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Guide to Power System Earthing Practice_v10 (Jun 08) Page i Guide to Power System Earthing Practice June 2008 MEL-R168 Rev 10 (previously MCL-R1234)

Guide to Power System Earthing Practice_rev 10 (Jun 08)

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  • Guide to Power System Earthing Practice_v10 (Jun 08) Page i

    Guide to Power System Earthing Practice

    June 2008

    MEL-R168 Rev 10 (previously MCL-R1234)

  • Guide to Power System Earthing Practice_v10 (Jun 08) Page i

    Quality Assurance Record

    Project Description: System of Supply Working Party - Review of ECP 35

    Document Number: MEL-R168 Rev 10

    Project Reference: EEA 106

    File name: EEA Guide to Power System Earthing Practice Rev 10

    Date of Issue: 23 June 2008 Prepared by: Bruno Lagesse in association with members of the EEA System of Supply Working Party

    This report will be reviewed and approved by the EEA System of Supply Working Party.

    Revisions

    Rev Date Description 00 1-06-2007 First draft 01 22-06-2007 Second draft 02 24-08-2007 Third draft Risk management section added 03 17-10-2007 Fourth draft Distribution section significantly updated 04 20-11-2007 Fifth draft Risk management section significantly updated 05 10-12-07 Sixth draft issued to Gerald Irving only with track changes to section 7 06 12-12-07 Section 7 completed. Draft for approval for industry consultation 07 14-12-07 Final draft for industry consultation

    08 17-04-08 Section 2.3.2 and Section 7 completely edited. Parts of section 3 and Appendix A has been updated. 09 23-05-08 Details on Cost Benefit Analysis added 10 23-06-08 Section 7.15 deleted. Small changes to risk assessment flowchart.

  • Guide to Power System Earthing Practice_v10 (Jun 08) Page i

    DISCLAIMER

    EEA NZ Electricity Networks Guide to Power System Earthing Practice has been prepared by representatives of the electricity industry for the purpose of providing principles on general earthing practices for use by the generation, transmission and distribution sectors of the electricity industry. EEA NZ Electricity Networks Guide to Power System Earthing Practice sets out general earthing practices considered appropriate for the electricity industry; it is expected that the electricity networks companies will develop their own procedures to implement these practices. Although the EEA NZ Electricity Networks Guide to Power System Earthing Practice is recommended by industry representatives, it is not legally binding. As such, the Electricity Engineers Association and the industry representatives involved in formulating EEA NZ Electricity Networks Guide to Power System Earthing Practice can accept no liability or responsibility for any injury, loss, damage, or any other claims caused by, or resulting from any inaccuracy in, or incompleteness of the EEA NZ Electricity Networks Guide to Power System Earthing Practice.

    COPYRIGHT 2008 Copyright is owned by the Electricity Engineers Association of New Zealand (Inc.) (EEA).

    All rights reserved. No part of this work may be reproduced or copied in any form or by any means (graphic, electronic or mechanical, including photocopying, recording, taping, or information retrieval systems) without the written permission of the copyright owner.

  • Guide to Power System Earthing Practice_v10 (Jun 08) Page ii

    FOREWORD

    The electricity supply industry has both general and specific safety responsibilities placed on it by the Health and Safety in Employment (HSE) Act 1992, the Electricity Act 1992 and Regulations made under those Acts. The industry recognises those legal responsibilities and has therefore developed EEA NZ Electricity Networks Guide to Power System Earthing Practice as industry-wide safe earthing guidelines. EEA NZ Electricity Networks Guide to Power System Earthing Practice does not override any legislative requirements. This Guide was produced for the Electricity Engineers Association of New Zealand by the following working group members in consultation with engineers from the electrical power supply industry in New Zealand.

    Mr. T Scott, Orion NZ Ltd (Chairman) Mr. D Abercrombie, Vector Ltd Mr. P Berry, EEA Mr. R Griffiths, Westpower Ltd Mr. S Hirsch, Orion NZ Ltd Mr. G Irving, Transpower NZ Ltd Mr. B Lagesse, Mitton Electronet Ltd Mr. W Lowe, Energy Safety Mr. A Marshall, Opus International Consultants Ltd (representing Telecom NZ Ltd) Mr. M OBrien, NZCCPTS Mr. G Ryan, Transpower NZ Ltd

    Comments for the revision of this Guide are welcomed and should be forwarded to: EEA Guide to Power System Earthing Practice - Convenor PO Box 5324 Wellington New Zealand

  • Guide to Power System Earthing Practice_v10 (Jun 08) Page iii

    CONTENTS

    PREFACE........................................................................................................................................................1 INTRODUCTION .............................................................................................................................................1 SECTION 1 SCOPE, PURPOSE, INTERPRETATIONS, GLOSSARY AND NUMBERING.......................2

    1.1 PURPOSE.....................................................................................................................................2 1.2 SCOPE.........................................................................................................................................2 1.3 INTERPRETATIONS........................................................................................................................2 1.4 RELEVANT ACTS AND REGULATIONS .............................................................................................3 1.5 RELEVANT STANDARDS AND DOCUMENTS .....................................................................................4 1.6 GLOSSARY OF ABBREVIATIONS USED IN THIS GUIDE.......................................................................4

    SECTION 2 GENERAL REQUIREMENTS..................................................................................................6 2.1 GENERAL.....................................................................................................................................6 2.2 DESIGN REQUIREMENTS FOR EARTHING SYSTEMS.........................................................................6 2.3 HAZARDS AND ELECTRICAL CONCEPTS .........................................................................................6 2.4 EPR RISK MANAGEMENT ...........................................................................................................11 2.5 ACCEPTABLE STEP AND TOUCH VOLTAGE LIMITS.........................................................................11 2.6 CRITICAL DESIGN PARAMETERS..................................................................................................12 2.7 EPR VOLTAGES TRANSFERRED ONTO THIRD PARTY ASSETS.......................................................13 2.8 TYPES OF EARTH ELECTRODES ..................................................................................................13 2.9 MATERIALS OF EARTH ELECTRODES AND CORROSION CONSIDERATIONS......................................14 2.10 JOINTS OF EARTH ELECTRODES..................................................................................................15 2.11 CURRENT RATING OF CONDUCTORS AND JOINTS .........................................................................15 2.12 HAZARD MITIGATION ..................................................................................................................17 2.13 SWITCHGEAR OPERATING MECHANISMS......................................................................................18 2.14 SURGE ARRESTERS ...................................................................................................................18 2.15 STATION FENCING .....................................................................................................................19 2.16 CONNECTION POINTS FOR TEMPORARY EARTHS..........................................................................20 2.17 EARTH ELECTRODE ENHANCEMENT ............................................................................................20 2.18 TESTING AND MAINTENANCE.......................................................................................................20

    SECTION 3 EPR RISK MANAGEMENT ...................................................................................................22 A. PROBABILISTIC METHOD ......................................................................................................................22

    3.1 RISK IDENTIFICATION AND ANALYSIS............................................................................................24 3.2 RISK EVALUATION CRITERIA .......................................................................................................25 3.3 COST BENEFIT ANALYSIS AND MITIGATION ..................................................................................26 3.4 PROBABILISTIC RISK MANAGEMENT FLOWCHART .........................................................................28 3.5 PERMISSIBLE STEP AND TOUCH VOLTAGE LIMITS .........................................................................29

    B. DETERMINISTIC METHOD......................................................................................................................30 3.6 EXPOSURE DEFINITIONS .............................................................................................................31 3.7 PERMISSIBLE TOUCH VOLTAGES .................................................................................................32 3.8 PERMISSIBLE STEP VOLTAGES....................................................................................................33 3.9 REFERENCES.............................................................................................................................36

    SECTION 4 RISK MITIGATION MEASURES ...........................................................................................37 4.1 EARTHING SYSTEM IMPEDANCE REDUCTION................................................................................37 4.2 GRADIENT CONTROL CONDUCTORS ............................................................................................38 4.3 NEUTRAL EARTHING RESISTORS.................................................................................................38 4.4 RESONANT EARTHING (PETERSEN COILS, ARC SUPPRESSION COILS, EARTH FAULT NEUTRALISER EARTHING) ..........................................................................................................................................39 4.5 OVERHEAD EARTH WIRES (OHEW)............................................................................................39 4.6 CABLE SCREENS........................................................................................................................40 4.7 SURFACE INSULATING LAYER......................................................................................................41 4.8 SEPARATION OF HV AND LV EARTHING .......................................................................................42 4.9 TT SYSTEM OF SUPPLY ..............................................................................................................43 4.10 INTERFERENCE WITH SERVICES ..................................................................................................44 4.11 OTHER MITIGATION MEASURES...................................................................................................44

