services up to 100A

EDS 08-0133 Underground Services to Unmetered Street Furniture

EDS 08-0136 LV Network Design

EDS 08-0143 Large Customer LV Supplies (future issue)

The Distribution Code (http://www.dcode.org.uk/).

The Electricity Safety, Quality and Continuity Regulations (ESQC) 2002 as amended (2006)

BS 951:1999 Electrical Earthing. Clamps for Earthing and Bonding. Specification

BS 7671:2008 incorporating Amendment No 1: 2011 Requirements for Electrical Installations (IEE Wiring Regulations Seventeenth Edition)

BS 7430 Code of Practice for Earthing

BS 7361 Cathodic Protection

BS 7375:1998 Distribution of Electricity on Construction and Building Sites

BS 7909 Code of Practice for Temporary Electrical Systems for Entertainment and Related Purposes



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BS EN 50122-1 Railway Applications Fixed Installations Protective Provisions Relating to Electrical Safety and Earthing

BS EN 50122-2 Railway Applications Fixed Installations Protective Provisions Against the Effects of Stray Currents caused by DC Traction Systems

BS EN 61140 Protection Against Electric Shock. Common Aspects for Installation and Equipment

BS EN 61851 Electric Vehicle Conductive Charging System. General Requirements

BS EN 61558-2-4 Safety of Power Transformers, Power Supply Units and Similar. Part 2.4: Particular Requirements for Isolating Transformers for General Use

BS EN 62305 Protection Against Lightning

ENA ER G12/4 Requirements for the Application of Protective Multiple Earthing to Low-voltage Networks

ENA ER G87/1 Guidelines for the Provision of Low-voltage Connections to Multiple Occupancy Buildings

ENA ER P24 AC Traction Supplies to British Rail

ENA ETR 123 Guidelines for Managing the Interfaces between Utility Services and Light Rapid Transit Systems

Electrical safety in quarries http://www.hse.gov.uk/quarries/electricity.htm

Guidance for the Design, Construction, Modification and Maintenance of Petrol Filling Stations published jointly by the Association for Petroleum and Explosives Administration and the Institute of Petroleum

IET Code of Practice on Electric Vehicle Charging Equipment Installation (available Spring 2012)



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4 Definitions1 Caravan** A trailer leisure accommodation vehicle, used for touring, designed to meet the requirements for the construction and use of road vehicles (also see Motor Caravan).

Class I Equipment** Equipment in which protection against electric shock does not rely on basic insulation only, but which includes means for the connection of exposed-conductive-parts to a protective conductor in the fixed wiring of the installation (refer to BS EN 61 140).

Class II Equipment** Equipment in which protection against electric shock does not rely on basic insulation only, but in which additional safety precautions such as supplementary insulation are provided, there being no provision for the connection of exposed metalwork of the equipment to a protective conductor, and no reliance upon precautions to be taken in the fixed wiring of the installation (refer to BS EN 61140).

Combined Neutral and Earth (CNE) A cable where the neutral and protective functions are combined in a single conductor.

Customer/Consumer Any person who has responsibility for premises connected by agreement to distribution networks owned by UK Power Networks.

Customer's Installation The electrical apparatus under the control of the customer on the customers premises together with the wiring connecting this apparatus to the supply terminals. A cut-out and meter shall not form part of the customers installation (unless additional metering is supplied by the customer, e.g. landlords supplies).

Customer's Premises Any area or building occupied by the customer.

Distributing Main (or Main) A low voltage electric line which connects a source of voltage to one or more service lines or directly to a single customers installation.

Distribution Network Operator See distributor.

Distributor* A person who owns or operates a network, except for a network situated entirely offshore or where that person is an operator of a network within the meaning of Part I of the Railways Act 1993.

Earth Electrode A metal rod, plate or strip conductor buried in the earth for the purpose of providing a connection with the general mass of earth.

1 *Definitions taken from the Electricity Safety, Quality and Continuity Regulations 2002. **Definitions taken from BS 7671.



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Earth Loop Impedance (ELI) or Earth Fault Loop Impedance (EFLI)** The impedance of the earth fault current loop starting and ending at the point of earth fault. This impedance is denoted by the symbol Zs. The part of the earth fault loop impedance which is external to the customer installation is denoted by the symbol Ze.

Earthing Systems (refer to Appendix B for further details) Protective Multiple Earthing (PME) PME is the most common form of earthing provided at new installations. A single conductor for neutral and earthing functions is utilised and an earth terminal is provided at the customers installation. The customers earthing may be connected to this terminal providing the relevant requirements in BS 7671 are satisfied. In some cases it is not appropriate to provide a PME earth terminal, either due to the nature of the distribution system or due to the type of installation itself.

Cable Sheath Earth/Separate Continuous Earth Wire Cable sheath earth and separate continuous earth wire arrangements are often found on supplies to older properties, or occasionally where PME facilities are unavailable. Separate conductors for neutral and earthing functions are utilised and an earth terminal is provided at the customers installation. The customers earthing may be connected to this terminal providing the relevant requirements in BS 7671 are satisfied.

Independent Local Earthing (TT) Independent local or TT earthing is common in older installations and is also used where PME cannot be provided. An earth terminal is not provided and the customer is responsible for providing the earth electrode system (buried earth rods and conductor). Where local earthing is employed the installation normally has to be protected by a residual current device (RCD) in order to comply with BS 7671.

Earthing Terminal The main earth terminal for an installation is usually located close to the incoming service. If provided by UK Power Networks as part of a PME supply (TN-C-S) this will be directly connected to the supply neutral conductor at the supply terminals.

Earth Potential Rise (EPR) or Rise of Earth Potential (ROEP) EPR or ROEP is the potential (or voltage) rise that occurs on any metalwork due to the current that flows through the ground when an earth fault occurs on the HV or LV network.

Electric Line* Any line which is used or intended to be used for carrying electricity for any purpose and includes, unless the context otherwise requires:

a) any equipment connected to any such line for the purpose of carrying electricity. b) any wire, cable, tube, pipe, insulator or other similar thing (including its casing or coating)

which surrounds or supports, or is associated with, any such line.

Equipotential Bonding An electrical connection maintaining various exposed conductive parts and extraneous conductive parts at substantially the same potential (voltage).

Exhibition** Event intended for the purpose of displaying and/or selling products etc, which can take place in any suitable location, either a room, building or temporary structure.

