As 1768 Lightning Protection

AS 1768—1991 NZS/AS 1768—1991

Australian StandardR New Zealand Standard

Accessed by CLOUGH ENGINEERING on 28 Mar 2002

Lightning protection

AS 1768—1991/NZS/AS 1768—1991

This Standard was prepared under a joint arrangement by Standards Australia and the Standards Association of New Zealand. It was approved for publication on behalf of the Council of Standards Australia on 18 September 1991 and on behalf of the Standards Council of New Zealand on 6 September 1991. It was published on 9 December 1991. The following organizations are represented on the Committees responsible for this Standard: Standards Australia Committee EL/24, Protection Against Lightning Association of Consulting Engineers Australia Australian Corrosion Association Australian Electrical and Electronic Manufacturers Association Australian Institute of Petroleum Building Owners and Managers Association of Australia Confederation of Australian Industry Department of Defence Department of Minerals and Energy, N.S.W. Department of Administrative Services—Australian Construction Services Electricity Supply Association of Australia Institution of Engineers Australia Public Works Department, N.S.W. Railways of Australia Committee Telecom Australia University of Queensland Standards Association of New Zealand

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Standards Association of New Zealand Electrotechnical Board, 60/–, was responsible for coordinating the New Zealand participation.

Review of Standards. To keep abreast of progress in industry, Joint Australian/New Zealand Standards are subject to periodic review and are kept up to date by the issue of amendments or new editions as necessary. It is important therefore that Standards users ensure that they are in possession of the latest edition, and any amendments thereto. Full details of all Joint Standards and related publications will be found in the Standards Australia and Standards New Zealand Catalogue of Publications; this information is supplemented each month by the magazines ‘The Australian Standard’ and ‘Standards New Zealand’, which subscribing members receive, and which give details of new publications, new editions and amendments, and of withdrawn Standards. Suggestions for improvements to Joint Standards, addressed to the head office of either Standards Australia or Standards New Zealand, are welcomed. Notification of any inaccuracy or ambiguity found in a Joint Australian/New Zealand Standard should be made without delay in order that the matter may be investigated and appropriate action taken.

This Standard was issued in draft form for comment in Australia as DR 90070 and in New Zealand as DZ 6110.

AS 1768—1991 NZS/AS 1768—1991

Australian StandardR New Zealand Standard

Lightning protection

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In Australia First published as AS MC1—1969. Revised and redesignated AS 1768—1975. Second edition 1983. Third edition 1991. In New Zealand First published as NZS/AS 1768—1991.

PUBLISHED JOINTLY BY:

STANDARDS AUSTRALIA (Standards Association of Australia), 1 The Crescent, Homebush, NSW, Australia STANDARDS NEW ZEALAND Level 10, Standards House, 155 The Terrace, Wellington 6001 New Zealand ISBN 0 7262 7132 2

PREFACE

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This Standard is issued as a joint Standard under the terms of the Memorandum of Understanding between Standards Australia and the Standards Association of New Zealand with the objective of reducing technical barriers to trade between the two nations. It was prepared by the Standards Australia Committee on Protection against Lightning and, in Australia, it supersedes AS 1768–1983. This Standard is intended to provide authoritative guidance on the principles and practice of lightning protection for a wide range of structures and systems, but excludes those owned or operated by public utilities and statutory authorities. It is not intended for mandatory application but, if called up in a contractual situation, compliance with this Standard requires compliance with all relevant clauses of the Standard. Alternative methods of protection to those described in this Standard will be the subject of future consideration. In general, it is not economically possible to provide total protection against all the possible damaging effects of lightning, but the recommendations in this Standard will reduce the probability of damage to a low level, and will minimize any lightning damage that does occur. Guidance is given to methods of enhancing the level of protection against lightning damage, if this is required in a particular situation. Following a review of submissions relating to AS 1768–1983, several changes and additions have been made to this Standard. Information is given on the protection of persons and equipment within buildings from the harmful effects of lightning strikes to the building, or to electrical power or communication services entering the building from remote sites. Revised recommendations are given relating to the compatibility of materials used in lightning protection systems, especially from the point of view of minimizing galvanic corrosion of components. In addition, changes have been made to recommendations for protection of the sides of tall buildings. Unless it has been specified that lightning protection must be provided, the first decision to make is whether the lightning protection is needed. Section 2 provides guidance to assist in this decision. Section 3 provides advice on the protection of persons from lightning, mainly relating to the behaviour of persons when not inside substantial buildings. Once a decision is made that lightning protection is necessary, Section 4 will provide details on interception lightning protection for the building or structure. This includes information on the size, material, and form of conductors, the positioning of air terminations and downconductors, and the requirements for the earth terminations. Persons and equipment within buildings can be at risk from the indirect effects of lightning and Section 5 gives recommendations on the protective measures that may need to be applied. Section 6 describes methods of lightning protection of various items not covered in earlier sections, such as communications aerials, chimneys, boats, fences, and trees. A new clause has been included on methods for protecting domestic dwellings, where a complete protection system may not be justified, but some protection is considered desirable. Section 7 sets out recommendations for the protection of structures with explosive or highly flammable contents. Section 8 gives advice on inspecting, testing, and maintaining lightning protection systems. A number of appendices are included which provide additional information and advice. The appendices form an integral part of this Standard unless specifically stated otherwise, i.e. appendices identified as ‘informative’ only provide supportive or background information and are therefore not an integral part of this Standard.

E Copyright — STANDARDS AUSTRALIA/STANDARDS NEW ZEALAND Users of Standards are reminded that copyright subsists in all Standards Australia and Standards New Zealand publications and software. Except where the Copyright Act allows and except where provided for below no publications or software produced by Standards Australia or Standards New Zealand may be reproduced, stored in a retrieval system in any form or transmitted by any means without prior permission in writing from Standards Australia or Standards New Zealand. Permission may be conditional on an appropriate royalty payment. Australian requests for permission and information on commercial software royalties should be directed to the head office of Standards Australia. New Zealand requests should be directed to Standards New Zealand. Up to 10 percent of the technical content pages of a Standard may be copied for use exclusively in–house by purchasers of the Standard without payment of a royalty or advice to Standards Australia or Standards New Zealand. Inclusion of copyright material in computer software programs is also permitted without royalty payment provided such programs are used exclusively in–house by the creators of the programs. Care should be taken to ensure that material used is from the current edition of the Standard and that it is updated whenever the Standard is amended or revised. The number and date of the Standard should therefore be clearly identified. The use of material in print form or in computer software programs to be used commercially, with or without payment, or in commercial contracts is subject to the payment of a royalty. This policy may be varied by Standards Australia or Standards New Zealand at any time.

CONTENTS Page SECTION 1 1.1 1.2 1.3 1.4

SCOPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . APPLICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . REFERENCED DOCUMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . DEFINITIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 2 2.1 2.2 2.3

4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14

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5.1 5.2 5.3 5.4 5.5 5.6

6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6.10

13 13 13

14 14 14 18 21 23 24 24 24 26 27 27 29 30

PROTECTION OF PERSONS AND EQUIPMENT WITHIN BUILDINGS

SCOPE OF SECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NEED FOR PROTECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MODES OF ENTRY OF LIGHTNING IMPULSES . . . . . . . . . . . . . . . . . . GENERAL CONSIDERATIONS FOR PROTECTION . . . . . . . . . . . . . . . . PROTECTION OF PERSONS WITHIN BUILDINGS . . . . . . . . . . . . . . . . PROTECTION OF EQUIPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 6

8

PROTECTION OF BUILDINGS

SCOPE OF SECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ZONES OF PROTECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . METHODS OF PROTECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MATTERS TO BE CONSIDERED WHEN PLANNING PROTECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MATERIALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FORM AND SIZE OF CONDUCTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . JOINTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FASTENERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . AIR TERMINATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . DOWNCONDUCTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TEST LINKS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . EARTH TERMINATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . EARTHING ELECTRODES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . METAL IN AND ON A STRUCTURE . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 5

7 7

BEHAVIOURAL PRECAUTIONS FOR PERSONAL SAFETY

SCOPE OF SECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PERSONAL CONDUCT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . EFFECT ON PERSONS AND TREATMENT FOR INJURY BY LIGHTNING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 4

5 5 5 5

ANALYSIS OF NEED FOR PROTECTION

NEED FOR PERSONAL PROTECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . NEED FOR PROTECTION OF BUILDINGS AND CONTENTS . . . . . . . NEED FOR PROTECTION OF PERSONS AND EQUIPMENT WITHIN BUILDINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 3 3.1 3.2 3.3

SCOPE AND GENERAL

33 33 33 34 36 37

PROTECTION OF MISCELLANEOUS STRUCTURES AND PROPERTY

SCOPE OF SECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . STRUCTURES WITH RADIO AND TELEVISION AERIALS . . . . . . . . . STRUCTURES NEAR TREES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PROTECTION OF TREES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CHIMNEYS, METAL GUY–WIRES OR CABLES . . . . . . . . . . . . . . . . . . . PROTECTION OF MINES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PROTECTION OF BOATS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MISCELLANEOUS STRUCTURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PROTECTION OF HOUSES AND SMALL BUILDINGS . . . . . . . . . . . . .

