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SS 551 : 2009

(ICS 13.260; 91.140.50)



(Formerly CP 16)

Published by SPRING Singapore 2 Bukit Merah Central Singapore 159835 SPRING Singapore Website: Standards Website:


SS 551 : 2009

(ICS 13.260; 91.140.50)



(Formerly CP 16)

All rights reserved. Unless otherwise specified, no part of this Singapore Standard may be reproduced or utilised in any form or by any means, electronic or mechanical, including photocopying and microfilming, without permission in writing from SPRING Singapore at the address below: Head Standardisation Department SPRING Singapore 2 Bukit Merah Central Singapore 159835 Telephone: 62786666 Telefax: 62786667 Email: [email protected]

ISBN 978-981-4278-27-0

SS 551 : 2009


Page Foreword CLAUSES 1 1.1 1.2 2 3 3.1 3.2 3.3 3.4 4 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 5 5.1 5.2 6 6.1 6.2 6.3 6.4 6.5 6.6 Scope and normative references Scope Normative references Definitions Design considerations ­ General System earthing Equipment earthing Factors involved in effective earthing Soil resistivity Earth-electrodes Effect of shape on electrode resistance Resistance of common types of earth-electrodes Selection of a material for an earth-electrode or a buried uninsulated earthing conductor Miscellaneous types of electrodes Earthing cathodically protected structures Selection of an earthing conductor and connection to an electrode Current density at the surface of an earth electrode Potential gradient around earth electrodes Generator sets General Low voltage generators Substations General General earthing arrangement of substations Substation earth-electrodes Earthing conductors for substations and industrial installations operating at similar voltages Earthing of sheath and/or armour of cables Miscellaneous matters in substations 10 10 10 11 16 16 16 16 17 19 19 19 28 30 33 34 39 39 44 44 44 53 53 55 56 57 62 65 9


SS 551 : 2009

Page 7 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10 8 8.1 8.2 8.3 9 10 11 11.1 11.2 11.3 12 13 13.1 13.2 13.3 13.4 13.5 14 14.1 14.2 14.3 14.4 15 15.1 16 Consumers' electrical installations General Earthing of installations Exchange of information Main equipotential bonding Installations and locations of increased shock risk Protective conductors Industrial and commercial installations Impedance earthing of the supply to large mobile or transportable plant Construction sites Marinas Temporary scaffolding and similar metallic structures Metallic structures assembled by means of bolted joints or screw clamps Lightning protection Other temporary structures Telecommunication circuits and equipment Lightning protection and earthing Street lighting and other electrically supplied street furniture General TN-S systems Protection arrangements Hazardous areas (potentially explosive atmospheres) Earthing of other equipment and installations General Fences Pipes Uninterruptible power supplies (UPSs) Secondary circuits of instrument transformers Earthing of conductors for safe working General Safety earthing Precautions relating to apparatus and cables Safety earthing of low voltage conductors Inspection and test ____________________________________________ Periodic inspection Documentation 65 65 66 67 67 67 68 69 72 72 72 73 73 73 74 74 74 74 74 75 75 75 76 76 76 76 76 76 76 76 77 78 78 78 78 82


SS 551 : 2009

Page ANNEXES A B (informative) Typical calculations for various types of earth electrodes (informative) Measures on earthing systems to reduce the effects of high frequency interference 84 87

TABLES 1 2 3 4 5 6 7 8 9 10 11 12 13 Examples of soil resistivity (-m) Factors for parallel electrodes arranged in line Factors for electrodes arranged in a hollow square Minimum sizes of components for earth-electrodes Coefficients for strip or round conductor Type and minimum dimensions of earth-electrode materials ensuring mechanical strength and corrosion resistance Suitability of materials for bonding together Geometric mean distance z for closely spaced reinforcing rods Earth fault current densities for 1 s duration for earthing conductors with initial conductor temperature of 30 oC Earth fault currents (in kA) for copper strip earthing conductors Earth fault currents (in kA) for aluminium strip earthing conductors Values of K and Values of current densities for earthing conductors 17 22 23 24 26 29 30 33 36 37 38 38 58

FIGURES 1 2 3 4 5 6 7 8 9 10 11 12 Effect of buried length of rod or pipe electrode on calculated resistance for soil resistivity of 100 -m (assumed uniform) Effect of inter-electrode spacing on combined resistance Calculated curves of resistance of 12.5 mm diameter driven rod electrodes Resistance of horizontal strip electrodes Ground surface potentials around a single rod and three rods in line Potential gradient in the vicinity of a horizontal strip electrode Potential distribution between rod electrodes Single low voltage standby generator (without paralleling facility) Low voltage standby generators with neutrals connected Low voltage standby generators with neutral earthing transformers Low voltage standby generators with star-point switching

21 21 25 27 40 41 42 47 48 49 50

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


SS 551 : 2009

Page 13 14 15 16 17 Simplified illustration of single-core cable sheath bonding systems Earthing arrangement and protective conductors for consumers' installations Measurement of earth electrode resistance Earth resistance curves Measurement of earth resistivity 64 66 80 81 82


SS 551 : 2009


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 both industrial and commercial installations; Integration of earthing and lightning protection system; Dedicated section on street lighting and electrically-supplied street furniture, which incorporates a myriad 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 earthing systems.

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. SPRING Singapore shall not be held responsible for identifying any or all of such patent rights.

NOTE 1. Singapore Standards are subject to periodic review to keep abreast of technological changes and new technical developments. The changes in Singapore Standards are documented through the issue of either amendments or revisions. Compliance with a Singapore Standard does not exempt users from legal obligations.



SS 551 : 2009

Code of practice for earthing




Scope and normative references



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.


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 BS 1377 BS 4363 BS 6423 BS 6626 Threaded pipe fittings in malleable cast iron and cast copper alloy Methods of test for soils for civil engineering purposes Specification for distribution assemblies for reduced low voltage electricity supplies for construction and building sites Code of practice for maintenance of electrical switchgear and controlgear for voltages up to and including 1 kV 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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