Exposed Conductive Part A conductive part (metalwork) of equipment which can be touched and which is not normally live but which can become live when the basic insulation fails.



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Extraneous Conductive Part A conductive part (metalwork) liable to introduce a potential (voltage), generally earth potential, into the site but not forming part of the electrical installation, e.g. pipework, scaffolding etc.

High-voltage (HV) HV refers to any voltage above 1000V. The HV network typically refers to the 22kV, 20kV, 11kV or 6.6kV secondary distribution system.

Independent Distribution Network Operator (IDNO) Any electricity distributor other than the ex-Public Electricity Suppliers (PES) distribution network operators.

HOT Site A HOT site is a grid, primary or secondary substation where the earth potential rise (EPR) is greater than 430V or 650V (for high reliability circuits).

Low-voltage (LV) LV refers to any voltage less than 1000V. The LV network refers to the 400V distribution system.

Marina** Facility for mooring and servicing of pleasure craft with fixed wharves, jetties, piers or pontoon arrangements capable of berthing more than one pleasure craft.

Mobile/Transportable Unit** A vehicle and/or mobile or transportable structure in which all or part of an electrical installation is contained, which is provided with a temporary supply by means of, for example, a plug and socket-outlet.

Motor Caravan** Self-propelled leisure accommodation vehicle, used for touring, that meets the requirements for the construction and use of road vehicles (also see Caravan).

Multi-service Any electric line through which energy may be supplied to two, three or four adjacent customers from any distributing main or substation.

Residual Current Device (RCD) An RCD is a current operated device which measures the imbalance between phase and neutral currents, and if this leakage current exceeds a preset level will operate to interrupt the current flow. Typical domestic RCDs have a 30mA operating threshold.

Secondary Substation An HV/LV substation typically transforming 22kV, 20kV, 11kV or 6.6kV to 400V.

Second Fix All the work after the plastering required to complete a building, i.e. electrical fixtures connected to the cables, sinks/baths connected to the pipes and doors fitted into doorframes.

Separate Neutral and Earth (SNE) A cable where the neutral and protective functions are provided by separate conductors.

Service Line* (or Service)



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Any electric line which either connects a street electrical fixture, or no more than four customers installations in adjacent buildings, to a distributing main.

Service Termination The cut-out where the service cable terminates.

Show** Display or presentation in any suitable location, either a room, building or temporary structure.

Stand** Area or temporary structure used for display, marketing or sales.

Street Electrical Fixture* A permanent fixture which is or is intended to be connected to a supply of electricity and which is in, on, or is associated with a highway.

Supplier* A person who contracts to supply electricity to consumers.

UK Power Networks (Operations) Ltd UK Power Networks (Operations) Ltd consists of three electricity distribution networks as follows:

Eastern Power Networks plc (EPN). London Power Network plc (LPN). South Eastern Power Network plc (SPN).



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5 Customer LV Installations

5.1 Design Criteria

The Electricity Safety, Quality and Continuity Regulations 2002 Regulation 24(4) (refer to Appendix A) state that a distributor shall make an earthing terminal available when installing a new low voltage connection or replacing an existing connection unless it is inappropriate for reasons of safety.

An earth terminal can usually be provided if the following criteria are satisfied:

The installation complies with the industry guidance in ENA ER G12. The installation complies with the equipotential bonding requirements of BS 7671 to

reduce any dangerous touch potentials to a minimum. All services to a building with a steel-frame or shared metallic services are from a single

point on the combined neutral/earth network to eliminate neutral current diversion.

5.2 Earth Terminal

UK Power Networks' standard is to provide an earth terminal from a PME system wherever it is appropriate. However there are situations where it may be inappropriate to offer an earth terminal from a PME system (refer to Section 5.3 for further details) and one of the following alternatives should be used:

TT earthing system. TN-S earthing system where permitted in this document.

Note: The construction of the low voltage network to PME requirements and the provision of an earth terminal from a PME system at the cut-out does not automatically imply that it is appropriate or safe to release the earth terminal for use by the customer. This decision is the responsibility of the meter operator acting on behalf of UK Power Networks.

All LV distribution networks shall be assumed to be PME even if they were originally constructed using SNE cables. Therefore if an SNE service is provided to a customer, their installation will still be subject to a possible rise in voltage in the event of a broken neutral on the main and therefore all new installations shall meet the full requirements for equipotential bonding specified in BS 7671.

The general bonding requirements detailed in Table 2 shall be applied and the appropriate warning label(s) shall be installed as detailed in Table 3.

Generally only one service and earth terminal shall be provided to a customer or a building. Multiple services to a single customer or building are not recommended practice since this causes problems due to neutral current diversion (refer to Section 7.8.1) and uncertainty when isolating the supplies. However in the rare circumstance that more than one service is required, the requirements of Section 7.8.4 shall be satisfied.

For further information on LV network design and underground services up to 100A refer to EDS 08-0136 and EDS 08-0129 respectively.



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5.3 Special Situations

Table 1 details where a PME earth terminal may not be provided. Unless specified otherwise in this document these installations shall use a TT earthing system which shall consist of an independent earth electrode and RCD protection. The TT earthing system shall be segregated by a minimum of 2m from any PME earthing system. The earthing system and protection for this type of installation is the responsibility of the customer.

Note: In these situations it is not acceptable to provide an SNE service from a PME main as the earth conductor will still experience a rise in voltage in the in the event of a broken neutral on the main.

Table 1 Special Situations Use of PME Earth Terminal

Situation PME Earth Terminal

Refer to the document listed for the PME criteria or alternative earthing system(s)

Exhibitions, shows and stands No Refer to BS 7671:2008 Section 711

Fairgrounds, amusement parks and circuses

No Refer to BS 7671:2008 Section 740

Boats, marinas and similar locations No Refer to BS 7671:2008 Section 709

Petrol filling stations No Refer to Guidance for the Design, Construction, Modification and Maintenance of Petrol Filling Stations published jointly by the Association for Petroleum and Explosives Administration and the Institute of Petroleum for further information

Permanent buildings associated with the above installations e.g. living accommodation, office, restaurant, shop etc

Yes Provided the building is electrically separated and its electrical installation complies with the bonding requirements of BS 7671.