    SECTION 5 HV A.C. STATIONS ...............................................................................................................46 5.1 INTRODUCTION...........................................................................................................................46 5.2 DESIGN REQUIREMENTS FOR HV A.C. STATION EARTHING SYSTEMS ............................................46

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    5.3 DESIGN ASPECTS.......................................................................................................................46 5.4 IMPORTANT DESIGN PARAMETERS ..............................................................................................47 5.5 SOIL RESISTIVITY .......................................................................................................................48 5.6 MAXIMUM EARTH FAULT CURRENT..............................................................................................48 5.7 MAXIMUM EARTH FAULT DURATION.............................................................................................48 5.8 TOUCH AND STEP VOLTAGE HAZARDS.........................................................................................49 5.9 MITIGATION OF EPR HAZARDS ...................................................................................................49 5.10 TRANSFERRED VOLTAGES ..........................................................................................................50 5.11 430 V, 650 V AND 2,500 V EARTH POTENTIAL RISE (EPR) CONTOURS ........................................51 5.12 EQUIPMENT EARTHING CONDUCTORS .........................................................................................52 5.13 JOINTS FOR EQUIPMENT EARTHING CONDUCTORS.......................................................................52 5.14 DISCONNECTORS AND EARTH SWITCHES.....................................................................................53 5.15 REINFORCED CONCRETE PADS AND HOLDING-DOWN BOLT CAGES ..............................................53 5.16 BUILDINGS .................................................................................................................................53 5.17 FENCES .....................................................................................................................................53 5.18 LIGHTNING SHIELDING AND LIGHTING ..........................................................................................54 5.19 PORTABLE EARTHING CONNECTIONS ..........................................................................................54 5.20 CONTROL CABINET EARTHS/ ODJBS ..........................................................................................54 5.21 EARTHING OF CABLES WITHIN THE EARTH GRID ............................................................................55 5.22 TRANSFORMER NEUTRAL EARTHING ...........................................................................................55 5.23 GENERATOR NEUTRAL EARTHING ...............................................................................................56 5.24 VOLTAGE TRANSFORMERS AND CAPACITOR VOLTAGE TRANSFORMERS ........................................56 5.25 VT/CT SECONDARY CIRCUITS.................................................................................................56 5.26 400/230 V SYSTEM....................................................................................................................56 5.27 CONDUCTOR AND JOINT SPECIFICATION......................................................................................57 5.28 EARTHING OF CABLE WITHIN THE EARTH GRID.............................................................................57 5.29 FEEDER CABLES ........................................................................................................................57 5.30 OHEW........................................................................................................................................58 5.31 POWER STATIONS, CUSTOMER SUBSTATIONS AND INDUSTRIAL INSTALLATIONS...............................59 5.32 INSTALLATION AND COMMISSIONING ............................................................................................59 5.33 TESTING AND MAINTENANCE.......................................................................................................59

    SECTION 6 DISTRIBUTION CENTRES AND EQUIPMENT ....................................................................61 6.1 INTRODUCTION...........................................................................................................................61 6.2 DESIGN REQUIREMENTS FOR DISTRIBUTION CENTRES AND EQUIPMENT EARTHING SYSTEMS ........61 6.3 DESIGN ASPECTS.......................................................................................................................61 6.4 RELIABLE DETECTION AND CLEARANCE OF HV EARTH FAULTS.....................................................62 6.5 EPR RISK MANAGEMENT ...........................................................................................................65 6.6 CONTROL OF DANGEROUS EPR IMPRESSED ON THIRD PARTY ASSETS AND PERSONNEL..............71 6.7 SEGREGATED HV AND LV EARTHING ..........................................................................................73 6.8 EARTHING SYSTEMS FOR DISTRIBUTION CENTRES AND EQUIPMENT .............................................74 6.9 CONNECTION OF NEUTRAL TO EARTH..........................................................................................74 6.10 EARTHING OF FITTINGS AT DISTRIBUTION CENTRES .....................................................................74 6.11 EARTHING OF FITTINGS AT DISTRIBUTION EQUIPMENT..................................................................75 6.12 SAFETY WHILE OPERATING DISCONNECTORS..............................................................................75 6.13 EARTHING CONNECTIONS ...........................................................................................................76 6.14 LOW VOLTAGE EARTHING CONDUCTORS ASSOCIATED WITH LV SYSTEMS ....................................76 6.15 CONNECTIONS TO EARTHING ELECTRODES .................................................................................76 6.16 SURGE ARRESTERS ...................................................................................................................77 6.17 SOIL RESISTIVITY .......................................................................................................................77 6.18 TYPICAL EARTHING ARRANGEMENTS...........................................................................................77 6.19 TESTING AND MAINTENANCE.......................................................................................................78

    SECTION 7 OVERHEAD ELECTRICAL LINES 50 KV AC AND ABOVE................................................79 7.1 INTRODUCTION...........................................................................................................................79 7.2 CORRIDOR MANAGEMENT...........................................................................................................79 7.3 STEEL LATTICE STRUCTURES .....................................................................................................79 7.4 STEEL AND CONCRETE POLES ....................................................................................................81 7.5 WOOD POLES ............................................................................................................................82 7.6 ELECTRODES & COUNTERPOISE EARTHING .................................................................................83 7.7 OVERHEAD EARTH WIRE .............................................................................................................84 7.8 LIGHTNING SURGE ARRESTERS ..................................................................................................84 7.9 GUY WIRE INSULATORS..............................................................................................................85 7.10 CLEARANCE OF EARTH FAULTS...................................................................................................85

  • Guide to Power System Earthing Practice_v10 (Jun 08) Page v

    7.11 TOWER FOOTING RESISTANCES..................................................................................................87 7.12 EPR ASSESSMENT.....................................................................................................................89 7.13 LIGHTNING .................................................................................................................................93 7.14 VOLTAGES IMPRESSED ONTO OTHER CIRCUITS OR UTILITIES .......................................................94

    APPENDIX A VOLTAGE LIMITS ..............................................................................................................97 APPENDIX B TYPICAL EARTHING ARRANGEMENTS .......................................................................105

  • Guide to Power System Earthing Practice_v10 (Jun 08) Page 1

    PREFACE This guide has been written to provide guidance based upon current industry best practice and international standards. INTRODUCTION This guide is intended to provide general guidance on acceptable methods for ensuring the safety of earthing systems associated with high voltage power systems and provide a means of compliance with relevant safety legislation.

  • Guide to Power System Earthing Practice_v10 (Jun 08) Page 2

    SECTION 1 SCOPE, PURPOSE, INTERPRETATIONS, GLOSSARY AND NUMBERING 1.1 PURPOSE

    The purpose of this guide is to give guidance and advice on safe earthing practices for high voltage a.c. power systems adequate to meet the requirements of electricity safety legislation.

    1.2 SCOPE

    1.2.1 This document provides guidance on power system earthing in general as set out in sections 1 to 4, and includes specific sections for:

    (a) High voltage (HV) a.c. stations. (b) Distribution centres, equipment and lines. (c) High voltage a.c. transmission lines.

    1.2.2 This guide does not apply to:

    (a) Low Voltage (LV) earthing on consumers installations. (b) Systems not operated at a normal frequency of 50 Hz.

    1.3 INTERPRETATIONS

    In this guide, unless the context otherwise requires, the following definitions apply:

    1.3.1 Disconnector means any disconnector, earth switch, air break switch (ABS), air break isolator (ABI), sectionaliser, etc.

    1.3.2 Distribution centre means any substation from which electricity is supplied direct at low or high

    voltage to an electrical installation that is to a consumer or end user. The distribution centre may consist of one or more transformers on a pole, on the ground, underground, or in a building; and includes the enclosure or building surrounding the transformer(s) and switchgear, if any, but does not include HV a.c stations.

    1.3.3 Distribution equipment means pole or pad mounted equipment such as lightning arresters, ring

    main unit (RMU), capacitors, reclosers, regulators and disconnectors (ABS, ABI, sectionaliser, etc) on a distribution network other than distribution centres.

    1.3.4 Distribution system means that portion of an electricity supply system from where electricity at low

    or high voltage is conveyed from a distribution centre, to the premises of consumers connected to that distribution centre, but does not include distribution or service mains.

