ECS 06-0023 Secondary Distribution Network …library.ukpowernetworks.co.uk/library/asset/bc81106a-1cd...When entering a site where earth connections have been stolen, safety precautions

Document Number: ECS 06-0023

Version: 4.0

Date: 28/07/2014

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ENGINEERING CONSTRUCTION STANDARD

ECS 06-0023

SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION

Network(s): EPN, LPN, SPN

Summary: This standard provides guidance for field staff on the application of earthing to the secondary distribution HV and LV underground cable and overhead line networks.

Owner: Stephen Tucker Date: 28/07/2014

Approved By: Steve Mockford Approved Date: 04/08/2014

This document forms part of the Company’s 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 author or owner of the current issue.

Circulation

UK Power Networks External

All UK Power Networks G81 Website

Asset Management Contractors

Capital Programme ICPs/IDNOs

Connections Meter Operators

HSS&TT

Network Operations

UK Power Networks Services

Other

Secondary Distribution Network Earthing Construction Document Number: ECS 06-0023

Version: 4.0

Date: 28/07/2014

© UK Power Networks 2014 All rights reserved 2 of 55

Revision Record

Version 4.0 Review Date 28/07/2019

Date 28/07/2014 Author Stephen Tucker

Revised to align and ensure consistency with EDS 06-0014, EDS 07-0102, the Overhead Line manual and other earthing standards. Feedback from Network Operations also incorporated. The main changes include:

Earth bar introduced (Section 6.3.2).

New EDS 07-0102 designs incorporated, including integral and basement substation design with embedded mesh (Section 6.4.4).

Lower rebar connection removed from all earthing arrangements (Section 6.4).

Earth grate introduced as option for emergency asset replacement (Section 6.4.7).

Preformed operator earth mat introduced for pole-mounted equipment (Section 7.2.5).

Use of aluminium earth conductor and copper/aluminium transition shown on pole-mounted earthing arrangement drawings (Section 7.3).

PME earth electrodes clarified (Section 8.2.1).

Simple rod and conductor electrode options added (Section 10).

Materials (Section 12) and connections (Section 13) updated.

Standard earth resistance measurement methods added (Section 15).



All sections revised. Pole, LV and NetMap sections added. Document reviewed for publishing on G81 website




Date 20/10/2010 Author Mariann Mulligan

Document rebranded



Original


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Contents

1 Introduction ............................................................................................................. 5

2 Scope ....................................................................................................................... 6

3 Abbreviations .......................................................................................................... 6

4 Definitions ................................................................................................................ 6

5 Theft of Earthing ...................................................................................................... 7

5.1 Use of Aluminium ...................................................................................................... 7

5.2 Sites where Theft has Occurred ................................................................................ 7

6 Secondary Distribution Substations ...................................................................... 8

6.1 New Substations ....................................................................................................... 8

6.2 Substation Refurbishment and Asset Replacement/Enhancement ............................ 8

6.3 Installation Requirements .......................................................................................... 9

6.4 Typical Earthing Arrangements ............................................................................... 17

7 Overhead Lines ..................................................................................................... 26

7.1 Design Overview ..................................................................................................... 26

7.2 Installation Requirements ........................................................................................ 26

7.3 Earthing Arrangements ............................................................................................ 29

8 LV Earthing ............................................................................................................ 39

8.1 Design Overview ..................................................................................................... 39

8.2 Installation Requirements ........................................................................................ 39

9 Customer Installations .......................................................................................... 43

9.1 Earth Terminal ......................................................................................................... 43

9.2 Connection to Supply Neutral Conductor at the Cut-out .......................................... 43

9.3 Connection to Cable Sheath/Armouring at the Cut-out ............................................ 43

9.4 Earth Fault Loop Impedance ................................................................................... 43

10 Earth Electrode Options ........................................................................................ 44

11 NetMap Earthing Information System .................................................................. 45

12 Materials ................................................................................................................. 46

13 Connection Techniques ........................................................................................ 47

14 Warning Labels ...................................................................................................... 49

15 Measurements ....................................................................................................... 50

15.1 Overview ................................................................................................................. 50

15.2 Test Equipment ....................................................................................................... 50

15.3 Earth Resistance Measurement using the Fall-of-Potential Method ......................... 50

15.4 Earth Resistance Measurement using the Comparison Method .............................. 52

15.5 Earth Resistance Measurement using a Clamp Meter ............................................. 52


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16 References ............................................................................................................. 53

Appendix A – Purpose of Earthing .................................................................................. 54

Appendix B – LV Earthing Systems ................................................................................. 55


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1 Introduction

This standard provides guidance for field staff on the application of earthing for:

New secondary substations (Section 6.1).

Asset replacement or a material alteration at secondary substations (Section 6.2).

Fencing and door replacement at secondary substations (Section 6.3.5).

Pole-mounted equipment and overhead networks (Section 7).

LV networks (Section 8).

Customer installations (Section 9).

This standard also includes information on earth electrode options (Section 10), NetMap earthing maps (Section 11), materials (Section 12), connection techniques (Section 13), warning notices (Section 14) and measurements (Section 15).

This standard is not a substitute for the design standards which cover the earthing design in detail. In general the earthing design should be provided by the project designer/planning engineer and this standard can then be used to install the design.

The earth resistance for all new secondary substations shall be specified by the designer/planning engineer – the generic ‘1 ohm’ rule is no longer applicable.

The earthing system at all new secondary substations shall, as a minimum, consist of a ring electrode around the site (except basement or integral sites where this is not practicable), a minimum of two earth rods, connections to the rebar and bonding between all equipment and main earth terminal, unless a site specific design has been provided by a competent earthing specialist.

When carrying out asset replacement, fence/door replacement or other works at a secondary substation the earthing shall be reviewed and enhanced if required to ensure that it complies with the latest standards.

The ring electrode around a secondary substation shall use bare copper conductor, buried directly in the soil; insulated or covered conductor that has previously been used at some installations shall not be used.

The minimum size of bonding conductor shall be 35mm2 aluminium or 16mm2 copper.

Aluminium shall be used for all earthing and bonding above ground where practicable and where it is vulnerable to theft. Bare aluminium shall not be used below ground.

The use of aluminium tape around the inside walls of the substation as previously used in London is not required unless it is specified as part of the earthing design (e.g. embedded mesh integral and basement substation designs).

A dedicated earth bar shall be used for marshalling all earth connections.

Connection of lower rebar in a standard 3m x 3m plinth is no longer required.

An LV earth terminal shall only be provided if it is appropriate and safe to do so and should be specified within the design along with the type of earthing system to be used.

This standard supersedes all legacy documentation on earthing associated with the HV and LV networks.


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2 Scope

This standard applies to the construction of earthing for secondary distribution substations, 11/6.6kV overhead networks and LV networks.

3 Abbreviations

Term Definition

CNE Combined Neutral Earth cable (refer to Section 0)

Ellipse UK Power Networks Asset Register

EPR Earth Potential Rise (refer to Section 0)

NetMap UK Power Networks GIS system

PME Protection Multiple Earthing (refer to Section 0)

ROEP Rise of Earth Potential (refer to Section 0)

SNE Separate Neutral Earth cable (refer to Section 0)

4 Definitions

This section defines some of the terms used in this guide. Further definitions can be found in EDS 06-0012.

Cable - Combined Neutral Earth (CNE) A CNE cable has a combined neutral and earth metallic outer sheath with a PVC covering and is typically 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 is a fourth core and the earth conductor forms a protective sheath.

COLD Site A COLD site is a grid, primary or secondary substation where the earth potential rise (EPR) is less than 430V. Note: Higher limits may apply at grid or supergrid substations with fast acting protection.

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

Earth Potential Rise (EPR) or Rise of Earth Potential (ROEP) EPR 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. 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.

Installing a buried grading electrode around the site and equipment and bonding exposed metalwork together helps to reduce potential differences between a person's hands and feet such that any current flowing through the body is kept to a safe level.


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Step, Touch and Transfer Potentials The step potential is the potential difference between a person’s feet assumed to be 1m apart. The touch potential is the potential difference between a person’s hands and feet when standing up to 1m away from any earthed metalwork they are touching. The transfer potential is the potential transferred by means of a conductor between an area with a significant rise of earth potential and an area with little or no rise of earth potential, and results in a potential difference between the conductor and earth in both locations.

Substation Earthing Database The substation earthing database contains the classification (HOT or COLD) of all grid and primary substations together with the details of the EPR and other relevant earthing information. Refer to EDS 06-0002 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 customer’s installation. The customer’s 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.

5 Theft of Earthing

5.1 Use of Aluminium

The theft of copper earthing continues to be a significant national problem. Therefore the earthing system shall be constructed to ensure that it is secure and not vulnerable to theft. Aluminium shall be used wherever practicable for all new or replacement earthing above ground at all secondary substations and pole-mounted sites.

Note:

Aluminium shall not be used for earthing conductors that are in direct contact or buried within the ground.

Small sections of copper/aluminium shall be avoided as multiple joints in close proximity may be problematic.

In some cases it may be prudent to replace existing copper with aluminium.

Where the use of aluminium is impractical or not suitable (e.g. connections to rebar) then copper may be used as a last resort provided it is suitably protected from theft, e.g. installed in tubing filled with resin, buried in concrete or pinned etc.

5.2 Sites where Theft has Occurred

When entering a site where earth connections have been stolen, safety precautions are necessary to reduce the risk of shock – refer to EOS 09-0067 for further information.


