EDS+06 0014+Secondary+Substation+Earthing+Design

8/14/2019 EDS+06 0014+Secondary+Substation+Earthing+Design

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Document Number: EDS 06-0014

Version: 2.0

Date: 18/03/2013

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ENGINEERING DESIGN STANDARD

EDS 06-0014

SECONDARY SUBSTATION EARTHING DESIGN

Network(s): EPN, LPN, SPN

Summary: This standard details the earthing design requirements for secondary distributionsubstations.

Originator: Stephen Tucker Date: 18/03/2013

Approved By: Barry Hatton Approved Date: 18/04/2013

Review Date: 18/03/2018

This document forms part of the Companys Integrated Business System and its requirements are mandatory throughout UKPower Networks. Departure from these requirements may only be taken with the written approval of the Director of AssetManagement. 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 useof this document it should be read in full.)

Version Date Details Originator

2.0 01/03/2013 Document completely revised Stephen Tucker

1.2 03/08/2011 Reclassification of document from EarthingDesign Manual Section 4

Stephen Tucker

1.1 11/01/2011 Version 1 (previously Earthing Manual Section 5)withdrawn and interim guidance provided

Stephen Tucker

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


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Secondary Substation Earthing Design Document Number: EDS 06-0014

Version: 2.0

Date: 18/03/2013

UK Power Networks 2013 All rights reserved 2 of 44

Contents

1 Introduction ............................................................................................................. 52 Scope ....................................................................................................................... 63 Abbreviations .......................................................................................................... 64 Definitions ................................................................................................................ 75 Design Criteria ......................................................................................................... 86 Design Procedure .................................................................................................... 96.1 Step 1Information Requirements ........................................................................... 96.2 Step 2Design Assessment ..................................................................................... 96.3 Step 3Detailed Design (if required) ...................................................................... 126.3.1 Overview ................................................................................................................. 126.3.2 Calculate EPR ......................................................................................................... 146.3.3 Calculate Transfer Voltage ...................................................................................... 166.3.4 Determine Touch Voltage ........................................................................................ 177 Standard Secondary Substation Earthing Arrangements .................................. 187.1.1 GRP and Brick-Built Substations (COLD Site) ......................................................... 197.1.2 GRP and Brick-Built Substations (HOT Site) ........................................................... 207.1.3 Compact (including Micro and Padmount) Substations ............................................ 217.1.4 Integral and Basement Substations ......................................................................... 227.1.5 Customer HV Supplies and Associated Substations ................................................ 247.1.6 Existing Outdoor Substations .................................................................................. 278 Earthing Requirements ......................................................................................... 288.1 General ................................................................................................................... 288.2 Electrode System .................................................................................................... 288.3 Bonding ................................................................................................................... 298.3.1 Equipment ............................................................................................................... 298.3.2 Metallic Fences, Gates and Doors ........................................................................... 298.3.3 Ancillary Metalwork.................................................................................................. 328.3.4 Ducting and Ventilation Shafts ................................................................................. 338.4 Cables ..................................................................................................................... 338.5 Combined HV/LV Earths ......................................................................................... 338.6 Additional Requirements for HOT Sites ................................................................... 348.6.1 LV Earth .................................................................................................................. 348.6.2 Neutral-Earth Link ................................................................................................... 348.6.3 Warning Notices for Segregated Earths ................................................................... 348.6.4 Lighting and Socket Supplies .................................................................................. 34


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8.6.5 Street Lighting Columns .......................................................................................... 349 Special Situations.................................................................................................. 359.1.1 Substation Refurbishment and Asset Replacement/Enhancement .......................... 359.1.2 Supplies to National Grid and HOT Sites ................................................................. 359.1.3 Supplies from HOT Sites ......................................................................................... 359.1.4 Supplies to Higher Voltage Substations ................................................................... 369.1.5 Substations near Livestock/Horses or other High Risk Locations ............................ 369.1.6 Mobile Phone Base Stations Associated with Transmission Towers ........................ 379.1.7 Substations Located Near Tower Lines ................................................................... 379.1.8 Substations Located Near Railways ........................................................................ 379.1.9 Substations Located Near Telephone Exchanges ................................................... 379.1.10 IDNO Substations .................................................................................................... 379.1.11 HV Generator Connections ...................................................................................... 379.1.12 Customers Lightning Protection .............................................................................. 3710 Earthing Design Assessment ............................................................................... 3810.1 External Connection Providers ................................................................................ 3810.1.1 Overview ................................................................................................................. 3810.1.2 Earthing Drawing ..................................................................................................... 3810.1.3 Earthing Report ....................................................................................................... 3810.1.4 Further Information .................................................................................................. 3810.2 UK Power Networks ................................................................................................ 3811 References ............................................................................................................. 39Appendix AUK Power Networks Supporting Data ...................................................... 40Appendix BTypical Electrode Systems ....................................................................... 43Appendix CEarthing Design Form ............................................................................... 44Appendix DEarthing Design Assessment Form ......................................................... 44Appendix ESecondary Substation Earthing Design Tool .......................................... 44 Appendix FEarthing Design Example .......................................................................... 44Appendix GStandard Secondary Substation Arrangement Voltage Profiles ............ 44


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Figures

Figure 6-1Initial Earthing Assessment Flowchart ............................................................. 11Figure 6-2Earthing Design Procedure Flowchart ............................................................. 13Figure 6-3Fault Current Path for Cables .......................................................................... 14Figure 6-4Transfer Voltage ............................................................................................. 16Figure 7-1COLD Site Earthing Design ............................................................................. 19Figure 7-2HOT Site Earthing Design ............................................................................... 20Figure 7-3Compact/Micro Earthing Design ...................................................................... 21Figure 7-4Standard Design Approach for Integral and Basement Substation .................. 23Figure 7-5Typical HV Supply and Customer Substation Arrangement for a COLD Site ... 25Figure 7-6Typical HV Supply and Customer Substation Arrangement for a HOT Site ..... 26Figure 7-7Typical Earthing associated with Asset Replacement...................................... 27Figure 8-1Metallic Fence Earthing Examples .................................................................. 30Figure 8-2Metallic Door Earthing ..................................................................................... 31Figure 8-3Typical Earthing associated with Fence and Gate Replacement ..................... 32Figure 9-1LV Supply from a HOT Site using an Insulating Duct and a Remote Earth ...... 36Figure 9-2Options for Supplies to High Voltage Substations from Secondary

Substations ........................................................................................................ 36Tables

Table 7-1Standard Secondary Substation Drawings ....................................................... 18Table 8-1Earth Electrodes ............................................................................................... 29Table 8-2Bonding Conductors ......................................................................................... 29Table A-1Data Sources for Earthing Design Calculations................................................ 40Table A-2Resistances Values for Standard Secondary Substation Earthing

Arrangements .................................................................................................... 40Table A-3Touch and Step Voltages for Standard Secondary Substation Earthing

Arrangements .................................................................................................... 41Table A-4Summary of Touch Voltages Related to EPR (for chippings) for Standard

Substation Arrangements based on the Limits in Table A-5 ............................... 41Table A-5Maximum Acceptable Touch Voltages (based on ENA TS 41-24 Figure 2) ...... 42Table A-6Protection Operation Time ............................................................................... 42Table B-11, 10 and 20 Earth Electrode Values ........................................................ 43


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1 Introduction

This standard (previously Section 4 of the Earthing Design Manual) details the earthingdesign requirements for secondary distribution substations. It is intended to provideguidance for UK Power Networks Designers/Planning Engineers and external connectionproviders to ensure that designs meet the requirements of new standards.

The earthing arrangements have been developed to cover common situations, supported bystandard design rules. Methods of calculation are given to allow optimal designs to beproduced. There will be some situations where standard arrangements are not suitable, andit is the responsibility of the Designer/Planning Engineer to exercise a degree of judgement,and to seek help from an earthing specialist (refer to EDS 06-0001) if the appropriateness ofa standard arrangement is in doubt.