    1.3.5 Earthed means electrically connected to the general mass of the earth. 1.3.6 Earth electrode means a conducting element or electrically bonded group of conducting elements

    in electrical contact with the earth and designed for dispersing electric currents into the earth. 1.3.7 Earth fault current path means the complete loop through which earth fault current flows. It

    includes system plant as well as dedicated earth connections and the main body of the earth.

    1.3.8 Earth grid means a system of interconnected bare conductors buried in the earth providing a common earth for fittings. The grid may be specifically designed to control surface potential gradients.

    1.3.9 Earth grid voltage rise (EGVR) means the voltage rise to remote earth on a metallic structure

    connected to an earthing system during an earth fault. 1.3.10 Earth grid return current means the portion of earth fault current which flows through the earthing

    system. 1.3.11 Earth potential rise (EPR) means a rise in potential on the earth surface relative to reference earth.

  • Guide to Power System Earthing Practice_v10 (Jun 08) Page 3

    1.3.12 Earth impedance in respect of an earth electrode system means the ohmic impedance at system frequency between the electrode system and the general mass of earth.

    1.3.13 Earthing conductor means a conductor connecting any part of an earth electrode to fittings

    required to be earthed. 1.3.14 Earthing system means all conductors, electrodes, clamps or other connections used to provide a

    path to earth. 1.3.15 High voltage (HV) means voltage exceeding 1,000 volts a.c. or 1,500 volts d.c. 1.3.16 Hazard means a source of risk or harm. In this case, an earth potential rise condition that could

    cause harm to a person in the vicinity (or nearby plant). 1.3.17 HV a.c. station means a HV station which has a controlled access area and a specific earth grid.

    This includes Transpower grid connection points, zone substations, HV switching stations, generating stations (including switchyards), air insulated indoor substations, gas-insulated substations (GIS), etc, but does not include distribution centres and distribution equipment.

    1.3.18 Mitigation means a measure or measures taken to reduce the hazard or risk. 1.3.19 Multiple earthed neutral (MEN) System means a system of earthing in which the earthing

    conductor within an electrical installation is connected to the neutral as well as to an earthing electrode. In this system, the distribution system neutral is earthed at the point of supply at a distribution centre, and at one or more points along the distribution or service mains, and provides a continuous electrical path between the consumer and the distribution centre earthing point.

    1.3.20 Normal Location means any urban or rural areas other than Special Locations. 1.3.21 Risk means a function of both the probability of an event and the consequence of that event. 1.3.22 Risk assessment means the determination that a given level of risk is tolerable or otherwise. 1.3.23 Special Location means any urban or rural area where a significant gathering of people may occur

    particularly situations and/or where people may not be wearing footwear. Special Locations could be found in areas such as within a schools grounds or within a childrens playground, or within a public swimming pool area, or at a popularly used beach or water recreation area, or in a public thoroughfare.

    1.3.24 Stations means substation or generating station. 1.3.25 Step voltage means the difference in surface potential experienced by a person bridging a

    distance of 1 metre with the persons feet apart, without contacting any other earthed object. 1.3.26 System voltage means the difference of potential normally existing between conductors, or

    between conductors and earth (phase-to-phase in a multi phase system and phase-to-earth in a single phase system).

    1.3.27 Telecommunications system means all plant that is part of a telecommunications network. This

    includes cables, aerial lines, pillars, exchange equipment, and customers fixed telecommunications wiring and attached equipment (e.g. PABXs, phones, etc).

    1.3.28 Touch voltage means voltage which will appear between any point of contact with uninsulated

    metalwork and any point on the surface of the ground within a horizontal distance of 1 metre from the vertical projection of the point of contact with the uninsulated metalwork.

    1.4 RELEVANT ACTS AND REGULATIONS

    This Guide was written to comply with the New Zealand Electricity Act and the New Zealand Electricity Regulations. The Electricity Regulations are currently under review.

  • Guide to Power System Earthing Practice_v10 (Jun 08) Page 4

    1.5 RELEVANT STANDARDS AND DOCUMENTS

    AS 2067-1984 Switchgear assemblies and ancillary equipment for alternating voltages above 1 kV.

    AS/NZS 3835 Earth .potential rise Protection of telecommunication network users, personnel and plant Parts 1 & 2.

    AS/NZS 4853 Electrical hazards on metallic pipelines. AS/NZS 60479-1 Effects of current on human beings and livestock. Part 1: general aspects

    (Equivalent to IEC 60479-1:1994). BS 7354 Code of practice for design of high voltage open terminal stations. BS EN 50341 Overhead electrical lines exceeding a.c. 45 kV. EEA NZ electricity networks guide to risk based earthing system design. ENA EG1 Substation earthing guide. ENA C(b)1 Guidelines for design and maintenance of overhead distribution and transmission

    lines. IEEE Std 80 Guide for safety in a.c. substation grounding. IEEE Std 81 Guide for measuring earth resistivity, ground impedance and earth surface

    potentials of a ground system. IEEE Std 81.2 Guide for measurement of impedance and safety characteristics of large,

    extended or interconnected grounding systems. IEEE Std 142 Recommended practice for grounding of industrial and commercial power

    systems. IEEE Std 524a IEEE Guide to grounding during the installation of overhead transmission line

    conductors. IEEE Std 665 Standard for generating station grounding. IEEE Std 837 Qualifying permanent connections used in substation grounding. IEC 61936-1 Power installations exceeding 1 kV a.c. IEC 60364-4-44 Electrical installations of buildings Part 4-44 Protection for safety Protection

    against voltage disturbances and electromagnetic disturbances. IEC 60479-1:2005 Effects of current on human beings and livestock. Part 1: general aspects. ITU-T K33 Limits for people safety related to coupling into telecommunications system from

    a.c. electric power and a.c. electrified railway installations in fault conditions. ITU-T K53 Values of induced voltages on telecommunications installations to establish

    telecom and a.c. power and railway operators responsibilities. NZCCPTS Application guide for earth potential rise. NZCCPTS Application guide for neutral earthing resistors/reactors. NZCCPTS Application guide for SWER HV power lines. NZCCPTS Application guide for cable separations Minimum separations between power

    and telecommunication cables. NZCCPTS Application guide for costs apportioning. NZCCPTS Application guide for cable sheath bonding. NZCCPTS Fundamentals of calculation of earth potential rise in the underground power

    cable distribution network. SM-EI Safety manual electricity industry (two manuals).

    1.6 GLOSSARY OF ABBREVIATIONS USED IN THIS GUIDE

    ABI Air Break Isolator ABS Air Break Switch ALARP As Low As Reasonably Practical CBA Cost Benefit Analysis CBR California Bearing Ratio CVT Capacitor Voltage Transformer DA Data Acquisition EPR Earth Potential Rise EGVR Earth Grid Voltage Rise HV High Voltage > 1 kV a.c. Hz Hertz Ib Permissible body current limit kg Kilograms KV Kilo-volts (1,000 volts) LV Low Voltage 1 kV MEN Multiple Earthed Neutral

  • Guide to Power System Earthing Practice_v10 (Jun 08) Page 5

    NER Neutral Earthing Resistor NET Neutral Earthing Transformer NPV Net Present Value NZECP New Zealand Electrical Code of Practice ODJB Outdoor Junction Box OHEW Overhead Earth Wire PVC Polyvinyl chloride RCD Residual Current Device RMU Ring Main Unit SWER Single Wire Earth Return T Time TFR Tower Footing Resistance TT Terra-Terra system of supply VoSL Value of Statistical Life VT Voltage Transformer

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    SECTION 2 GENERAL REQUIREMENTS 2.1 GENERAL

    Depending on access, location and exposure levels, metal structures, and equipment that may be livened to dangerous voltage levels as a result of an earth fault should be bonded to earth. This can be achieved by permanent connections to electrodes in contact with the general body of earth. Power system earthing is typically required to ensure that earth faults associated with the power system are detected so that the earth fault protection devices are effectively operated to disconnect the supply. When a fault on a high voltage power system causes current to flow to earth, the earthing system should also ensure that the voltage difference between conducting parts that may be momentarily livened, and which may be contacted by a person does not carry a significant risk. Hazardous voltages between conductive parts may typically appear between the hand and one or both feet (i.e. touch voltages), or between the two hands (i.e. reach touch voltages), or between one foot and the other (i.e. step voltages). Such voltage differences can occur within power system stations, and also on metallic structures along the length of, or close to power lines, under earth fault current conditions. Earthing in conjunction with other mitigation measures can be used to control dangerous voltage differences to acceptably safe levels. During earth fault conditions voltage differences will exist between station equipment and the main body of earth. These voltage differences may need to be controlled, to ensure that insulation breakdown or failure does not occur on apparatus connected to points outside the station. Cable sheaths, metallic pipes, fences, etc which are connected to the station earthing system will transfer earth fault voltages from the station earth electrode to the remote points. Similarly, cable sheaths, metallic pipes, etc. which are connected to remotely earthed structures but isolated from the station earth electrode will transfer the earth fault voltage of the remote structure into the station.