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6 Secondary Distribution Substations

6.1 New Substations

The earthing design for all new ground-mounted secondary (20kV, 11kV, 6.6kV, 3kV and 2kV) substations shall be carried out by the designer/planning engineer to ensure that the design criteria detailed in EDS 06-0014 are fulfilled. The designer/planning engineer shall provide the following for each project:

An earthing design/construction form specifying the required HV earth resistance, whether the HV and LV earths should be combined or separated and the COLD/HOT status of the site.

An earthing drawing either based on one of the standard UK Power Networks’ arrangements in Section 6.4 or a bespoke arrangement.

Note: It is no longer acceptable to use the generic ‘1 ohm’ rule. In many situations a substation may be acceptable with a resistance value greater than 1Ω. However, in some situations (in particular when supplied from a solidly earthed grid/primary substation, or if an overhead line section makes up part of the circuit), a value less than 1Ω may be required to achieve an acceptable site.

Using the available information the installer shall:

1. Install the earthing as detailed on the provided drawing. Note: Although any additional electrode should be specified in the earthing design it is acceptable for the installer to use different combinations of earth rod, conductor and mats provided that the required earth resistance is achieved and the installation satisfies the requirements detailed in this document.

2. Apply the earthing requirements shown in Section 6.3 to the earthing arrangement. 3. Select the appropriate materials, connections and warning labels from Sections 12, 13

and 14. 4. Measure the value of the HV and LV (if required) earth resistance in accordance with

Section 15 to confirm that the required value has been achieved. 5. Commission the earthing system in accordance with ECP 11-0503.

Any queries or changes to the earthing design shall be referred back to the designer/planning engineer.

6.2 Substation Refurbishment and Asset Replacement/Enhancement

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

The earthing should, where possible, be based around the standard arrangements detailed in Section 6.4; Section 6.4.7 includes some specific examples of earthing enhancement during asset replacement.

Metallic fences, gates and doors require particular attention to ensure they are correctly bonded in accordance with Section 6.3.5.


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6.3 Installation Requirements

6.3.1 General

The sections below detail the common earthing requirements that shall be applied to all secondary substations, unless a site specific design has been provided by an earthing specialist.

If the site is designed with separate HV and LV earths, care is required to ensure that all earthed metalwork is more than 2m from any other metalwork and that separate HV/LV earths are not inadvertently combined.

6.3.2 Earth Bar

All earth connections shall be labelled and connected via separate connections to a dedicated earth bar (GRP/brick-built designs) or marshalling bar (integral/basement designs) which in turn shall be connected to the main transformer/switchgear earth terminal to allow:

Operational personnel to determine if the earthing is intact when entering the substation.

The earthing to be easily identified.

The earth resistance to be measured using a clamp meter.

Note: At new enclosed substations the HV earth bar within the LV cabinet/pillar shall not be used to marshal the earthing connections as access, particularly at IDNO substations, to the LV cabinet/pillar is not always available. However it is acceptable to use the HV earth bar within the LV cabinet/pillar at existing outdoor sites for all earth connections to prevent theft.

Refer to Sections 6.3.11 and 6.3.14 for combined and separated HV/LV earth configurations.

Earth Bar Marshalling Bar


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6.3.3 HV Earth Electrodes

Bare copper cable or tape and copper clad rods (refer to Section 12) shall be used as earth electrodes. Section 10 provides electrode options to obtain various resistance values. The minimum sizes of earth electrode are shown below.

Function Source Fault Level

Bare Stranded Hard Drawn Copper Conductor

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

Up to 15kA 2 x 70mm2 25mm x 6mm

Earth Rod Electrode Any 1m or 1.2m Copper Clad Rods (16mm or 19mm diameter)

Note:

Earth electrode should be installed underneath the HV cable (or LV cable) at a depth of at least 600mm to provide physical protection and surrounded by 150mm of fine texture non-corrosive soil which has been firmly consolidated. If the indigenous soil is hostile to copper (i.e. known to be overly acidic or alkaline) suitable surrounding soil should be imported.

In areas of high soil resistivity additional earth electrodes may be specified in the design consisting of either horizontal conductor, long vertical earth rods or a mixture of both.

Where difficult installation problems are envisaged or found (such as boulder clay beneath the ground) vertical earth rods may need to be installed in pre-drilled holes and backfilled with appropriate earthing compound (refer to Section 12), rather than driven in by mechanical means.

When laying stranded conductor, care should be taken to avoid distorting and opening the individual strands as this increases the probability of corrosion.

6.3.4 Equipment Bonding

All current carrying items of equipment including the HV switchgear, LV pillar/cabinet/board and LV ACB shall be bonded to the main earth terminal using an independent connection. The minimum sizes of the bonding conductor (refer to Section 12) are shown below.

Function Source Fault Level

Covered Stranded Cable Tape

Any Up to 8kA 70mm2 Copper 25mm x 3mm Copper

Up to 12kA 120mm2 or 2 x 70mm

2 Copper 25mm x 4mm Copper

Up to 15kA 2 x 70mm2 Copper 25mm x 6mm Copper

Above Ground Bonding

Up to 8kA 120mm2 Aluminium 25mm x 6mm Aluminium

Up to 15kA 2 x 120mm2 Aluminium 40mm x 6mm Aluminium

All other non-current carrying items of equipment (e.g. control units, RTUs, battery chargers etc.) shall be bonded to the main earth terminal using a minimum of 35mm2 covered stranded aluminium cable, 16mm2 covered stranded copper cable or equivalent.


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6.3.5 Metallic Fences and Gates

The diagrams below show typical earthing arrangements for metallic fences within 2m of accessible earthed equipment and more than 2m away from the equipment or other earthed metalwork.

Gate

Metallic Fence within 2m of Equipment

Metallic Fence more than 2m away from Equipment or Equipment in GRP/Brick Enclosure

Duplicate connections

to the HV earth or main

earth terminal

Gate

An earth grading electrode should be:

Installed outside the fence at a distance of 300-

500mm away or, if not practical, directly

underneath/just inside the fence (as shown)

70mm2 bare stranded copper conductor or

25mm2 x 4mm bare copper tape earth

Installed at a depth of 500mm (300mm

minimum) below ground level

Connected to each fence post

Buried connection between gate posts:

§ 35mm2 aluminium or 16mm

2 copper covered

stranded cable

Flexible connection between gate and gate posts:


2 copper covered

stranded cable or 16mm2 tinned copper braid

To eliminate stray voltages a single driven earth

rod shall be installed:

§ At each corner fence post

§ At each gate post and connected to the

fence/gate

Buried connection between gate posts:


2 copper covered

stranded cable

Flexible connection between gate and gate posts:


2 copper covered

stranded cable or 16mm2 tinned copper braid

Note: Where the substation fence is bonded to the HV earth, no other metallic fencing or conducting material shall be abutted to the fence or within 2 metres of it. Otherwise any voltage rise at the substation may be transferred to a remote location far from the substation where it will be impossible to protect against dangerous touch voltages. Insulating fence panels or stand-off insulators can be used to achieve this requirement. Metallic third party fences should not be within simultaneous touching distance (2m) of metalwork/fences connected to the HV earth. If necessary a ‘floating’ (isolated) section of fence or non-conductive barrier should be introduced outside the substation boundary (refer to


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ECS 06-0022 for further information on the use of insulated panels and insulated stand-offs). If this is not practicable then specialist advice should be sought.

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 is unlikely that a fence earthing system will exist. It is not sufficient simply to replace wooden panelling with metallic, nor is it sufficient to merely bond metallic fence panels together above ground without a buried electrode system.

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

6.3.6 Metallic Doors

Generally, due to the small size of most secondary substations, metallic doors should be bonded to the HV earth as shown below unless a site specific design is provided.

Note: The ‘grading electrode’ outside the substation doors may be omitted if specified in the design and only if there is concrete or tarmac outside the substation and the substation is designed with combined HV/LV earthing and is a COLD site.

1m

To HV

Earth

To HV

Earth

500mm

Metallic

Doors

Connection between door and door

frame: 35mm2 aluminium or 16mm

2

copper covered stranded cable or

16mm2 tinned copper braid

Connection between door frame

and HV earth: minimum 35mm2

aluminium or 16mm2 copper

covered stranded cable

Unless specified otherwise an earth grading

electrode shall be:

Installed outside the door at a distance

of 1m in front of the door and 500mm

beyond each door frame

70mm2 bare stranded copper conductor

or 25mm2 x 4mm bare copper tape earth

Installed at a depth of 300-500mm

below ground level

Covered with a 100mm thickness of

concrete to protect against damage or

theft

Connected to the HV earth at each end

However if metallic doors are installed more than 2m away from other earthed metalwork and are not connected to the HV earth an independent earth electrode shall be installed and connected to each door (refer to EDS 06-0014 for further details).

6.3.7 Ancillary Metalwork

All other exposed and normally un-energised metalwork inside the substation perimeter boundary (e.g. ventilation ducts, staircases, louvres etc) within 2m of other earthed metalwork shall be bonded to the HV earth using 35mm2 covered stranded aluminium cable or 16mm2 covered stranded copper cable to avoid any potential differences between different items of metalwork.

Note: Metal frames and other metallic parts that form part of a GRP enclosure do not need to be bonded to the HV earth.


