New Zealand Technical Report Low voltage supply earthing systems SNZ TR 6120:2014
COMMITTEE REPRESENTATION This technical report was prepared under the supervision of the LV Supply Earthing Systems Review Committee that the Standards Council established under the Standards Act 1988. The committee consisted of representatives of the following nominating organisations: Chorus Limited Electricity Engineers Association New Zealand Electrical Institute ACKNOWLEDGEMENT Standards New Zealand gratefully acknowledges the contribution of time and expertise from all those involved in developing this technical report, particularly Peter Browne and Alan Marshall. Opus International is also acknowledged for the use of Figures 1 and B1. COPYRIGHT The copyright of this document is the property of the Standards Council. No part of this document may be reproduced by photocopying or by any other means without the prior written permission of the Chief Executive of Standards New Zealand, unless the circumstances are covered by Part 3 of the Copyright Act 1994. Standards New Zealand will vigorously defend the copyright in this standard. Every person who breaches Standards New Zealand s copyright may be liable, under section 131(5) (a) of the Copyright Act 1994, to a fine not exceeding $10,000 for every infringing copy to which the offence relates, but not exceeding $150,000 for the same transaction, or to imprisonment for a term not exceeding 5 years. Those in breach under section 131(5) (b) of the Copyright Act 1994 may be liable to a fine not exceeding $150,000 or to imprisonment for a term not exceeding 5 years. If there has been a flagrant breach of copyright, Standards New Zealand may also seek additional damages from the infringing party, in addition to obtaining injunctive relief and an account of profits. Published by Standards New Zealand, the trading arm of the Standards Council, Private Bag 2439, Wellington 6140. Telephone: (04) 498 5990; Fax: (04) 498 5994; Website: www.standards.co.nz. AMENDMENTS No. Date of issue Description Entered by, and date
New Zealand Technical Report Low voltage supply earthing systems ISBN (Print) 978-1-77551-333-9 ISBN (PDF) 978-1-77551-334-6
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CONTENTS Committee representation Acknowledgement Copyright IFC IFC IFC Section 1 INTRODUCTION 1 2 ABBREVIATIONS AND ACRONYMS 2 3 REFERENCES 3 4 SUMMARY AND RECOMMENDATIONS 4 4.1 Summary 4 4.2 Recommendations 5 5 LOW VOLTAGE SUPPLY EARTHING SYSTEMS 6 5.1 Definitions 7 5.2 Consideration of alternatives to MEN system 8 5.3 MEN system 10 5.4 TT system 13 6 TELECOMMUNICATIONS PLANT AND EQUIPMENT 17 7 APPLICATION OF MEN AND TT SYSTEMS PERFORMANCE IN FAULT SITUATIONS 18 7.1 Mal-operation of LV feeder protection 18 7.2 Mal-operation of service protection 19 7.3 Broken LV PEN conductor in the street reticulation 20 7.4 Transposition of service phase and neutral 21 7.5 Broken service neutral 21 7.6 Broken earth conductor to earth electrode 22 7.7 HV to LV fault or contact 23 7.8 HV to earth fault at distribution substation (short term) 25 7.9 HV earth fault at distribution substation (continuous) 27 7.10 Flashover in electrical installation or equipment/appliance 27 7.11 Electric shock to persons in or near electrical installations 28 8 APPLICATION OF MEN AND TT SYSTEMS PERFORMANCE IN SUPPLY LOCATIONS 30 8.1 Urban 30 8.2 Rural conventional distribution 32 8.3 Rural SWER distribution 33 8.4 Rural dairy shed 33 8.5 Factory with own substation 34 COPYRIGHT Standards New Zealand iii
9 CONCLUSIONS AND RECOMMENDATIONS 37 9.1 Conclusions 37 9.2 Recommendations 38 Appendix A Proposed requirements for a TT system 39 B Effect of separation of HV and LV earth electrodes 40 C Reduction of earth potential rise 42 Table 1 Summary fault situations 29 2 Summary supply locations 36 C1 Time/current zones for a.c. 15 Hz to 100 Hz for hand to feet pathway summary of zones of Figure C1 44 Figure 1 IEC LV supply earthing system types 6 B1 Percentage reductions of EPR for various separations of earth electrodes 41 C1 Conventional time/current zones of effects of a.c. currents (15 Hz to 100 Hz) on persons for a current path corresponding to left hand to feet 44 iv COPYRIGHT Standards New Zealand
New Zealand Technical Report Low voltage supply earthing systems 1 INTRODUCTION The supply of electricity to electrical installations at voltages that are above extra low voltage requires the adequate management of risks of serious harm to persons, mainly due to shocks, and significant damage to property, mainly through ignition of combustible material. Consequently, there is an onus on regulators, the electrical industry, and the electricity supply industry (ESI) to ensure that the safety-related risks are managed sufficiently well that a reasonable person will consider the residual risks to be acceptable. Leaving aside the supply of electricity at high voltage, which requires very effective management of the associated safety related risks, the supply of electricity to electrical installations at low voltage (LV) (defined in the ESRs as standard low voltage ) still requires the exercise of care to ensure that the supply is as safe as may reasonably be expected. This implies that, if any fault develops that may increase the risk of serious harm to persons or significant damage to property, some protective action, such as the blowing of a fuse or the opening of a circuit breaker (CB) or residual current device (RCD), will take place to remove the supply or mitigate the effects of the fault to an acceptable level. Central to the requirement for protection is the need for the supply system to be provided, or not provided, with a reference to earth. Various internationally recognised earthing systems for LV supplies are described in this report. In New Zealand, there is a mandatory requirement that electrical installations operating at standard low voltage must be connected to a multiple-earthed neutral (MEN) system, defined and described in detail in this report. However, the MEN system does not deliver perfect safety in all situations and locations and the use of other earthing systems should be considered as alternatives to the MEN system where their characteristics are such as to better manage the safety related risks in certain fault situations or locations. This report includes the deliberations, decisions, and recommendations of a Low Voltage Supply Earthing Systems Committee established by Standards New Zealand specifically to examine the MEN system and any alternatives that the regulator might permit to be used if convinced that their use will better manage the risks associated with LV supply. The committee was charged with considering the likely performance of the MEN system and its alternatives for various foreseeable fault situations and supply locations and making any recommendations that it wishes the regulator to consider. For the avoidance of doubt, the report does not advocate any widespread conversion of supplies to electrical installations from the MEN system to an alternative system but rather the selective use of the alternative system where there are clear benefits in the reduction of risks of serious harm or significant damage. In addition, other than pointing out the need to amend the ESRs and AS/NZS 3000 should government decide to permit the use of an alternative system in addition to the MEN system, the report does not include proposals for amending wording since these will be considered and recommended by other parties. COPYRIGHT Standards New Zealand 1
2 ABBREVIATIONS AND ACRONYMS The following abbreviations and acronyms are used in this report: AS/NZS BS CB ES ECC EEA EPR ESI Joint Australian and New Zealand Standard British Standard Circuit breaker Energy Safety (section of WorkSafe New Zealand) Earth continuity conductor Electricity Engineers Association Earth potential rise Electricity Supply Industry ESRs Electricity (Safety) Regulations 2010 HV IEC IT LV MEN NER NZS PEN PV RCD SWER TN TN-C TN-C-S TN-S TT High voltage International Electrotechnical Commission International IT system of supply Low voltage Multiple-earthed neutral Neutral earthing resistor/reactor New Zealand Standard Combined protective earthing and neutral conductor Photovoltaic Residual current device Single wire earth-return International TN system of supply International TN-C system of supply International TN-C-S system of supply International TN-S system of supply International TT system of supply 2 COPYRIGHT Standards New Zealand
3 REFERENCES The following publications are either cited directly in this report or else contain content that is relevant to the object of the report. Reference should be made to such publications for additional background and information. AS/NZS 3000:2007 BS 7430:2011 BS 7671:2008 IEC 60364-4-41 Ed 5 IEC 60364-4-44 Ed 2 Electrical installations (known as the Australian/New Zealand wiring rules) Code of practice for protective earthing of electrical installations Requirements for electrical installations Low-voltage electrical installations Part 4-41: Protection for safety Protection against electric shock Low-voltage electrical installations Part 4-44: Protection for safety Protection against voltage disturbances and electromagnetic disturbances IEC 60479-1 Ed 4 Effects of current on human beings and livestock Part 1: General aspects IEC 60479-2 Ed 3 Effects of current on human beings and livestock Part 2: Special aspects Cobben and Diuven EEA EEA EEA Schneider Electric Schneider Electric Schneider Electric Schneider Electric Advantages of earthing according to the TN system Guide for High Voltage Single Wire Earth-Return Systems Guide to Power System Earthing Practice Review of Low Voltage Earthing Systems for Installations and Distribution Supply Transformers Cahier technique no. 114 Residual current devices in LV Cahier technique no. 172 System earthings in LV Cahier technique no. 173 Earthing systems worldwide and evolutions Electrical installation guide 2010 Chapter E COPYRIGHT Standards New Zealand 3
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