    2.2 DESIGN REQUIREMENTS FOR EARTHING SYSTEMS

    The performance of the earthing system(s) shall satisfy the safety and functional requirements of the high voltage power system, including lines, substations and the associated fittings and equipment. The earthing system may be used jointly or separately for the protective or functional purposes according to the requirements of the power system. The design, selection and installation of the earthing systems shall be such as to ensure:

    2.2.1 Performance Requirements

    The performance requirements for an earthing system include: (a) Proper functioning of electrical protective devices. This entails reliable detection of HV earth

    faults and either clearing the fault or minimising the resulting fault current. (b) Manage the risks associated with step and touch voltages in accordance with Electricity

    Regulations, applicable standards and guidelines. (c) Manage the risks associated with EPR transferred onto third party plant, staff and users (i.e.

    telecommunications, railways, pipelines, etc.) in accordance with Electricity Regulations, applicable standards and guidelines.

    2.2.2 Functional Requirements

    The functional requirements for an earthing system include: (a) Earth fault currents and earth-leakage currents can be carried without danger and without

    exceeding design limits for thermal, thermo-mechanical and electro-mechanical stresses. (b) The value of earthing impedance is in accordance with the protective requirements and is

    continuously effective over the planned lifetime of the installation with due allowance for corrosion and mechanical constraints.

    2.3 HAZARDS AND ELECTRICAL CONCEPTS

    2.3.1 Sources of hazards

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    Electrical hazards in the form of touch, step or transferred voltages can appear on the metal structures or equipment associated with, or nearby high voltage power systems, due to one or a combination of the following factors: (a) Electrical insulation failure, or mechanical failure or both, causing earth fault current to flow, and

    EPR to occur. (b) Human error, resulting in accidental livening of station equipment, and/ or lines circuits. (c) Electromagnetic induction. (d) Static charges induced on de-energised lines due to atmospheric conditions. (e) Lightning strikes to in-service/ de-energised lines.

    In addition to the above, electrical interaction can occur between power system earthing and nearby third party systems. This interaction may involve EPR or transferred EPR, stress to the insulation of telecommunication circuits, induced voltages, or the creation of voltage differences between the EPR of power system earthing and the independent earthing (either local or remote) of other systems such as private generating plant, or telecommunication systems. The consequences of such differential voltages may involve both insulation breakdown and component failure (e.g. electronic equipment). In some cases common HV and LV network earths, may be of particular concern as detailed in section 4.8. The widespread use of power operated digital electronic equipment for modern telecommunication equipment, such as cordless telephones, etc., has increased the possibility of damage to such power/telecommunications equipment and of hazard to the users. This Guide does not include detailed guidance on issues of EPR transferred onto third party systems. For the telecommunication systems, detailed guidance is available from a series of publications issued by the New Zealand Committee for the Co-ordination of Power and Telecommunication Systems (NZCCPTS). The NZCCPTS publications are listed in section 1.5 of this guide, and provide detailed information on assessing the likely degree of hazard involved and suitable means of mitigating possible hazard. For pipelines, detailed guidance can be obtained from AS/NZS 4853, Electrical hazards on metallic pipelines. It should also be noted that the voltage-time safety criteria for telecommunications equipment and users, differs from the step and touch values used in earthing design. The relevant criteria for telecommunications equipment and users are detailed in the Electricity Regulations. Similar criteria are typically used by the railway industry.

    2.3.2 Earth potential rise (EPR) An earth fault will result in an EPR. During an earth fault, there is significant current flowing from the power source into the fault point. This current then returns to the source through the ground surrounding the fault point or earth mat. The soil has inherent resistivity and the current flowing through this resistance causes voltages to appear on the soil surface and consequently an EPR. The value of this at the earth mat is determined by the resistance between the earth mat and the remote earth as well as the magnitude of the earth fault current. The soil surface voltages are highest at the fault location or the source substation earth mat, and reduce as the distance from the fault location or the source substation earth mat increases. Equipotential contours reflect all the locations that would have the same voltage on the soil surface during an earth fault. The closer the contours are to each other, the steeper the voltage gradients are. This results in: a higher touch voltage; higher step voltages close in, but lower step voltages further out; smaller step voltage hazard zone; lower EPR in the nearby soil; smaller EPR hazard zones; less problems with EPR hazard to nearby other utility plant (e.g. telecommunications plant). If a human or animal contacts two different voltages simultaneously, a voltage difference will be applied across the body. This will cause a current to flow in the body. The current that may be

  • Guide to Power System Earthing Practice_v10 (Jun 08) Page 8

    harmful is influenced by a number of factors including fault duration, the contact area, the body current path and the impedance characteristics of the skin and the body. Hand-to-hand or more typically hand-to-foot voltages are known as touch voltages. A touch voltage occurs when the surface a person is touching and the surface a person is standing on, or a second location that they are touching, are at different voltages. Hence touch voltages occur where there is contact with a conductive structure where a current path occurs through the body to a location at a different potential during an earth fault. Foot-to-foot voltages are known as step voltages. A step voltage occurs when a stride is taken and the soil surface under each foot is at different voltages. A step voltage can only be experienced when both feet are in simultaneous contact with the ground and each foot is on a different voltage contour. This results in a current path through the body from foot to foot during an earth fault. The third category of EPR voltages are transfer voltages. Voltages can appear on any long metallic object during an earth fault, that are in electrical contact with the soil surface and that passes across the EPR voltage contours. Typical examples are wire fences, telephone wires or gas industry pipelines. There are two separate consequences. Firstly, the metallic object will attain the EPR voltage of the soil surface that it is in contact with. It may conduct or transfer this voltage from the area close to the fault point to a location some distance away. A significant touch voltage may then occur through a person touching the metallic object whilst standing on a soil surface well beyond the immediate influence of the fault. Secondly, the reverse can also occur, where the metallic object may conduct or transfer a low voltage into an area close to the fault point. The soil surface may have attained a high EPR voltage as a result of the fault. A significant touch voltage may then occur, again through a person touching the metallic object but in this instance with the person standing close to the fault point.

    2.3.3 Electric field (capacitive) coupling

    Electric field (capacitive) voltages typically can be coupled onto an insulated metallic object in an electric field from an energised circuit. A typical example of electric field coupling is the voltage that appears on a de-energised overhead circuit running alongside an energised circuit. When contact is first made with the isolated object, the capacitor will discharge and the final voltage on the object is likely to be low. As long as the stored energy is not very large the discharge current will be low. However, if the stored energy is large, such as on a relatively long de-energised circuit in parallel with an energised circuit, the discharge current may be high and dangerous. Bonding the isolated object to earth will effectively discharge capacitive coupled voltages. Capacitive coupling is rarely an issue for the public. Electric utilities employees working on de-energised circuits or equipment have to take necessary precautions such as applying temporary earthing to ensure that capacitive coupled voltages are minimised.

    2.3.4 Magnetic field induction

    Currents (steady state or earth fault currents) flowing through a circuit in parallel with metallic conductors can cause hazardous voltages to be magnetically induced into the parallel metallic conductor. Induced voltages may be a hazard to telecommunications equipment and personnel and must be limited to electrically safe values.

    Induced voltages may also be a hazard in gas, oil or other pipelines, where they run parallel to high voltage transmission or distribution lines. Hazards arise to personnel inspecting and maintaining such pipelines. Induced voltages may also be hazardous to the public on fences or other metallic conductors which run parallel to power lines. Magnetic field induction is not considered in more details in this Guide. Further information about magnetic field induction may be obtained from:

  • Guide to Power System Earthing Practice_v10 (Jun 08) Page 9

    (a) CJC5-1997, Coordination of Power and Telecommunications Low Frequency Induction, Standards Australia, 1997.