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6.3.8 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 the HV earth, they could transfer potential outside the substation zone and may pose a risk to the general public. Generally it is impractical to install measures to control touch and step potentials where these vents emerge. Therefore one of the following approaches, in order of preference1, shall be taken to minimise risk to the public:

Bond the ducts and ventilation shafts to the HV earth (unless the EPR is greater than 430V), and install them such that they are out of reach (i.e. 3m above ground or other foothold) where they emerge from the substation..

Leave the ducts and ventilation shafts un-bonded, and install them such that there is no possibility of a simultaneous touch contact between the ducts and other earthed metalwork. This may require the installation of insulated ducts or barriers. A warning label can be installed as a further precaution.

6.3.9 HV Cables

All cable earth screens shall be bonded to the main earth terminal in accordance with Section 4 of the 11kV Cable Jointing manual.

6.3.10 LV Cables

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

1 The risk to the public can be reduced by leaving the ducts and ventilation shafts un-bonded. However this may

introduce a touch potential risk to staff inside the substation since the ducts and vents may act as a remote earth and will therefore be at a different potential to HV earth during fault conditions; the risk of an HV fault happening occurring while staff are on site and bridging a gap between the HV earth and the duct is thought to be extremely small and is outweighed by the risk to public which may occur if the systems are bonded.,


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6.3.11 Combined HV/LV Earths

If the EPR is less than 430V (COLD site) an LV earth is not required. The LV neutral-earth link in the LV cabinet, pillar or board, that bonds the LV neutral to the substation HV earth, shall be in place so that the HV and LV earths are combined.

Note: Where an LV ACB is mounted directly on to the transformer and an LV cabinet or board is not present the transformer earth terminal shall be bonded to the neutral using the appropriate bonding conductor from Section 6.3.4 as shown below.

L1

HV Switchgear

E

HV Electrode Resistance (as specified)

HV Cables

LV Cables

HV/LV Neutral-

Earth Link in

Place

LV Distribution Board

L2

L3Transfomer

N

Main Earth

Terminal

Earth bonding

or electrode

Connection to

Reinforcement

Rebar/Mesh

Duplicate Connections

to HV Earth Mat

Additional HV

Earth Electrode

Earth Bar Located in Cable Pit or

Marshalling Bar Mounted on Wall

Equipment

and Ancillary

Metalwork

Bonding

LV Cabinet

Neutral

Bar

Earth

Bar

Removable Link

LV Board

Neutral/

Earth Bar

Transformer

Earth Stud

Neutral – Earth link

LV Cabinet ACB


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6.3.12 Separate HV/LV Earth Additional Requirements

In addition to the requirements detailed in Section 6.3, the requirements detailed in this section shall be applied at sites with separate HV and LV earths.

Note: If a supply is provided to a HOT site (e.g. grid or primary substation) or a National Grid substation refer to EDS 08-0121 for additional earthing requirements.

6.3.13 LV Earth

The LV earth electrode shall be:

Selected from Section 10 to provide a resistance of 20Ω or less.

Separated from any HV electrode by at least 8m.

Installed under an LV cable in the cable trench wherever practicable to enhance its security.

Connected to the LV neutral bar in the LV pillar/cabinet using 70mm2 covered copper conductor (also laid under an LV main cable) in accordance with the LV Cable Jointing manual.

6.3.14 Neutral-Earth Link

The HV/LV neutral-earth link shall be removed for a separated HV/LV earth as shown below.

L1

HV Switchgear

E

HV Electrode Resistance (as specified)

HV Cables

LV CablesHV/LV Neutral-

Earth Link

Removed

LV Distribution Board

L2

L3Transfomer

N

Main Earth

Terminal

8m minimumLV Electrode Resistance

(not to exceed 20Ω)

PVC covered conductor

within 8m of bare HV

electrode or metallic

sheathed HV cables

Earth bonding or

electrode


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Connection to

Reinforcement

Rebar/Mesh

Duplicate Connections

to HV Earth Mat

Additional HV

Earth Electrode

Equipment

and Ancillary

Metalwork

BondingHV Earth in LV

Cabinet/Board

Separate

LV Earth

LV Cabinet

Neutral

Bar

HV Earth

Bar

Link Removed

Neutral bar

LV earth to be connected here Neutral – Earth link

As Delivered Removed

Transformer

Earth Stud

Earth Bar Located in Cable Pit or

Marshalling Bar Mounted on Wall

6.3.15 Warning Notices for Separate HV/LV Earths

Where the HV and LV earths are separated, warning labels (EDS 07-0009.118 and EDS 07-0009.117) as detailed in Section 14 shall be installed as required.

6.3.16 Lighting and Socket Supplies

Care shall also be taken with lighting and socket supplies to avoid operator contact between different earthing systems. Therefore at sites with separate HV and LV earths:

Light switches and conduits shall preferably be plastic; metallic light switches and conduits shall not be installed within 2m of any metalwork bonded to the HV earth.

All 13A sockets shall be disconnected or removed (for disconnection of sockets in LV cabinets refer to Schneider instruction EM-MS-06-005-r3 and EM-MS-06-009-r0).

RTU supplies shall be provided via a 500VA isolation transformer with a 5kV insulation rating (refer to Schneider instruction2).

6.3.17 Street Lighting Columns and Street Furniture

New substations with separate HV and LV earths shall not be installed within 2m of street lighting columns or other street furniture

2 The Schneider LV cabinet design is due to be modified in 2014 to enable the isolation transformer to be fitted

on-site as required.


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6.4 Typical Earthing Arrangements

The general principles for the construction of a substation earthing system are as follows:

Bare copper clad earth electrodes using the minimum sizes specified in 6.3.3.

A ring of bare earth electrode buried around the perimeter of the substation (or alternatively buried around the inside of the substation) at a depth of 500-600mm (except at integral and basement substations where this is not achievable).

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

Two connections from the ring onto a dedicated earth bar or main earth terminal.

An earth electrode passing underneath any HV switchgear or LV operating position and connected to the other electrode or an insulated or earthed operator platform.

Connections to the reinforcement rebar or mesh.

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

All items of HV equipment and exposed metalwork bonded to the main earth bar/terminal.

The section contains typical earthing arrangements based on the above principles, showing the installation of the HV and LV (if required) earthing, for the following types of installation:

New GRP, brick-built, compact, micro pad-mount and outdoor substations.

Integral, basement and raft substations.

Customer HV supplies and substations.

Existing outdoor substations.

The standard substation design drawings contained in EDS 07-0102 (and listed below) also include an associated earthing arrangement (on sheet 2).

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 Elevated Substation with Fully-bunded Plinth EDS 07-0102.21

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

Freestanding Brick-built Substation for a Single Transformer without/with ACB and LV Board (4m x 5m)

EDS 07-0102.05-06

Integral Substation for a Single Transformer without/with ACB and LV Board EDS 07-0102.07-08

Basement Substation EDS 07-0102.09

Micro Substation EDS 07-0102.10

Compact Substation EDS 07-0102.11

Fenced Outdoor Substation with Micro and Ring Main Unit EDS 07-0102.12

Fenced Outdoor Substation with Micro and Extensible Switchgear EDS 07-0102.13


Version: 4.0

Date: 28/07/2014


6.4.1 GRP and Brick-Built Substations (Combined HV/LV Earths)

The general earthing arrangement for GRP or brick-built substations with a combined HV/LV earth (COLD site) is shown below (refer to EDS 07-0102 for specific designs).

Primary Fault Level Bare Copper Conductor

Up to 8kA

Up to 12kA

Up to 15kA

70mm2

120mm2 (or 2 x 70mm

2)

2 x 70mm2

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

1 - 2.4m earth rods at rear corners

4 - HV electrode connecting each side of outer loop to main earth terminal

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

5 - Connection to reinforcement rebar/mesh

RMU

Transformer

Note: Not all equipment

bonding is shown

Main Earth Bar/Terminal

6 - Neutral/Earth link in place

6

1

4

1

34

5

22

Combined HV/LV Earths

Neutral/Earth Link In

To HV

Earth

To LV Cable

Neutral

LV CNE Cable

LV

Bare Copper Tape

25mm x 3mm

25mm x 4mm

25mm x 6mm


Version: 4.0

Date: 28/07/2014


6.4.2 GRP and Brick-Built Substations (Separate HV/LV Earths)

The general earthing arrangement for GRP or brick-built substations with a separate HV/LV earth (HOT site) is shown below (refer to EDS 07-0102 for specific designs).

LV CNE Cable

LV earth connection – 70mm2 PVC

covered stranded copper conductor

HV/LV Separation

(8m minimum)

LV Earth Connection (Insulated)

LV Earth Electrode/Rods

(max resistance 20Ω)

Bare Copper Conductor Bare Copper Tape

70mm2

120mm2 (or 2 x 70mm

2)

2 x 70mm2

25mm x 3mm

25mm x 4mm

25mm x 6mm


1 - 2.4m earth rods at rear corners

4 - HV electrode connecting each side of outer loop to main earth terminal

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 – 70mm2 bare

stranded copper conductor

HV Earth Electrode LV Earth


RMU

Transformer

7 - Warning labels

Note: Not all equipment

bonding is shown

6 - Neutral/Earth link removed

6

1

4

1

7

73

4

5

22

Separate HV/LV Earths

Neutral/Earth Link Out

To HV

EarthTo LV

Earth

To LV Cable

Neutral

LV

Main Earth Bar/Terminal

Primary Fault Level

Up to 8kA

Up to 12kA

Up to 15kA


Version: 4.0

Date: 28/07/2014


6.4.3 Compact (including Micro and Pad-mount) Substations

The earthing arrangement for a compact/micro substation without an enclosure is shown below. These are generally supplied via the overhead line network and therefore the HV and LV earths are generally separated as shown and detailed in Section 6.3.12. However if the EPR is less than 430V, the HV and LV earths may be combined. Refer to EDS 07-0102 for specific designs.