Historically, the earthing at a secondary distribution substation may have entailed just one ortwo earth rods achieving an earth resistance value of 1 . This is no longer acceptable forthe following reasons:

The widespread use of polymeric sheathed cables. These do not contribute locally to thepotential grading (i.e. reducing the touch voltage) because of their insulated sheath. Thiswas a useful by-product of the older lead sheathed, steel wire armoured cables thathelped increase local safety.

It has been demonstrated that the previous assumptions that it was safe to bond the HVand LV earths when a 1 combined earth resistance was achieved, and that asubstation was SAFE if the EPR did not exceed 430V are no longer valid.

Following the publication of BS EN 50522 earthing design now requires greaterconsideration than before and the following changes are included in this standard:

Additional precautions (e.g. a ring electrode around the site and plant) are generallyrequired to achieve a SAFE site with acceptable touch and step voltages and a COLDsite allows the HV and LV earths to be combined wherever practicable.

The required value of earth resistance to achieve a SAFE and a COLD site should becalculated for each site and may be higher or lower than 1.

Compact or micro (pad-mounted) substations require a ring electrode buried aroundthem.

Metallic fencing, if connected to the HV earth, requires a buried grading electrode aroundthe site.

It is necessary in some cases to install bare earth wire alongside buried polymericsheathed cables to achieve the required earth resistance.

The remote earth contribution provided by polymeric type cables may help in reducingthe overall earth resistance.

Arrangements based upon earth rods are no longer acceptable unless they achieve avery low earth resistance and/or have additional standard safety features such as havingthe floor rebar bonded.

The contribution of vertical steel piles can be included as a supplement to the mainsystem, provided they are bonded via a welded connection.

Earthing arrangements have been developed and are presented in this document to cover

each of the standard substation designs and are included on the civil substation constructiondrawings. Conductor sizing is recommended based on a common approach to all threenetwork areas.


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2 Scope

This standard applies to all new secondary substations and existing secondary substationswhere a material alteration is to take place, e.g. switchgear replacement, fencingreplacement etc.

This standard is effective from 1st May. All internal quotations and external designsubmissions should comply with the updated standard from this date but olderdesigns may be accepted up until 1st July. All designs submitted after 1st July shallcomply with this standard.

The earthing design for pole-mounted equipment, LV networks (including LV overheadnetworks) and customer installations are covered respectively in EDS 06-0015,EDS 06-0016 and EDS 06-0017.

3 Abbreviations

Term Definition

CDL UK Power Networks Intranet Central Document Library

CNE Combined neutral earth (refer to Section4 for definition)

Ellipse UK Power Networks asset register

EPR Earth potential rise (refer to Section4 for definition)

IDNO Independent Distribution Network Operator

NetMap UK Power Networks graphical information system (GIS)SNE Separate neutral earth (refer to Section4 for definition)


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4 Definitions

This section defines the main terms used in this standard (refer to EDS 06-0012 for a full listof earthing terms and definitions).

Cable - Combined Neutral Earth (CNE)A CNE cable has a combined neutral and earth metallic outer sheath with a PVC coveringand is used in a PME (protective multiple earthing) LV earthing system.

Cable - Separate Neutral Earth (SNE)An SNE cable has separate neutral and earth conductors. Generally the neutral conductor isa fourth core and the earth conductor forms a protective sheath.

COLD SiteA COLD site is a grid, primary or secondary substation where the earth potential rise (EPR)is less than 430V or 650V (for high reliability protection with a fault clearance less than200ms).

HOT SiteA HOT site is a grid, primary or secondary substation where the earth potential rise (EPR) isgreater than 430V or 650V (for high reliability protection with a fault clearance less than200ms).

Earth Potential Rise (EPR)EPR is the potential (voltage) rise that occurs on any metalwork due to the current that flowsthrough the ground when an earth fault occurs on the HV or LV network. Note:Some current

will flow through the cable sheath back to the source and some will flow through the ground,it is only the current that flows through the ground that causes the earth potential rise (refertoFigure 6-3). Historically this has also been called Rise of Earth Potential (ROEP)

Step, Touch and Transfer VoltagesThe step voltage is the potential difference between a persons feet assumed to be 1mapart. The touch voltage is the potential difference between a persons hands and feetwhen standing up to 1m away from any earthed metalwork they are touching. The transfervoltage is the potential transferred by means of a conductor between an area with asignificant earth potential rise and an area with little or no earth potential rise, and results ina potential difference between the conductor and earth in both locations.

Substation Earthing DatabaseThe substation earthing database contains the classification (HOT or COLD) of all grid andprimary substations together with the details of the earth potential rise (EPR) and otherrelevant earthing information. Refer to EDS 06-0002 for further details.


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5 Design Criteria

Substation earthing provides the following function:

To pass the fault current during an earth fault back to the system neutral and operate thesource protection.

To prevent dangerous potentials appearing at the substation and causing danger to staffor the public.

To prevent dangerous potentials appearing on the customers LV neutral/earth.

To comply with the requirements for substation LV earthing for PME systems.

To satisfy these requirements the following design criteria shall be used for secondarysubstation earthing design:

To operate the protection a maximum HV electrode earth resistance of 10is required.To manage both the earth resistance and the touch voltages appearing on equipment aring electrode is required enclosing and bonded to all equipment.

To prevent dangerous voltages appearing on the LV system the earth potential rise shall,as far as reasonably practicable, be limited to 430V1which will usually require a relativelylow value of earth resistance. Provided this limit is maintained the HV/LV earths can becombined; otherwise the HV/LV earths shall be segregated and the site classified as aHOT site.

Where a separate LV earth is required, a maximum LV earth resistance of 20 isrequired in accordance with ENA ER G12.

Refer to EDS 06-0012 for a more detailed explanation of earth potential rise, the voltage

limits and associated calculations.

1Provided the HV protection operates within 1 second.


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6 Design Procedure

This standard uses a three step process for earthing design:

Step 1information gathering (Section6.1).

Step 2initial design assessment process (Section6.2).

Step 3detailed design process (Section6.3).

6.1 Step 1Information Requirements

The following information is required to design a secondary substation earthing system:

Source grid/primary substation earth fault level and earth resistance value (if available).

Earth fault level at the new secondary substation.

Source substation classification (HOT/COLD) and the associated earth potential rise forHOT sites.

Details of the cable or overhead line network between the source and the new secondarysubstation including lengths, types, and the cable sheath cross-section and material(where appropriate) etc.

Distance of the secondary substation from the source substation.

Soil resistivity at secondary substation location.

Fault clearance time for an earth fault at the new substation (detailed design only).

Refer toAppendix A for UK Power Networks data sources.

Where required UK Power Networks will provide this information to enable an externalconnection provider to design a suitable secondary substation earthing system.

6.2 Step 2Design Assessment

An initial earthing design assessment for a secondary substation shall be carried out usingthe procedure detailed below. If this process does not provide a satisfactory design thedetailed design process detailed in Section6.3 shall be used.

1. If any of the special situations listed below apply to the proposed secondary substationrefer to Section9 before assessing the design. A standard earthing system may not beappropriate, in which case an earthing specialist may be required to carry out a bespoke

design.

Substations associated with a customerssubstation and/or plant and equipment.

Substation refurbishment and asset replacement/enhancement (Section9.1.1).

Supplies to/from HOT sites (see Section9.1.2 and9.1.3).

Secondary substations associated with higher voltage substations (Section9.1.4).

Substations near livestock, racehorses etc, or other high risk locations (Section9.1.5).

Supplies to mobile phone masts (Section9.1.6).

Substations located near tower lines (Section9.1.7).

Substations located near railways (Section9.1.8).

Substations located near to telephone exchanges (Section9.1.9).IDNO substations (Section9.1.10).

HV generator connections (Section9.1.11).

Customers lightning protection(Section9.1.12).