    (b) AS/NZS 4853:2000, Electrical hazards on metallic pipelines. 2.3.5 Lightning strikes

    Even though lightning activity in New Zealand is typically low compared to many other regions of the world, lightning is still considered a significant source of hazards to employees. Lightning overvoltages and currents can travel a long way over overhead lines and affect personnel working on earthing systems. It is impractical to provide adequate protection to personnel in the form of earthing and equipotential bonding during lightning conditions because lightning surges typically have high current magnitude and rate of rise. This Guide does not cover lightning protection issues in detail. Information on Insulation coordination and lightning protection may be obtained from: (a) IEC 60071 Insulation Coordination (multiple parts). (b) Andrew R Hileman, Insulation Coordination for Power Systems, Marcel Dekker Inc., 1999. (c) IEEE Std 998-1996 IEEE Guide for Direct Lightning Stroke Shielding of Substations. (d) IEEE Std 1313-1993 IEEE standard for power systems - insulation coordination (three parts). (e) IEEE Std 1410-2004 IEEE guide for improving the lightning performance of electric power

    overhead distribution lines. Note: All personnel are required to stop handling all conductors including those associated with any earthing system until the lightning hazard has passed. This is a requirement from SM-EI 3.702.

    2.3.6 Touch Voltage

    Touch voltage is the voltage generated during an EPR event which may appear between any point of contact with uninsulated metalwork and any point on the surface of the ground within a horizontal distance of one metre from the vertical projection of the point of contact with the uninsulated metalwork. Touch voltages typically appear between a hand and one or both feet of a person touching a temporarily livened earthed structure while standing on the ground surface one metre away from the structure (see Figure 1). A touch voltage may also appear between the two hands of a person simultaneously touching two earthed structures that are temporary livened. This is termed the reach touch voltage and may only be an issue if one or both objects are not bonded to the grid. For a HV a.c. station with an earth grid, the maximum touch voltage which can develop in the mesh of the grid is termed the mesh voltage. Because of the equipment and structures in a HV a.c. station, it is possible for someone to be touching structures or items of equipment including mobile plants while standing at the centre of a mesh.

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    Figure 1: Touch and Step Voltages around a Substation

    2.3.7 Step Voltage

    Step voltage is the difference in surface potential experienced by a person bridging a distance of one metre with the persons feet apart, without contacting any other earthed object. Examples of a step voltage are shown in Figure 1.

    2.3.8 Transferred Voltage

    The transferred voltage is a special case of touch voltage whereby a voltage is either transferred to the substation from a remote point or is transferred from the substation to the remote point (see Figure 2). In that case, the touch voltage may be approaching the full EGVR. Where voltage rises on the earthing system are transferred by metalwork such as neutral conductors of a MEN system, water pipes, and the like to locations remote from the installation, allowance may be made for voltage drop in these conductors. Otherwise, the transferred potential should be regarded as being equal to the EGVR.

    Figure 2: Example of Transferred Voltage

    Voltages may also be transferred to third party plant and equipment via the potential rise in the ground. Additional information on the transfer of hazardous voltages on third party assets is provided in section 2.7.

    VTransferred EGVR

    IF RGrid

    V = EGVR

    Remote Earth

    HV A.C. Substation

    Substation Fence

    Third Party Fence

    Touch Voltage

    Touch Voltage

    Touch Voltage

    Reach Touch

    Voltage

    Step Voltage

    Step Voltage

    HV a.c. station

    Substation Fence

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    2.3.9 Hazards to equipment

    While humans (and animals) are susceptible to electric current, plant and equipment are also susceptible. Any plant such as data and communications cables and equipment may be severely damaged by high voltage gradients appearing on the earthing systems during an earth fault. Limits for equipment can vary significantly and it is difficult to provide specific values. Modern telecommunications equipment (cordless telephones, facsimile machines, multiplex equipment) is susceptible to damage from excessive voltage. Close liaison between the network operator and third parties shall be undertaken at the early stages of any development or alterations to either partys network.

    2.4 EPR RISK MANAGEMENT

    The occurrence of earth faults on power systems causing hazardous voltage differences and the presence of human beings in simultaneous contact with these voltage differences are probabilistic in nature. The concept of electrical safety formulated by The Electricity Regulations is that, there is no significant risk of injury or death to any person, or of damage to any person or property, as a result of the use of the works, electrical installations, or associated equipment, or of the passage of electricity through those works, electrical installations, fittings, electrical, electrical appliances, or associated equipment as the case may be. For a dangerous situation to arise, a power system earth fault must be coincident with a person being at a location exposed to a consequential hazardous voltage. Fortunately few human electric shock incidents have been recorded in these situations to date. A low earth resistance is not always necessary to provide a safe earthing system. The earthing system design is required to keep the voltage gradients across the earthing system under earth fault conditions within safe levels to prevent danger to persons or equipment. Traditionally, an earthing system with a low overall earth resistance was considered to be safe but there is not a simple relationship between the resistance of the earthing system (e.g. "10 ") and the magnitude of shock voltage that can arise in any particular situation. Appropriate analysis is therefore required that takes into account all the necessary factors and includes risk assessment. Earthing system design and testing can show the existence of possible hazardous voltages. The risks associated with these hazardous voltages should be identified and evaluated against given criteria to determine whether the risk needs to be mitigated. To manage the risk from EPR events, either of the following two methods may be used: (a) The Probabilistic Method; or (b) The Deterministic Method. In section 3, the risk management concepts are developed further.

    2.5 ACCEPTABLE STEP AND TOUCH VOLTAGE LIMITS

    The hazard to human beings is that a current will flow through the region of the heart that is sufficient to cause the heart to go into ventricular fibrillation. The current limits, for power-frequency purposes, are derived from an established international standard such as IEEE Std 80 or IEC 60479-1 (AS/NZS 60479.1). The current limits need to be translated into voltage limits for comparison with the calculated step and touch voltages, taking into account the impedance present in the body current path. The voltage limits should take into account the following factors: (a) The proportion of the human body current flowing through the region of the heart. (b) The human body impedance for the current path. (c) The contact resistance between the human body contact points and conductive surfaces in the

    return path (e.g. soil (at remote earth potential), earth electrode).

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    (d) The duration of the current flow through the human body. 2.6 CRITICAL DESIGN PARAMETERS

    2.6.1 The following design parameters are critical as they form the basis for the calculations and assumptions which define the earthing systems required at locations where mitigation is required to achieve electrical safety.

    (a) Design fault currents (b) Design fault duration (c) Site soil resistivity

    2.6.2 Design fault current

    Prior to carrying out any earthing grid design, it is necessary to accurately establish the realistic earth return fault current. Often only a small proportion of the prospective earth fault current may return via the earth grid proper. In some cases, fault current is diverted from the grid via cable screens, overhead earth wires or other bonded conductors such as pipelines. Some of the earth fault current may also circulate within an earth grid and not contribute to the EGVR. Therefore, before calculating the EGVR, touch voltages and step voltages, it is important to first calculate the realistic earth grid return current.

    2.6.3 Design fault duration

    For the calculation of allowable step and touch voltages, primary protection clearing time shall be used. For thermal rating, guidelines are given in section 2.11.

    2.6.4 Soil Resistivity

    The soil resistivity can vary significantly with soil moisture content. This is an important aspect that needs to be considered when designing earthing systems. From a protection point of view, earthing systems should be designed based on the highest value of soil resistivity likely to be encountered on the site. However, the effect of soil resistivity variation on step and touch voltages depends on many factors and no simple guideline can be applied. Data on soil resistivity variation with "seasons" is not available for New Zealand. In many areas of New Zealand, where the soil moisture content is relatively constant due to regular rainfalls, "seasonal" resistivity variation may not be significant. However, significant seasonal changes in soil moisture content in other areas may result in significant soil resistivity variation and where possible, these should be taken in consideration. For areas where significant seasonal variation in soil moisture content is expected, a conservative value of soil resistivity should be used for a design. For these situations, designs should check the sensitivity of safety levels to soil resistivity variations. The Wenner method is the most commonly used method to measure soil resistivity. It also has the advantage of being one of the simplest methods to use and is recommended. The raw data obtained from the soil resistivity measurements is difficult to interpret and is not very useful for the design of earth electrodes. The data needs to be converted into a model which is representative of the soil resistivity at the site. Computer software can be used for this purpose. When conducting soil resistivity tests it is important to carry out enough measurements so that an accurate soil resistivity model of the site can be derived. Measurements at a minimum number of 12 probe separations are recommended to ensure an accurate soil resistivity model can be derived. The larger probe separations should be in proportion to the size of the earth electrode/grid. The soil resistivity model will give an indication of the structure of the soil at the site. If lower soil resistivity layers are evident from the model, then the use of deep driven rods may be considered.