Note: If the compact is installed in a GRP enclosure the standard arrangements for a GRP shown in 6.4.1 and 6.4.2 shall 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 20Ω)

Separate HV/LV Earths

Neutral/Earth Link Out

2 2

4 - HV electrode connecting each side of outer loop to the main earth terminal

9

9

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

9 - Warning labels7 - Neutral/Earth link removed

7

7

Additional HV Earth

Electrode/Rods

(as specified )

LV

Neutral

HV

Earth

To HV Earth

Electrode

1 1

3

4


Neutral/Earth Link In

2 2

5 5

8

8

8 - Neutral/Earth link in place

Separated

HV/LV Earths

Combined

HV/LV Earths


6

6

LV earth connection – 70mm2 PVC

covered stranded copper conductor

Bare Copper Conductor Bare Copper Tape

70mm2

120mm2 (or 2 x 70mm

2)

2 x 70mm2

25mm x 3mm

25mm x 4mm

25mm x 6mm

LV earth electrode – 70mm2 bare

stranded copper conductor

HV Earth Electrode LV Earth

Primary Fault Level

Up to 8kA

Up to 12kA

Up to 15kA


Version: 4.0

Date: 28/07/2014


6.4.4 Integral, Basement and Raft Substations

The standard arrangements shown in Section 6.4.1 and 6.4.2 should be used wherever possible. However where the substation is situated within a building or on a raft, it is usually impracticable to install one of these arrangements. Instead a standard approach should be applied using earth rods installed through the substation floor or in the basement, external electrodes underneath the HV cable, vertical piles and an embedded mesh within the floor screed (to control the touch and step potentials). Some examples are shown below.

It is very important to consider the earthing prior to construction if a satisfactory earthing system is to be designed. Refer to EDS 07-0102 for specific designs and EDS 06-0014 for further information.

1a

RMU

Transformer

LV

1a - Main earth rods (length to be determined by calculation based on

target resistance value and soil resistivity)

2 - Connections to mesh embedded within concrete floor screed - two per

each sheet of 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 ring above floor level/below door tread to aid

connections

6 - Doorframe and door bonding

6

5 - Main equipment bond (not all equipment 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 through

Substation Floor into Soil

Substation

(c) Earth Rod Installation through

Basement into Soil

5

(d) Embedded Mesh within Floor

Screed

2

2


Version: 4.0

Date: 28/07/2014


6.4.5 Customer HV Supplies and Substations

Some typical arrangements for customer substations with combined and separate HV/LV earths are shown below. Refer to Section 7 in EDS 06-0017 for further information including the interconnection of UK Power Networks and customer earthing systems.

Note: UK Power Networks is not responsible for the LV earth and the decision to combine or separate the HV and LV earths is the customer’s responsibility. Care is required if LV supplies (from the customer) are to be introduced into the UK Power Networks substation and is generally only permissible for combined earthing systems.

1

2

5

53

3

MU

Additional

electrode/rods

(if required )

1

1 1

2 2

1

3

3

UK Power Networks

Substation

MU

Customer

Substation

Additional

electrode/rods

(if required )

1

UK Power Networks

Substation

LV

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


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

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

6 - Neutral-earth link in place

Note:

Equipment

bonding not

shown

Note:

Equipment

bonding not

shown


1

2

1

3

3

Additional

electrode/rods

(if required )

Customer

Substation

HV

6

Customer LV SNE Cables

LVHV

6


8 - Warning labels

7 - HV cable screen insulated from earth

8

7

4

4

4


TX

TXRMU

3

3

4

RMU


Version: 4.0

Date: 28/07/2014


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


1 1

2

5

5

2

1

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

3

3

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

Additional

electrode/rods

(if required )

1

1 1

2 2

1

3

3

UK Power Networks

Substation

Customer

Substation

Additional

electrode/rods

(if required )

1

UK Power Networks

Substation

Customer

Substation

HV/LV

Separation

(8m minimum)

LV Earth Cable (Insulated)

LV Earth Cable (Insulated)

LVHV

6

Note:

Equipment

bonding not

shown

Note:

Equipment

bonding not

shown



LVHV

6


HV/LV

Separation

(8m minimum)

8

6 - Neutral-earth link removed

8 - Warning labels

7 - HV cable screen insulated from earth

4

4

Separated HV/LV Earths

TX

TXMU

3

3

4

RMU

3

3

MU

7

4

RMU


Version: 4.0

Date: 28/07/2014


6.4.6 Outdoor Substations

Outdoor secondary substations are no longer constructed except in Areas of Outstanding Natural Beauty when a close boarded fence substation is required. Refer to EDS 07-0102.12-13 for specific designs.

6.4.7 Asset Replacement

Earthing needs to be considered during asset replacement work at existing sites. The installation of a complete earthing system based on a standard arrangement is rarely practical. However the opportunity to enhance the earthing should be taken and the earthing installed should seek to achieve as much of the following as possible, using the excavations that are necessary for the remedial work:

Buried bare electrode around the equipment at a depth of around 600mm and connected to the main earth bar. Note: It is especially important to ensure that there is bare electrode under the operator’s standing position – especially if metallic sheathed cables are replaced with plastic cables (even short lengths) during a switchgear change.

An embedded and bonded mesh or grate under the operator’s standing position if buried electrode is not practical.

One or two substantial earth rods connected to the buried earth electrode or the main earth bar.

Bonding of all equipment to the main earth bar.

If a metallic fence and/or gates are present the requirements of Section 6.3.5 shall also be applied.

Some typical examples for outdoor substations are shown below. If only the LV pillar is being replaced the installation of a buried electrode system is unlikely to be practical but the pillar shall be bonded to the main earth bar. The more equipment alterations and associated excavations that 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 in the previous sections.

For basement and integral substations the standard approach outlined in Section 6.4.4 may be used and is more likely to provide a more practical solution.

For emergency asset replacement or where the installation of buried electrode is not practical a steel grate plate3 should be installed in front of the switchgear where the operator would stand and connected to the main earth terminal using duplicate connections. The grate should be securely embedded in the shingle or bolted to the concrete floor.

Some typical examples are shown overleaf.

3 The steel grate option is being trialled at the time of writing; contact the document author for further details.


Version: 4.0

Date: 28/07/2014


HV

Sw

itch

ge

ar

Transformer

LV

Pilla

r

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

Fault LevelBare Copper

Conductor

Bare Copper

Tape

Up to 8kA

Up to 12kA

Up to 15kA

70mm2

2 x 70mm2

2 x 70mm2

25mm x 3mm

25mm x 4mm

25mm x 6mm

HV Earth Electrode

3 RMU

LV

Pilla

r

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

RMU

Transformer

LV

Steel Grate under

Operator Position

(d) Use of Steel Grate


Version: 4.0

Date: 28/07/2014


7 Overhead Lines

This section provides an overview of the main overhead line and pole-mounted equipment earthing requirements; refer to EDS 06-0015 for more detailed information. All overhead earthing shall be constructed in accordance with the Overhead Line manual.

Note: It is particularly important during asset replacement or pole refurbishment that the earthing is brought up to the latest standards and separated HV and LV earths are not accidentally combined.

7.1 Design Overview

The following design criteria are used for all new and refurbished pole earthing:

Transformers, switchgear with control units, switchgear with operating handles at low level, cable terminations and surge arresters shall always be earthed.

Poles with only cross-arms and hook-stick operated equipment may be left unearthed. However where any item of equipment on the pole is connected to earth all steelwork on the pole (including cross arms, pilot insulator pins, trussing gear, high voltage cable boxes etc) shall be connected to the main earth.

The maximum HV earth resistance value shall be 10Ω.

The maximum LV earth resistance value shall be 20Ω.


7.2.1 General

The following general requirements apply to all pole earthing:

All above ground earth conductors shall be insulated for a minimum of 3m above ground level and mechanically protected (in accordance with the Overhead Line manual) for a minimum of 2m above ground level.

The HV earth electrode shall be installed at the base of the pole except at locations where it is necessary for an operator to carry out switching operations. At these locations the HV earth electrode shall be installed on the opposite side of the pole to the operating position and 5m away from the operating position. Any earth conductor within 5m of the operating position shall be insulated.

The HV earth electrode system shall be installed at a minimum depth of 1m.

The main earth conductor/electrode shall be insulated for a minimum of 1m below ground level.

Bare HV and LV earth electrodes buried in the ground shall be separated by a minimum of 8m.

HV and LV earthing systems on the same pole shall be segregated by a minimum of 120° and the insulation integrity maintained throughout.

HV and LV earth conductors buried in the ground shall be separated by a minimum distance of 100mm.

Earthing conductors associated with surge protection shall be kept as straight as possible with no sharp bends.

An earth mat shall be installed for all switchgear operated from ground level.

Note: Any deviation from these arrangements, or failure to maintain the local earthing systems, can result in operators being placed at unacceptable risk.


Version: 4.0

Date: 28/07/2014


7.2.2 Location

The electrode system shall preferably be installed alongside hedges or in the pavement area and the following locations shall be avoided:

Where persons could stand whilst operating any pole-top equipment.

Near metal fences, fence posts, buried cables, pipes.