Mobile and transportable vehicles and units that contain an electrical installation for outside broadcast2, medical services, advertising, catering services etc

No Refer to BS 7909 and BS 7671:2008 Section 717

Construction and demolition sites No Use either TN-S or TT refer to Section 7.1

Mines and quarries No Use either TN-S or TT refer to Section 7.2

Railway installations Yes Refer to Section 7.3

Farms and horticultural premises Yes Refer to Section 7.4

Sports pavilions and swimming pools Yes Refer to Section 7.5

Street lighting Yes Refer to Section 7.7.1

Electric vehicle charging points Yes Refer to Section 7.7.2.1

Street furniture Yes Refer to Section 7.7.2.2

Multiple occupancy buildings Yes Refer to Section 7.8

Metal-clad buildings Yes Refer to Section 7.9

2 The blanket approval previously given for outside broadcast vehicles (such as those owned by the BBC or IBA companies) to use a PME earthing system has been withdrawn.



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Situation PME Refer to the document listed for the PME Earth criteria or alternative earthing system(s) Terminal

Conversion of older properties into flats

Yes Refer to Section 7.10

Cathodic protection installations Yes Refer to Section 7.11

Communication stations Yes Refer to Section 7.12

Mobile Phone Base Stations and Masts associated with Substations

Yes Refer to Sections 7.13 and 7.14

5.4 Bonding Requirements

The main bonding requirements and minimum size of bonding connectors for PME installations are given in Table 2.

Table 2 General Bonding Requirements

Connection Type Copper or Copper Equivalent Bonding Conductor

At customers premises between service neutral and main earthing terminal

Note: The main earth terminal is usually built into the cut-out

16mm2 or half the size of the neutral meter tail whichever is the larger

At customers premises between the main earthing terminal and the earth bar of the consumer unit

Note: This is the responsibility of the consumer

16mm2 or half the size of the neutral meter tail whichever is the larger

At customers premises the main bonding conductor between the main earthing terminal and all metal structures, metal pipes and other extraneous conductive parts

Note: This is the responsibility of the consumer

Refer to BS 7671:2008 Table 54.8



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5.5 Warning Labels

In some circumstances it is necessary to install a warning label where specified in this document. Table 3 details the warning labels that are available.

Table 3 Warning Labels

Situation/Location Materials Code3/ Reference

Specification Label

PME/PNB Earth

Next to the cut-out refer to Section 5.2

21758H

EDS 07-0009.130

100mm x 50mm adhesive label

PNB Earth

On the PNB earth where it enters the cut-out refer to Section 5.2

EDS 07-0009.9 70mm x 20mm tie on label

Multiple Supplies

Next to the cut-out when multiple supplies are provided to single customer or building refer to Section 7.8.4

EDS 07-0009.119 100mm x 50mm adhesive label

Multiple Supplies

At each end of the bonding conductor between the cut-out earth terminals refer to Section 7.8.4

BS 951 SAFETY ELECTRICAL CONNECTION DO NOT REMOVE

Connection of two earthing systems

At the removal link between two separate earthing systems refer to Section 8


3 UK Power Networks logistics material code.



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Materials Code3/ Situation/Location Specification Label Reference

Street Furniture

Next to the cut-out refer to Section 7.7.2.2




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6 Earth Fault Loop Impedance The earth fault loop impedance requirements are covered by EDS 06-0004 which contains guidance on:

Typical earth fault loop impedance values. Earth fault loop impedance measurements. Customer installations and situations requiring further investigation. Supplies to street furniture. Relevant legislation.

7 Special Situations This section describes a number of special situations where an alternative earthing system may be required and also includes arrangements for some other special situations. This section is generally based on the guidance given in ENA ER G12/4 Section 6.2. The following special situations are included:

Construction and demolition sites. Mines and quarries. Railway installations. Farms and horticultural premises. Swimming pools. Sports pavilions. Street lighting and road signs. Street furniture including electric vehicle charging points. Freestanding metallic pillars. Multiple occupancy buildings. Metal-clad buildings. Conversion of older properties into flats. Cathodic protection installations. Communication stations. Lightning protection systems. LV generators. HOT sites. Inset networks.

7.1 Construction and Demolition Sites

This section details the types of earthing systems that can be used for temporary supplies to construction and demolition sites. The transition from a temporary to a permanent supply shall be taken into account and both supplies considered during the design and planning stages. UK Power Networks and the customer shall work together to ensure that the customer's expectations can be satisfied.

The earthing requirements described in this section are based on the latest draft of ENA ER G12/4. The distribution of electricity on construction and building sites is also covered in BS 7375.



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7.1.1 Background

Construction and demolition sites have particular earthing issues due to the amount of exposed conductive parts (e.g. scaffolding, cranes etc) which are in contact with the ground and in effect providing an earthed surface. The very process of construction and demolition means that bonding is not always in place. If another earth from an electricity supply is introduced into the site there may be a potential (voltage) difference between the two earthing systems. If this occurs there is a possibility of simultaneous contact with the two earthing systems and an electric shock. The situation is exacerbated by the presence of earthed conducting surfaces and a higher risk of both installation fault and broken supply neutral conditions.

7.1.2 Temporary Supplies

A PME earth terminal shall not be provided to construction or demolition sites because it is not possible to verify that the installation continuously complies with the bonding requirements of BS 7671. The following alternatives are available:

TT earthing system. TN-S (SNE) earthing system from a dedicated transformer. TN-S (SNE) earthing system via an isolating transformer. The protection requirements for each of these are covered in BS 7671.

If the temporary supply is for a substation, advice should be sought from Asset Management (refer to EDS 06-0001).

7.1.2.1 TT Earthing System with RCD Protection

The preferred option is a TT earthing system as shown in Figure 1.

HV : LV

DNO Secondary Substation

Construction Site

Other Customer Supplies

L1L2L3

N/E

L1L2L3NE

CustomerDNO

RCD

Figure 1 Temporary Building Supply with a TT Earthing System

The supply shall be protected in accordance with BS 7671 and shall include a residual current device (RCD) on the customers side of the cut-out. There should be no extraneous conductive parts before and/or enclosing the RCD.

The earth electrode shall be a minimum of 2m away from any PME earth electrode or exposed LV metallic cable sheath and a minimum of 8m away from any HOT substation and associated cables to limit the effect of transfer potential between earthing systems.



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All metering equipment shall be contained within a non-conductive enclosure, i.e. not metal-clad. If unavoidable, please consult the meter operator for further information.