42 42 42 42 43 43 44 45 46 47

Page SECTION 7 7.1 7.2 7.3 7.4 7.5

SCOPE OF SECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . GENERAL CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . AREAS OF APPLICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . EQUIPMENT APPLICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SPECIFIC OCCUPANCIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 8 8.1 8.2 8.3 8.4 8.5

PROTECTION OF STRUCTURES WITH EXPLOSIVE OR HIGHLY–FLAMMABLE CONTENTS 48 48 48 48 49

INSTALLATION AND MAINTENANCE PRACTICE

WORK ON SITE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . INSPECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TESTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RECORDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MAINTENANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

53 53 53 53 53

APPENDICES

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A THE NATURE OF LIGHTNING AND THE PRINCIPLES OF LIGHTNING PROTECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B NOTES ON EARTHING ELECTRODES AND MEASUREMENT OF EARTH IMPEDANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C THE CALCULATION OF LIGHTNING DISCHARGE VOLTAGES AND REQUISITE SEPARATION DISTANCES FOR ISOLATION OF A LIGHTNING PROTECTION SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . D WAVESHAPES FOR ASSESSING THE SUSCEPTIBILITY OF EQUIPMENT TO TRANSIENT OVERVOLTAGES DUE TO LIGHTNING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E ALTERNATIVE DETERMINATION OF INDEX E BASED ON LIGHTNING FLASH DENSITY/ENERGY DATA . . . . . . . . . . . . . . . . . . F REFERENCED DOCUMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

54 66

77

84 88 94

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STANDARDS AUSTRALIA/STANDARDS ASSOCIATION OF NEW ZEALAND Australian/New Zealand Standard Lightning protection SECTION 1 SCOPE AND GENERAL 1.1 SCOPE This Standard sets out guidelines for the protection of persons and property from hazards arising from exposure to lightning. The recommendations specifically cover the following applications: (a) The protection of persons, both outdoors, where they may be at risk from the direct effects of a lightning strike, and indoors, where they may be at risk indirectly as a consequence of lightning currents being conducted into the building. (b) The protection of a variety of buildings or structures, including those with explosive or highly-flammable contents, and mines. (c) The protection of sensitive electronic equipment (e.g. facsimile machines, modems, computers) from overvoltages resulting from a lightning strike to the building or its associated services. The nature of lightning and the principles of lightning protection are discussed and guidance is given to assist in a determination of whether protective measures should be taken. The recommendations in this Standard do not apply to the protection of large scale power or communications systems, nor do they apply to the protection of special structures such as oil and gas platforms. 1.2 APPLICATION This Standard does not override any statutory requirements but may be used in conjunction with such requirements. Compliance with the recommendations of this Standard will not necessarily prevent damage or personal injury due to lightning but will reduce the probability of such damage or injury occurring.

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1.3 REFERENCED DOCUMENTS The documents referred to in this Standard are listed in Appendix F. 1.4 DEFINITIONS For the purpose of this Standard, the definitions below apply. 1.4.1 Air termination—a conductor or rod of a lightning protection system, positioned so as to intercept a lightning discharge, which establishes a zone of protection. 1.4.2 Air termination network—a network of air terminations and interconnecting conductors which forms the part of a lightning protection system which is intended to intercept lightning discharges. 1.4.3 Base conductors (base tapes)—conductors placed around the perimeter of a structure near ground level interconnected to a number of earth terminations to distribute the lightning currents amongst them. 1.4.4 Bond (bonding conductor)—a conductor intended to provide electrical connection between the lightning protection system and other metalwork and between various metal parts of a structure or between earthing systems. 1.4.5 Downconductor—a conductor which connects an air termination with an earth termination. 1.4.6 Earth impedance (Z)—the electrical impedance of an electrode or structure to earth, derived from the earth potential rise divided by the impulse current to earth causing that rise. It is a relatively complex function and depends on— (a) the resistance component (R) as measured by an earth tester; (b) the reactance component (X), depending on the circuit path to the general body of earth; and (c) a modifying (reducing) time-related component depending on soil ionization caused by high current and fast rise times. 1.4.7 Earth potential rise (EPR)—the increase in electrical potential of an earth electrode or earthed structure, with respect to distant earth, caused by the discharge of current to the general body of earth through the impedance of that electrode or structure. 1.4.8 Earthing boss (terminal lug)—a metal boss specially designed and welded to process plant, storage tanks, or steelwork to which earthing conductors are attached by means of removable studs and nuts or bolts. 1.4.9 Earthing conductor—the conductor by which the final connection to an earth electrode is made.

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1.4.10 Earthing electrodes (earth rods or ground rods)—those portions of the earth termination which make direct low resistance electrical contact with the earth. 1.4.11 Earthing resistance—the resistance of the lightning protection system to the general mass of earth, as measured from a test point. 1.4.12 Earth termination (earth termination network)—that part of a lightning protection system which is intended to discharge lightning currents into the general mass of the earth. All parts below the lowest test link in a downconductor are included. 1.4.13 Explosive gas atmosphere—a mixture of flammable gas, vapour or mist with air in atmospheric conditions in which, after ignition, combustion spreads throughout the unconsumed mixture that is between the upper and lower explosive limits. NOTE: The term refers exclusively to the danger ari sing from igniti on. Where danger from other causes such as toxicit y, asphyxiati on, and radioactivit y may arise this is specifi call y menti oned.

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1.4.14 Finial—a term not used in this Standard owing to its confusion with architectural application but occasionally used elsewhere in other Standards as referring to short vertical air terminations. 1.4.15 Hazardous area—an area where an explosive atmosphere is, or may be expected to be present continuously, intermittently or due to an abnormal or transient condition (see the AS 2430 or NZS 6101 series). 1.4.16 Joint—a mechanical and electrical junction between two or more portions of a lightning protection system. 1.4.17 Lightning flash (lightning discharge)—an electrical discharge in the atmosphere involving one or more electrically charged regions, most commonly in a cumulonimbus cloud, taking either of the following forms: (a) Ground flash (earth discharge) — a lightning flash in which at least one discharge channel reaches the ground. (b) Cloud flash — a lightning flash in which the discharge channels do not reach the earth. 1.4.18 Lightning flash density—the number of lightning flashes of the specified type occurring on or over unit area in unit time. This is commonly expressed as per square kilometre per year (km –2 year –1). The ground flash density is the number of ground flashes per unit area and per unit time, preferably expressed as a long-term average value. 1.4.19 Lightning protection system—a system of conductors and other components used to reduce the injurious and damaging effects of lightning. 1.4.20 Lightning strike—a term used to describe the lightning flash when the attention is centred on the effects of the flash at the attachment point, rather than on the complete lightning discharge. 1.4.21 Lightning strike attachment point—the point on the ground or on a structure where the lower end of the lightning discharge channel connects with the ground or structure. 1.4.22 Lightning stroke—a term used to describe an individual current impulse in a complete ground flash. 1.4.23 Side flash—a discharge occurring between nearby metallic objects or from such objects to the lightning protection system or to earth. 1.4.24 Striking distance (d s)—the distance between the tip of the downward leader and the eventual strike attachment point at the moment of initiation of an upward intercepting leader. 1.4.25 Structure or object—any building or construction, process plant, storage tank, tree, or similar, on or in the ground. 1.4.26 Surge arrestor—a protective device, usually connected between any conductor of a system and earth, which limits surge voltages by diverting surge current to earth when a given voltage is exceeded. 1.4.27 Test link—a joint designed and situated so as to enable resistance or continuity measurements to be made. 1.4.28 Thunderday—a calendar day during which thunder is heard at a given location. The international definition of lightning activity is given as the number of thunderdays per year (also called ‘isoceraunic level’ or ‘ceraunic level’). 1.4.29 Zone of protection—the portion of space within which an object or structure is considered to be protected by a lightning protection system.