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    2.7 EPR VOLTAGES TRANSFERRED ONTO THIRD PARTY ASSETS

    During a HV phase to earth fault at a HV earthing system including HV conductive pole and LV MEN system that is bonded to the HV earthing system, the resultant EGVR on the HV earthing system can present a hazard to third party network plant, customers and personnel, by either of the following mechanisms:

    2.7.1 Hazard from nearby HV Earthing Systems or HV Conductive Poles Third party plant such as telecommunications network plant in the road reserve (e.g. buried cable, pits, pillars, pedestals, joints, cross-connect cabinets, electronic cabinets) or railway signalling circuits or pipelines all are effectively referenced to remote earth. This means that EPR in the ground may stress the insulation of any adjacent telecommunications network plant, railway signalling assets and pipeline protective coatings to the full value of that EPR.

    2.7.2 Hazard arising from Common HV/LV Earthing Systems

    When a HV phase to earth fault occurs at a distribution transformer which has a common HV/LV earthing system, the resultant EGVR appears on both the distribution transformer HV earthing system and on the neutral of the LV MEN system supplied by that transformer. This means the earth potential in all buildings supplied by the distribution transformer will rise to the level of the HV earthing system EGVR (minus a small amount of volt drop along the neutral). Any mains-powered third party equipment in those building which is also connected to a remote earth will be stressed by virtually the full EGVR. The main category of third party mains-powered equipment affected in this way is telecommunication equipment including equipment located in residential dwellings such as fax machines, answer machines, cordless phones, and, most commonly, computer modems. This equipment will be connected to a (remote) Telephone Exchange earth reference via the telecommunications network copper cable pairs, and hence will be stressed by the EGVR on the LV MEN system. For limiting interference to telecommunication networks Electricity Regulation 58 deems EPR or induced voltages not likely to be hazardous where they do not exceed:

    650 Vrms for fault durations 0.5 s 430 Vrms for fault durations > 0.5 s

    Additional information on EPR transfer to third party plant may be obtained from the following publications: (a) AS/NZS 4853:2000, Electrical hazards on metallic pipelines. (b) AS/NZS 3835:2006, Earth potential rise Protection of telecommunication network users,

    personnel and plant Parts 1 & 2.

    2.8 TYPES OF EARTH ELECTRODES

    Only the following types of earth electrodes may be used: (a) Vertical rods or pipes driven not less than 1.8m into the ground. (b) Horizontal grid or mesh. (c) Horizontal bare buried conductors. (d) Electrodes embedded in foundations. (e) Metal reinforcement in concrete or other earth conductors in concrete.

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    Table 1: Brief Guide on Selecting Earthing Electrode Designs Description Example Simple driven rod Domestic or light industrial MEN earths Array of driven rods, horizontal conductors or rings

    Roadside ground or pole mounted distribution transformers

    Interconnection of a number of separate earthing systems

    Distribution and associated LV MEN systems

    Buried grid of horizontal conductors with or without driven rods.

    HV a.c. stations

    Interconnection of any of the above with other large conductive structures (dams, foundations)

    Power station

    The design of the electrode should take into consideration the type and moisture content of the soil. The type and embedded depth of the earth electrodes should be such that soil drying and freezing will not increase the earth resistance of the earth electrodes above the required value. Where practicable, the earth electrodes should be embedded below permanent moisture level, except for electrodes which are used for gradient control. Typically, in New Zealand, a burial depth of at least 500 mm for horizontal conductors is recommended to minimise the effects of changes in temperature and soil moisture content. In many situations, this depth is also adequate to avoid freezing of the soil surrounding the buried earth conductor. Greater burial depth should be considered in areas where freezing can occur for a significant portion of the year. Such areas are typically associated with higher ground. The addition of driven rods to an HV a.c station earth grid usually has a small effect on the impedance of the earth grid unless the driven rods reach lower soil layers with a reduced resistivity. Driven rods should be separated by at least a distance equal to the length of the rods. Additional rods enclosed within rows of other rods are ineffective in reducing the overall impedance.

    2.9 MATERIALS OF EARTH ELECTRODES AND CORROSION CONSIDERATIONS

    In areas where corrosion is likely to be severe, the electrodes should be of hard drawn copper, copper clad or stainless steel, or other metal of such nature or so treated as to be not less resistant to corrosion than hard drawn copper, or copper clad or stainless steel. In areas where corrosion is not severe, galvanized or plain steel electrodes may be used. Aluminium shall not used as a buried electrode. Copper is by far the most common metal used for earthing systems. It has a high conductivity and has the advantage that it does not generally suffer from corrosion problems. Copper clad or copper bonded steel is usually used for driven rods. The minimum thickness of the copper coating/sleeve shall be 250 m (micron) to minimise the risk of rapid corrosion of the copper bonded or copper clad steel rods. Unfortunately, copper is often responsible for causing galvanic corrosion of other metals such as steel which are buried in the vicinity of copper. Corrosion can have a significant impact on the integrity of both the buried electrode and the earthing connections. The design, selection of materials, and construction of the earth electrodes shall take into consideration the possible deterioration and increase of resistance due to corrosion over the expected life of the installation. There are many causes of corrosion of earthing conductors and rods which include the following: (a) Uneven distribution of moisture in the vicinity of the electrode. (b) The acidity and chemical content of the soil, as well as the presence of foreign materials

    including cinders, scrap metal or organic material. (c) The presence of stray electric current particularly d.c. (d) The interconnection of dissimilar metals in the soil or above ground where moisture is present.

    The latter is among the most common causes of corrosion of earth electrodes. For example, the connection of copper electrodes to galvanised steel water pipes may cause rapid corrosion of the water pipes.

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    The resistivity of the soil, as an electrolyte, is an important factor associated with corrosion. Soils having resistivities lower than approximately 15 -m are likely to cause severe corrosion. Corrosion should be slight in soils having resistivities higher than approximately 200 -m. The mitigation of corrosion is complex and it is not possible to lay down rigid rules for good practice. If corrosion problems are encountered or are anticipated, these should be investigated on a case by case basis. In areas where a considerable quantity of buried galvanised steel or structural steel is present near a copper earth electrode, stainless steel may be an attractive alternative to copper. The use of concrete to encase the earth electrode may be used to mitigate corrosion. Conductive concrete may also be used. Concrete encased galvanised steel electrodes and steel reinforced foundations can be effective earth electrodes. Connections which are above ground should be protected from moisture using a waterproof compound. Copper earthing connections should also be tin plated before being protected from moisture.

    2.10 JOINTS OF EARTH ELECTRODES

    All buried connections, crossings and joints of earth electrodes should be welded using suitable exothermic products and moulds or by brazing. Compression or wedge type fittings may also be used underground provided these have met the requirements of IEEE Std 837. Bolted connections shall not be used underground. Exothermic products used for welding earthing conductors shall comply with the requirements of IEEE Std 837. Exothermic mixtures shall only be used with the manufacturer approved moulds. Exothermic mixtures from a supplier shall not be used in conjunction moulds from a different supplier. Exothermic welding shall only be performed by operators who have been specifically trained by a suitably qualified representative of the equipment supplier or any accredited training provider. Operators who have not carried out exothermic welds in the last six months should attend a training/refresher course before attempting to weld. Brazed joints above or below ground are acceptable. It is recommended to provide additional mechanical retention before brazing a joint. Mechanical retention shall be provided to ensure that enough brazing material flows into the interface between the two metals to fill the gap.

    2.11 CURRENT RATING OF CONDUCTORS AND JOINTS

    The conductor used for earthing of primary plant must be rated to withstand short circuit currents without damage or deterioration. When selecting a fault clearing time to be used for rating buried earth conductors, the following should be considered: (a) All earthing conductors forming the station or distribution transformer earth electrodes shall

    meet the requirements of IEEE Std 80 conductor sizing factors, and a factor of safety as per section 11.3, of IEEE Std 80 in determining the conductor size. A factor of safety is required to take into account the long duration these conductors are expected to be in service and relied upon, and the corrosive nature of the ground soil in which they are installed.