The developer/contractor is responsible for maintaining the RCD and the site earth. It is also the developers/contractors responsibility to ensure that RCD settings comply with BS 7671.

7.1.2.2 TN-S from a Dedicated Transformer

If the site has a dedicated UK Power Networks secondary substation that only supplies the customer it will usually be possible to provide a TN-S earth terminal directly from the transformer neutral see Figure 2. This arrangement will also enable easier transition to a permanent supply when it is required.

Note: If the transformer supplies other customers or interconnects with other parts of the LV network, this arrangement is not permitted. Before connecting the construction supply it is necessary to first permanently disconnect:

The other customers or LV network. All relevant cables. All earth connections apart from the substation LV earth. If there is any doubt one of the other options shall be used.

HV : LV

DNO Secondary Substation

L1L2L3NE

Construction Site

L1L2L3NE

CustomerDNO

Figure 2 TN-S Earth from a Dedicated Transformer

7.1.2.3 TN-S Earthing via an Isolating Transformer

If the site doesn't have a dedicated transformer, i.e. the transformer supplies other customers or other parts of the LV network, it is still possible to provide a TN-S supply by using a 1:1 isolating transformer as shown in Figure 3. The neutral of the isolating transformer can be used to provide a TN-S earthing system within the boundary of the site.

The isolating transformer should be - and comply with BS EN 61558-2-4. The transformer enclosure and core shall be connected to the site earth. The transformer shall be protected against primary winding faults with a residual current device (RCD) on the customers side of the cut-out. The setting of the RCD shall ensure that the voltage rise on the site earth is less than 50V.

Note: Additional RCDs may be required on the secondary side of the isolating transformer for the sub-circuit protection to satisfy the requirements of BS 7671.



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The customers earth electrode shall be a minimum of 2m away from any PME earth electrode or exposed LV metallic cable sheath and a minimum of 8m away from any HOT substation and associated cables to limit the effect of transfer potential between earthing systems.

The developer/contractor is responsible for maintaining the RCD, the isolating transformer, the LV supply and the site earth. It is also the developers/contractors responsibility to ensure that RCD settings comply with BS 7671.

Figure 3 TN-S Earth from an Isolating Transformer

7.1.3 Transition to a Permanent Supply

Although it is preferable, it is not always practical to remove the temporary building supply before the permanent supply is required. Therefore a permanent supply using PME may be provided to a building if the following criteria are satisfied:

The building installation shall satisfy the requirements of BS 7671. It is not possible for a person to make simultaneous contact between any conductive

parts bonded to the PME earth and conductive parts outside the building. (Provided that they cannot be touched at the same time these conductive parts outside the building will not be classified as extraneous conductive parts, as defined by BS 7671.) Note: For a building other than a metal-clad building, it will usually be possible to satisfy this requirement even if scaffolding is still in place on the outside of the building provided the scaffolding is not bonded to the TT earth.

Any isolating transformer used to provide a temporary supply shall be removed from the site.

Any TT earth including equipment bonded to it shall be a minimum of 2m from the permanent supply earth including equipment bonded to it.

The second fix shall be completed or, alternatively, a phased handover agreed with the meter operator.

The site shall be in a reasonable condition and satisfy the meter operator's requirements. Note: These requirements also apply to an existing building that is undergoing construction works with scaffolding in place.



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7.2 Mines and Quarries

A PME earthing terminal may be provided for use in mine/quarry permanent buildings (e.g. permanent offices and canteens) provided that the electrical installation complies with the bonding requirements of BS 7671. A PME earthing terminal may not be provided to amenity shower blocks unless an earth grid is installed.

Supplies to underground shafts, the production side of quarries or associated amenity shower blocks shall use a TT earthing system and shall consist of an independent earth electrode and RCD protection. The TT earthing system shall be segregated by a minimum of 2m from any PME earthing system. The supply and installation of earthing system and protection is the responsibility of the customer.

If the site has a dedicated secondary substation that only supplies the customer an alternative is to provide a TN-S earth terminal directly from the transformer neutral.

Also refer to HSE internet publication 'Electrical Safety in Quarries' at http://www.hse.gov.uk/quarries/electricity.htm

Note: It is the responsibility of the customer to ensure that the installation conforms to the requirements of BS 7671.

7.3 Railway Installations

This section details the requirements for providing PME earth terminals to operators of railway systems and is based on the requirements from the latest draft ENA ER G12/4.

7.3.1 General

The following criteria apply to all supplies to railway installations with a PME earth terminal:

All installations shall comply with the requirements of BS 7671, including equipotential bonding for PME conditions.

The housing at the intake position shall not expose a member of the general public to dangerous touch potentials.

Metallic enclosures containing LV equipment are not permitted at the intake position or where they may expose a member of the general public to dangerous touch potentials. Note: Network Rail has been given a temporary dispensation by the ENA to enable them to use of metallic enclosures provided the earthing requirements of Section 7.7.2.3 are adopted.

The above measures do not necessarily provide full protection against touch potentials for railway personnel and it is the responsibility of the railway operator to assess and control such risks.

7.3.2 LV Supplies at Traction Supply Points

A PME earth terminal shall not be provided at traction supply points associated with AC traction systems. The requirements for LV auxiliary supplies at these locations are detailed in ENA ER P24.



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7.3.4

7.3.3 LV Supplies for Sites with both AC and DC Traction Systems

A PME earth terminal shall not be provided to sites that have both AC and DC traction systems unless it can be demonstrated that:

There is no transfer track between the AC and DC systems. The DC system is not connected to earth. The DC supply is segregated by at least 1m from the PME earth. The requirements for LV supplies to AC traction systems (Section 7.3.4) and DC traction

systems (Section 7.3.5) are satisfied.

LV Supplies Associated with AC Traction Systems at Locations other than Traction Supply Points

Specific information about the compliance of individual railway operators is detailed in Appendix C.

A PME earth terminal may be provided to premises and trackside cubicles associated with railway lines using an AC traction system subject to the railway operator confirming that the following criteria are satisfied:

1. If there is DC traction in the vicinity the AC and DC traction systems shall be fully segregated such that there can be no earth connection to the DC system and there shall be no sections of transfer track between the AC and DC systems.