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2. Select a standard earthing arrangement for the proposed secondary substation (e.g. unit,GRP etc) from Section7.

3. Use the flowchart in Figure 6-12

to determine whether a standard design is SAFE,whether the site is HOT or COLD together with the maximum earth resistance andwhether a separate (segregated) LV earth is required.

4. The detailed design procedure in Section6.3 should be followed if:

The earth resistance is thought to be too onerous.

A HOT site has been achieved and a COLD site is required.

A HOT site has been achieved and HV/LV earth segregation is not possible due topresence of HV/LV PILC cables or the situation.

5. Determine the electrode requirements fromAppendix B and check fi achievable on site.

6. Select the earthing electrode/conductor sizes and other earthing requirements fromSection8.

7. Complete the earthing design form inAppendix C.

8. The details of any HOT secondary substation shall also be sent to UK Power NetworksAsset Management ([email protected]) so that they can berecorded in the Substation Earthing Database. BT shall also be notified of any HOTsubstation within 10m refer to Section9.1.9 further details.

2The earthing assessment flowchart is based on the assumption that for cable connected substations with a

maximum earth fault level of 1500A and a 1earth resistance the maximum current that will flow through groundis approx 25%, therefore the EPR will be less than 430V and the site will be SAFE and COLD.
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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START

Special situation?Yes

No

Overhead site?

Yes

Refer to Section 10

Located

within grid or primary

site?

COLD Site

Use combined HV/LV earth

connected to site earthYes

No

Entirely

cable fed from source

substation?

No

Grid or primary site HOT?

HOT Site

Use combined HV/LV earth

connected to site earth

(not suitable for providing

external supplies)

Source

substation HOT?

Yes

No

SAFE HOT Site

Use standard design with

segregated HV/LV earths

HV 1 ohm and LV 20 ohm

Substation

first on the feeder or within 100m

of source substation?

No

Yes Yes

SAFE COLD Site

Use standard design withcombined HV/LV earth with

a resistance of 1 ohm

Determine electrode andother earthing requirements

Standard earthing

arrangement?

No

Carry out detailed earthing

design

Yes

No

Refer to Section 7

Yes

Yes

No

Refer to

EDS 06-0015

Earth fault current


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6.3 Step 3Detailed Design (if required)

6.3.1 Overview

For the detailed design it is necessary to follow the process outlined in the flowchart inFigure 6-2 to calculate the actual EPR (Section6.3.2)and/or determine the earth resistanceto limit the EPR to less than 430V (Section6.3.2)so that a standard earthing arrangementwith combined HV/LV earths can be installed. The process will also identify whetheradditional earth electrodes are required to supplement those in standard earthingarrangements.

If the EPR is greater than 430V a standard earthing arrangement can still be used but withsegregated HV/LV earths. However in this instance it will also be necessary to calculate thetouch voltage (Section6.3.4)to determine whether it is within the acceptable limits and thesubstation is SAFE. The touch and step voltages for each standard earthing arrangementhas been calculated as a percentage of the EPR and are given for each standardarrangement inTable A-3 (Appendix A). It is possible to infer from these a maximum EPRthat is acceptable without modification. The acceptable EPR is dependent on the normalfault clearance time as given in Table A-4 and Table A-5 (Appendix A) for the standardarrangements.

An earthing design calculator (Appendix E) is available to assist with the various calculationsdetailed in Sections 6.3.2,6.3.3 and6.3.4.The earthing design calculator also includes aform to record the earthing design decisions for inclusion in the overall substation designdocumentation.

Note:If a non-standard design is required an earthing specialist shall be employed to carryout the appropriate design and calculations.


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START

Standard substation

arrangement?

No

Yes

Obtain soil resistivityObtain source substation

earth resistance and EPR

Obtain circuit details

Calculate EPR

EPR >430V?Yes

No

Required

resistance

achievable?

No

Source

substation HOT?

NoSAFE HOT Site


segregated HV/LV earths,

calculated HV resistance

and 20 ohm LV resistance

Transfer voltage > 430V?

No

Yes

Yes

SAFE COLD Site


combined HV/LV earth with

calculated HV resistance

Determine electrode and

other earthing requirements

Touch

voltage acceptable for normal

protection clearance

times?

Calculate required resistance

to limit EPR to less than 430V

Yes

Yes

Employ earthing specialist

to carry out design

Calculate touch voltage

Calculate touch voltage

Calculate EPR for achievable

resistance

Calculate transfer voltage

No

Determine secondarysubstation

earth resistance

Calculate substation earth

fault level (or use source

substation value)

Figure 6-2Earthing Design Procedure Flowchart


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6.3.2 Calculate EPR

1. Determine the source substation earth resistance (or assume 0.5 if not available).

Grid and primary substation earthing data is available from the Substation EarthingDatabase3. Refer to EDS 06-0002 for further information.

2. Estimate the substation HV electrode resistance (R) using Table A-2 (for the standardsubstation type) and the soil resistivity. This table has been produced using computersimulation of the standard earth electrode designs (given in Section 4) to generateexpected resistance values depending on the resistivity of the soil.

Soil resistivity data is available in NetMap4. Refer to EDS 06-0018 for further

information.

3. Determine the earth fault current at the secondary substation (If). If the earth fault level atthe secondary substation is not available the earth fault level at the source substation willprovide a good (but pessimistic) approximation.

Refer to EDS 08-0134 for further Information on obtaining earth fault levels.

4. Calculate the percentage (%Igr) of fault current that will flow through the ground.

For overhead supplied sites this is 100%.For entirely cable supplied sites, a ground return current of 25% of the total earthfault current can be assumed as a first estimate.

Note:It is likely that there are several types of cable between the secondary substationand the source substation. The initial calculation should be based on the smallest sizecable. Modelling each cable separately will provide a more accurate value and a lowervalue of EPR but will need to be calculated by an earthing specialist if it is required.

If11kV

Igr11kV

F33kV

Majority of fault current returns

through cable sheath (Isheath11kV)

but a small percentage (Igr11kV)

returns through the ground to the

source (primary) substation

Isheath11kV

Primary 33/11kVSubstation

EPR11kV= Igr11kV x Rsecsub

RPrimSub

If11kV

Secondary 11kV/415V Substation

RSecSub

If11kV

Figure 6-3Fault Current Path for Cables

3UK Power Networks maintains a Substation Earthing Database which is available from the UK Power Networks

Intranet (Applications > Reporting Centre > Substation Earthing Database).4UK Power Networks GIS system.


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5. Use this percentage (%Igr) to calculate the value of the ground return current Igr:

Igr= %Igr If

6. Use the calculated value of Igrand the estimated value of RSecSubto calculate the EPR forthe site:

EPR = Igr RSecSub

7. Assess the calculated EPR value. If the EPR calculated is less than 430V the site can beassumed COLD and combined HV/LV earthing can be installed provided that themeasured value of HV electrode resistance is less than the value calculated above.

8. If the EPR is greater than 430V then further calculation is required to determine therequired resistance needed to limit the EPR under fault conditions to 430V or less. The

value of RSecSubcan be calculated using the equation below. However as the value ofresistance will change the proportion of current returning through earth it will benecessary to repeat the steps above to calculate the final EPR. Note: It may benecessary to repeat this several times to determine actual values of resistance and EPR.

RSecSub= EPR Igr

9. Using the results of the calculations above determine the additional earth electroderequirements usingTable B-1 (Appendix B).

10. If it is not considered realistic to achieve a low enough earth resistance to limit the EPRto 430V the design shall be based on reasonably practical installation with a low

resistance to minimise the EPR. The site shall be classified as HOT and segregated HVand LV earthing installed. It is also necessary to calculate the touch voltage as detailedSection 6.3.4. Note: Further design work may be required to achieve a COLD site ifHV/LV earth segregation is not possible due to presence of HV/LV PILC cables or thesituation.

