SINGAPORE STANDARD Code of practice for earthing

SS 551 : 2009 (ICS 13.260; 91.140.50)

SINGAPORE STANDARD

Code of practice for earthing (Formerly CP 16)

Published by

SS 551 : 2009 (ICS 13.260; 91.140.50)

SINGAPORE STANDARD

Code of practice for earthing

ISBN 978-981-4278-27-0

SS 551 : 2009

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This Singapore Standard was approved by the Electrical and Electronic Standards Committee on behalf of the Standards Council of Singapore on 6 October 2009. First published as CP 16, 1981 First revision, 1991 Second revision and renumbering as SS 551, 2009 The Electrical and Electronic Standards Committee appointed by the Standards Council consists of the following members:

Name Capacity

Chairman : Mr Renny Yeo Ah Kiang Member, Standards Council

Deputy Chairman : Er. Peter Leong Weng Kwai Member, Standards Council

Secretary : Mr Lee Toon Huat SPRING Singapore

Members : Mr David Chen Singapore Manufacturers’ Federation

Prof Choi San Shing Nanyang Technological University

Mr Chong Weng Hoe TUV SUD PSB Pte Ltd

Mr Mazad Khan Jeaudeen Singapore International Chamber of Commerce

Er. Adeline Koh Lian Suan Association of Consulting Engineers Singapore

Mr Kwek Chin Hin National Environment Agency

Er. Prof Liew Ah Choy National University of Singapore

Er. Lim Ah Hee Housing & Development Board

Er. Lim Say Leong Singapore Business Federation

Er. Kenneth Liu Association of Consulting Engineers Singapore

Er. Ng Eng Kiong Individual Capacity

Er. Ng Kim Leong Institution of Engineers Singapore

Mr Ng Kin Ming Singapore Electrical Contractors and Licensed Electrical Workers Association

Mr Michael Ong SPRING Singapore

Er. Ong Ser Huan Institution of Engineers Singapore

Mr K Seshadri Individual Capacity

Mr Sim Wee Meng Land Transport Authority

Mr Tan Boon Chong Singapore Manufacturers’ Federation

Er. Tan Hak Khoon Energy Market Authority

Er. Andrew Tan Lee Ann SP PowerGrid Ltd

Dr Wong Woon Kwong Agency for Science, Technology and Research

Mr Jimi Wong Yick Chee Singapore Electrical Trades Association

Prof Yeo Tat Soon National University of Singapore

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The Technical Committee on Earthing, appointed by the Electrical and Electronic Standards Committee and responsible for the preparation of this standard consists of representatives from the following organisations:

Name Capacity

Chairman : Er. Ng Eng Kiong Member, Electrical and Electronic Standards Committee

Deputy Chairman : Er. Wang Hee Weng Singapore Electrical Contractors and Licensed

Electrical Workers Association

Secretary : Mr Allan Koh SPRING Singapore

Members : Assoc Prof Chang Che Sau National University of Singapore

Mr Eng Cher Suah Singapore Electrical Trades Association

Er. Malvin Ho Hock Choon Land Transport Authority

Mr Lee Chee Whye Singapore Polytechnic

Mr Thomas Lee Eng Kiang JTC Corporation

Mr Lim Choon Poh SP Services Ltd

Er. Lim Say Leong Singapore Manufacturers’ Federation

Er. Timmy Mok Kam Tim Institution of Engineers Singapore

Mr Stephen Mok Soon Huan Building and Construction Authority

Er. Ong Seng Wei SP PowerGrid Ltd

Mr Caleb Seow Hong Energy Market Authority

Er. Soh Swee Seng Housing & Development Board

Mr Teh Gek Huat Ngee Ann Polytechnic

Assoc Prof Wang Peng Nanyang Technological University

Co-opted Member : Er. David Chan Tat Wai Individual Capacity

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Contents Page

Foreword 9 CLAUSES

1 Scope and normative references 10

1.1 Scope 10

1.2 Normative references 10

2 Definitions 11

3 Design considerations – General 16

3.1 System earthing 16

3.2 Equipment earthing 16

3.3 Factors involved in effective earthing 16

3.4 Soil resistivity 17

4 Earth-electrodes 19

4.1 Effect of shape on electrode resistance 19

4.2 Resistance of common types of earth-electrodes 19

4.3 Selection of a material for an earth-electrode or a buried uninsulated earthing conductor 28

4.4 Miscellaneous types of electrodes 30

4.5 Earthing cathodically protected structures 33

4.6 Selection of an earthing conductor and connection to an electrode 34

4.7 Current density at the surface of an earth electrode 39

4.8 Potential gradient around earth electrodes 39

5 Generator sets 44

5.1 General 44

5.2 Low voltage generators 44

6 Substations 53

6.1 General 53

6.2 General earthing arrangement of substations 55

6.3 Substation earth-electrodes 56

6.4 Earthing conductors for substations and industrial installations operating at similar voltages 57

6.5 Earthing of sheath and/or armour of cables 62

6.6 Miscellaneous matters in substations 65

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Page 7 Consumers’ electrical installations 65