    (b) A long established New Zealand practice of rating buried conductors for 3 s for the expected

    worst case short circuit current may be used as this is considered to meet the requirements of IEEE Std 80. Alternatively, a lesser time than 3 s may be used, but only if two reasonably independent protection systems (that is 100% redundancy) will ensure fault clearance occurs in the lesser time, even if any one item of the protection systems fails to operate, and provided that the requirements of (a) above are satisfied. Protection system include relays, CTs, VTs, d.c. supplies, communications systems (where appropriate) and CBs.

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    Note: The term reasonably independent secondary protection systems is not intended to imply a requirement for primary equipment to be duplicated, and only applies to secondary equipment including current transformer cores, relay and communication systems, and in combination with a highly reliable/ duplicated secondary d.c. / a.c. power supplies.

    Earthing conductors also need to be physically robust. The buried earth conductors in an earth grid can be rated for lower fault currents, as the fault current will disperse into the ground. Typically the buried conductors are rated for 70 % of the highest prospective fault current. Additional information is given in section 5. For conductor rating calculations, ambient temperatures of 20C should be used for buried conductors and 30C for above ground conductors. For bolted or compression joints, the maximum temperature which the earthing conductor shall be allowed to reach is 250C. A maximum temperature of 400C is allowed for earthing conductors which are welded or brazed. Figure 3 and Figure 4 show the conductor ratings for various sized conductors and various fault durations (i.e. 0.5 s to 3 s) for both bolted and welded connections. Further details on conductor ratings can be obtained from IEEE Std 80.

    0

    5000

    10000

    15000

    20000

    25000

    30000

    35000

    40000

    45000

    50000

    0 20 40 60 80 100 120 140 160 180 200

    Conductor Size (sq mm)

    Faul

    t Cur

    rent

    (A)

    0.5 s

    1.0 s

    2.0 s

    3.0 s

    Figure 3: Copper Conductor Ratings for Bolted Connections (250C)

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    0

    5000

    10000

    15000

    20000

    25000

    30000

    35000

    40000

    45000

    50000

    0 20 40 60 80 100 120 140 160 180 200

    Conductor Size (sq mm)

    Faul

    t Cur

    rent

    (A)

    0.5 s

    1.0 s

    2.0 s

    3.0 s

    Figure 4: Copper Conductor Ratings for Welded Connections (400C)

    2.12 HAZARD MITIGATION

    Once hazards associated with an earthing system are identified, mitigation shall be considered. Some typical mitigation options are summarised in Table 2. These are presented as a guide only. It is important that local conditions and all alternative options are considered during the planning of risk mitigation. Various mitigation options are discussed in more detail in section 4.

    Table 2: Mitigation Options Mitigation Options Advantages Disadvantages Comments Reduction in earthing system resistance

    Can reduce EGVR and associated touch and step voltages.

    May require extensive additional earthing at significant expense (Note 1).

    Often only effective if earth resistance is reduced to at least 40-50 % of power system source impedance. Should be investigated at early stages to check viability. Can be very effective in significant urban areas by bonding neutrals from adjacent MEN systems to create an extensive earthing system.

    Installation of gradient control conductors

    Easy to implement. Can extend step voltage hazards further out.

    Very effective. Practical in most situations. Extensively used for HV a.c station earthing.

    NER Limits earth fault currents. Limits induced voltage into telecommunications circuits.

    Cost, although offset by lower ratings for cable sheaths.

    Usually very effective for zone substations. Reduces the risks on distribution centres especially when the NER impedance is high relative to the distribution centre MEN earth impedance.

    Resonant earthing (Petersen coils)

    Eliminates EPR hazards. Improves system reliability.

    Cost. Extensively and successfully used in Europe. Significant system and operation changes.

    OHEW Can greatly reduce EPR and induced voltages.

    Cost, additional pole loading, may create more frequent EPR hazard around towers or poles.

    Can be very effective.

    Cable screen bonding

    Can greatly reduce EPRs and induced voltages.

    May transfer EPR to other areas.

    Can be very effective. Requires proper analysis to confirm suitability.

    Crushed rock Can reduce touch and step voltage hazards significantly.

    Not easy to specify correctly and installation requires care.

    Very effective especially for substation earthing. Preferred method for substations. Should be considered as part of substation designs. May not be effective for lines 66 kV and above.

    Asphalt Can reduce touch and step voltage hazards significantly.

    Asphalt requires integrity checks.

    Very effective especially for substation earthing and lower voltage distribution system. May not be effective for lines 66 kV and above.

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    Mitigation Options Advantages Disadvantages Comments Separation of HV and LV earthing for Distribution Centres

    Eliminate the hazards for the LV installation and for third party services supplied to the customer installation

    Sometimes difficult to implement. Not commonly used in NZ at present. Provides no protection from HV line to LV line contact

    Can be effective. Maintaining the integrity of the separation may be difficult to achieve in practice. The integrity may be compromised by LV electrode encroachment on the separation distance and by connections to other LV neutrals.

    TT System Easy to implement. Not presently used in NZ. Will require special dispensation from the Regulator.

    May be difficult to maintain the integrity of the system. Other contractors who do not know about the system may change it back to a MEN system.

    Install physical barriers or fences

    Low cost, suited to smaller areas of hazards.

    Requires maintenance.

    Very effective.

    Isolation of specific metallic conductors such as fences

    Minimal cost provided few conductors require isolation.

    Requires regular integrity checks.

    Very effective. Must ensure all conductors located.

    Alternative power or telecommunications route

    Can offer significant risk reductions even with lower physical protection.

    May involve additional planning issues and costs.

    Dependent upon risks associated with new route. Should always be considered.

    Isolation (telecommunications)

    Low cost where few customers affected.

    High cost where many customers affected.

    Very effective.

    Reduce fault clearance times

    May be easy to implement.

    May require significant protection review and upgrade.

    Only likely to be useful if hazardous voltages do not exceed tolerable levels significantly.

    Note 1: Since this may push out any EPR contours, there is a greater chance of affecting third party plant (e.g. telecommunications networks). This may also push touch and step hazards further out possibly into new more sensitive areas (e.g. a childrens playground).

    2.13 SWITCHGEAR OPERATING MECHANISMS

    Operating handles of earth switches and disconnectors may be a significant source of EPR hazards if the handles are not sufficiently earthed. The manual operation of an earth switch or disconnector may cause hazardous currents to flow through the earth switch or disconnector operating mechanism. Since this operation requires the presence of an operator near the structure, the operator may be subjected to hazardous touch and step voltages. For earth switches or disconnectors located within earth grids, it is relatively easy to protect the operator against hazardous voltages. If the earth grid has been designed to be safe from touch and step voltage hazards, there is no risk to the operator. However, the operator may still be in a position to receive a significant non-fatal electric shock. For this reason, additional safety measures are usually taken to further limit touch voltages for the operator. An equipotential zone is created for the operator by providing an earth mat (operator mat) where the operator would be standing to operate the switch or disconnector. The operator mat is bonded to the operating handle but is not bonded directly to the earth grid. In addition, it is advantageous to bond the operating rod/shaft and the mechanism box to the support stand or directly to the earth grid. The use of insulating gloves may also be considered. For earth switches or disconnectors on a distribution network, it is more difficult to protect the operator against hazardous voltages. The installation of a buried gradient control conductor under the area where the operator will be standing could be considered. Alternatively, a driven rod is installed under the position where the operator will be standing. The buried gradient control conductor or the driven rod shall be bonded to the earth switch mechanism. These measures will help to mitigate touch voltages on the operating handles but in most cases are unlikely to be enough. The use of insulating gloves is recommended.

    2.14 SURGE ARRESTERS

    Earthing requirements for surge protection is different to earthing requirements for the control of EPR hazards. For surge protection especially from lightning, the inductance of an earthing conductor can have a significant effect on the overvoltage seen by an item of equipment. Because high frequencies are involved in a lightning surge, even a straight piece of earthing conductor can have a significant inductance. Also, the distance between the equipment and surge arresters can have a significant effect on the overvoltage at the equipment.