11. If the source substation is HOT it is necessary to calculate the transfer voltage asdetailed in Section6.3.3.

12. The details of any HOT secondary substation shall also be sent to UK Power NetworksAsset Management ([email protected] ) so that they can berecorded in the Substation Earthing Database. BT shall also be notified of any HOTsubstation within 10m refer to Section9.1.9 further details.

13. Determine the electrode requirements fromAppendix B and check if achievable on site.

14. Select the earthing electrode/conductor sizes and other earthing requirements fromSection8.

15. Complete the earthing design form inAppendix C.


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6.3.3 Calculate Transfer Voltage

If the secondary substation is entirely cable fed from a HOT source substation (i.e. the EPR

at the source substation is greater than 430V) it is necessary to calculate the transfer voltagefrom the source substation as illustrated inFigure 6-4. Previously, the rule of thumb wasthat the first substation out from a HOT site should be treated as HOT however thismethodology is not valid in all cases.

Note: The transfer potential calculation is not necessary if an overhead line section isincluded in the circuit.

If the new substation is not the first substation on the circuit it is not valid to assume that thetransfer potential from the source will be of no significance. However, accurate calculation iscomplex, therefore it is sufficient to simply disregard intermediate substations and to use thetotal circuit length.

RSecSubEPRSourceSub

Primary 33/11kV

Substation

Secondary 11kV/415V

SubstationRCircuit

VTransfer

Figure 6-4Transfer Voltage

1. Calculate the transfer voltage using the following formula.

SecSubCircuit

SecSub

imSubPrTransferZZ

ZEPRV

2. If the transfer voltage is greater than 430V the secondary substation shall be classifiedas HOT and segregated HV and LV earthing installed.

3. Update the earthing design form (Appendix C).

4. The details of any HOT secondary substation shall also be sent to UK Power NetworksAsset Management ([email protected] ) so that they can berecorded in the Substation Earthing Database. BT shall also be notified of any HOTsubstation within 10m refer to Section9.1.9 further details.


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6.3.4 Determine Touch Voltage

If the EPR is greater than 430V it is also necessary to assess the touch voltage for the

proposed design. Touch and step voltages for each of the standard earthing arrangementsexpressed as a percentage of the EPR are given inTable A-4 (Appendix A)which gives thetouch voltage percentages for the standard earthing designs. Furthermore a set ofacceptable touch and step voltage limits, based on fault clearance times and substationsurface type, are given in Table A-5 (Appendix A). A comparison of the calculated touchvoltage against the acceptable limits can then be completed without further calculation.

Note:If the design does not use a standard arrangement then the earthing electrode systemshall be modelled by an earthing specialist to determine the touch and step voltage.

1. To calculate the touch voltage and determine whether it is acceptable the followinginformation is required:

Proposed earthing arrangement for the secondary substation.

Substation surface type.

Calculated value of EPR.

Fault clearance time.

What fault clearance time to use?

The fault clearance time is the sum of the protection relay (source or upstream) and thecircuit-breaker operating times. A value of 1s can be used for 11kV circuits but is likely tobe pessimistic and provide onerous touch voltage limits.

Alternatively the actual protection clearance time can be calculated using Table A-6(Appendix A)and the circuit-breaker operating time of either 100ms (oil) or 50ms (SF6orvacuum).

2. Calculate the touch voltage using the following formula:

VT= EPR %

where % is the percentage of the EPR where the maximum touch voltage can occurobtained fromTable A-4 (Appendix A).

3. Check whether the touch voltage is greater than the acceptable limits in Table A-5(Appendix A). If the touch voltage exceeds the limits further work is required to reducethe substation resistance and hence the EPR or an earthing specialist should beemployed to carry out a bespoke design.

4. Update the earthing design form (Appendix C).

5. A design cannot be accepted or approved if the touch and step voltages are outside thelimits.


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7 Standard Secondary Substation Earthing Arrangements

This section details the earthing arrangements for the standard secondary substationdesigns. The arrangements include HV and LV (if required) earthing for the following typesof ground-mounted substation:

New COLD site design for GRP, brick-built and outdoor substations (Section7.1.1).

New HOT site design for GRP brick-built and outdoor substations (Section7.1.2).

Compact or micro pad-mounted substations without an enclosure (Section7.1.3).

Integral and basement substations (Section7.1.4).

Customer substations (Sections7.1.5).

Existing outdoor substations (Section7.1.6).

These earthing arrangements have been incorporated into the standard substation design

drawings contained in EDS 07-0102. A full list of the available drawings is given inTable 7-1.

Table 7-1Standard Secondary Substation Drawings

Description Drawing No

GRP Unit/Package Substation with Standard Plinth EDS 07-0102.01

GRP Unit/Package Substation with Fully-bunded Plinth EDS 07-0102.02

GRP Micro Substation with Standard Plinth EDS 07-0102.03

GRP Compact Substation with Standard Plinth EDS 07-0102.04

GRP Metering Substation with Standard Plinth EDS 07-0102.16

Freestanding Brick-built (3.6m x 3.6m) Unit/Package Substation EDS 07-0102.18

Freestanding Brick-built Substation for a Single Transformer EDS 07-0102.05

Freestanding Brick-built Substation for a Single Transformer with LV ACB/LVBoard

EDS 07-0102.06

Integral Substation for a Single Transformer EDS 07-0102.07

Integral Substation for a Single Transformer with ACB and LV Board EDS 07-0102.08

Micro Substation EDS 07-0102.10

Compact Substation EDS 07-0102.11

Fenced Outdoor Substation with Micro and Ring Main Unit EDS 07-0102.125

Fenced Outdoor Substation with Micro and Extensible Switchgear EDS 07-0102.135

Connection Techniques EDS 07-0102.25

The electrode resistance of the standard arrangements in a range of uniform soil conditionscan be found in Table A-2 (Appendix A). Note: The resistance value of the standalonesubstation electrode is unlikely to be low enough to give a COLD site. In most casesadditional electrodes will be required to provide a lower value of earth resistance.

Plots showing the touch and step voltages across the sites with standard earthing installedare included in Appendix G.

5Only for use in Areas of Natural Outstanding Beauty (ANOB).


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7.1.1 GRP and Brick-Built Substations (COLD Site)

The general earthing arrangement for GRP, brick-built and outdoor6 substations with a

combined HV/LV earth (COLD site) is shown below. Refer to EDS 07-0102.01-04,16 (GRP),EDS 07-0102.05-06,18 (brick-built) and EDS 07-0102.12-13 (outdoor) for specific designs.

Fault Level Bare Copper Conductor Bare Copper Tape

Up to 8kA

Up to 12kA

Up to 15kA

70mm2

120mm2(or 2 x 70mm

2)

120mm2(or 2 x 70mm

2)

25mm x 3mm

25mm x 4mm

25mm x 6mm

3 - HV electrode around the outer edge of foundation buried at a depth of 500-600mm

1 - 2.4m earth rods at rear corners

6 - HV electrode underneath the foundations (or in the cable trench) required for larger brick-built

substations

4 - HV electrode connecting each side of outer loop to switchgear/transformer earth terminal

5 - HV electrode connecting front of outer loop to switchgear/transformer earth terminal passing underwhere an operator stands when using the HV switchgear

2 - Alternative internal 2.4m earth rods in place of external ones for brick-built substations

Additional HV Earth

Electrode/Rods

(as required to achieve

earth resistance)

HV Earth Electrode

7 - Connection to reinforcement rebar/mesh

RMU

Transformer

Note:Not all equipment

bonding is shown

Main Earth Terminal

8 - Neutral/Earth link in place

8

1

4

1

34

7

56

22

COLD Site

Neutral/Earth Link In

To HVEarth

To LV Cable

Neutral

LV CNE Cable

LV

Figure 7-1COLD Site Earthing Design

6Outdoor secondary substations are not generally used for new build, however they may be used in specific

situations, e.g. Areas of Outstanding Natural Beauty, when GRP and brick-built designs are not suitable.


