BS7430 earthing&lighting protection.pdf

BRITISH STANDARD

BS7430:1998

Code of practice for

Earthing

ICS 13.260; 91.140.50

BS7430:1998

Committees responsible for this

British Standard

The preparation of this British Standard was entrusted to Technical

Committee GEL/600, Earthing, upon which the following bodies were

represented:

BEAMA Ltd.

BEAMA Electrical Cable and Conductor Accessory Manufacturers’ Association

BG plc

British Broadcasting Corporation

British Telecommunications plc

Copper Development Association

Electrical Contractors’ Association

Electrical Installation Equipment Manufacturers’ Association (BEAMA Ltd.)

Electricity Association

Engineering Equipment and Materials Users’ Association

Institute of Petroleum

Institution of Lighting Engineers

Institution of Incorporated Executive Engineers

National Federation of Master Steeplejacks and Lightning Conductor

Engineers

National Inspection Council for Electrical Installation Contracting

Department of Trade and Industry (Engineering Inspectorate)

Department of the Environment, Transport and the Regions

(HighwaysAgency)

Health and Safety Executive

Ministry of Defence

This British Standard, having

been prepared under the

directionof the General

Electrotechnical Standards

PolicyCommittee, was

publishedunder the authority

ofthe Standards Committee

andcomes into effect on

15November1998

First published as CP1013:1965

First published as BS7430,

January1992

Second edition, November1998

Amendments issued since publication

Amd. No.

Date

Comments

The following BSI references

relate to the work on this

standard:

Committee reference GEL/600

Draft for comment98/260937 DC

ISBN 0 580 28229 5

BS7430:1998

Contents

Page

Committees responsible

Inside front cover

Foreword

iii

1 Scope

2 Normative references

3 Definitions

4 Supply system earthing

5 Electrical equipment earthing

6 System and equipment protection

7 Factors involved in effective earthing

8 Soil resistivity

9 Effect of shape on electrode resistance

10 Resistance of common types of earth electrodes

11 Selection of a material for an earth electrode or a buried

uninsulated earthing conductor

12 Miscellaneous types of electrode

13 Earthing cathodically protected structures

14 Selection of an earthing conductor and connection to an electrode

15 Current density at the surface of an earth electrode

16 Potential gradient around earth electrodes

17 Periodic inspection and testing of the earthing

system and measurement of resistance of installed electrodes

and earthing conductors

18 Generating plant

19 Power stations and substations

20 Earthing associated with overhead power lines

21 Consumers’ electrical installations

22 Temporary scaffolding and similar metallic structures

23 Telecommunication circuits and equipment

24 Lightning protection and earthing

25 Mines and quarries

26 Street lighting and other electrically supplied street furniture

27 Hazardous areas (potentially explosive atmospheres)

28 Earthing of conductors for safe working

29 Electric traction

Annex A (informative) Typical calculations for various

types of earth electrodes

Bibliography

Figure 1 — TN systems

Figure 2 — TT and IT systems

Figure 3 — Effect of buried length of rod or pipe electrode on calculated

resistance for soil resistivity of100 7 ·m (assumed uniform)

Figure 4 — Effect of inter-electrode spacing on combined resistance

Figure 5 — Calculated and experimental curves of resistance

of12.5mm diameter driven rod electrodes

Figure 6 — Resistance of horizontal strip electrodes

Figure 7 — Effect of spacing on combined resistance of two

horizontal strip electrodes

Figure 8 — Ground surface potentials around a

single rod and three rods in line

BS7430:1998

Page

Figure 9 — Potential gradient in the vicinity of a

horizontal strip electrode

Figure 10 — Potential distribution between rod electrodes

Figure 11 — Measurement of earth electrode resistance

Figure 12 — Earth resistance curves

Figure 13 — Measurement of earth resistivity

Figure 14 — Single low voltage standby generator

(without paralleling facility)