2. The equipment being supplied and all other equipment to which it is bonded shall not have any risk of coming into contact with the traction supply or (where equipment is at risk of contact with the traction supply) a bonding conductor shall be provided between the PME earth terminal and the traction return circuit, which shall be of an adequate size to carry traction fault current using one of the following options:

Direct bonding as shown in Figure 4 where the LV electrical equipment is either directly bonded to the traction return circuit, or in contact with a conductive structure.

Indirect bonding as shown in Figure 5. 3. If there is a traction supply at the railway station, the traction supply and LV supply shall

not originate from the same source (grid/primary) substation.

4. The earth potential rise during fault conditions shall be less than:

430V for faults with a duration greater than 0.2s but less than 3s. 650V for faults with a duration less than or equal to 0.2s.

Note: An earth potential rise of 484V for 0.25s is deemed acceptable.

5. The rise of voltage on the traction rail due to traction return current shall not exceed 25V under frequent traction peak starting or running current conditions.

6. The railway operator shall provide a drawing of the proposed installation in order to demonstrate that a suitable path exists for traction fault current.

Two methods of bonding are employed for equipment associated with AC electrified railways. LV electrical equipment which is either a) in contact with a conductive structure directly bonded to the traction return circuit, or b) is directly bonded to the traction return circuit is classed as directly bonded. In all other cases equipment is indirectly bonded. The two methods of bonding are illustrated in Figure 4 and Figure 5.



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Booster

transformer25kV catenary

Traction supply

DNO intake

L

N

L

N

E

Equipment in electrical contact with traction return circuit

L

N

E

Direct bonding to traction return circuit

Bonding to traction return circuit by

agreement with DNO

Overhead return conductor

Running rail

Figure 4 Direct Bonding for Equipment in Contact with the Traction Return Circuit

Booster

transformer25kV catenary

Overhead return conductor

Traction supply

DNO intake

L

N

Equipment not in contact with traction return circuit but at risk of becoming

live from 25kV overhead lineL

N

E

Bonding to traction return circuit

Running rail

Figure 5 Indirect Bonding for Equipment not in Contact with the Traction Return Circuit.

7.3.5 LV Supplies Associated with DC Traction Systems

These requirements seek to minimise the risk of electrolytic corrosion of earthing systems due to stray DC currents. They are based on a recognition that, if stray currents exist, there will be paths electrically closer to the traction system which will take larger stray currents than will flow through an LV earthing system. In this case corrosion of cable sheaths, structures and earthing systems, which are subject to regular inspections, will quickly become apparent to the railway operator. These measures will also ensure that for PME systems no external voltage is impressed on the neutral/earth conductor.

Refer to Appendix C for information on the construction of DC traction systems.



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A PME earth terminal may be provided to premises and trackside cubicles associated with railway lines using DC traction systems provided the following criteria are satisfied:

1. If there is an AC traction system in the vicinity, the systems shall be fully segregated such that there can be no earth connection to the DC system and there shall be no sections of transfer track between the AC and DC systems.

2. Neither pole of the traction supply shall be directly connected to earth and any connection to earth is solely for the purpose of the detection of earth fault conditions.

3. The traction supply (3rd rail or overhead) and return (running rails and/or 4th rail) rails are insulated from earth in accordance with the requirements of BS EN 50122-2.

4. The LV supply, including the protective earthing conductor, and all earthed metal associated with it shall be segregated from all DC conductors by the maximum practicable distance, subject to a minimum distance of 1m in accordance with BS EN 50122-2.

5. There shall be no evidence of corrosion on railway equipment which may be due to stray DC current. In the event that the railway operator detects corrosion due to stray DC current on any of their equipment following the provision of an LV supply they shall advise UK Power Networks4.

6. The voltage between the running rails and earth shall satisfy the requirements of BS EN 50122-1 i.e. less than 5V.

7. The conductance between the running rails and earth shall satisfy the requirements of BS EN 50122-2 i.e. less than 0.55 S/km per track.

7.3.6 Other Electrified Systems

Refer to ETR 123 for supplies to Light Rapid Transit Systems.

Requirements for the provision of earthing terminals to premises and equipment at the trackside of operators of other traction systems should be referred to the earthing specialist (refer to EDS 06-0001).

7.3.7 Assessment Process

To enable the request for a PME supply associated with a railway installation to be properly assessed, the railway operator shall be requested to complete the application form in Appendix D. The form and flow chart shown in Appendix E can then be used to assess the application.

4 If UK Power Networks are advised by Network Rail that corrosion has been detected the supply shall be re-assessed using the assessment process in Section 7.3.7 and if appropriate the earth terminal shall be withdrawn and Network Rail advised to use a TT earthing system.



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7.4 Farms and Horticultural Premises

A PME earthing terminal may be provided for use in farms and horticultural premises provided that:

There is no extraneous metalwork. The building electrical installation complies with the bonding requirements of BS 7671. The building is not occupied by animals or, if it is occupied by animals, a bonded earth

grid is installed in the floor.

If these conditions cannot be satisfied a TT earthing system shall be used and shall consist of an independent earth electrode and RCD protection. The TT earthing system shall be segregated by a minimum of 2m from any PME earthing system or any building with a PME earthing installation. Furthermore earthing systems shall not be mixed within a building. The supply and installation of earthing system and protection is the responsibility of the customer.

Note: It is the responsibility of the customer to ensure that the installation conforms to the requirements of BS 7671. BS 7671:2008 (Regulation 705.411.1) requires all final circuits to incorporate RCD protection regardless of type of earthing system used.

7.5 Swimming Pools

A PME earthing terminal may be provided for use in swimming pools provided that the electrical installation complies with the bonding requirements of BS 7671. Electrical supplies to swimming pools and other basins are covered in Section 702 of BS 7671:2008.


7.6 Sports Pavilions

A PME earthing terminal may be provided for use in sports pavilions and similar locations provided that the electrical installation complies with the bonding requirements of BS 7671 and either:

No shower area exists or is likely to exist, or A bonded earth grid is installed in the floor of the shower area. If these conditions cannot be satisfied a TT earthing system shall be used and shall consist of an independent earth electrode and RCD protection. The TT earthing system shall be segregated by a minimum of 2m from any PME earthing system. The supply and installation of earthing system and protection is the responsibility of the customer.