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7.1.2 GRP and Brick-Built Substations (HOT Site)

The general earthing arrangement for GRP, brick-built and outdoor7 substations with a

separate HV/LV earth (HOT site) is shown below. Refer to EDS 07-0102.01-04,16 (GRP),EDS 07-0102.05-06,18 (brick-built) and EDS 07-0102.12-13 (outdoor) for specific designs.

LV CNE Cable

LV earth connection - 70mm2PVC

covered stranded copper conductor

HV/LV Separation

(8m minimum)

LV Earth Connection (Insulated)

LV Earth Electrode/Rods

(max resistance 20)


Up to 8kA

Up to 12kA

Up to 15kA

70mm2

120mm2(or 2 x 70mm

2)

120mm2(or 2 x 70mm

2)

25mm x 3mm

25mm x 4mm

25mm x 6mm


1 - 2.4m earth rods at rear corners

6 - HV electrode underneath the foundations (or in the cable trench) required for larger brick-built

substations


5 - HV electrode connecting front of outer loop to switchgear/transformer earth terminal passing under

where an operator stands when using the HV switchgear

2 - Alternative internal 2.4m earth rods in place of external ones for brick-built substations

Additional HV Earth

Electrode/Rods

(as required to achieve

earth resistance)

LV earth electrode - 70mm2bare

stranded copper conductor

HV Earth Electrode LV Earth


RMU

Transformer

9 - Warning labels

Note:Not all equipment

bonding is shown

Earth Terminal

8 - Neutral/Earth link removed

8

1

4

1

9

93

4

7

56

22

HOT Site

Neutral/Earth Link Out

To HVEarth

To LV

Earth

To LV Cable

NeutralLV

Figure 7-2HOT Site Earthing Design

7Outdoor secondary substations are not generally used for new build; however they may be used in specific

situations, e.g. Areas of Outstanding Natural Beauty, when GRP and brick-built designs are not suitable.


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7.1.3 Compact (including Micro and Padmount) Substations

The earthing arrangement for a compact/micro substation without an enclosure is shown

below. These are generally installed on the overhead network and therefore the HV and LVearths shall be segregated as shown. However if the compact/micro substation is suppliedfrom the source by continuous cable and the site is shown by calculation to be COLD the HVand LV earths may be combined.

Refer to EDS 07-0102.10, 11 and 15 for specific designs. Note:If the compact substation isinstalled in a GRP enclosure the standard arrangements shown in Sections7.1.1 and7.1.2shall be used.

Additional HV Earth

Electrode/Rods

(as specified )

LV CNE Cable

HV/LV Separation

(8m minimum)

LV Earth Connection

1 - 2.4m earth rods at rear corners 500mm behind plinth

LV

Neutral

HV

Earth

To LV Earth

Electrode

To LV Cable

Neutral and Sheath

To HV Earth

Electrode

3 - HV electrode in a ring around the substation, extending 500mm on all sides, buried at a depth of 500mm and

connected to the earth rods

2 - 2.4m earth rods at front corners 500mm in front of the plinth

1 1

3

4

LV Earth Electrode/Rods

(max resistance 20)

HOT Site

Neutral/Earth Link Out

2 2


LV earth connection - 70mm2PVC

covered stranded copper conductor


Up to 8kA

Up to 12kA

Up to 15kA

70mm2

120mm2(or 2 x 70mm2)

120mm2(or 2 x 70mm2)

25mm x 3mm

25mm x 4mm

25mm x 6mm

LV earth electrode - 70mm2bare

stranded copper conductor

HV Earth Electrode LV Earth

8

8

5

5 - HV electrode in two places between the earthing ring and the earth terminal, passing directly underneath the

positions where an operator is required to stand to open the front cover and carry out operations

5

8 - Warning labels6 - Neutral/Earth link removed

6

6

Additional HV Earth

Electrode/Rods

(as specified )

LV

Neutral

HV

Earth

To HV Earth

Electrode

1 1

3

4

COLD Site

Neutral/Earth Link In

2 2

5 5

6

7

7 - Neutral/Earth link in place

HOT Site

COLD Site

Figure 7-3Compact/Micro Earthing Design


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7.1.4 Integral and Basement Substations

The standard arrangements shown Section7.1.1 and7.1.2 should be used where possible.

However where the substation is situated within an existing or even a new property, it isoften impracticable to install one of the standard arrangements but a standard approach isnecessary. This will use earth rods installed through the floor or side walls, externalelectrodes and vertical piles and the rebar in the floor slab to control touch voltages.

It is not usually possible to segregate the HV and LV earths, so it is important to achieve aCOLD site (EPR less than 430V) so that they can be combined. If a COLD site is notpossible or if the building or its electrical supply will interact with Network Rail, LondonUnderground or other electrified travel infrastructure, a bespoke design is necessary,involving an earthing specialist.

The earthing design should include the following elements which are illustrated in Figure

7-4 (a):

Install 2 to 4 vertical earth rods through the substation floor (Figure 7-4 (c)) or thebasement (Figure 7-4 (c)), in each case directly into natural soil, to achieve a sufficientlylow earth resistance for a low EPR and a COLD site.

Bond the reinforcing mesh (rebar) in the concrete floor slab of the substation, or install amesh metal floor or a thin concrete layer with embedded mesh, to control the touchvoltages around UK Power Networks equipment.

The following options, where practical, may be used to supplement the above:

Install at least 20m (ideally 50m) of bare copper electrode at a depth of approximately500mm along the cable route under the HV cable, direct into natural soil.

Install bare copper electrode in the soil at a depth of approximately 500mm, adjacent orup to 1m away from the outer walls of as many sides of the UK Power Networks part ofthe building as possible. Wherever practicable, this shall include the wall adjacent to theHV switchgear.

Incorporate the steel in vertical piles near the substation into the designthis must be ofwelded type and only the electrically continuous length should be included in thecalculation.

The standard approach outlined above should cover the majority of integral and basementsubstations; however advice from an earthing specialist should be sought at an early stage

for more complex installations.


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1a

RMU

Transformer

LV

1a - Main earth rods on outer wall

2 - Bond to reinforcing mesh (rebar) in concrete floor slab or thin concrete layer with embedded mesh

1b

1b 1a

2

Main Earth Terminal

3

3 - Main earth bar

1c

1b - Optional additional earth rods to help achieve overall low resistance

2

4

4 - Wall- mounted earth bar above floor level/below door tread to aid connections

6 - Door bonding

5

5 - Main equipment bond (not all equipment bonding shown)

1c - Optional bare earth electrode laid with incoming HV cables to help achieve overall low resistance

(a) Overall Earthing Arrangement

Substation

Basement

(b) Earth Rod Installation into Soil

Substation

(c) Earth Rod Installation through

Basement into Soil

Figure 7-4Standard Design Approach for Integral and Basement Substation


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7.1.5 Customer HV Supplies and Associated Substations

The earthing system for an HV supply and any associated substation will consist of parts

provided by the customer and parts provided by UK Power Networks. The objective is todesign an earthing system that satisfies the safety requirements with an acceptable degreeof redundancy and, wherever possible, a COLD site classification (EPR less than 430V).

Therefore the customer shall provide an HV earthing system for the installation, irrespectiveof the earthing provided by UK Power Networks. The earthing system should normallyconsist of copper earth electrodes (tapes and rods) and steel reinforcement piles or rebar inthe vicinity of the substation. In the majority of cases the earthing systems can beinterconnected, especially when the resulting earth resistance is low enough to achieve aCOLD site. In this case it may also be possible to use the same earthing system to providethe LV earth. This is illustrated in Figure 7-5.The aim of the design is to ensure that UKPower Networks and customer earthing systems shall each be adequate to ensure safety in

the absence of the other system. The customer system shall not be reliant on UK PowerNetworks earth terminal for safety since the integrity of either system can be subject toexternal influences.