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SECTION 2 ANALYSIS OF NEED FOR PROTECTION

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2.1 NEED FOR PERSONAL PROTECTION A hazard to persons exists during a thunderstorm. Each year, a number of persons are struck by lightning particularly when outdoors in an open space such as an exposed location on a golf course, or when out on the water. Others receive electric shocks attributable to lightning when indoors. In built-up areas protection is frequently provided by nearby buildings, trees, power lines or street lighting poles. Persons within a substantial structure are normally protected from direct strikes, but may be exposed to a hazard from conductive materials entering the structure (e.g. power, telephone, or TV antenna wires) or from conductive objects within the structure which may attain different potentials. Measures for the protection of persons within buildings or structures are set out in Section 5. Lightning strikes direct to a person or close by may cause death or serious injury. A person touching or close to an object struck by lightning may be affected by a side flash, or receive a shock due to step, touch or transferred potentials, as described in Appendix A. When moderate to loud thunder is heard, persons out of doors should avoid exposed locations and should seek shelter or protection in accordance with the guidance for personal safety provided in Section 3, particularly if thunder follows within 15 s of a lightning flash (corresponding to a distance of less than 5 km). 2.2 NEED FOR PROTECTION OF BUILDINGS AND CONTENTS 2.2.1 Factors governing decision whether or not to protect A decision to provide lightning protection may be taken without any risk assessment, for example, where there is a desire that there be no avoidable risk. In such cases a clear statement should be made that a lightning protection system should be installed in accordance with this Standard. The object of this Clause is to give guidance on those factors which are capable of assessment in terms of the likelihood of the structure being struck and the consequences of any such strike. The use of the structure, the nature of its construction, the value of the contents, and the prevalence of thunderstorms in the area can all be considered in making the assessment. Where it is thought that the consequential effects will be small and that the effect of a lightning flash will most probably be merely slight damage to the fabric of the structure, it may be economic not to incur the cost of protection but to accept the risk. Even though this decision is made, it is suggested that a calculation is still worth making so as to give some idea of the magnitude of the risk that is being taken. The variety of structures is so great that any method of assessment may lead to anomalies and those who have to decide on protection should use their judgement. For example, a steel-framed building may be found to have a low risk but as the addition of an air termination and earthing system will give greatly improved protection, the small extra cost of doing so may often be worthwhile. A low risk value may arise for chimneys made of brick or concrete. However, where such chimneys are free-standing or where they project for more than 5 m above the adjoining structure, they will require protection regardless of the value of the risk index. In determining how far to go in providing lightning protection for specific cases, or whether it is needed at all, it is necessary to take into account a number of factors. With some structures there will be little doubt as to the need for protection; examples of such structures are— (a) those in or near which large numbers of persons congregate; (b) those concerned with the maintenance of essential public services; (c) those in areas where lightning is prevalent; (d) very tall or isolated structures; and (e) structures of historic or cultural importance. Although structures of large area are more likely to be struck than smaller ones, the cost-effectiveness of structure protection is not strongly dependent on this characteristic for non-flammable structures. However, the need to protect electronic equipment and to protect persons against potential differences associated with metallic services increases with the building area (see Section 5). Any structure which is entirely within a zone protected by an adjacent object or objects (whether protected or not) should be deemed to be protected (see Clause 4.2). 2.2.2 Risk index In Tables 2.1 to 2.5, index figures are given opposite headings denoting the relative degree of importance or risk in each case.

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The risk index, R, is obtained from the equation: R=A+B+C+D+E .... 2.2.2 the index figures A, B, C, D and E being obtained from the tables. The higher the risk index the greater the need for protection, and vice versa. Table 2.6 shows an assessment of the risk associated with various values of the risk index, R. The table also provides guidance on the need for protection. Examples of risk index calculations for different structures are given in Table 2.7. The risk index equation has been determined empirically. The equation has been applied to a variety of cases and, despite the incompleteness of present knowledge of lightning phenomena, it has been found to lead to recommendations which, in general, accord with commonly accepted practice in Australia. Attention is called to the above factors and their importance without pre-empting the purchaser’s right to determine whether or not lightning protection should be provided. However, to avoid ambiguity where a purchaser specifies lightning protection in accordance with this Standard and provides no further guidance, protection should be provided wherever the risk index, evaluated as described in this Clause, is equal to or greater than 13. The factors are set out in Clauses 2.2.3 to 2.2.5. 2.2.3 Value and nature of building and contents The value and nature of the building and contents are obviously vital factors in deciding whether the expense of protection is warranted. In addition to direct losses caused by fires, damage to equipment and buildings and killing of livestock, indirect losses such as interruption to business services and farming should also be taken into account when assessing the need for protection (see also Clause 2.3). 2.2.4 Relative exposure The relative exposure of a particular building will be an element in determining whether the expense of protection is warranted. In closely built-up towns and cities, the hazard is not so great as in the open country. In the latter, farm buildings are in many cases the most prominent targets for lightning in a large area. In hilly or mountainous districts, a building upon high ground is usually subject to a greater hazard than one in a valley or otherwise sheltered area. 2.2.5 Frequency and severity of thunderstorms The frequency of occurrence of thunderstorms varies significantly depending on location. Moreover, the severity of lightning storms, as distinct from their frequency of occurrence, is known to be much greater in some areas than in others. Hence, the need for protection varies across the country, although not necessarily in direct proportion to thunderstorm frequency. A few severe thunderstorms in a season may make the need for protection greater than a relatively large number of storms of lesser activity. Data on the average yearly distribution of days with thunderstorm activity are given— (a) in Figure 2.1 for Australia; and (b) in Figure 2.2 for New Zealand. Thunderday information is of limited usefulness in assessing the need for protection but may be the only information available on which such an assessment can be made. Lightning detection systems are in use at a limited number of sites in Australia. Such systems detect the number of ground flashes within a specified area and, in some cases, the peak current of each discharge, thus providing a more meaningful indication of the lightning activity at a given location. Ground flash data covering a region of South East Queensland and North East New South Wales are provided in Appendix E. A procedure for determining a value of the index E in Equation 2.2.2, based directly on ground flash data rather than thunderday data (see Table 2.5), is under consideration. See details given in Appendix E. 2.3 NEED FOR PROTECTION OF PERSONS AND EQUIPMENT WITHIN BUILDINGS As explained in Clause 2.1, persons and equipment within buildings can be at risk from lightning currents and associated voltages which may be conducted into the building as a consequence of a lightning strike to the building or associated services. Some equipment (e.g. electronic equipment, including computers) is especially susceptible to damage from overvoltages in the electricity supply caused by lightning and such damage may occur even when the lightning strike is remote from the building, e.g. from a surge conducted into the building via the electricity supply. Measures may therefore need to be taken to protect persons and equipment within buildings and Section 5 provides further advice on this subject. The measures recommended in Section 5 can be implemented even when a lightning protection system for the building structure has not been provided. The decision as to whether to provide protection specifically directed to equipment will depend on the value placed on that equipment and on the cost and inconvenience which might result from the equipment being out of service for an extended period. The risk index determined from Clause 2.2.2 will provide guidance on the likelihood of a building being subject to a lightning strike with consequent risk of damage occurring to equipment within the building. However, since damage to equipment can result from lightning strikes to adjacent properties or to power or signal lines some distance away, the index value may not be a sufficient indicator of the risk. The incidence of damage occurring to similar equipment within buildings in the vicinity may provide a better guide to the need to protect.