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7.7 Street Lighting and Street Furniture

7.7.1 Street Lighting and Road Signs (up to 500W)

A PME earthing terminal may be provided to supply street lighting and road signs with a load of 500W or less owned by Network Operators or Street Lighting Authorities. Private street lighting installations are not permitted to combine the neutral and earth function therefore SNE cables shall be used throughout their installation. The supply to street lighting authority and private installations is usually via a pillar. The following earthing requirements shall be applied to all street lighting installations:

An earth electrode shall be installed at the last lamp column position. However the earth electrode may be omitted for single lamp columns provided the distributor neutral conductor is earthed either at or beyond the service joint position (refer to Figure 6). Note: It is not permissible to consider metallic lamp columns to be earth electrodes.

An earth electrode shall also be installed at the feeder pillar, feeder column or at the end of the service supplying a Street Lighting Authority or private street lighting network (refer to Figure 6).

The minimum size of bonding conductor shall be 6mm2 for a standard street lighting cut-outs (up to 25A) and 16mm2 for other cut-outs up to 100A.

Small isolated metal parts (e.g. doors in concrete lamp columns) which are unlikely to come into contact with earthed metalwork do not need to be bonded.

Segregated from any HOT substation by 2m. The earth terminal and any external metalwork of street lighting fittings mounted on

buildings or wooden poles shall be connected to the PME earth terminal unless the fitting is Class II insulated.

Figure 6 Examples of Street Lighting Earth Electrode Positions

For further details on all other aspects of street lighting refer to EDS 08-0133.



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Electrical supplies to outdoor lighting installations, highway power supplies and street furniture are covered in Section 559.10 of BS 7671:2008.

7.7.2 Street Furniture (not covered by 7.7.1)

This section covers street furniture accessible to the public including electric vehicle charging points communication and cable television electrical distribution cabinets etc, with a load above 500W. Larger fenced off installations are excluded.

7.7.2.1 Electric Vehicle Charging Points

The permissible options for supplies to electric vehicle charging points are given in Table 4. The installation shall comply with the requirements detailed in the IET Code of Practice on Electric Vehicle Charging Equipment Installation All exposed-conductive-parts of the charging point shall be bonded to the earthing system and the incoming supply to the charging point shall be protected by an RCD. The customer is responsible for maintaining the RCD and any TT earth.

Table 4 Electric Vehicle Charging Point Earthing Options

Electric Vehicle Charging Point Location Earthing System

On-street TT

Inside Use existing earthing system (TN-C-S or TT)

Domestic Installations (including small offices and shops where the electrical supply and installation are similar to those at domestic premises) Outside TT

Inside Use existing earthing system (TN-C-S or TT)

Industrial and Commercial Installations (including installations on or adjacent to premises, public and private car parks both single-storey, multi-storey and those associated with multiple occupancy buildings) Outside TT

Note: The IET Code of Practice on Electric Vehicle Charging Equipment Installations also includes a risk assessment to cover the situation where different earthing systems, e.g. PME and TT, may co-exist at the same premises and various checklists to assess an installation.



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7.7.2.2 Other Street Furniture

The practice of installing equipment in metallic enclosures is discouraged due to the risk of any earthed metalwork becoming live in the event of a broken neutral and the difficulty in managing the step and touch potentials around the enclosure. Therefore all street electrical fixtures should be Class II (double-insulated) construction (as defined in BS 7671), e.g. public telephones, ticket machines, bollards etc, or housed in a Class II (double-insulated) enclosure wherever possible.

Where the street electrical fixture or enclosure is of Class I construction (as defined in BS 7671), a PME earth terminal may be provided without significant risk to the public provided that the electrical installation:

Complies with the bonding requirements of BS 7671. Complies with the earth electrode and labelling requirements detailed in Table 5. Is segregated from any HOT substation by 2m. Alternatively, or if these requirements cannot be satisfied, a TT earthing system shall be used and shall consist of an independent earth electrode and RCD protection. The TT earthing system shall be segregated by a minimum of 2m from any PME earthing system. The supply and installation of earthing system and protection is the responsibility of the customer.

Table 5 PME Maximum Load and Earth Electrode Resistance Values for Class I Street Furniture

Connection Maximum Single-phase Load or Three-phase

Overall Load Unbalance

Earth Electrode Requirements

Labelling Requirements

Balanced three-phase

No load unbalance Not required n/a

500W Maximum 100 earth electrode5

Refer to Table 3

2kW Maximum 20 earth electrode5

Refer to Table 3

Unbalanced three-phase or single-phase

>2kW Grading electrode (refer to Section 7.7.2.3)

n/a

5 These values are based on ENA ER G12/4 and have been selected to limit the touch voltage to 100V which is deemed to be safe.



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7.7.2.3 Freestanding Metallic Feeder Pillars

Where freestanding metallic pillars are used and the rules for street furniture detailed in Section 7.7.2.2 are not satisfied (e.g. metering cubicles supplying IDNOs and Network Rail), the following requirements shall be applied and are shown in Figure 7:

The pillar shall be surrounded by a 70mm2 bare copper conductor grading electrode buried at a depth of approx 500mm and located approx 500mm away from all sides of the pillar.

The pillar and the grading electrode shall be connected to the earth terminal. The pillar shall be at least 2m clear of other earthed metalwork not connected to the

same earth to avoid touch potential risks arising from inadvertent contact between the pillar and the other earthed metalwork.

Where the pillar is located within 2m of a secondary substation the pillar grading electrode shall be connected to the secondary substation earthing via duplicate connections.

Pill

ar

Figure 7 Metallic Pillar with a Grading Earth Electrode

Network Rail has been given a temporary dispensation by the ENA to use metallic cabinets provided the earthing design shown in Figure 8 is adopted. For further details refer to drawing EDS 07-0102.14. Note: This dispensation will apply until Network Rail has exhausted their current stock of metallic cabinets.

Figure 8 Metallic Pillar Earthing at Network Rail Sites



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7.8 Multiple Occupancy Buildings

7.8.1 Overview

This section is based on ENA ER G87. A multiple occupancy building is defined in ENA ER G87 as a building occupied by more than one customer some typical examples are flats and industrial units (Figure 9). EDS 08-0118 covers all of the other design aspects associated with multiple occupancy buildings including fire fighting supplies.

Note: The requirements detailed in this section may be applied from the date this document is issued; however they will not become mandatory until EDS 08-0118 is issued, until this time legacy standards may be used if appropriate.