7.1 General 65

7.2 Earthing of installations 66

7.3 Exchange of information 67

7.4 Main equipotential bonding 67

7.5 Installations and locations of increased shock risk 67

7.6 Protective conductors 68

7.7 Industrial and commercial installations 69

7.8 Impedance earthing of the supply to large mobile or transportable plant 72

7.9 Construction sites 72

7.10 Marinas 72

8 Temporary scaffolding and similar metallic structures 73

8.1 Metallic structures assembled by means of bolted joints or screw clamps 73

8.2 Lightning protection 73

8.3 Other temporary structures 74

9 Telecommunication circuits and equipment 74

10 Lightning protection and earthing 74

11 Street lighting and other electrically supplied street furniture 74

11.1 General 74

11.2 TN-S systems 75

11.3 Protection arrangements 75

12 Hazardous areas (potentially explosive atmospheres) 75

13 Earthing of other equipment and installations 76

13.1 General 76

13.2 Fences 76

13.3 Pipes 76

13.4 Uninterruptible power supplies (UPSs) 76

13.5 Secondary circuits of instrument transformers 76

14 Earthing of conductors for safe working 76

14.1 General 76

14.2 Safety earthing 77

14.3 Precautions relating to apparatus and cables 78

14.4 Safety earthing of low voltage conductors 78

15 Inspection and test ____________________________________________ 78

15.1 Periodic inspection 78

16 Documentation 82

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Page ANNEXES

A (informative) Typical calculations for various types of earth electrodes 84

B (informative) Measures on earthing systems to reduce the effects of high frequency interference 87 TABLES

1 Examples of soil resistivity (Ω-m) 17

2 Factors for parallel electrodes arranged in line 22

3 Factors for electrodes arranged in a hollow square 23

4 Minimum sizes of components for earth-electrodes 24

5 Coefficients for strip or round conductor 26

6 Type and minimum dimensions of earth-electrode materials ensuring mechanical strength and corrosion resistance 29

7 Suitability of materials for bonding together 30

8 Geometric mean distance z for closely spaced reinforcing rods 33

9 Earth fault current densities for 1 s duration for earthing conductors with initial conductor temperature of 30 oC 36

10 Earth fault currents (in kA) for copper strip earthing conductors 37

11 Earth fault currents (in kA) for aluminium strip earthing conductors 38

12 Values of K and β 38

13 Values of current densities for earthing conductors 58 FIGURES

1 Effect of buried length of rod or pipe electrode on calculated resistance for soil resistivity of 100 Ω-m (assumed uniform) 21

2 Effect of inter-electrode spacing on combined resistance 21

3 Calculated curves of resistance of 12.5 mm diameter driven rod electrodes 25

4 Resistance of horizontal strip electrodes 27

5 Effect of spacing on combined resistance of two horizontal strip electrodes _____________ 27

6 Ground surface potentials around a single rod and three rods in line 40

7 Potential gradient in the vicinity of a horizontal strip electrode 41

8 Potential distribution between rod electrodes 42

9 Single low voltage standby generator (without paralleling facility) 47

10 Low voltage standby generators with neutrals connected 48

11 Low voltage standby generators with neutral earthing transformers 49

12 Low voltage standby generators with star-point switching 50

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Page 13 Simplified illustration of single-core cable sheath bonding systems 64

14 Earthing arrangement and protective conductors for consumers’ installations 66

15 Measurement of earth electrode resistance 80

16 Earth resistance curves 81

17 Measurement of earth resistivity 82

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Foreword

This Singapore Standard was prepared by the Technical Committee on Earthing under the purview of the Electrical and Electronic Standards Committee. This standard is the result of the revision of Singapore Standard CP 16 : 1991 and has been renumbered as SS 551.