Figure 15 — Low voltage standby generators with neutrals connected

Figure 16 — Low voltage standby generators with

neutral earthing transformers

Figure 17 — Low voltage standby generators with star-point switching

Figure 18 — Single high voltage standby generating set not

suitable for parallel operation with incoming mains supply

Figure 19 — Single high voltage standby generating set suitable

for parallel operation with incoming mains supply

Figure 20 — Multiple high voltage standby generating sets

with neutral earthing transformer suitable for parallel operation

with each other and with the incoming mains supply

Figure 21 — Multiple high voltage standby generating

sets suitable for parallel operation with each other and

with the incoming mains supply

Figure 22 — Low voltage standby generating sets with

generator transformers giving a high voltage supply and

suitable for parallel operation with the incoming mains supply

Figure 23 — Simplified illustration of single-core cable sheath

bonding systems

Figure 24 — Earthing arrangement and protective conductors

for consumers’ installations

Figure 25 — Method of supplying electricity to caravan

pitch electrical equipment

Table 1 — Examples of soil resistivity

Table 2 — Factors for parallel electrodes arranged in line

Table 3 — Factors for electrodes arranged in a hollow square

Table 4 — Minimum sizes of components for earth electrodes

Table 5 — Coefficents for strip or round conductor electrodes

Table 6 — Recommended materials for the manufacture

of earthing components

Table 7 — Corrosion resistance of some electrode

materials related to soil parameters

Table 8 — Suitability of materials for bonding together

Table 9 — Geometric mean distance z for closely spaced reinforcing rods

Table 10 — Earth fault current densities for1s duration for

earthing conductors with initial conductor temperature of30 C 26

Table 11 — Earth fault currents for copper strip earthing conductors 27

Table 12 — Earth fault currents for aluminium strip earthing conductors 28

Table 13 — Values of K and ¶ 28

Table 14 — Values of current densities for earthing conductors

BS7430:1998

Foreword

This British Standard has been prepared by Technical Committee GEL/600. It

supersedes BS7430:1991 which is withdrawn. This edition incorporates various

changes necessary to ensure that the standard remains authoritative in the light

of recent developments, but it does not constitute a comprehensive revision.

Since the preparation of the previous edition of BS7430, the Electricity at Work

Regulations (Northern Ireland) were introduced in1991, and the Electrical

Equipment (Safety) Regulations were introduced in1994.

Protective multiple earthing (PME) is widely adopted in the UK for public

electricity supply at low voltage and this British Standard contains

recommendations on appropriate earthing practices for consumers’ installations.

However, this standard describes all three types of electrical supply systems

standardized by the International Electrotechnical Commission (IEC).

The development of international electrical installation standards by the

International Electrotechnical Commission (IEC) and their adoption by the

European Committee for Electrotechnical Standardization (CENELEC) have

been recognized in BS7671, Requirements for electrical installations (IEE Wiring

Regulations, Sixteenth Edition). This edition of BS7430 has been adapted to align

with the latter document.

BS7430 gives recommendations for earthing temporary structures. The clause

on generation covers standby generators, machines operating in parallel with an

incoming supply, asynchronous machines and both three-phase and single-phase

portable generators.

BS7671, Requirements for electrical installations (IEE Wiring Regulations,

Sixteenth edition) is not statutory, but compliance may satisfy the requirements

of certain statutory regulations. It is advisable therefore to comply with BS7671

where appropriate.

As a code of practice, this British Standard takes the form of guidance and

recommendations. It should not be quoted as if it were a specification and

particular care should be taken to ensure that claims of compliance are not

misleading.

A British Standard does not purport to include all the necessary provisions of a

contract. Users of British Standards are responsible for their correct application.

Compliance with a British Standard does not of itself confer immunity

from legal obligations.

Summary of pages

This document comprises a front cover, an inside front cover, pages i to iv,

pages1to82, an inside back cover and a back cover.

This standard has been updated (see copyright date) and may have had

amendments incorporated. This will be indicated in the amendment table on

theinside front cover.

iii

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