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TABLE 2.1 INDEX FIGURE A (TYPE OF STRUCTURE) Usage and contents Protecti on not justi fi ed having regard to nature of building, occupancy and contents Structure and contents inert , occupati on infrequent, e.g. domesti c outbuil ding, farm shed, roadside hoarding, metal chimney or mast Structure containing ordinary equipment or a small number of people, e.g. domesti c dwell ing, store, shop, small factory, rail way stati on, tent or marquee Structure or contents of fair import ance, e.g. water tower, store wit h valuable contents, offi ce, factory or residenti al buil ding, non-metalli c chimney or mast Cinema, church, school, boat, historical monument of medium importance, densely populated marquee Museum, art gallery, stadium, entert ainment complex, telephone exchange, computer centr e, air craft hangar, airport terminal, air port contr ol tower, li ghthouse, industri al plant, power station, historical monument or tr ee of major import ance Petr ol and gas installati on, hospital Explosives building

Value of index A −10 0 1 2 3

4 5 15

TABLE 2.2 INDEX FIGURE B (CONSTRUCTION) Constructi on Full y metalli c str ucture, electr ically continuous Reinforced concrete or steel frame with metall ic roof Reinforced concrete or steel frame with concrete or other non-metall ic roof Cott age or small buil ding of timber or masonry with metall ic roof Large area building of timber or masonry with metall ic roof Small buil ding of timber or masonry wit h non-metall ic roof Large area building of timber or masonry with non-metall ic roof Large tent or marquee of fl ammable material Membrane str uctures with metall ic frames

Value of index B 0 1 2 3 4

TABLE 2.3 INDEX FIGURE C (HEIGHT)

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Height of structure, m Exceeding 0 6 12 17 25 35 50 70 100 140 200

Not exceeding 6 12 17 25 35 50 70 100 140 200

Value of index C 0 2 3 4 5 6 7 8 9 10 11

TABLE 2.4 INDEX FIGURE D (SITUATION) Situation On the flat, at any elevation Hill side up to three-quarters of the way up, or mountainous country up to 1000 m Mountain top above 1000 m

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TABLE 2.5 INDEX FIGURE E (LIGHTNING PREVALENCE) Average thunderdays per year* Value of index E Exceeding Not exceeding 0 2 0 2 4 1 4 8 2 8 16 3 16 32 4 32 64 5 64 6 * See thunderday data in Figures 2.1 and 2.2. NOTE: See Appendix E for an alternative procedure, which is stil l under development, for the determination of the index E based on ground fl ash data in li eu of thunderday data.

TABLE 2.6 ASSESSMENT OF RISK AND NEED FOR PROTECTION Risk index, R (R = A + B + C + D + E) 14

Assessment of risk Negligible Small Fair Medium Great Very great

Need for protection Not needed Not needed Might be advisable Advisable Strongly advisable Essential

TABLE 2.7 EXAMPLES OF THE CALCULATIONS FOR EVALUATING THE NEED FOR PROTECTION Example

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10 m high domestic dwelli ng, brick wall s, non-metall ic roof located on hill side—15 thunderdays 15 m high domestic dwelli ng, brick wall s, non-metall ic roof located on hill side—30 thunderdays 20 m high historic tr ee on fl at—60 thunderdays 15 m high aircraft hangar, steel frame with metall ic roof, located in hill y countr y at 1000 m —15 thunderdays 10 m high church, bri ck wall s, metal roof, located on hill side—30 thunderdays 24 m high offi ce building, reinforced concrete, located on fl at—15 thunderdays 40 m high offi ce building, reinforced concrete, located of fl at—30 thunderdays 16 m high wooden masted yacht on open sea—10 thunderdays 20 m high brick chimney located on fl at— 30 thunderdays 10 m high marquee located on fl at— 40 thunderdays

Index values A B C D Type of Constructi on Height Situation structure (Table 2.1) (Table 2.2) (Table 2.3) (Table 2.4) 1 3 2 1

E Prevalence (Table 2.5) 3

R=A+B +C+D +E

Assessment of risk

Protecti on

10

Negligible

Not needed

1

3

3

1

4

12

Fair

Might be advisable

3

3

4

0

5

15

Very great

Essential

4

1

3

2

3

13

Medium

Advisable

3

2

2

1

4

12

Fair

Might be advisable

2

2

4

0

3

11

Small

Not needed

2

2

6

0

4

14

Great

Strongly advisable

3

3

3

0

3

12

Fair

Might be advisable

2

3

4

0

4

13

Medium

Advisable

3

4

2

0

5

14

Great

Strongly advisable

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NOTES: 1 Contours on the map join locati ons having the same number of thunderdays per year, a thunderday being a day on which thunder is heard. 2 A colour copy of this map is avail able fr om the Bureau of Meteorology.

FIGU RE 2.1 AV ER AG E ANN UA L THUN DE RD AY MAP OF AU STRA LIA

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NOTES: 1 Contours on the map join locati ons having the same number of thunderdays per year, a thunderday being a day on which thunder is heard. 2 The above data are based on information contained in the Meteorological Off ice Note No. 82, Frequency of Thunderstorms in New Zealand, publi shed by the New Zealand Meteorological Service.

FIGU RE 2.2 AV ER AG E ANN UA L THUN DE RD AY MAP OF NE W ZEA LAND

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SECTION 3 BEHAVIOURAL PRECAUTIONS FOR PERSONAL SAFETY 3.1 SCOPE OF SECTION This Section provides guidance for personal safety during thunderstorms and mainly applies to behaviour when outdoors. Measures for the protection of persons which should be incorporated in lightning protection systems for buildings and structures are outlined in other sections.

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3.2 PERSONAL CONDUCT Persons seeking protection from lightning should observe the following precautions: (a) Seek shelter in a substantial building with at least normal headroom or within a totally enclosed, metal-bodied vehicle. Conventional fabric tents offer no protection; small sheds offer uncertain protection. (b) If on open ground, remote from shelter, crouch down, singly, with feet together. Footwear or a layer of any non-absorbing material, such as a plastics sheet, offers some protection against ground currents, should there be a nearby lightning flash. (c) If in an open boat keep a low profile. Additional protection is gained by anchoring under relatively high objects such as jetties and bridges, provided that no direct contact is made with them. Avoid isolated buoys and pylons. (d) Avoid riding horses or bicycles, or riding in any open vehicle such as a tractor or beach buggy, or in any enclosed vehicle with a non-metallic roof. (e) Avoid swimming or wading. (f) Persons in an exposed position during the approach of a thunderstorm are advised to seek shelter. If the time interval between a lightning flash and hearing the thunder becomes less than 15 s, move quickly to a protected location as there is immediate danger of a lightning strike nearby. (g) Avoid high ground and isolated trees. If the vicinity of a tree cannot be avoided, seek a position just beyond the spread of the foliage. (h) Avoid touching or standing close to tall metal structures, wire fences and metal clothes lines. (i) Avoid handling substantial metallic objects, and remove metal objects from the hair or head covering. (j) Limit the use of telephones when a thunderstorm is overhead. (k) Avoid contact with electrical appliances and metal objects, e.g. stoves, refrigerators, metal window frames, sinks, radios and television sets. (l) If the use of household appliances or the telephone is unavoidable, keep clear of other appliances and metal objects, and keep any such use brief. 3.3 EFFECT ON PERSONS AND TREATMENT FOR INJURY BY LIGHTNING* The severity of the injuries inflicted on a person by lightning depends on the fraction of the total lightning current that flows through the person’s body and the path of the current through or over the body. The worst situation is where the person is struck on the upper part of the body, so all the current must flow through the trunk, where the heart and lungs are the vitally significant organs, or over the skin. A less dangerous situation is where the person is subjected to step or touch potentials, and only a small fraction of the total current passes through the body, although the pathway taken by this fraction is still important. The effects of lightning include burns to the skin, which are usually superficial, damage to various bodily organs and systems, unconsciousness, but, most dangerously, cessation of breathing and cessation of heart beat. Independently of these electrically related effects, temporary or permanent hearing impairment may be experienced as a consequence of the extremely high sound pressure levels associated with a nearby lightning strike. In the first-aid treatment of a patient injured by lightning, it is essential that breathing be restored by artificial respiration and blood circulation be restored by external cardiac massage, if appropriate. These procedures should be continued until breathing and heart beat are restored, or it can be medically confirmed that the patient is dead. It should also be noted that the usual neurological criteria for death may be unreliable in this situation. There is no danger in touching a person who has been struck by lightning. Lightning strike victims are sometimes thrown violently against an object, or are hit by flying fragments of a shattered tree, so first-aid treatment may have to include treatment for traumatic injury. Subsequent treatment of a lightning strike patient is a specialized area with important differences from the treatment of injuries inflicted by electric power current. For example, the nature of the burns, and the extent of damage to underlying muscle tissue tends to be severe with electric power current, but mild with lightning current. Neurological and cardiac injuries also are different, and follow different courses.