Near camping or caravan parks where persons may be bare foot.

Across main farm gates.

Near drinking troughs or areas where livestock regularly congregate.

Across open fields where there is a risk of damage by ploughing or drainage works.

7.2.3 Earth Electrodes

The minimum earth electrodes are detailed below.

Function Network Fault Level Electrode

Earth Electrode EPN Up to 4kA 35mm² Bare Hard Drawn Stranded Copper Cable

SPN Up to 8kA 70mm² Bare Hard Drawn Stranded Copper Cable

Rod Electrode EPN/SPN All 1m or 1.2m Copper Clad Earth Rods

7.2.4 Earthing and Bonding Conductors

The minimum size of earthing and bonding conductors are detailed below.

Function Network Fault Level Earthing and Bonding Conductors

Earth Conductor EPN Up to 4kA 50mm2 Covered Stranded Aluminium Cable

35mm2 Covered Stranded Copper Cable

SPN Up to 8kA 120mm2 Covered Stranded Aluminium Cable


Bonding Conductor

EPN/SPN All 35mm2 Covered Stranded Aluminium Cable


7.2.5 Earth Mat

A preformed earth mat (preferred and shown) or an earth mat constructed of bare conductor shall be:

Approximately 1m x 1m in size.

Installed directly below where the operator will stand when operating the switchgear.

Installed at a depth of 300mm below ground.

Connected to the switch handle or control unit.

Segregated from all other earthing conductors where possible.

Protected above ground by a cable guard.

Embedded below ground in earthing compound (two bags below and above) to protect against theft (preformed mat only). Further details will be included in the revised Overhead Line manual.

1000mm

10

00m

m


Version: 4.0

Date: 28/07/2014


7.2.6 Surge Arresters

Where surge arresters are installed:

The main earth conductor between the surge arrester and the electrode system shall be as short and straight as possible with no sharp bends.

Except at locations where it is necessary for an operator to carry out switching operations, the electrode shall be installed at the base of the pole.

At locations where it is necessary for an operator to carry out switching operations the earth electrode shall be installed 5m away from the pole to avoid unacceptable step potentials close to the operator. Any earth conductor within 5m of the operating position shall be insulated. The insulated conductor shall be installed inside a PVC duct to provide additional mechanical protection and insulation. It also serves to maintain the conductor in a slow bend which improves lightning performance.

The main earth conductor shall be insulated to a depth of 1m below ground level.

The earth electrode resistance value shall not exceed 10Ω.

7.2.7 HV Steelwork

All HV pole-top steelwork supporting live equipment e.g. cross-arms, equipment supports etc., shall be bonded together using the bonding conductor detailed in 7.2.4. The bonded steelwork shall only be earthed if it is necessary to earth other equipment on the pole.

7.2.8 H-Poles

Where H-poles are fitted with cross-bracing equipment, the lowest cross-brace shall be not less than 3m above the ground; where a cross-brace is found to be less than 3m above ground it shall be repositioned. On an earthed pole the cross-brace shall be bonded to the pole-top steelwork and on an unearthed pole it shall remain unbonded and unearthed.

All cable supports accessible from ground shall be bonded together and to a separate earth on the opposite side to the main HV earth.

7.2.9 LV Steelwork

LV pole-mounted equipment including voltage regulators, static balancers, metalclad fusegear, street lighting brackets, CNE cable sheaths, SNE cable sheaths and armours shall be bonded to the neutral earth conductor using the bonding conductor detailed in 7.2.4.

Reel insulator supports, pole bolts, D-irons and stay make-offs do not require bonding.

7.2.10 Anti-Climbing Device

The anti-climbing device shall not be bonded to any other steelwork or connected to earth.

7.2.11 Stays

All stays attached to a pole shall have a fully rated insulator or insulator(s) fitted in the stay in accordance with the Overhead Line manual. This is to prevent the bottom of the stay becoming live in the event of a pole-top insulation failure.

HV stays: the pole-top make-off of all HV stays shall be bonded to the steelwork in accordance with the Overhead Line Manual.

LV stays: the pole top make-offs of LV stays shall not be bonded to the steelwork.


Version: 4.0

Date: 28/07/2014


7.3 Earthing Arrangements

The standard earthing arrangements detailed in this section satisfy the design principles and earthing requirements outlined in the previous sections and include the following:

Pole-mounted transformers with LV overhead line and LV cable.

Switches, reclosers and sectionalisers.

Handle-operated air-break switch disconnectors.

Hook-stick operated equipment.

Note: The arrangements demonstrate the earthing principles but should be constructed in accordance with the Overhead Line manual.

7.3.1 Pole-Mounted Transformer with HV Overhead Line and LV Overhead Line

Ground Level

Min 8m Separation

1m

2.4

m m

in

All Steelwork

Connected to

HV Earth

HV Earth

Transformer

Earth

Terminal

Insulated for a

Minimum of 3m

Above Ground Level

Mechanical

Protection for a

Minimum of 2m

LV Earth Electrode

(to achieve max 20Ω)

HV Earth Electrode


LV Earth

L1

L2

L3

N

N

L

Covered Aluminium

Earth Conductor

Bare Earth Conductor

Phase Conductor

Bare LV Overhead Line

Key:

Covered Copper Earth

Conductor

Bi-metallic Splice(150-300mm above ground)Bi-metallic Splice

(150-300mm above ground)

Neutral Conductor

Bi-metallic Splice(100mm below lowest crimp)

Note: Anti-climbing

Guards Not Shown

LV Network either

ABC or Open Wire


Version: 4.0

Date: 28/07/2014


7.3.2 Pole-Mounted Transformer with HV Overhead Line and LV Cable

Ground Level

8m Separation

1m

2.4

m m

in

LV

Earth

LV CNE Cable

(PVC Covered)

Bond to LV CNE

Cable Shealth

All Steelwork

Connected to

HV Earth

HV

Earth

N

Transformer

Earth Terminal

L

LV Earth Electrode

(to achieve max 20Ω)HV Earth Electrode


Covered Aluminium

Earth Conductor


Phase Conductor

Key:


Conductor

Neutral Conductor

Bi-metallic Splice(150-300mm above ground)

Insulated for a

Minimum of 3m

Above Ground Level

Mechanical

Protection for a

Minimum of 2m



Note: Anti-climbing

Guards Not Shown

Note: Where a metallic sheathed hessian served LV cable is connected to a pole-mounted transformer the cable shall be replaced with a plastic sheathed cable for a minimum length of 8m to ensure HV/LV earth segregation is maintained..


Version: 4.0

Date: 28/07/2014


7.3.3 Handle-Operated Air-Break Switch Disconnector (ABSD)

Plan View

Ground Level

5m Separation

1m

2.4

m m

in

All Steelwork

Connected to

HV Earth

HV

Earth

Insulated for a Minimum of

3m Above Ground Level

Mechanical Protection for a

Minimum of 2m

30

0m

m

Switch

Handle

Earth

MatPVC Duct Installed from

Pole to Earth Rod

Operating Rod

Insulator

HV Earth Electrode


Covered Aluminium

Earth Conductor


Key:


Conductor




Version: 4.0

Date: 28/07/2014


7.3.4 Handle-Operated Air-Break Switch Disconnector (ABSD) with Cable Termination

Ground Level

5m Separation

All Steelwork

Connected to

HV Earth

HV

Earth




Minimum of 2m

30

0m

m

Switch

Handle

Operating Rod

Insulator

HV

Cable

Earth

MatPVC Duct Installed from

Pole to Earth Rod

Plan View

1m

2.4

m m

in

HV Earth Electrode


Covered Aluminium

Earth Conductor


Key:


Conductor




Version: 4.0

Date: 28/07/2014


7.3.5 Hook-Stick Operated ABSD

Ground Level

All Steelwork

Bonded Together

Key:


Conductor


Version: 4.0

Date: 28/07/2014


7.3.6 Hook-Stick Operated ABSD with Cable Termination (including Surge Arresters)

Ground Level

5m Separation

All Steelwork

Connected to HV

Earth

HV

Earth

PVC Duct Installed from

Pole to Earth Rod

1m

2.4

m m

in

HV Earth Electrode


Covered Aluminium

Earth Conductor


Key:


Conductor

Insulated for a Minimum

of 3m Above Ground

Level

Mechanical Protection

for a Minimum of 2m

HV

Cable




Version: 4.0

Date: 28/07/2014


7.3.7 Hook-Stick Operated Equipment with a Pole-Mounted Transformer

Ground Level

LV

Earth

Fuses

or LinksAll Steelwork

Connected to

HV Earth

NTransformer

Earth

Terminal

L

5m Separation

PVC Duct Installed from

Pole to Earth Rod

1m

2.4

m m

in

min 8m Separation LV Earth Electrode

(to achieve max 20Ω)HV Earth Electrode


Covered Aluminium

Earth Conductor


Phase Conductor

Key:


Conductor

Neutral Conductor

LV CNE Cable

(PVC Covered)

Bond to LV CNE

Cable Shealth





Minimum of 2m


HV

Earth


Note: Anti-climbing

Guards Not Shown


Version: 4.0

Date: 28/07/2014


7.3.8 Cable Termination (including Surge Arresters)

Ground Level

All Steelwork

Connected to

HV Earth

HV

EarthHV

Cable1

m2

.4m

min




Minimum of 2m

HV Earth Electrode


Covered Aluminium

Earth Conductor


Key:


Conductor




Version: 4.0

Date: 28/07/2014


7.3.9 Pole-Mounted Switches, Reclosers and Sectionalisers with Low-level Control Unit

Plan View

Ground Level

1m

2.4

m m

in

HV

Earth

30

0m

m

Earth Mat

HV Earth Electrode



Covered Aluminium

Earth Conductor


Key:


Conductor

Direction of Supply


Version: 4.0

Date: 28/07/2014


7.3.10 Pole-Mounted Switches, Reclosers and Sectionalisers with High-level Control Unit

Ground Level

HV

Earth




Minimum of 2m

Ground Level

5m Separation

PVC Duct Installed

from Pole to Earth Rod

1m

2.4

m m

in

All Steelwork

Connected to

HV Earth

Note: VT Neutral

Earth Not Shown

HV Earth Electrode


Covered Aluminium

Earth Conductor


Key:


Conductor



Note: Anti-climbing

Guards Not Shown

Direction of Supply


Version: 4.0

Date: 28/07/2014


8 LV Earthing

This section provides an overview of the main LV earthing requirements; refer to EDS 06-0016 for further information. Appendix B includes the definitions of the various LV earthing system types.