The situation is more complex if the HV earthing system is classified as HOT in which caseone of the following design options shall be used:

Extend the HV earth or reduce the earth fault current to achieve a COLD site, if this ispossible at a reasonable cost. One option for substations on new networks (such as inLondon) is to interconnect the earthing with existing 11kV sites that have metallicsheathed cables or connect onto abandoned sheathed cables.

Interconnect the HV and LV earths and operate as a HOT site with the necessarymeasures in place. This is only really practical at an isolated location such as astandalone factory or office, a wind or solar farm, generating station or National Gridsite.

Segregate the HV and LV earthing systems at all points by a minimum of 8m and ensurethat they cannot be interconnected. Precautions will also be required to ensure that aperson cannot contact both earth systems simultaneously (refer toFigure 7-6).

Segregate the UK Power Networks HV earth from both of the customer earths. This isdifficult to achieve, is not a desirable solution and generally requires a special design.

Options to achieve it include introduction of a span of unearthed overhead line or cablesheath insulation joints between the site and the UK Power Networks system. Howeverthe working practices (such as isolation and earthing for work on the HV system) needcareful consideration in this situation.

Note:If the site is HOT the transfer voltage requires special consideration especially if thereare metallic boundary fences or metallic buildings in the vicinity.


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1

2

6

63

3

4 MU

Additional

electrode/rods

(if required )

1

1 1

2 2

1

3

3 3

3

4

UK Power Networks

Substation

MU

Customer

Substation

Additionalelectrode/rods

(if required )

1

UK Power Networks

Substation

COLD Site

COLD Site

LV

1 - 2.4 m earth rods at 2 corners of substation (alternatively they can be installed internally)

4 - Additional HV electrodes to control touch/step voltages if required


3 - HV electrode connecting outer loop to switchgear/transformer earth terminal

6 - Interconnection via a link between UK Power Networks and Customer substations

7 - Neutral-earth link in place

Note:

Equipment

bonding not

shown

Note:

Equipment

bonding not

shown


1

2

1

3

3

4

Additional

electrode/rods

(if required )

Customer

Substation

HV

7

Customer LV SNE Cables

LVHV

7


4

9 - Warning labels

8 - HV cable screen insulated from earth

9

8

5

TX

5

TX

5

RMU

5

RMU

Figure 7-5Typical HV Supply and Customer Substation Arrangement for a COLD Site


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1 - 2.4 m earth rods at 2 corners of substation (alternatively they can be installed internally)

4 - Additional HV electrodes to control touch/step voltages if required


1 1

2

6

6

2

1

3 - HV electrode connecting outer loop to switchgear/transformer earth terminal

3

3 3

3

4 4

6 - Interconnection via a link between UK Power Networks and Customer substations

RMU MU

Additional

electrode/rods

(if required )

1

1 1

2 2

1

3

3 3

3

4 4

UK Power Networks

Substation

MU

Customer

Substation

Additional

electrode/rods

(if required )

1

UK Power Networks

Substation

Customer

SubstationHOT Site

HOT Site

HV/LV

Separation

(8m minimum)

LV Earth Cable (Insulated)

LV Earth Cable (Insulated)

8LV

HV

7

Note:

Equipment

bonding not

shown

Note:

Equipment

bonding not

shown



LV

HV

7


HV/LV

Separation

(8m minimum)

9

7 - Neutral-earth link removed

9 - Warning labels

8 - HV cable screen insulated from earth

5

TX

5

TX

5

RMU

5

Figure 7-6Typical HV Supply and Customer Substation Arrangement for a HOT Site


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7.1.6 Existing Outdoor Substations

Outdoor secondary substations are no longer constructed except in Areas of Outstanding

Natural Beauty. For these a close boarded fence is required refer to EDS 07-0102.12-13for specific designs.

Earthing needs to be considered during asset replacement work at existing outdoor sites.The installation of a complete earthing system based on a standard arrangement is rarelypractical. However the opportunity to enhance the earthing should not be overlooked. Theearthing installed should seek to achieve as much of the following as possible, using theexcavations that are necessary for the remedial work:

Buried bare electrode around the equipment at a depth of around 0.5m and connected tothe main earth bar. Note:It is especially important to ensure that there is bare electrodeunder the operators standing position especially if metallic sheathed cables are

replaced with plastic cables (even short lengths) during a switchgear change.One or two substantial earth rods connected to the buried earth electrode or the mainearth bar.

Bonding of all equipment to the main earth bar.

If a metallic fence and/or gates are present the requirements of Section8.3.2 shall alsobe applied.

Some typical examples are shown in Figure 7-7. If just the LV pillar is being replaced theinstallation of a buried electrode system is unlikely to be practical but the pillar shall bebonded to the main earth bar. The more equipment alterations and associated excavationsthat are taking place, the more the earthing can be improved until a stage is reached where

it is close to one of the standard arrangements shown in the previous sections.

HV

Switchgear

Transformer

LVPillar

3 - Buried bare copper earth electrode in front of the switchgear

where an operator stands when using the switchgear

1 - 1.2 m earth rods connected to the buried electrode

2 - Buried bare copper earth electrode ring around and the

switchgear and any other equipment

1

2


Up to 8kA

Up to 12kA

Up to 15kA

70mm2

120mm2(or 2 x 70mm

2)

120mm2(or 2 x 70mm

2)

25mm x 3mm

25mm x 4mm

25mm x 6mm

HV Earth Electrode

3 RMU

LVPillar

1

2

3

Transformer

1

2RMU

3

Transformer

LV

1

1

1

(a) Switchgear Replacement Only(b) Switchgear and Transformer

Replacement

(c) Switchgear, Transformer and LV

Replacement

4 - Connection to equipment earth terminal

Figure 7-7Typical Earthing associated with Asset Replacement


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8 Earthing Requirements

This section details the general earthing requirements for all new and modified earthinginstallations.

If combined HV/LV earthing is installed then the requirements in Sections8.1 to8.5 shallapply. The HV and LV earthing should be bonded together as described in Section8.5.

If it has been determined that segregated HV and LV earthing is to be installed then therequirements of Section8.6 shall also apply in addition to section8.1 to8.5.

8.1 General

The theft of copper earthing continues to be a significant national problem. Therefore theearthing system shall be designed to ensure that it is secure and not vulnerable to theft. To

aid this aluminium conductor or tape shall be used for all above ground earthing whereverappropriate and practicable.

All earth connections shall be connected to the main transformer/switchgear earth terminalor a dedicated earth bar and not the HV earth bar within the LV cabinet/pillar for thefollowing reasons:

To enable UK Power Networks to easily determine if the earthing is intact when enteringthe substation as access to the LV cabinet/pillar is not always available.

To enable the earth resistance to be correctly measured in the future using a clamp-on-type meter.

8.2 Electrode System

The earth electrode system shall provide the basic functional earthing for the site so that it isSAFE without any contribution from the network to which it is to be connected. The earthelectrode system shall therefore consist of the following:

Bare copper earth electrodes using the minimum sizes specified inTable 8-1.

A ring of earth electrode buried around the perimeter of the substation (or alternativelyburied around the inside perimeter of the substation) at a depth of 500-600mm.

A minimum of two earth rods installed on two corners of the substation (or alternativelyinternally) and connected to the outer ring.

Two connections from the outer ring onto the main transformer/switchgear earth terminalor dedicated substation earth bar.

An earth electrode passing underneath any switchgear or LV operating position andconnected to the outer electrode. This may be omitted if it can be shown that rebar (orequivalent) is providing this function.

Connections to the rebar or reinforcement mesh. Note:The rebar shall not extend whereit might be within 2m of LV metalwork or other earthed metalwork if the substation isHOT.

Additional electrode and rods, as necessary, to enable the required earth resistance tobe achieved.