Figure 9 Multiple Occupancy Building Examples

It has been common practice for many years to provide connections to individual premises from a PME system and for individual premises to be given a PME earth terminal. However this is no longer acceptable due to the issues outlined below:

The provision of a suitable end-of-main electrode which may be impractical, particularly in multi-storey buildings.

The problems caused by the flow of neutral current (refer to Appendix F) through the building steelwork.

Having to apply continuous ratings to steel wire armour on cables used for the building network to cater for diverted neutral current.

The need for equipotential bonding between the intake positions. The following sections only apply to multiple occupancy buildings that have a steel-frame or shared metallic services. Where these criteria do not apply all services may be provided with a PME earth terminal, subject to the relevant planning standards.



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7.8.2 New Building Design Single Intake and Single Service

Supplies to new multiple occupancy buildings shall wherever possible be designed with a single intake position and be provided with an earth terminal from a PME system as shown in Figure 10 and Figure 11. The appropriate main equipotential bonding connections to structural steelwork and to metallic services shall be made at this point of connection.

All cables from the intake to individual customers shall be SNE6.

At an individual customers installation the main equipotential bonding between metallic services, extraneous metalwork and the earth terminal shall be carried out in accordance with BS 7671. This will ensure that no harmful potentials appear between earthed and extraneous metalwork within the customers premises under fault conditions. For bonding purposes, the customers installation shall be considered to be TN-C-S if a PME earth terminal is provided at the intake position.

A single intake position with a single service avoids the problems outlined above. However if it is not possible to design on the basis of a single-intake/service position, two separate intakes may be provided in accordance with Section 7.8.3 or multiple services to a single intake in accordance with 7.8.4.

INTAKE

CO

New CNE Service

LV Network

11kV400V

LV Cabinet

Secondary Substation

PME

Single Intake Position Supplies

to Multiple Flats/Units

Ownership Boundary

CustomerUKPN

Metering and customer equipment not shown refer to EDS 08-0118

Building

Flat/Unit 1

Flat/Unit 2

Flat/Unit n

CO Cut-out PME End-of-main Electrode CNE cable SNE cable

Figure 10 Single Intake Supply Arrangement New PME Service from Secondary Substation

6 If SNE cable is unavailable CNE cable with a separate earth conductor is an acceptable alternative.



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11kV400V

LV Cabinet

LV Network

PME Main

PME

PME Main

Ownership Boundary

CustomerUKPN


INTAKE

COPME

Single Intake Position Supplies

to Multiple Flats/Units

Building

New CNE Service


Flat/Unit 1

Flat/Unit 2

Flat/Unit n

Figure 11 Single Intake Supply Arrangement - New PME Service from PME Main

7.8.3 New Building Design Multiple Intakes and One Service

It is not always possible to design using a single intake position particularly for a horizontal run of steel-framed residential, industrial or commercial units. Individual PME connections shall not be used in these situations due to the risks associated with significant neutral current flow through structural steelwork and fixings under fault or other conditions and the difficulty associated with providing a bond between intake positions.

Therefore one of the following options shall be used:

Freestanding pillar provide a PME connection to a freestanding distribution pillar/cabinet and use a SNE7 cable to supply each unit (refer to Figure 12a and Figure 12b). Note: The neutral and earth conductors of the SNE cables shall not be bonded together at any point other than the pillar as this would result in a path for neutral current to be diverted into the building structure.

Secondary substation use a SNE7 cable to supply each unit directly from the local secondary substation (refer to Figure 12c).

Individual service alternatively an individual service may be provided to each unit using a TT earthing system. Although this will eliminate the problems it has other disadvantages that may be unacceptable to the customer.

7 If SNE cable is unavailable CNE cable with a separate earth conductor is an acceptable alternative.



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LV Network

11kV400V

Dedicated New Services single or multi-core installed as

SNE Cables to Each Intake Position

Intake 2

Intake n

CO

LV Cabinet

Intake 1

Secondary Substationon Customer Site

CO

CO

Supplies to Multiple Intake

Positions

CustomerUKPN

Ownership Boundary

New CNE Service11kV400V

LV Cabinet

LV Network

New SNE Services


PME

CustomerUKPN

Ownership Boundary

INTAKE

CO

New CNE Service11kV400V

(a)

(b)

(c)

LV Cabinet

LV Network

New SNE Services


PME

CustomerUKPN

Ownership Boundary

INTAKE

CO


Flat/Unit 1

Flat/Unit 2

Flat/Unit n

Building




Intake 2

Intake n

CO Intake 1

CO

CO

Supplies to Multiple Intake

Positions

Supplies to Multiple Flats/

Units

Building

Site Boundary

Site Boundary

Site Boundary

Building

Figure 12 Multiple Intake Supply Arrangements



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7.8.4 Multiple Services to a Single Customer or Building

Multiple CNE services to a single customer or building are not recommended due to the problems associated with neutral current diversion (refer to Appendix F) and with the isolation of the supplies. If there is no alternative to providing an additional service or an additional service has already been provided the following requirements (shown in Figure 13) shall be satisfied for PME earthing:

The services shall be from the same secondary substation and ideally from the same main.

The services shall be positioned in the same intake room/cabinet and have the same size neutral conductors.

The earth terminals of each service shall be bonded8 together using a copper conductor covered in green/yellow PVC with a blue marker at each end (to indicate that it's carrying current). The size of the bonding conductor shall be not less than that of the associated service neutral conductor. The bonding conductor shall have a warning label (BS 951) fitted to each end as detailed in Table 3.

A warning label (EDS 07-0009.119) shall be installed next to each cut-out as detailed in Table 3.

Each service to the building shall be treated as a separate supply within the customers installation and the equipotential bonding to other services (gas, water etc) installed accordingly. This ensures that equipotential bonding is maintained if one of the services is removed in the future.

If the above requirements cannot be satisfied:

Only one earth terminal shall be provided to a customer or building from the network and all other services shall use a TT earth.

The electrical installations (including equipotential bonding) supplied from each service shall be entirely isolated from one another.

CO

PME

PME

COCNE Service 1

CNE Service 2


11kV400V

LV Cabinet

LV Network

Customer SINGLE Intake

Room

Ownership Boundary

CustomerUKPN

CO Cut-out PME End-of-main Electrode Warning Label CNE cable SNE cable


Flat/Unit 1

Flat/Unit 2

Flat/Unit n

Building

Figure 13 Multiple Services to a Single Customer or Building

8 The neutral/earth bonding conductor prevents excessive current flowing through the customers bonding in the event of a broken neutral conductor.