* For a more comprehensive treatment of the subject covered by this Clause, see the foll owing publi cati on: ANDREWS C.J., COOPER M.A., DARVENIZA M. and MACKERRAS D. (Eds) Lightning injury: Electrical, medical and legal aspects . CRC Press. Baton Range, Flori da. (In publication.) COPYRIGHT

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SECTION 4 PROTECTION OF BUILDINGS 4.1 SCOPE OF SECTION This Section sets out recommendations for installation practices and for the selection of equipment to prevent or to minimize damage or injury which may be caused by a lightning discharge. The recommendations apply generally to the protection of buildings and structures. Recommendations for the protection of particular structures and property are given later in this Standard. 4.2 ZONES OF PROTECTION 4.2.1 Basis of recommendations Some parts of a structure are exposed to direct lightning strikes while other parts lie within zones of protection established by higher parts of the structure. Protection against direct lightning strikes is achieved by installing a lightning protection system in such a way that its air terminations establish zones of protection enclosing the whole structure. The recommendations that follow are based on the ‘rolling sphere’ technique of determining zones of protection. Using this technique a sphere of specified radius is theoretically brought up to and rolled over the total building. All sections of the building which the sphere touches are considered to be exposed to direct strokes. Sections of the building which cannot be touched by the sphere are considered protected by other sections of the building. A sphere of 45 m radius has been selected to provide a high degree of protection to conventional buildings, this being designated as ‘standard protection’. A sphere of smaller radius may be used to establish zones of protection where a higher degree of protection is desired. NOTE: A sphere of 20 m radius is recommended for the protection of structures with explosive or highly flammable contents (see Clause 7.2.2).

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The influence of variation in sphere radius on the protection provided is discussed in Paragraph A7, Appendix A. For unusual or complex building forms, the ‘rolling sphere’ technique may be used directly in determining both zones of protection and air termination configurations. 4.2.2 Required protection Air terminations should be installed on parts of the structure most likely to be struck such as the outermost edges of the roof (especially the corners of an elevated roof), at tops of towers, and on parapets, ridges and chimneys which protrude above the general roof level, in accordance with Clause 4.9.2. The zones of protection established by air terminations on higher parts of a structure should be determined having regard to the following: (a) Air terminations which do not exceed 45 m above ground are considered to protect lower sections of structure where these lie in the space beneath an arc of 45 m radius and where the arc passes through the highest point of the building and is tangential to the ground (see Figure 4.1(a)). (b) Air terminations or structures in excess of 45 m are considered to protect only those lower sections of the structure which lie in the space beneath an arc of 45 m radius which is tangential both to the air termination or side of the building and to the ground, as shown in Figure 4.1(b). For buildings in excess of 45 m, direct strikes to the side of the structure above the 45 m level may be anticipated. However, these are less probable than strikes to the top of the building and are also likely to be of a lesser magnitude. Roofs of structures and protruding parts of structures which do not lie within the zones of protection established by air terminations on higher parts of the structure should be protected by additional air terminations. Air terminations of height h above a flat roof or horizontal plane are considered to protect points on that plane up to a horizontal distance r from a horizontal air termination conductor or to a horizontal radius r from a vertical air termination rod, where r is given by: . . . . 4.2 r = (90h − h2) where r and h are in metres. A simple array of such vertical rods at spacing distances d metres from the nearest adjacent rods on a flat roof or horizontal plane is considered to protect the whole surface within the boundary of the array provided that d ≤ r 2. Table 4.1 and Figure 4.2 illustrate the protective zones established by air terminations on a flat non-conducting roof with a parapet on one side. 4.3 METHODS OF PROTECTION 4.3.1 Structural steel-framed buildings Buildings with structural steel framing may be protected by the installation of metal air terminations at the high parts of the building, the air terminations being connected to the steel framing and the framing earthed in the vicinity of the foundation. A typical system is shown in Figure 4.3 (see also Clause 4.14.1).

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TABLE 4.1 HEIGHT AND SPACING OF AIR TERMINATIONS TO PROTECT A FLAT ROOF metres Height of air termination

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h 0.5 1.0 2.0 4.0 8.0

Horizontal distance for which roof is protected r 6.7 9.4 13.3 18.5 25.6

Maximum spacing distance for array d 9.5 13.3 18.8 26.2 36.2

FIGURE 4.1 ZONE OF PROTECTION ESTABLISHED BY AIR TERMINATIONS ON THE HIGHER PARTS OF A STRUCTURE

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NOTE: The hatched areas show the zones of protection established by each air termination. In the top view the zones are in the plane of the roof.

FIGURE 4.2 ZONES OF PROTECTION ON A FLAT ROOF

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4.3.2 Buildings without structural steel frames 4.3.2.1 General The required conditions of protection for non-metallic buildings are generally met by placing metal air terminations on the uppermost parts of the building or its projections, with conductors connecting the air terminations to each other and to ground. By this means a relatively small amount of metal properly positioned and distributed can be made to afford a satisfactory degree of protection and, if desired, the material may be placed so as to give minimum interference to the appearance of the building. A typical system is shown in Figure 4.4. Additional methods utilizing the individual characteristics of particular types of building construction are given in Clauses 4.3.2.2 to 4.3.2.4, and in Figure 4.3. 4.3.2.2 Structures with continuous metal Structures containing continuous metal, e.g. metal within a roof, wall, floor or covering may, if the amount and arrangement of the metal is adequate in terms of the recommendations of Clauses 4.10 to 4.14, utilize such metal as part of the lightning protection system. 4.3.2.3 Metal-roofed buildings For buildings which are roofed, or roofed and clad, with metal, it may be possible to dispense with some air terminations and to cater for any upper portions of the building which are susceptible to damage by earthing such metal. 4.3.2.4 Reinforced concrete buildings The following recommendations apply to the use of steel reinforcement in reinforced concrete buildings as part of the lightning protection system (see also Paragraph A5.5.2, Appendix A): (a) General As far as possible, the steel reinforcement should be made electrically continuous in all concrete elements having a structural purpose, e.g. columns, beams and also in non-structural concrete elements, e.g. concrete wall panels, where the element, or a part of it, if dislodged, could endanger persons below. Where steel reinforcing elements are not in physical contact with each other, lightning discharges may cause cracks in the vicinity of the gaps in reinforcement. Where insulating gaps cannot be avoided, the building should be treated in the same way as one of non-conducting materials. Where the steel reinforcement is used as the downconductor system, an effective electrical connection should be made from the air termination system to the steel reinforcement at the top of the building. Such connections should be made, by means such as welding or clamping, to the vertical and horizontal bars in as many places as necessary to ensure a multiplicity of conductive paths for the discharge of lightning current. NOTE: Steel reinforcement which is overlapped and tied by means of wire is not considered to provide an effective electrical connection for this purpose but such joints are acceptable elsewhere as part of the downconductor system where current sharing is assured.

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Modern reinforced concrete structures frequently involve several structural techniques including in situ reinforced concrete, prestressed reinforced concrete and precast concrete; recommendations for these are listed in Items (b), (c) and (d). (b) In situ reinforced concrete The metal rods in the columns of a reinforced concrete structure cast in situ are occasionally welded at splice points, thus providing definite electrical continuity. Where very tall columns are involved, a spliced connection between rods is frequently achieved by a mechanical clamping device or threaded ferrule which also provides a high degree of electrical continuity. Most frequently, however, the rods are tied together by steel tie wire at splice points, but despite the fortuitous nature of the metallic connection, the very large number of rods and crossing points assures a subdivision of the total lightning current into a multiplicity of parallel discharge paths. Experience shows that with this splicing technique the rods can also be readily utilized as part of the lightning protection system without thermal or mechanical damage to the structure. Particular recommendations on the size, material or number of tie wires are not given in this Standard, normal building practice being relied upon to provide adequate continuity. Normal building practice also ensures the multiple conducting paths continue into the building foundations (see Note). The foundations are deep in the mass of earth and the resistivity of concrete is generally comparable with that of clay or other moderately conductive ground. Hence, except in soils of low resistivity, the resistance to ground from the foundation reinforcement is often lower than can be obtained economically with driven rods, because of the much greater surface area. Concrete foundations themselves constitute a satisfactory earth termination network but their use, as such, precludes the inclusion of base conductors. It is desirable, however, that a metallic connection to the reinforcing be installed, in a position suitable for the bonding of metallic services associated with the building. NOTE: Conductive paths may not be ensured if special building techniques are used, e.g. grouting reinforcing bars into drilled holes in concrete after it has set, using an insulating epoxy-based material.

(c) Prestressed reinforced concrete Prestressed reinforced concrete is used most commonly in the horizontal structural elements in a building, such as the beams and floor slabs, and only rarely in vertical elements such as columns. Consequently, the principal reason for avoiding insulating gaps in prestressed concrete relates to side flashing rather than to the ability of the reinforcement to carry a lightning discharge to ground. Details of the treatment of prestressed concrete in order to avoid side flashing are given in Clause 4.14, and the principles described in that clause should be used in the rare instance where vertical prestressed elements, such as prestressed columns, occur in a building.