This Code was revised to give an updated guidance on the principles and practices of earthing. The main changes are:

Earthing for substations, generator sets and consumers’ electrical installations which include bothindustrial and commercial installations;

Integration of earthing and lightning protection system;

Dedicated section on street lighting and electrically-supplied street furniture, which incorporates amyriad of electrically operated equipment and products;

Emphasis on earthing of conductors for safe working;

Elaboration on periodic inspection and testing to maintain the integrity and reliability of earthingsystems.

The earthing and bonding requirements of electrical installations in buildings are covered in depth in Singapore Standard Code of Practice for Electrical Installations, SS CP 5 : 1998.

This Code is an adoption of BS 7430 : 1998 (except for Clause 2 which is aligned to SS CP 5 : 1998) and relevant clauses of Draft BS 7430 : 2007 and is implemented with the permission of the British Standards Publishing Ltd.

Attention is drawn to the possibility that some of the elements of this Singapore Standard may be the subject of patent rights. Enterprise Singapore shall not be held responsible for identifying any or all of such patent rights.

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Code of practice for earthing 1 Scope and normative references 1.1 Scope 1.1.1 General This Code gives guidance on the methods which may be adopted to earth an electrical system for the purpose of limiting the potential (with respect to the general mass of the earth) of current-carrying conductors forming part of the system, and non-current-carrying metalwork associated with equipment, apparatus, and appliances connected to the system. The former object is normally essential to the proper operation of the system, and this aspect is generally known as "system earthing". The latter concerns the safety of human life, of animals and of property, and this aspect is sometimes known as "equipment earthing". The term "earthing" is used in this regard whether or not reliance is placed on the earth itself as a low impedance return path for earth fault current. 1.1.2 Exclusion from scope This Code applies to land-based installations; it does not apply to trains, ships, aircraft or offshore installations, nor does it deal with the earthing of medical equipment or special problems encountered with solid-state electronic components and equipment due to their sensitivity to static electricity. This standard does not address electromagnetic compatibility requirements for earthing, nor does it give recommendation for functional earthing. 1.1.3 Materials, appliances and components Materials, appliances and components should comply with the relevant Singapore Standards, International Electrotechnical Commission Publications, British Standards or other equivalent national standards. 1.2 Normative references The following referenced documents are indispensable for the application of this Code. For dated references, only the edition cited applies. For undated references, the latest edition of the document (including any amendments) applies. BS 143 and 1256 Threaded pipe fittings in malleable cast iron and cast copper alloy BS 1377 Methods of test for soils for civil engineering purposes BS 4363 Specification for distribution assemblies for reduced low voltage electricity

supplies for construction and building sites BS 6423 Code of practice for maintenance of electrical switchgear and controlgear for

voltages up to and including 1 kV BS 6626 Code of practice for maintenance of electrical switchgear and controlgear for

voltages above 1 kV and up to and including 36 kV

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BS 6701 Telecommunications equipment and telecommunications cabling. Specification for installation, operation and maintenance

BS 6867 Code of practice for maintenance of electrical switchgear for voltages above 36 kV

BS EN 10025-1 Hot rolled products of structural steels. General technical delivery conditions

BS EN 1011-4 Welding. Recommendations for welding of metallic materials. Arc welding of aluminium and aluminium alloys

BS EN 13636 Cathodic protection of buried metallic tanks and related piping

BS EN 15112 External cathodic protection of well casing

IEC 60079-0 Explosive atmospheres - Part 0: Equipment - General requirements

IEC 60309-1 Plugs, socket-outlets and couplers for industrial purposes – Part 1: General requirements

IEC 60364-1 Low-voltage electrical installations – Part 1: Fundamental principles, assessment of general characteristics, definitions

IEC 61140 Protection against electric shock – Common aspects for installation and equipment

IEC 61557-1 Electrical safety in low voltage distribution systems up to 1000 V a.c. and 1 500 V d.c. – Equipment for testing, measuring or monitoring of protective measures – Part 1: General requirements

IEC 61558-1 Safety of power transformers, power supplies, reactors and similar products – Part 1: General requirements and tests

SS 97-1 Residual current operated circuit-breaker without integral overcurrent protection for household and similar uses (RCCBs) – General rules

SS 322 Earthing and bonding clamps

SS 538 Code of practice for maintenance of electrical equipment of electrical installations

SS CP 5 Code of practice for electrical installations

SS CP 33 Code of practice for lightning protection

SS CP 88 Code of practice for temporary electrical installations – Construction and building sites

TIA-J-STD-607-A Commercial building grounding (earthing) and bonding requirements for telecommunications