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7.8.6

7.8.5 Existing Buildings

Wherever possible the requirements for new buildings detailed in Section 7.8.2 or Section 7.8.3 shall be applied to existing buildings, however, if the requirements cannot be satisfied then the requirements for multiple services detailed in Section 7.8.4 shall applied.

End-of-main Earth Electrode

Where a PME service is provided to a multi-occupancy building an end-of-main earth electrode with a maximum resistance of 100 shall be installed adjacent to the point of entry to the building of the incoming service cable. The associated earthing conductor may be run into the building alongside the service cable and connected to the neutral at the intake position (refer to EDS 06-0016 for the electrode details).

7.9 Metal-clad Buildings

Where metal-clad buildings incorporate a steel-frame that utilises steel vertical beams that are within the foundations, the steel-frame will provide a good connection with the earth which will effectively limit the earth potential rise.

A PME service may be provided to a metal-clad building provided the following criteria are satisfied:

The metal cladding is bonded to the steel-frame. The supply is either three-phase with less than 40%9 unbalance or the supply is single-

phase and the frame to earth impedance is less than 20.

7.10 Conversion of Older Properties into Flats

There is an increasing trend to convert older properties, particularly in city areas, into flats which then require changes to the electrical supply. Very often these properties have concrete floors and the ground floor flat may be equipped with a shower and it is often impractical for the builder to install a bonded earth grid in the shower area. Section 7.6 describes the risks associated with showers located at ground floor level and where an earth grid is required.

The absence of an earth grid should be discussed with the customer, and a TT earthing system with an independent earth electrode and RCD protection considered as a more appropriate alternative to a PME earthing system.

7.11 Cathodic Protection Installations

The usual source of power for cathodic protection installations is a mains supply via a transformer rectifier unit. The preferred arrangement for cathodic protection installations is a TT earthing system which shall consist of an independent earth electrode and RCD protection. The TT earthing system shall be segregated by a minimum of 2m from any PME earthing system. The supply and installation of earthing system and protection is the responsibility of the customer..

However a PME earthing terminal may be given provided there is no electrical connection between the primary and secondary of the transformer rectifier unit and that the electrical

9 Source ENA ER G12/4.



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installation complies with the bonding requirements of BS 7671. Cathodic protection is covered by BS 7361.

7.12 Small Radio Stations Requiring a Functional Earth

Some communication stations require an independent earth electrode for functional/lightning purposes. Where such an earth is installed its earth resistance may be comparable or less than that of the DNO earthing system. On a PME network, in the event of an open circuit neutral the customer earth electrode may carry most of the diverted neutral current. However the size of the customer earthing and bonding connections may be insufficient for this current, particularly where the service size is small.

If this is the case a TT earthing system and RCD protection shall be used for either the whole of the installation or the part of the installation supplying the radio/communication equipment and any associated metalwork to prevent the possible neutral current diversion described above. In the latter case the TT installation earthing shall be segregated by a minimum of 2m from any PME earthing system and any earthing/bonding in the remainder of the customer installation. The supply and installation of earthing system and protection is the responsibility of the customer..

Where disruption due to possible RCD nuisance tripping would be unacceptable, an alternative to the TT earthing system is to size the earthing and bonding accordingly (i.e. the customer earthing and bonding shall not be less than the cross-sectional area of the service neutral).

7.13 Mobile Phone Masts

A PME earthing terminal may be provided for a mobile phone mast supply provided the rules for street furniture detailed in Section 7.7.2 are satisfied.

However if a functional earth is also required the installation shall be treated as a radio transmitter and a TT earthing system consisting of an independent earth electrode and RCD protection shall be used. The TT earthing system shall be segregated by a minimum of 2m from any PME earthing system. The supply and installation of earthing system and protection is the responsibility of the customer..

7.14 Mobile Phone Base Stations and Masts Associated with Substations

Positioning a mobile phone base station (MPBS), mast or other communication tower adjacent to or within a substation has touch, step and transfer potential implications as there are two separate earthing systems in close proximity to each other. Furthermore, the ground in and around the substation will rise in potential under earth fault conditions and it is necessary to ensure that the MPBS and/or mast is not situated within the 430V contours, since this voltage could be collected by the masts earthing system and impressed on the supply neutral.

Therefore there is a need to limit touch and step potentials, and to prevent transfer potentials onto the LV network. The rules in the following sections may be applied to MPBS and masts associated with secondary distribution substations and pole-mounted sites to satisfy this.

However, if these rules, and in particular the specified segregation, cannot be satisfied a detailed earthing study shall be carried out to calculate the earth potential rise, the extent of the voltage contours and the interaction with any metalwork. A specialised design is required to ensure that touch and step potentials are managed and that dangerous potentials are not transferred into the LV network.



Version: 3.1

Date: 22/08/2012

For MPBS and masts associated with grid and primary substations refer to EDS 06-0013.

7.14.1 MPBS and Masts Adjacent to a COLD Secondary Substation

A PME earthing terminal may be provided to a COLD site (combined HV/LV earth) as the earth potential rise is within safe limits and transfer potential is not an issue. However, the MPBS or mast cannot be assumed to be bonded to the PME earth terminal and at times may exist at a different potential to the metalwork at the substation. Therefore the mast and any metalwork connected to it shall be positioned at least 2m away from any metalwork connected to or associated with the substation (refer to Figure 14).

Figure 14 COLD Substation with Adjacent MPBS or Mast

7.14.2 MPBS and Masts Adjacent to a HOT Secondary Substation/Pole-mounted Site

A PME earthing terminal may be provided at a HOT site (segregated HV/LV earth) however additional precautions are needed to prevent danger resulting from the high voltages appearing on the metalwork and the rise in earth potential under earth fault conditions. Therefore the MPBS, mast and any associated metalwork shall be positioned at least 8m away from the following (refer to Figure 15):

Any metallic sheath cables in contact with the ground. Any other HV or LV earth electrodes. Any metalwork connected to or associated with the substation. Note: Metallic fences around the substation shall be assumed to be part of the

substation.

Figure 15 HOT Substation with Adjacent

EDS+06-0017+Customer+Installation+Earthing+Design

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