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Although prestressed concrete affords a large reduction in the cross-sectional area of steel reinforcement compared with conventionally reinforced concrete, calculations indicate that prestressed cables of 10 mm diameter or larger, will not be damaged thermally by lightning and that thermal effects become negligible when several cables are connected in parallel. (d) Precast concrete Where electrical continuity is required through precast concrete elements, the structural connection details, e.g. attachment plates, threaded ferrules, bolt or dowel connections, should be carefully examined from an electrical continuity standpoint. In most cases, the attachment device will be a metallic one and continuity can be achieved by simply welding the attachment device to electrically continuous reinforcement within the precast concrete element. 4.3.3 Structures with flammable or explosive atmosphere Structures in which very small induced sparks present an appreciable element of danger, such as structures which contain explosive atmospheres of flammable vapour or gas and structures in which easily ignitable fibres or materials producing combustible flyings are stored, e.g. cotton, grain or explosives, usually require much more than the standard protection. Such structures can be protected by tall conducting masts earthed at the bottom end, by bonding as detailed in Clause 4.14.2.2, or by overhead earthed wires (for further details see Section 7). 4.4 MATTERS TO BE CONSIDERED WHEN PLANNING PROTECTION 4.4.1 Structures to be erected For structures that are to be erected, the matter of lightning protection should be considered in the planning stage, as the necessary measures can often be effected in the architectural features without detracting from the appearance of the building. In addition to the aesthetic considerations, is usually less expensive to install lightning protection during construction than afterwards. 4.4.2 Design considerations 4.4.2.1 General considerations The structure or, if the structure has not been built, the drawings, should be examined with due regard to all the relevant details of this Standard and in particular to the following: (a) Metal used in the roof, walls, framework or reinforcement above or below ground, e.g. sheet piling, to determine the suitability of such metal in place of, or for use as, components of the lightning protection system.

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NOTE: For a non-metallic roof, the position of any conduit, piping, water mains or other earthed metal immediately beneath the roof should be noted, as this may inadvertently attract a discharge if not shielded by an adjacent roof or structure, or downconductor on or above the roof.

(b) Available positions for downconductors providing the required number of low impedance paths from the air termination network to the earth termination; this is particularly important for internal downconductors. (c) The nature and resistivity of the soil as revealed by trial bore holes for foundation purposes or soil resistivity tests with, where economically practicable, the driving and testing of a trial earth rod electrode with the object of designing a suitable earth termination. (d) Services entering the structure above and below ground. (e) Radio and television receiving aerials. (f) Flag masts, roof level plant rooms, e.g. lift motor rooms, ventilating plant and boiler rooms, water tanks and other salient features. (g) The construction of roofs to determine methods of fixing conductors with special regard to maintaining weatherproofing of the structure. (h) Possible penetration of waterproofing membrane where earth terminations are to be sited beneath the structure. (i) The provision of holes through, or fixing to, reinforced concrete. (j) The provision of bonding connections to steel frame, reinforcement rods or internal metalwork, and for any holes through the structure, parapets, cornices, and the like, to allow for the free passage of the lightning conductor. (k) The choice of metal most suitable for the conductor, e.g. aluminium conductors for structures where aluminium is employed externally. (l) Accessibility of test joints; protection by non-metallic casing from mechanical damage or pilferage and hazard to persons; lowering of flagmasts or other removable objects; facilities for periodic inspection, especially on tall chimneys. (m) The preparation of an outline drawing incorporating the foregoing details and showing the positions of the main components to form a basis for the record drawing recommended in Clause 8.4. (n) Requirements for the coordination of the structure’s lightning protection earthing and the earthing of power and communication services.

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FIGUR E 4.3 TYP ICAL LIGHTNING PROTECTION SYS TEM USING METAL IN OR ON A STRUC TURE

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FIGU RE 4.4 TYPICA L SYS TEM EM PLOY ING HORIZONTAL AN D VE RTICAL AIR TER MINA TION NETWORK

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4.4.2.2 Route for conductors Conductors should be installed with a view to offering the least impedance to the passage of discharge current between the air terminals and ground. The most direct path is the best (see Clause 4.10.2). The impedance to earth is approximately inversely proportional to the number of widely separated paths, so that from each air terminal there should be as many paths to earth as practicable. The number of paths is increased and the impedance decreased by connecting the conductors to form a cage enclosing the building. 4.4.2.3 Trouble-free installation Since a lightning conductor system, as a general rule, is expected to remain in working condition for long periods with little attention, the mechanical construction should be strong, and the materials used should offer resistance to corrosion. 4.4.2.4 Economy of installation Economy of installation can be effected by keeping the variety of equipment to a minimum, avoiding the use of unusual air terminal ornaments and similar features, and taking advantage of constructional features of the building as far as practicable. 4.5 MATERIALS 4.5.1 General Copper is recommended for its conductivity and durability; however, alternative materials may be used if suitable for the environment in which they are installed and are otherwise satisfactory for the purpose (see Clause 4.6). Typical materials from which the component parts of lightning protection systems may be chosen are given in Table 4.2 (see also Clause 4.5.2). Where insulating coatings are used, due regard should be given to their durability and non-flammability. For the protection of conductors at the tops of chimneys, see Clause 4.5.2.2(a). 4.5.2 Corrosion 4.5.2.1 Basic considerations The materials used in lightning protection systems should be resistant to corrosion resulting from the environment in which they are installed. This includes the effects of atmospheric, soil or water-borne electrolytes or contaminants, and of contact with those metals or alloys which will lead to galvanic corrosion in the presence of moisture. Corrosion resulting from contact of dissimilar metals can exist where a conductor is held by fixing devices on or against external metal surfaces of a building or structure. Corrosion of this nature can also arise where water passes over a relatively cathodic metal such as copper carrying small amounts of copper corrosion product which is deposited as a fine film of metallic copper on relatively anodic metals such as aluminium, zinc or steel. This causes destructive galvanic corrosion of the latter metals which are commonly used in building cladding or roofing. The metallic components of the lightning protection system should therefore be compatible with the metals used externally on the structure over which these components pass or with which they may make contact. The components of lightning protection systems may be constructed from a variety of materials as described in Clauses 4.5.2.2 and 4.5.2.3. 4.5.2.2 Air terminations and downconductors Specific recommendations for air terminations and downconductors are given in Clauses 4.9 and 4.10 respectively. Account should be taken of the principles outlined in Clause 4.5.2.1 in the selection of materials for those components. Where there is a risk of metallic building elements being contaminated by corrosion products, e.g. from copper conductors, the use of insulated conductors should be considered. Such insulation may need protection against ultraviolet radiation, e.g. by enclosure in conduit or by the application of appropriate paints or coatings. Where insulated cables are used as downconductors, bonding should be effected at the specified intervals and bonding connections should be sealed against the ingress of moisture. Where structural steel or reinforcing bars form part of the downconductor system no further corrosion protection will normally be required. With the common conductor materials, several specific precautions are necessary as follows: (a) Bare copper Copper should be of the grade ordinarily used for commercial electrical work. NOTE: Where any part of a copper protective system is exposed to the direct action of chimney gases or other corrosive gases, it should be protected by a continuous coating of tin, lead or other material suitable for the environment to which it is exposed. Such a coating should extend at least 500 mm below the top of the chimney. The coating should not be removed at joints.

(b) Bare alloys Alloys of metals should be substantially as resistant to corrosion as copper under similar conditions. Galvanized iron may be used as part or the whole of the downconductor system provided it has adequate current-carrying capacity and is fastened with fittings having compatible corrosion characteristics. The galvanized iron may comprise the structural or decorative elements of the building subject to these requirements. (c) Bare aluminium or aluminium alloys Care should be taken not to use aluminium in contact with concrete, mortar, the ground, or in other situations where moisture may be retained causing the aluminium to deteriorate. Precautions should be observed at connections with dissimilar metals. In aluminium lightning protection systems, copper, copper-covered and copper alloy fixtures and fittings should not be used. Aluminium or aluminium alloy fixtures and fittings or non-metallic components of adequate strength and durability are required. Special arrangements will be needed at the ground termination for this class of system. Other materials may be used to the extent recommended elsewhere in this Standard.