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Table 8-1Earth Electrodes

Function Fault Level Bare Copper Stranded Cable Bare Copper Tape

HV Earth Electrode Up to 8kA 70mm2 25mm x 3mm

Up to 12kA 120mm2(or 2 x 70mm

2) 25mm x 4mm


2) 25mm x 6mm

Earth Rod Electrode Any 1m or 1.2m 16mm2copper clad rods

8.3 Bonding

8.3.1 Equipment

All current carrying items of equipment including the HV switchgear, LV pillar/cabinet/boardand LV ACB shall be bonded to the transformer (or switchgear) earth terminal using anindependent connection. The minimum size of the bonding conductors is detailed inTable8-2.

Table 8-2Bonding Conductors

Function Fault Level PVC Covered Stranded Cable Tape

Any Up to 8kA 70mm2Copper 25mm x 3mm Copper


2) Copper 25mm x 4mm Copper


2) Copper 25mm x 6mm Copper

Above GroundBonding

Up to 8kA 120mm2Aluminium 25mm x 6mm Aluminium

Up to 15kA 240mm2Aluminium 40mm x 6mm Aluminium

All other non-current carrying items of equipment (e.g. control units, RTUs, battery chargersetc.) shall be bonded to the main earth terminal using a minimum of 35mm2PVC coveredaluminium cable, 16mm2PVC covered stranded copper cable or equivalent8.

8.3.2 Metallic Fences, Gates and Doors

The general rule for metallic substation fences, gates and doors is that they shall be bonded

to the HV earth at all times, unless:

The fence, gate or door is situated more than 2m from any item of equipment or otherearthed metalwork bonded to the HV earth, or

A barrier exists sufficient to prevent simultaneous contact between the fence/gate/doorand the other earthed metalwork.

The site is HOT, in which case the fences, gates and doors can be a shock risk to thoseoutside the substation. Grading electrodes or independent earthing is required in suchsituations as described below. Alternatively the fence could be replaced with a non-metallic one.

Note:Due to the small size of most secondary substations, metallic fences, gates and doors

will nearly always be within 2m of the equipment and this approach should be adopted inmost cases.

8Minimum conductor sizes taken from BS EN 50522 while ENA TS 41-24 is being revised.


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If a fence, gate or door is not bonded to the HV earth for the reasons described above, itshall be provided with an electrode system sufficient to eliminate stray voltages. This shouldconsist of one or more rod electrodes, adjacent to the fence, gate or door.

8.3.2.1 Metallic Fences

If a metallic fence is installed within 2mof accessible earthed equipment (whether the site isHOT or otherwise):

It shall be connected to the HV earth; and

A grading electrode of 70mm2bare copper cable or 25mm x 4mm bare copper tape shallas a minimumbe installed under the fence line, or just inside (or outside), ideally at adepth of 500mm (300mm minimum) and connected to the fence; this is to protect staffand the public from dangerous touch potentials. Ideally, and if practicable the gradingelectrode should be installed outside the fence at a distance of 300-500mm away fromthe fence (in some situations the designer will specify an outside grading electrode, inwhich case an electrode underneath or inside the fence line is unacceptable).

Each metallic gate shall be bonded to the gatepost using flexible 16mm2PVC coveredstranded copper cable or tinned copper braid.

Each pair of gateposts shall be bonded together using flexible 16mm 2 PVC coveredcopper cable.

Gate

Gate post bonding - 35mm2aluminium or 16mm2copper PVC covered stranded cable

Fence grading buried electrode - 70mm2bare stranded copper conductor or 25mm2x

4mm bare copper tape

Gate

Fence within 2m of Equipment Fence more than 2m of away Equipment

To HV

Earth

To HV

Earth

1m fence earth rods

Gate bonding - 35mm2aluminium or 16mm2copper PVC covered stranded cable or

16mm2tinned copper braid

Figure 8-1Metallic Fence Earthing Examples

If metallic fencing is installed more than 2m away from the equipment or other earthed

metalwork, or the metalwork is completely contained in GRP or brick enclosure (and


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therefore not accessible9), the fencing shall notbe connected to the HV earth. Instead asingle earth rod shall be installed at each corner fence/gate post and connected to the fenceto eliminate stray voltages. At larger sites additional earth rods shall be installed at 5m

intervals, 1m either side of any overhead line crossing and connected to the fence.

Note:Where the substation fence is bonded to the HV earth no other metallic fencing orconducting material shall be abutted to the fence or within 2 metres of it. Otherwise this willexport the EPR from the site to a point which may be far remote from the substation where itwill be impossible to protect against dangerous touch potentials. Insulating fence panels orstand-off insulators can be used to achieve this requirement. Metallic third party fencesshould not be within simultaneous touching distance (2m) of metalwork/fences connected tothe HV earth. If necessary a floating (isolated) section of fence should be introducedoutside the substation boundary. If this is not practicable then specialist advice should besought.

8.3.2.2 Metallic Doors

If metallic doors are installed within 2mof equipment or other earthed metalwork:

A loop of 70mm2 bare copper cable or 25mm x 3mm bare copper tape shall as aminimum be installed directly under the door at a depth of 300mm to 500mm. Ifpracticable it should be outside the door, 1m from the door front and 500mm beyondeach door frame as shown below. Each end of the loop shall be connected to theexisting HV electrode using bare copper conductor. The complete loop shall be coveredwith a 100mm thickness of concrete to provide protection against damage or theft.

Alternatively, a steel or copper mesh may be installed in concrete at a depth of 200mm

to 300mm, covering the same area as above.Each metallic door shall be bonded to the framework using flexible 16mm2PVC coveredstranded copper cable or tinned copper braid.

The door framework shall be bonded to the HV earth using 35mm2 PVC coveredaluminium cable or 16mm2PVC covered copper cable.

HV earth electrode

16mm

2

PVC covered stranded copper cable or tinnedcopper braid

1m

To Main

Earth

To Main

Earth

500mm

Metallic

Doors

Loop of 70mm2bare copper conductor or 25mm x

4mm bare copper tape minimum buried 300-500mm

below ground level

35mm2PVC covered stranded aluminium cable

or16mm2PVC covered stranded copper cable

Figure 8-2Metallic Door Earthing

If metallic doors are installed more than 2maway from the equipment they shall not beconnected to the HV earth. Instead a 1m driven earth rod shall be installed at each doorhinge post and connected to the post to eliminate stray voltages.

9 In assessing this requirement, thought should be given to metalwork that is accessible when the substation

doors are open; if a 2m separation cannot be maintained on the door side of the GRP/b rick enclosure then it willbe necessary to bond the fence to HV steelwork to prevent dangerous hand-hand touch voltages.


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8.3.2.3 Fence, Gate and Door Replacement

The requirements of the previous sections also apply to fence, gate and door replacement;

however Figure 8-3 provides a more pragmatic approach that is more suited to replacementat existing substations.

Care should be exercised when replacing wooden fencing with a metallic type (e.g.Pallisade, Expamet, 358 etc) since its bonding requirements are more onerous, and it isunlikely that a fence earthing system will exist. It is notsufficient simply to replace woodenpanelling with metallic, nor is it sufficient to merely bond metallic fence panels togetherabove ground without a buried electrode system.

Note:Metallic fences even if painted or powder coated shall be considered as bare metalunless covered in an approved insulated coating that will not degrade over time.

Gate

3 - Bond between gate and gate posts - 35mm2aluminium or 16mm2copper PVC covered stranded

cable or 16mm2tinned copper braid

1 - Earth grading electrode either directly underneath the fence line (or just inside fence), ideally at a

depth of 500mm (300mm minimum) below ground level (not the level of any shingle) and connected

to each fence post - 70mm2bare stranded copper conductor or 25mm2x 4mm bare copper tape

2 - Bond between gate posts - 35mm2aluminium or 16mm2copper PVC covered stranded cable

4 - Connection to the HV earth or main earth terminal in two places

5 - Ensure that no other metallic fencing or conducting material is within 2m of the substation fence

2

3

3

4

4

5

1

Figure 8-3Typical Earthing associated with Fence and Gate Replacement

8.3.3 Ancillary Metalwork

All other exposed and normally un-energised metalwork inside the substation perimeter (e.g.ventilation ducts, staircases etc) within 2m of other earthed metalwork shall be bonded to themain earth using 35mm2PVC covered aluminium cable, 16mm2PVC covered copper cableor equivalent to avoid any potential differences between different items of metalwork 10.