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    Typically, surge arresters should be placed as close to the equipment as possible and should have short connecting leads to the equipment and to the earth electrode. Surge arresters shall not be earthed to an earth electrode which is separate from the equipment earth electrode. For distribution centre transformers, the best protection levels are achieved when the surge arresters are installed directly on the transformer tank. The earth path between the surge arresters and the transformer tank is then minimised and the protection effectiveness of the surge arresters is maximised. When this configuration is implemented, the fuses typically end up on the supply side of surge arresters relative to the transformer. Lightning surge current discharged by the arrester passes through the fuses and may result in nuisance operation of the fuses. The frequency of nuisance failures is dependent on the type and rating of the fuses. Surge arresters used for the protection of cable terminations should be earthed as directly as possible to the cable screens and to an earth electrode. The placement of surge arresters in HV a.c. stations is dependent on the presence of overhead earth wires on the lines. If overhead earth wires are used on the lines, surge arresters may be placed further from the equipment being protected provided the risk of backflashovers close to the substation on the lines is adequate. An insulation coordination study is usually carried out to strategically place surge arresters around the substation so that all or most of the equipment is protected. For HV a.c. stations where the lines do not have overhead earth wires, the risk of equipment failures is typically higher. Surge arresters should then be placed as close as possible to the transformers and protection of the other substation equipment can be achieved by the use of surge arresters at the station entrances. When surge arresters are installed on a steel structure and the structure is relied upon for the earthing of the arresters i.e. a transformer tank, it is necessary to ensure that the cross sectional area of the steel is adequate (steel is significantly less conductive than copper) and that a good connection is achieved on the steel structure. Paint films and rust on the steel structure shall be avoided.

    2.15 STATION FENCING

    During an EPR event at a substation surrounded by a metallic fence, touch voltage hazards on the fence may be significant. Therefore, the earthing of the fence is very important since the public generally has access to the fence. The design of the substation earthing system shall investigate hazardous touch voltages on the fence and the risk associated with these. There may also be step voltages outside the fence which may be hazardous to the public. The following options for earthing the fence should be reviewed as part of the design: (a) The fence is bonded to the earth grid and is either located within the earth grid or outside the

    earth grid. (b) The fence is located outside the earth grid and may be either earthed to a separate earthing

    conductor or earthed through the metallic support posts. Typical practice in New Zealand has been option (a) above. The fence is bonded to the earth grid and is either located within or outside the earth grid. Mitigation of touch voltages on a fence typically involves one or a combination of the following measures: (a) The reduction the earth grid impedance. (b) The installation of a strip of crushed rock or asphalt outside the fence. (c) The use of gradient control conductors. (d) The use of non-conductive (e.g. timber) fences.

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    Note that if a fence is located outside the earth grid and is bonded to the grid, then the addition of a gradient control conductor outside the fence effectively means that the fence is contained within the earth grid. To mitigate touch voltage hazards on the fence the option of locating the fence outside the earth grid may also be considered. This involves providing a separation distance between the fence and the earth grid, and bonding the fence either to a separate earth conductor or relying on the fence metallic supports for earthing. Touch voltage hazards are mitigated by placing the fence at or near an EPR contour which would result in acceptable touch voltages on the fence. A gradient control conductor located outside the fence and bonded to the fence can be added to limit touch voltages if required. For this option to work effectively, it is necessary to maintain the same separation distance between the fence and the earth grid around the whole perimeter of the earth grid. Also, the following should be considered: (a) It is necessary to maintain the separation distance between the fence and the earth grid at all

    times. The separation distance may be compromised by other services such as metallic water pipes or by other earth conductors added at a later stage.

    (b) The falling of an overhead live conductor on the line may cause additional hazards. The risk of a live overhead conductor falling on a fence is considered low.

    (c) Variation in the soil resistivity around the site may cause touch voltages to appear on the fence at various locations. This issue cannot be predicted by modelling and can only be verified by testing.

    2.16 CONNECTION POINTS FOR TEMPORARY EARTHS

    The provision of earthing points for the application of temporary earths inside HV a.c. stations shall be considered as part of the earthing design. The earthing points should be positioned to ensure that temporary earths can be safely applied to equipment. The provision for temporary earthing on distribution networks is beyond the scope of this document. Industry guidelines for temporary earthing on distribution networks exist and should be consulted.

    2.17 EARTH ELECTRODE ENHANCEMENT

    Methods of electrode enhancement include the encasement of the electrode in conducting compounds and the chemical treatment of the soil surrounding the electrode. These methods may be considered in certain circumstances as a possible solution to the problem of high electrode resistance to earth. They may also be applied in areas where considerable variation of electrode resistance is experienced due to seasonal climatic changes.

    2.17.1 Conductive Concrete and Other Compounds The use of conductive concrete and other compounds is a practical means of reducing the resistance of earth electrodes. It can also result in electrode resistance values that are less susceptible to fluctuation with temperature, humidity and soil moisture content than non-encased electrodes. In some circumstances, it may be the only practical way of reducing the electrode resistance to within acceptable limits.

    2.17.2 Chemical Treatment Chemical treatment of the soil surrounding an electrode should only be considered in exceptional circumstances where no other practical solution exists, as the treatment requires regular maintenance. Since there is a tendency for the applied salts to be washed away by rain, it is necessary to reapply the treatment at regular intervals. Chemicals should only be applied if these are approved for use by local authorities.

    2.18 TESTING AND MAINTENANCE

    Owners of works are required to take all practicable steps to maintain their earthing systems to meet the requirements for safety and functional operation and shall establish and operate administrative

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    systems (including records of checks undertaken) that provide periodic safety checks at reasonable intervals appropriate to the operating environment and operational risks. The asset owner should determine appropriate inspections and tests intervals based on its knowledge of its earth electrodes installation and design standards, and on its understanding of environmental conditions and assessment of risk eg soil conditions, theft of copper, etc.

    The earth impedance of an earthing system should be determined by testing at the time of installation to verify that the actual earth impedance is below its maximum desired value and also to establish a benchmark against which later measurements can be compared. Continuity tests carried out to verify the integrity of earthing connections between equipment and the earth grid and between the earth grid and the system neutrals should test to a common reference point (or several common reference points depending on the size of the substation) using a micro-ohmmeter. A maximum resistance of approximately 10 m per bond test should be obtained.

    When work has taken place that may have interfered with the earthing system, the system in that area shall be inspected and checked. All parts of the earthing system exposed by excavation should be inspected for damage or deterioration.

    Where there is any probability of significant corrosion of the buried earth grid, more frequent inspections of the earth grid and connections shall be carried out and replacements made where necessary.

    To enable the integrity of the earthing installation over a long period of time and its suitability for present fault levels to be assessed the following records shall be maintained: (a) Initial design calculations where applicable. (b) Results of periodic inspections and measurements. (c) Updating of fault level. (d) Drawings showing the earth electrode layout including location and size of all earth conductors

    and driven rods, and the location of all grid connections and/or joints.

    Additional guidelines are provided in sections 5, 6 and 7.

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    SECTION 3 EPR RISK MANAGEMENT

    Risk management is an internationally recognized tool used when designing systems and processes. New Zealand Electricity Regulations reflect an outcome-based approach to EPR safety involving risk management instead of requiring certain prescriptive criteria to be met as in the Deterministic Method. This enables Network Companies to design systems based on optimising costs but at the same time minimizing risks to the public. To manage the risk from EPR events, either of the following two methods may be used: (a) The Probabilistic Method (b) The Deterministic Method The Probabilistic Method identifies the types and extent of the region or area where an individual or a group of individuals is potentially at risk. It then evaluates the likelihood of a hazard event occurring when an individual or group of individuals is present. The Probabilistic Method is suitable as a general approach and may be applied to any locations. It is especially suitable for locations where hazard events are relatively rare and or where exposure would be typically very short. The Deterministic Method determines if hazardous step and touch voltages are present on the basis of internationally acceptable limits of body currents. Probabilities of exposure to the hazard and of the hazard occurring are not calculated. The method proceeds with the design of the earthing system to ensure calculated body currents are reduced to acceptable limits. The Deterministic Method has been adopted for controlled areas, such as substations, where faults are relatively frequent. It is also adopted elsewhere as a threshold beyond which harm is exceedingly unlikely to occur.

    A. PROBABILISTIC METHOD

    During earth faults on HV network assets, there may be some areas or zones on or around the structures where hazardous step and touch voltages occur. The risk associated with these hazardous voltages must be managed. This may require a change in design to eliminate or reduce the risk where required or in cases where the risk of harm is already acceptably low, no further action is required. The Probabilistic Method is an earthing system design process whereby the risk associated with hazardous voltages is identified and evaluated against given criteria to determine whether the risk needs to be mitigated. This method is comprehensively described in the EEA Guide to Risk Based Earthing System Design, and so here we will lim