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TABLE 4.2 TYPICAL MATERIALS FOR CURRENT-CARRYING COMPONENTS

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Material Castings Leaded gunmetal Aluminium alloy Bars and rods Copper, hard-drawn or annealed Phosphor-bronze Naval brass Aluminium bronze Aluminium Aluminium alloy Galvanized steel Stainless steel Tubes Copper Galvanized steel Strip Copper, annealed Aluminium Galvanized steel Stainless steel Stranded conductors Copper, hard-drawn Aluminium Galvanized steel Stainless steel Fixing bolts and screws for copper Phosphor-bronze Naval brass Aluminium bronze Common brass Stainless steel Fixing bolts and screws for aluminium and aluminium alloys Aluminium alloy Galvanized iron or steel

Standard

Grade or type

AS 1565 AS 1874

C92410 EA401 or AA607

AS AS AS AS AS AS

1567 1567 1567 1567 1866 1866 — AS 2837

110 518 464 627 1050 6063 or 6463A — —

AS 1432 or NZS 3501 AS 1074

— —

AS 1566 AS 1866 AS 1397 or NZS 3441 AS 1449

110 1200 — —

AS 1746 AS 1531.1 AS 1222.1 —

— — — —

AS 1567 AS 1567 AS 1567 AS 2738.2 AS 2837

518 464 627 272 303

BS 1473 AS 1214

HB30 —

4.5.2.3 The earth electrode system The design of the earth electrode system should assume that the earth electrode will be bonded, directly or fortuitously, to the following: (a) The multiple earthed neutral of the electricity supply (see AS 3000 or New Zealand Electrical Wiring Regulations). (b) The building structural steelwork or reinforcing material. (c) The communication service earth(s), if any. (d) The water supply pipes, if metallic. (e) Pipelines for gaseous or liquid fuels, if metallic. Some supply authorities attempt to isolate services (d) and (e) from (a), for galvanic corrosion control reasons, by inserting insulating spacers at the pipe entry. Consideration should be given to the fitting of surge arrestors across the insulating spacers, in consultation with the supply authority, to prevent arc discharge without prejudicing the corrosion control measures. The earth electrode system should be capable of satisfactory performance for the expected life of the lightning protection system under the corrosion conditions existing at the site when bonded to— (i) copper-based earthing systems (in most electrical installations); (ii) steel-based structural material; (iii) communication service earths which may be stainless steel, galvanized iron, copper or lead; and (iv) other metallic services, e.g. steel or copper pipes for water or gas. There are two hazards which arise from the bonding of other electrodes or service lines to the multiple earthed neutral (MEN) of the electrical supply. Firstly, if the earthing system of the electrical supply is copper-based (as is mostly the case) it will cause progressive galvanic destruction of less cathodic metals, such as steel, to which it is connected. Secondly, the electricity supply has many loads connected to it that generate a direct current component; this direct current is an electrolytic hazard to other earthing systems to which the supply system earth is bonded. The amount of direct current which can be generated by each appliance is limited by AS 3100 and NZS 6200, but it is still sufficient to place at risk some types of electrodes. In particular, steel rods clad with copper or stainless steel suffer premature failure when this small amount of direct current perforates the cladding, initiating a process of self-destruction of the rod core. COPYRIGHT

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It will be clear that the selection of any common metal or alloy for the earth electrode system places either itself or other systems or services at some risk from galvanic corrosion. For lower-cost installations the use of one of the common metals or alloys may be satisfactory. A list of these, with comments relating to their corrosion performance, is provided in Table 4.3. The extent to which the material combination ‘can be damaging’ is related to soil moisture, the type and nature of electrolytes present, and area and resistance relationships. Inherently, if such materials are used, a maintenance checking routine is essential (see Paragraph B9, Appendix B). Where soil conditions are particularly aggressive from a corrosion viewpoint (soil resistivity typically below 30 Ω.m, especially if combined with a pH value of less than 5.5), such as may exist in reclaimed marine areas, the use of an inert anode material (see AS 2832.1) may be necessary. Expert advice on the selection of an appropriate earth electrode system should normally be sought where such soil conditions exist. TABLE 4.3 CORROSION PERFORMANCE OF METALS AND ALLOYS USED AS EARTHING ELECTRODES Metal/alloy

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Galvanized iron or steel Solid copper Copper-clad steel

Deleterious effect of this metal/alloy on other bonded underground ferrous metals Nil Damaging Damaging

Solid stainless steel or nickel iron alloy

Generally acceptable

Stainless-steel-clad steel Bronze Brass

Generally acceptable Generally damaging Can be damaging

Zinc Aluminium Magnesium

Nil Nil Nil

Deleterious effect on this metal/alloy from bonding to MEN (copper-based) systems Damaging Nil Can be damaging— may be acceptable Can be damaging— may be acceptable Can be damaging May be acceptable May be acceptable— can be dezincified Damaging Extremely damaging Extremely damaging

4.6 FORM AND SIZE OF CONDUCTORS 4.6.1 Factors influencing selection The form and size of the conductors of the lightning protection system should be selected having regard to their— (a) electrical and thermal characteristics (see Clause 4.6.2); and (b) mechanical strength, if required, and the likelihood of corrosion (see Clause 4.6.3). Typical dimensions of current-carrying components of lightning protection systems are given in Table 4.4. 4.6.2 Electrical and thermal considerations Air terminations, downconductors and other conductors of the lightning protection system which may carry the full lightning current should have a cross-sectional area and electrical conductivity such that they are able to carry the expected current without deterioration and without attaining temperatures which may give rise to risk of fire. Copper conductors having a cross-sectional area of at least 35 mm 2 will normally be necessary for this purpose. Conductors of other materials may be used provided they satisfy the above criteria for current-carrying capacity and temperature rise. Conductors which, because of their arrangement in the lightning protection system, will carry only a proportion of the lightning current may have a cross-sectional area that is proportionately reduced but should be not less than one-fifth of the cross-sectional area needed to carry the full lightning current, or 6 mm2, whichever is the greater. Conductors of larger cross-sectional area than recommended above may be needed as indicated in Clause 4.6.3. 4.6.3 Mechanical strength and corrosion considerations Conductors of larger cross-section than those recommended in Clause 4.6.2 may be needed where— (a) a significant reduction of cross-sectional area is likely to be experienced in service due to the effects of corrosion; or (b) an increase in cross-sectional area or section of different shape (e.g. tubular instead of solid) is required to provide adequate mechanical strength, e.g. for air terminations (see Clause 4.9.1). Consideration should also be given to the use of a larger cross-sectional area than that recommended in Clause 4.6.2 in situations where inspection or repair of the conductor is unusually difficult.

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4.7 JOINTS 4.7.1 Effectiveness of joints The lightning protection system should have as few joints as possible. Joints and bonds should be mechanically and electrically effective, e.g. clamped, screwed, bolted, crimped, riveted or welded. Where overlapping joints are used, the length of the overlap should be not less than 20 mm for all types of conductor. Contact surfaces should first be cleaned then inhibited from oxidation with a suitable corrosion-inhibiting compound. 4.7.2 Protective covering Joints and bonds may be protected with bitumen or embedded in a plastics compound according to the local conditions. Particular attention should be given to joints of dissimilar metals. 4.8 FASTENERS Conductors should be securely attached to the building or other object upon which they are placed. Fasteners should be substantial in construction and not subject to breakage, and should be, together with the nails, screws, or other means by which they are fixed, of the same material as the conductors, or of such nature that there will be no serious tendency towards galvanic corrosion in the presence of moisture because of contact between the different parts. Fasteners should be spaced so as to give adequate support to the conductor. Downconductors should be fastened at spacings not exceeding 1.0 m on horizontal runs and not exceeding 1.5 m on vertical runs. The method of fastening should not result in a reduction of the conductor cross-section below the minimum recommended in Clause 4.6. NOTE: Plastics materials may be used for the fixing of conductors provided such materials are suitable for long term exposure to the outdoor environment (e.g. stabilized against the harmful effects of ultraviolet radiation) and otherwise satisfy the recommendations of this Clause.