8.1 Design Overview

All new low voltage mains and services shall be constructed to PME standards using combined neutral earth (CNE) cables and overhead lines to enable an earth terminal to be provided. Whenever major work (e.g. refurbishment, diversion etc.) is carried out on the low voltage distribution network it shall be brought up to PME standards where appropriate.


8.2.1 Additional PME Electrodes

In addition to the main LV neutral earth at or near the secondary substation or pole-mounted transformer, the neutral conductor shall be connected to earth (using the electrode specified in Section 8.2.2) as follows:

All pot-ends on underground cable networks.

All cut-outs above 100A for large services and multi-occupancy buildings. Note: The earth is usually placed in the cable trench outside the building.

Not more than every six spans on LV overhead line networks.

Note: Earth electrodes shall not be installed in any joint.

8.2.2 Earth Electrodes

The permitted earth electrodes are given below.

Note: The use of rod electrodes is preferred but due to practical difficulties, particularly in urban areas where damage can be caused to other services, cable electrodes are acceptable.

Cable Size Underground Cable Network Overhead Network

Up to 35mm

2

1.2m earth rod connected via 35mm2

covered copper cable or 2m of 35mm2

bare copper cable laid directly in trench underneath the LV cable


covered copper cable (below ground) and 95mm

2 covered aluminium cable (above

ground)

> 35mm2


covered copper cable or 2m of 70mm2

bare copper cable laid directly in trench underneath the LV cable


covered copper cable (below ground) and 95mm

2 covered aluminium cable (above

ground)


Version: 4.0

Date: 28/07/2014


8.2.3 Bonding Conductors

The minimum size of bonding connections are given below. Below ground, i.e. buried, earthing and bonding conductors shall be copper. Above ground bonding conductors may be copper, aluminium or corrosion protected steel of the appropriate cross sectional area.

Connection Type Bonding Conductor

Between supply neutral conductor and PME earth electrode

For cable sizes up to 35mm2: 50mm

2 covered

aluminium cable or 35mm2 covered copper

cable

For cable sizes greater than 35mm2: 95mm

2

covered aluminium cable or 50mm2 covered

copper cable

Between supply neutral conductor and link box or feeder pillar steelwork

Between sheath of SNE cable and neutral of CNE cable

At customer’s premises between service neutral and main earthing terminal

16mm2 or half the size of the neutral meter tail,

whichever is the larger.

Note: This is usually built into the cut-out

At customer’s premises between the main earthing terminal and the earth bar of the consumer unit

Note: The bonding between the main earthing terminal and the consumer unit is the responsibility of the consumer. It is given here for information only.

16mm2 or half the size of the neutral meter tail,

whichever is the larger

8.2.4 Earth Resistance Values

The resistance of the supply neutral conductor to the general mass of earth shall not at any point exceed 20Ω. This can be achieved using the values below.

Electrode Resistance Value Electrode Systems

Main LV Earth 20Ω Refer to Section 10 and 11

Additional PME Earth 100Ω


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8.2.5 Converting Existing Underground Cable Networks to PME

The opportunity shall be taken wherever possible to upgrade existing networks to PME. Any extension or modification shall use CNE cables and their typical application to existing networks is shown below.

SNE CNE

CNE SNE

SNE

CNE SNE

New CNE Service

See note 1

See note 2

See note 1

See note 2

See note 2

SNE

SNESNE

SNE SNE

S/S

S/S

S/S

CNE

S/S

S/S

LBS/S

Secondary substation

Link box

Separate neutral and earth cable

Combined neutral and earth cable

Transition joint – CNE neutral

connected to SNE neutral and shealth

PME earth electrode

Meets PME requirements

CNE

SNE

Key:

S/S

LB

Notes:

1. A non-insulated SNE cable in direct contact with

the ground will usually provide an adequate earth

electrode.

2. These sections of SNE cable will meet the PME

requirements if the sheath and neutral are bonded

at the end of main and an end of main electrode

added.See note 1

All reasonable precautions shall be taken to ensure that customers supplied via SNE cables are not adversely affected by repairs, modifications or additions to existing networks. When a CNE cable is introduced into a SNE network, existing customers may retain a SNE service provided:

A continuous metallic earth return path exists to the source substation; and

They are connected to a continuous length of non-insulated metallic sheathed cable in direct contact with the ground with an earth resistance of 10Ω or less. The table below specifies the length required to achieve this in different soil conditions.

If these conditions cannot be satisfied the service shall be converted to PME (provided the installation complies with the BS 7671 bonding requirements), or a TT earthing system shall be used.

Typical Soil Type Resistivity (Ωm) Length (m)

Loams, garden soils etc. 25 or less 8

Chalk 50 or less 15

Clay 100 or less 29

Marsh/Peat 200 or less 58

Sand/Gravel/Clay mix 300 or less 87

Slate/Shale/Rock 500 or less 115


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8.2.6 Converting Existing Customers to PME

Customers with an existing separate neutral and earth, connected to an SNE cable network can be converted to a combined neutral and earth (PME) provided the following requirements are satisfied:

The customer's installation complies with BS 7671.

A new PME cut-out is installed.

The SNE cable is replaced with a CNE cable (or the neutral and earth are combined at the service transition joint and at the cut-out).

An earth electrode is required at the service transition joint. However a length of SNE cable in direct contact with the ground will normally provide a suitable connection with earth and satisfy this requirement.

Note: It is not permitted to simply bond the neutral and earth at the cut-out.

8.2.7 Protective Neutral Bonding (PNB)

Although PME is preferred, protective neutral bonding (PNB) may provide a better solution in circumstances where it is not practical to install the LV earth at the transformer. In a PNB earthing system the LV neutral conductor is connected to an earth electrode at a point remote from the transformer at or near the customer's supply terminals. PNB arrangements may only be used if the following criteria are satisfied:

A maximum of four customers.

The connection to earth shall be made as close as possible to customer's supply terminals and no more than 40m from the furthest customer.

The earth electrode shall have a maximum resistance of 20Ω.

The earth electrode shall be a minimum of 8m from any HV earth or HV metallic sheath cable.

The metallic sheaths of any LV cables shall also be connected to the earth electrode at the customer’s supply terminals and to the LV neutral at the transformer.

The transformer tank and associated HV metalwork shall be connected to the HV earth electrode.

A PNB earth terminal shall be treated as a PME earth terminal and the appropriate labelling from Section 14 applied (EDS 07-0009.121 and EDS 07-0009.9).

Various PNB earthing arrangements are shown below.

HV : LV HV : LV

HV : LV HV : LV HV : LV

SNE Cut-outSNE Cut-outSNE Cut-out

CNE Cut-outCNE Cut-out


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9 Customer Installations

9.1 Earth Terminal

A PME earth terminal can usually be offered to customers, except in special situations, provided their installation complies with BS 7671. Customer installations including special situations and the provision of a PME earth terminal are covered in more detail in EDS 06-0017. The earth terminal shall only be provided as detailed in the design and it shall have the appropriate labelling from Section 14 applied (EDS 07-0009.130 and.9).

9.2 Connection to Supply Neutral Conductor at the Cut-out

If an earth terminal is being provided the customer's earth terminal shall be connected to the supply neutral terminal/conductor for CNE services or the earth terminal/conductor for SNE services at the cut-out using a copper conductor with a minimum cross-sectional area as specified in Section 8.2.3. Any bolted link between the neutral and the earth terminal shall be of equivalent cross-sectional area.

9.3 Connection to Cable Sheath/Armouring at the Cut-out

The metallic sheath and armouring of underground service cables shall be connected to the earth terminal by means of a copper conductor of minimum cross-sectional area as specified in Section 8.2.3. The connection to the cable sheath should be made by means of either an approved earthing clamp or a substantial sweated connection in accordance with the LV Cable Jointing manual.

Where service cables with a concentric neutral are used, the concentric neutral and any separate earth conductor shall be connected to the earth terminal, neutral terminal or neutral connector block, as appropriate.

9.4 Earth Fault Loop Impedance

Typical earth fault loop impedance values are shown below. For further information refer to EDS 06-0004 which also includes a flowchart to resolve situations where these values cannot be achieved and a dedicated form (UPN9017) to advise customers.