Note: Metal frames and other metallic parts that form part of a GRP enclosure may beexcluded.

10Minimum conductor sizes taken from BS EN 50522 while ENA TS 41-24 is being revised.


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8.3.4 Ducting and Ventilation Shafts

Metallic ducts and ventilation shafts passing through indoor secondary substations provide

an electrical path between the inside and outside of the substation. If they are bonded to theHV earth, they could transfer voltage outside the substation zone and may pose a risk to thegeneral public. Generally it is impractical to install measures to control touch and stepvoltages where these vents emerge.

Therefore one of the following approaches, in order of preference11, shall be taken tominimise risk to the public:

Bond the ducts and ventilation shafts to the HV earth (unless the site is HOT), and installthem such that they are out of reach where they emerge from the substation. To achievethis they shall be higher than 3m above ground or other foothold.

Leave the ducts and ventilation shafts un-bonded, and install them such that there is no

possibility of other metalwork (e.g. opening doors) making contact with the ducts or ventsand no possibility of a simultaneous touch contact between the ducts and the HVequipment that is normally operated. As a further precaution a warning label can beinstalled.

Use insulated ducts.

8.4 Cables

All HV cable earth screens shall be bonded to the transformer or switchgear earth terminal.

All LV cables shall be bonded as follows:

CNE cables- the outer sheath of the cable shall be connected to the neutral bar in theLV pillar/cabinet in accordance with Section 4 of the LV Cable Jointing manual.

SNE cables- the outer sheath and armouring shall be bonded together and connectedto the neutral bar in the LV pillar/cabinet/board. The neutral conductor shall beconnected to the neutral bar in the LV pillar/cabinet/board in accordance with Section 4of the LV Cable Jointing manual.

8.5 Combined HV/LV Earths

At COLD sites an LV electrode is not required. The LV neutral/earth link in the LV cabinet,pillar or board, that bonds the LV neutral/earth to the substation HV earth, shall be in place

so that the HV and LV earths are combined.

11

The risk to the public can be reduced by leaving the ducts and ventilation shafts un-bonded. However this mayintroduce a touch potential risk to staff inside the substation since the ducts and vents may act as a remote earthand will therefore be at a different potential to HV earth during fault conditions; the risk is the occurrence of an HVfault while staff are on site and bridging a gap between the HV earth and the duct. This risk is thought to beextremely small and is outweighed by the risk to public which may occur if the systems are bonded. It is likelythat duct fans etc or other fortuitous contact will provide connection to the LV earth in any case,


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8.6 Additional Requirements for HOT Sites

If the substation has an EPR greater than 430V it shall be classified as HOT and the

additional requirements detailed in this section shall be applied where necessary.

Extra care is required to ensure that all earthed metalwork is more than 2m from any othermetalwork and that separate HV and LV earths are not inadvertently combined.

8.6.1 LV Earth

A segregated LV earth electrode shall be:

Selected to provide a resistance of 20or less.

Segregated from any HV electrode by at least 8m.

Installed under an LV main cable in the cable trench wherever practicable to enhance itssecurity.

Connected to the LV neutral bar in the LV pillar/cabinet using 70mm2 PVC coveredcopper conductor (also laid under an LV main cable) in accordance with the LV CableJointing manual.

Note:If an existing substation with metallic sheathed HV and LV cables is being replacedwith a new one it may not be possible to segregate the HV and LV earthing and further workis required to achieve a COLD site

8.6.2 Neutral-Earth Link

The HV/LV neutral-earth link shall be removed.

8.6.3 Warning Notices for Segregated Earths

Where the HV and LV earths are segregated, warning labels as detailed in ECS 06-0023shall be installed next to the neutral-earth connection and on the site as required.

8.6.4 Lighting and Socket Supplies

Care shall also be taken with lighting and socket supplies to avoid operator contact betweendifferent earthing systems. Therefore at HOT sites:

Metallic light switches, 13A sockets and conduits shall not be installed within 2m of anymetalwork bonded to the HV earth.

13A sockets shall be disconnected or removed from LV fuse cabinets, LV pillars and LVboards.

RTU supplies shall be provided via an isolation transformer with a 5kV insulation rating.

ECS 06-0023 contains further details on the practicalities of carrying this out on site.

8.6.5 Street Lighting Columns

Where possible HOT new substations shall not be installed within 2m of street lighting

columns. However where this is impractical the columns shall be earthed via a separateearth rod installed adjacent to the column and shall not use the neutral/earth of a PMEservice.


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9 Special Situations

This section provides further details on specific earthing circumstances that may beencountered when designing secondary substation earthing.

Substation refurbishment and asset replacement/enhancement (Section9.1.1).

Supplies to/from HOT sites (see Section9.1.2 and9.1.3).

Secondary substations associated with higher voltage substations (Section 9.1.4).

Substations near livestock/horses or other high risk locations e.g. outdoor swimmingpools, showers, zoos, locations where footwear is not worn etc (Section9.1.4).

Supplies to mobile phone masts (Section9.1.4).

Substations located near tower lines (Section9.1.7).

Substations located near railways (Section9.1.8).

Substations located near to telephone exchanges (Section9.1.9).

IDNO substations (Section9.1.10).HV generator connections (Section9.1.11).

Customers lightning protection(Section9.1.12).

9.1.1 Substation Refurbishment and Asset Replacement/Enhancement

When work is carried out at substations, e.g. civil refurbishment, asset replacement orenhancement, the earthing shall be reviewed, and brought in line with current requirements;however the earthing enhancement should be proportional to the actual work being carriedout and be practical to install.

The earthing should, where possible, be based around the standard arrangements detailedin Section 7 (Section 7.1.6 includes some specific examples of earthing enhancement duringasset replacement at outdoor substations).

If lead sheathed cables are being replaced with polymeric types, any decommissioned leadcables should be retained if possible and bonded to the earthing system since these maymake an important contribution to the reduction of earth resistance and EPR at the site.

9.1.2 Supplies to National Grid and HOT Sites

For supplies to HOT sites and allNational Grid sites refer to EDS 08-0121 before carryingout the earthing design to determine a suitable supply arrangement.

9.1.3 Supplies from HOT Sites

Generally supplies shall notbe taken from a HOT site. HoweverFigure 9-1 shows a typicalarrangement which may be used, with care, to provide an LV supply from a HOT secondarysubstation provided the EPR does not exceed 2kV.

The LV earth electrode shall be installed outside the HOT zone area.

The LV cable and LV earth cable shall be PVC covered and installed in an insulated duct(both rated to withstand the maximum EPR of the site) inside the HOT zone. Note:A 650Vsheath withstand may be assumed for PVC covered cables in the absence of manufacturers

data.


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Any outgoing LV metallic sheathed/armoured cables shall, at the substation end, be isolatedfrom the substation metalwork and insulated to prevent touch contact or contact with the soil,for the length of their passage through the HOT zone. Also refer to Section9.1.2.

HOT Zone

Insulated Duct/

Cable Sheaths

HV Earth

HV Network

8m

HV Network

LV Earth

LV Supplies

Outside

HOT Zone

Figure 9-1LV Supply from a HOT Site using an Insulating Duct and a Remote Earth

9.1.4 Supplies to Higher Voltage Substations

Generally, when designing the earthing arrangements for a secondary substation sitedwithin, adjacent to or supplying any higher voltage substation the secondary substationearthing system can be an integral part of the higher voltage substation earthing system.The standard earthing arrangements for new grid and primary substations detailed inEDS 06-0013 include a provision for a secondary substation.

A number of acceptable arrangeme

Documents

EDS+06 0014+Secondary+Substation+Earthing+Design