Earth Terminal 230V 1Ø Up to 100A

400V 3Ø Up to 100A

400V 3Ø 200 to 300A

400V 3Ø Exceeding 300A

PME/PNB 0.35Ω 0.35Ω 0.2Ω 0.15Ω

Cable Sheath/Continuous Earth Wire 0.8Ω 0.8Ω


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10 Earth Electrode Options

The table below provides the resistance of varying lengths of earth rod based on the soil resistivity. For example a 6m rod in 100Ωm soil will give a resistance of 18.6Ω .

If multiple rods are used and separated by the twice the rod length, the approximate resistance can be found by dividing the resistance of a single rod by the number of rods. For example four 6m rods spaced 6m apart will give an approximate resistance of 18.6/4 ≈ 5Ω.

No of Rods

Rod Length (m)

Resistance (Ω)

25Ωm 50Ωm 100Ωm 150Ωm 200Ωm 300Ωm 400Ωm 500Ωm

1 1.2 17.9 35.8 71.6 107.4 143.2 214.7 286.3 357.9

2 2.4 10.1 20.2 40.4 60.6 80.8 121.2 161.5 201.9

3 3.6 7.2 14.4 28.7 43.1 57.4 86.1 114.9 143.6

4 4.8 5.6 11.2 22.5 33.7 45.0 67.5 90.0 112.5

5 6.0 4.6 9.3 18.6 27.9 37.2 55.8 74.3 92.9

6 7.2 4.0 7.9 15.9 23.8 31.8 47.7 63.6 79.5

7 8.4 3.5 7.0 13.9 20.9 27.8 41.7 55.6 69.6

8 9.6 3.1 6.2 12.4 18.6 24.8 37.2 49.6 62.0

9 10.8 2.8 5.6 11.2 16.8 22.4 33.6 44.8 56.0

10 12 2.6 5.1 10.2 15.3 20.4 30.6 40.8 51.1

11 13.2 2.3 4.7 9.4 14.1 18.8 28.2 37.6 47.0

12 14.4 2.2 4.4 8.7 13.1 17.4 26.1 34.8 43.6

13 15.6 1.9 3.8 7.7 11.5 15.4 23.1 30.8 38.5

14 16.8 1.8 3.6 7.2 10.9 14.5 21.7 28.9 36.2

15 18 1.8 3.6 7.2 10.7 14.3 21.5 28.7 35.8

16 19.2 1.7 3.4 6.8 10.2 13.5 20.3 27.1 33.9

17 20.4 1.6 3.2 6.4 9.6 12.8 19.3 25.7 32.1

The table below provides the resistance of varying lengths of 70mm2 copper earth conductor buried at a depth of 0.6m and a lattice earth mat based on the soil resistivity. For example, in 100Ωm soil a 50m conductor will give a resistance of around 3.9Ω or a 1m x 1m earth mat embedded in an earthing compound will give a resistance of around 14.66Ω.

Conductor/Mat Length

Resistance (Ω)

25Ωm 50Ωm 100Ωm 150Ωm 200Ωm 300Ωm 400Ωm 500Ωm

Conductor

10m 3.6 7.3 14.6 21.9 29.2 43.8 58.3 72.9

25m 1.8 3.5 7.0 10.5 14.0 21.0 28.0 35.0

50m 1.0 2.0 3.9 5.9 7.9 11.8 15.8 19.7

100m 0.5 1.1 2.2 3.3 4.4 6.6 8.8 11.0

150m 0.4 0.8 1.5 2.3 3.1 4.6 6.2 7.7

200m 0.3 0.6 1.2 1.8 2.4 3.6 4.8 6.0

250m 0.2 0.5 1.0 1.5 2.0 3.0 4.0 5.0

1m x 1m Lattice Mat in Soil

13.2 26.3 52.6 79.0 105.3 157.9 210.6 263.2

1m x 1m Lattice Mat in Earthing Compound

3.7 7.3 14.7 22.0 29.3 44.0 58.6 73.3


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11 NetMap Earthing Information System

NetMap4 includes several earthing maps showing the earthing options to achieve 1Ω, 10Ω and 20Ω earths. Brief details are shown below. For further information refer to EDS 06-0018.

The earthing maps are selected using the Object Control as shown. The various maps can be displayed by clicking the following buttons:

1 ohm earthing map.

10 ohm earthing map.

20 ohm earthing map.

An example of the 20Ω map and its associated properties (select area of map, right-click and select Edit Selection) is shown below. The map colour and traffic light provides a quick indication of the type of earthing installation required and the properties provide additional details.

4 NetMap is UK Power Networks GIS system.


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12 Materials

The most common earthing materials are listed below and a more extensive list can be found in EAS 06-0011.

Component Description Material Code5

Earth rods Earth rod extensible 1.2m x 19mm 02472U

Earth rod coupling 02475Y

Earth rod driving stud 02473E (standard) 02478D (special)6

Earth rod driving point 02479N

Earth mat/grate

Copper lattice earth mat 1m x 1m 03100V

Galvanised earth grate Not available see EAS 06-00117

Earth electrode

70mm2 Bare stranded (7/3.55mm) hard

drawn copper cable (to BS 7884) 05823E

Bare copper tape (to BS EN 13601) 25mm x 3mm 25mm x 6mm

02998J 03008R

Bonding PVC covered stranded soft drawn copper cable (to BS 6004)

16mm2 35mm

2 70mm

2 120mm

2

05847L 05864Y 05865J 05867D

Bonding (above ground only)

PVC covered stranded aluminium cable 35mm2 50mm

2 95mm

2 120mm

2

06209E 06210A 06211K 06212U

40mm x 6mm Bare aluminium tape/bar Tape 25m Coil Bar 4m Length

03004C 01303G

80mm aluminium lap joint 01629F

Aluminium right angle fishplate 01630B

40mm x 6mm plastic cleat self-locking 01619W

22mm2 tinned copper braid 03095Q

Earth Bar Earth bar 6-way with M10 studs 02477T

Connectors Earth rod U bolt clamp 02476J

NE13 brass connector 02829A

Brass Mechanical lug M10/M12 16mm2-70mm

2 120mm

2

02228U 02236J

Tinned copper compression lug M10/M12 35mm2 50mm

2 70mm

2 120mm

2

02365H

01354K

02364X

01371X

01372H

08421S

01373S

09023L

Bimetallic splice (copper/aluminium) 35mm2/50mm

2 70mm

2/120mm

2

09891N 08683E

Rebar clamp for 20-25mm rebar 01351F

2-hole earth point with 1500mm welded tail 01353A (for use with the above)

Consumables Castrol Rustilo 431 grease Not available see EAS 06-0011

Denso tape 50mm x 10m 03452G

Marconite earthing compound 25kg bag 03132R

5 UK Power Networks logistics material code.

6 The special earth rod driving stud (02478D) is supplied as an alternative drive head where the standard earth

rod driving head (02473E) is too large to fit into existing installation tools. 7 Material currently being sourced and will be added to the stores catalogue, EAS 06-0011 will be updated when

available.


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13 Connection Techniques

The table below outlines the approved bolted, mechanical and crimped connections for earthing; exothermic welded or brazed connections are also acceptable. Refer to ECS 06-0022 for all other approved connection techniques not included below.

Application Connection Technique

Copper conductor to earth rod using U-bolt clamp

Copper conductor to copper conductor using NE13 connector

Earth rod 02472U and U-bolt clamp 02476J Sicame NE13 02829A

Copper conductor to reinforcement rebar and mesh

U-bolt clamp 02476J on rebar

Earth point with tail 01353A U-bolt clamp 02476J on mesh

Conductor connection to 6-way Earth Bar (02477T) using lug

Copper conductor to aluminium conductor or tape using a bimetallic crimps (09891N or 08683E) and lugs (various)

Bimetallic crimp 09891N Aluminium tape 50mm x 6mm 03005M, clip to wall 01620S and bimetallic lug 02228U


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Application Connection Technique

LV Board Substation Earth Ring Installed No Substation Earth Ring

Connect the neutral on the LV board to the substation earth ring using 40mm x 6mm aluminium bar

Connect the neutral on the LV board to the main earth terminal using 120mm

2 covered copper

cable. A short length of 40mm x 6mm aluminium bar may be attached to the neutral to facilitate the connection

Earth link in pit (for segregation of customer earthing systems) using earth test pit and copper tape (02998J or 03008R)

Fence panel/post bonding

Tape to tape Exothermic welded or bolted connections in accordance with ECS 06-0022

Equipment connections Mechanical lugs

HV cables Refer to the 11kV Cable Jointing manual

LV cables Refer to the LV Cable Jointing manual

Overhead Lines Refer to the Overhead Line Manual

Notes:

All surfaces shall be cleaned before jointing.

All connections to metalwork shall be free from paint, cleaned and have a protective coating of neutral compound grease8 applied before and after connection.

All below ground connections shall be completely covered in Denso tape to prevent moisture ingress and corrosion

All joints involving dissimilar metals (e.g. copper and steel, copper and aluminium) shall be 100mm above ground level and completely covered in Denso tape to prevent moisture ingress and corrosion.

8 The use of neutral compound grease is particularly important for aluminium bolted joints to prevent surface

oxidation and corrosion.


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14 Warning Labels

Situation/Location Reference9

(Material Code10

) Specification Label

Separate HV and LV Earths

In LV cabinet or micro/compact

EDS 07-0009.118 100mm x 50mm adhesive label

Separate HV and LV Earths

Install in prominent position inside fence/enclosure

EDS 07-0009.117

(21515D)

170mm x 220mm

ABS sheet

PME/PNB Earth

Next to the cut-out

EDS 07-0009.130

(21758H)

100mm x 50mm adhesive label

PNB Earth

On the PNB earth where it enters the cut-out

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

Two Earthing Systems

At the removal link between connecting two separate earthing systems

EDS 07-0009.120 100mm x 50mm adhesive label

HOT Sites Refer to EDS 06-0121 for a complete set of HOT site labels

9 Refer to EAS 07-0021 for the availability of labels without a material code.

10 UK Power Networks logistics material code.


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15 Measurements

15.1 Overview

The HV and LV (if applicable) earth resistance values shall be measured before and after connection to the network to confirm that the design values have been achieved. This section includes various methods for measuring the earth resistance, for more detailed information, guidance and test equipment refer to ECS 06-0024.

All measurements shall be carried out in accordance with the Distribution Safety Rules and:

HV insulated gloves and dielectric footwear (yellow wellies) shall be worn when handling the test equipment.

All connections to the substation or pole-mounted equipment shall be made before connecting the leads to the test equipment.

The test leads shall be temporarily disconnected from the test equipment and the ends kept separated when moving probes during the test.

Communication shall be maintained between operators at all times.

These measures are intended to reduce the possibility, or consequence, of test leads becoming live under HV fault conditions.

This section includes the following test methods:

Earth resistance measurement using the fall-of-potential method.

Earth resistance measurement using the comparison method.

Earth resistance measurement using a clamp meter.

15.2 Test Equipment

The following approved earth resistance measurement equipment, leads and accessories are available from Norwich Instrument Services (NIS) and are compatible with the tests detailed in this standard:

DET4TD2 (battery) or DET4TR2 (rechargeable) is a basic four-terminal earth tester.

DET4TC2 (battery) or DET4TCR2 (rechargeable) is a four-terminal earth tester with selectable frequencies and greater sensitivity and also includes stakeless (clamp-on) testing capability. Note: The clamp-on kit is an optional accessory.

15.3 Earth Resistance Measurement using the Fall-of-Potential Method

The most common method of measuring the earth resistance is the fall-of-potential method using a four-terminal or three-terminal earth tester. Three points of contact are made with the soil: the earthing system under test (connected to C1 and P1), the current probe (C2) and the voltage probe (P2).


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The method described below is suitable for the measurement of small earth electrode systems associated with secondary substations and pole-mounted equipment. For larger systems additional measurement points are required as detailed in ECS 06-0024.

1. Connect terminals C1 and P1 to the HV or LV earthing system under test.

2. Place the C2 current probe 50 metres away from the substation or pole.

3. Take three measurements by placing the P2 potential probe in line with the C2 probe at 25 metres (50%), 31 metres (62%) and 35 metres (70%) away from substation or pole the as shown below11.

C1 P1 P2 C2

FOUR-TERMINAL

EARTH TESTER

50% 62% 70% 100%

25m 31m50m

35mP1

P2

C1

C2

Disconnect P2 terminal

when moving P2 probe

Operator to wear HV

rubber gloves

4. If the measured values are within 5% of the middle (31 metre) value and do not decrease with distance, the value at 31 metres is the overall earth resistance. If the measurements are more than 5% of the middle value the test should be repeated using a different transverse, i.e. relocate the C2 probe at 90 degrees to the first test and measure the potential using P2 along the same line. See examples below. If this does not provide a satisfactory value the probe spacing should be doubled to 50, 62, 70 and 100 metres and the test repeated again.

Example 1:

9.6 ohms measured at 25m



10 ohms x 0.95 (-5%) = 9.5 ohms and 10 ohms x 1.05 (+5%) = 10.5 ohm

The readings are within the range 9.5-10.5, therefore the resistance of 10 ohms is valid.

Example 2:




12 ohms x 0.95 (-5%) = 11.4 ohms and 12 ohms x 1.05 (+5%) = 12.6ohm

The readings are outside the range 11.4 to 12.6, therefore the resistance of 12.0 ohms is not valid and the test should be repeated.

11 Other distances may be used provided the percentage distances are maintained.


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15.4 Earth Resistance Measurement using the Comparison Method

If there is not sufficient space to carry out a fall-of-potential measurement (or if there is a likelihood of buried metallic services along the route as in many urban locations) it is possible to measure the earth resistance by comparison using an earth. This method uses the wider earthing system as a reference point and assumes it has a very low value.

A four-terminal earth tester is used to measure the earth resistance of a local earthing electrode.

1. Connect terminals C1 and P1 to the earthing system under test.

2. Connect terminals C2 and P2 to the reference earthing system as shown below.

3. The meter will display the sum of the reference earth and the test electrode; if the reference earth is a large cable network the dominant resistance will be that of the system under test. In other words, the actual value will be slightly less than the displayed value.

Earth Bar

Substation Earth Under Test

(Disconnected from Network)

C1 P1 P2 C2

FOUR-TERMINAL

EARTH TESTER

Reference Earth i.e.

Wider NetworkR1

RParallel

P1

C1

P2

C2

If RParallel << R1 then measured ‘earth loop’

resistance’ (RParallel + R1) approaches R1

15.5 Earth Resistance Measurement using a Clamp Meter

A clamp meter can also be used to carry out a comparative test by measuring between the bundled electrode system and the wider cable sheath network as shown below. This test can be used whether or not cables have been connected; however it is more accurate if the cables are connected. It is of most value if there is no easy way of measuring the contribution of the whole system. Note: This method relies on conductors being ‘bundled’ together to effectively reduce the test to a single point of connection between the earthing system and the reference earth. If this is not done, the test current will circulate around the local electrode system only and the result will be incorrect; readings below 0.5 ohms should be treated with suspicion.

Earth Bar

Clamp Type

Earth Tester

Substation Earth Under Test

(Connected to the Network)

Reference Earth i.e.

Wider Network

If RParallel << R1 then measured ‘earth loop’

resistance’ (RParallel + R1) approaches R1

R1

RParallel

Substation Connected to

Network via Cable Screens


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16 References

EDS 06-0002 HOT Sites (internal document only)

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

EAS 06-0011 Earthing Materials

EDS 06-0012 Earthing Design Criteria

EDS 06-0014 Secondary Substation Earthing Design

EDS 06-0015 Pole-mounted Equipment Earthing Design

EDS 06-0016 LV Network Earthing Design

EDS 06-0017 Customer Installation Earthing Design

EDS 06-0018 NetMap Earthing Information System (internal document only)

ECS 06-0022 Grid and Primary Substation Earthing Construction

ECS 06-0024 Earthing Testing and Measurements

EAS 07-0021 Signs and Labels for Operational Sites

EDS 07-0102 Secondary Substation Civil Design Standards

EDS 08-0121 Supplies to HOT Sites and National Grid Sites

EOS 09-0067 Theft of Substation Earthing (internal document only)

ECP 11-0503 Earthing Commissioning Procedure

11kV and LV Cable Jointing Manual

Overhead Line Manual

EM-MS-06-005-r3/ EM-MS-06-009-r0

Instructions for Disconnecting Auxiliary Circuits in a Fuse Cabinet for a Hot Site


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Appendix A – Purpose of Earthing

When an earth fault occurs on the electricity distribution network (see diagram below):

1. A large current will flow along the cable and return to the source via the cable sheath or combined neutral-earth conductor and the general mass of earth.

2. The current will flow until the upstream protection operates. 3. The current flowing through the earth can cause a considerable rise in voltage (known as

the earth potential rise or EPR) on the earth and any earthed metalwork near the fault - creating possible danger (touch and step voltages) to anyone in the vicinity.

4. This excessive rise in voltage may be transferred onto adjacent power and communication cables creating possible danger to anyone who might be in contact with them – this may be some distance from the actual fault.

Protection

Device

LoadEarth

Fault

Source

Transformer

Direction of

Current Flow

Some current returns

through the ground

Some current returns

through cable sheath

Therefore the purpose of earthing is to:

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

2. To prevent dangerous voltages appearing at the substation and causing danger to staff or the public.

3. To prevent dangerous voltages appearing on the customers LV neutral/earth. 4. To prevent damage to sensitive equipment (e.g. communications).

An EPR of less than 430V allows the HV and LV to be combined and is classified as a COLD site. The EPR can be limited by reducing the fault current or the earth resistance at the substation or site. If the EPR is greater than 430V the substation the HV and LV have to be separated and the site is classified as a HOT site.


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Appendix B – LV Earthing Systems

PME – a form of TN-C-S system where the neutral and earth are connected together at the transformer. The supply cables use a combined neutral and earth (CNE) cable and the customer’s installation uses a cable with a separate neutral and earth.

PNB – similar to PME except the neutral is earthed at the customer end rather than the transformer. This arrangement is only used to supply up to four customers in remote locations.

TN-S – the neutral and earth are connected together at the transformer and separate neutral and earth (SNE) cables are used throughout. Note: If any part of the network is converted to PME, or the earth and neutral conductors are bonded at any point beyond the transformer, the system is TN-C-S.

TT - the neutral and earth are connected together at the transformer. No earth terminal is provided to the customer. The customer has an independent earth electrode to which any exposed metalwork of the customer’s installation is connected. The earth loop impedance is relatively high for this arrangement and therefore a residual current device (RCD) is usually required to protect the customer's installation.

(PME)

Customer's Earth

RA

ECS 06-0023 Secondary Distribution Network …library.ukpowernetworks.co.uk/library/asset/bc81106a-1cd...When entering a site where earth connections have been stolen, safety precautions

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