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TABLE 4.4 TYPICAL SECTION DIMENSIONS OF MAIN CURRENT-CARRYING COMPONENTS Component Typical section dimensions (see Note) Air terminations Strip 20 mm × 3 mm 10 mm dia. Rods Stranded conductors 35 mm2 Downconductors Strip 20 mm × 3 mm Rods 10 mm dia. Stranded conductors 35 mm2 Galvanized materials 35 mm2 Earthing electrodes and base conductors Hard-drawn copper rods for direct driving into soft ground 12 mm dia. Hard-drawn or annealed copper rods for indirect driving or laying in ground 10 mm dia. Galvanized star stakes 25 mm × 19 mm × 19 mm Stainless steel 10 mm dia. Galvanized steel water pipe 12 mm dia. Galvanized steel or copper strip: Base conductors 30 mm × 5 mm Earth electrodes 30 mm × 3 mm Fixed connections (bonds) External: Strip 20 mm × 3 mm Rods 10 mm dia. Internal: Strip 20 mm × 1.5 mm Rods 6.5 mm dia. Standard flexible connections (bonds) External 70 mm2 Internal 35 mm2 NOTE: Where stainless steel is used and is likely to carry the full lightning current, section dimensions larger than those indicated above may be necessary to avoid excessive temperature rise.

4.9 AIR TERMINATIONS 4.9.1 General requirements An air termination may consist of a vertical rod as for a spire, a single horizontal conductor as on the ridge of a small dwelling, or a system of horizontal conductors with vertical rods for the protection of roofs of large horizontal dimensions (see Figure 4.4). Protection may also be provided with a horizontal overhead wire supported, if necessary, independently of the building to be protected or by a vertical air termination network (see Figure 4.2). Salient points of the structure should be incorporated in the air termination network.

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The upper portions of the downconductors on tall buildings should be regarded as a continuation of the air termination network and should be positioned so as to intercept side strikes to the building. Preference should be given to placing downconductors as near as possible to the exposed outer vertical corners of a building. All metallic projections, on or above the main surface of the roof should be bonded to, and form part of, the air termination network. In the case of aerials which have to be insulated from earth, a spark gap connection to earth or surge arrestor should be provided. If portions of a structure vary considerably in height, any necessary vertical air termination or air termination network of the lower portions should, in addition to their own downconductors, be bonded to the downconductors of the taller portions (see Figures 4.1 and 4.4). Air terminations may be of any form provided the section used and the means of attaching it to the building structure have adequate mechanical strength to withstand the expected wind loading and natural harmonic resonances. Where copper rod is used as a vertical air termination, Table 4.5 gives guidance on the maximum height which should be adopted. TABLE 4.5 GUIDE TO MAXIMUM HEIGHT FOR VERTICAL AIR TERMINATIONS COMPRISING COPPER RODS

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Diameter of rod mm ≥ 10 < 16 ≥ 16 < 21 ≥ 21 < 26 ≥ 26 < 31

Recommended maximum height m 1.0 1.5 2.0 2.5

4.9.2 Protection of roofs The parts of roofs most likely to be struck by lightning are parapets, the edges of flat roofs, chimneys, and the ridges and eaves of sloping roofs. Preference should be given to positioning the air terminations so as to protect these highly exposed parts. The height of a vertical air termination rod should be such that the tip will be not less than 0.5 m above the object to be protected. On large flat and gently sloping roof areas a number of vertical rods of greater than 0.5 m in height may be needed to establish a zone of protection over the whole roof area in accordance with Clause 4.2. Horizontal air termination conductors may be used to protect a planar roof surface. The roof will be deemed to be protected by air termination conductors spaced no more than 6 m apart on the roof surface, provided that the highly exposed edges or ridges forming the boundary of the surface are fully protected by the air termination network. Horizontal and vertical air termination conductors and interconnecting conductors of the air termination network should be located so as to constitute, as nearly as local conditions permit, an enclosing network which joins each air termination to each other and to all downconductors. Metal objects, such as gutters, should be bonded to the air termination network. In special circumstances, such as where it is desired to preserve the appearance of a historic building, the air termination conductors may be installed immediately underneath the cladding (e.g. tiles) of a non-conductive roof. However, it should be noted that, in the event of a lightning strike to the roof, the cladding will be punctured and may suffer some damage. 4.9.3 Protection of the sides of tall buildings 4.9.3.1 Influence of forms of construction Many buildings will have perimeter columns in which the reinforcement (or structural steel) is used as a part of the downconductor system. Where these columns on the external facade are no further than 10 m apart, no further protection will be required in respect of strikes to the side of the building. In the event of a strike to such a column or to isolated metal components such as small window frames, it is likely that a section of masonry cladding material may be dislodged. Where the risk of this is unacceptable, conductors should be installed on the external faces of the columns to receive the strikes. These conductors will take the form of lightning air terminations/downconductors and should be bonded at the bottom into the lightning protection system. Where suitable columns do not exist to receive strikes to the sides of buildings, vertical conductors should be installed for this purpose. These conductors should be spaced around the perimeter of the building at a spacing not exceeding 10 m or 30 m if the conditions of Clause 4.9.3.3 apply. 4.9.3.2 Curtain wall construction It has become commonplace for tall buildings to have external glass curtain walls, with the curtain wall external to perimeter columns. The majority have major glass elements contained (and restrained) within a metallic framework. This framework is often inherently connected, electrically, to the metal in the building structure via the standard connection details used to mechanically fix the curtain wall structure to the structural frame of the building itself. COPYRIGHT

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Where this inherent connection occurs and where the frame of the building is incorporated into the lightning protection system, no further bonding of the curtain wall to the lightning protection system is necessary. Some curtain wall designs incorporate a metallic framework which is not exposed externally. This framework is then not inherently available to receive direct strikes to the side of the building. For the curtain wall sections of this type above 45 m, where direct strikes to the side of the building are anticipated, special design and detailing modifications to the curtain wall should be made. The objective of these modifications should be to achieve a performance for receiving direct strikes equivalent to a curtain wall with an exposed framework. The modifications should provide exposed metalwork, suitable for receiving direct strikes, spaced at intervals of not more than 10 m vertically and 10 m horizontally, or 30 m horizontally if the conditions of Clause 4.9.3.3 apply. This exposed metalwork should be located to occur particularly at corners of the curtain wall where the probability of direct strikes is the highest. The provision of exposed metalwork for this purpose at less than 45 m above ground is not necessary, however, the curtain wall framework should be bonded to the lightning protection system at intervals not exceeding those recommended above. 4.9.3.3 Dispensation for large flat surfaces For tall buildings, application of the rolling sphere in accordance with Clause 4.2 will indicate that protection should be provided for the sides of the building above the height of the sphere radius (see Figure 4.1). However, large flat surfaces which are vertical or near vertical are less likely to form attachment points for lightning discharges than are external corners or other projections which provide electric field enhancement. Consequently, notwithstanding the protection that may be inferred as necessary for such surfaces in accordance with Clause 4.2, surfaces that are protected in accordance with the following recommendations will be deemed to be protected for the purposes of this Standard: (a) Downconductors should be provided on external corners and other external changes of direction where the plane of the principal surfaces subtends an angle of greater than 20° (see sketch).

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(b) Additional downconductors should be provided at the intervals necessary to comply with Clause 4.10.1. Surfaces that are inclined at 45° or more from the vertical should be treated as roofs and protected in accordance with Clause 4.9.2. 4.10 DOWNCONDUCTORS 4.10.1 Structures — General The number of downconductors should be one for every 30 m of perimeter. The perimeter should be measured by the ‘taut-string’ method (see Figure 4.4). A non-metallic structure exceeding 30 m in height should have at least two downconductors symmetrically spaced, bonded by a metal cap or by a conductor around the top. 4.10.2 Route The route followed by downconductors should be in accordance with the following recommendations: (a) Downconductors should be distributed around the outside walls of the structure. It is undesirable to locate downconductors in areas where persons are liable to congregate. The walls of light wells may be used for fixing downconductors, but lift shafts should not be used for this purpose. (b) Where the provision of suitable external routes for downconductors is impracticable or inadvisable, e.g. buildings of cantilever construction from the first floor upwards, downconductors may be housed in an air space provided by a non-metallic, non-combustible internal duct. Any covered recess or any vertical service duct running the full height of the building may be used for this purpose, provided that it does not contain any unarmoured or non-metal-sheathed service cable (see Clause 4.14.2.3). (c) Any extended metal running vertically through the structure should be bonded to the lightning downconductor at the top and bottom unless the clearances are in accordance with Clause 4.14.

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(d) A downconductor should follow the most direct path possible between the air termination and the earth termination. Right angle bends may be used when necessary but deep re-entrant loops should be avoided. (e) A structure on bare rock, protected in accordance with Clause 4.12.3.1, should be provided with at least two downconductors equally spaced. NOTE: The positioning and spacing of downconductors on large structures has often to be decided in practice by architectural considerations. However, their number should be governed by the recommendations above.

It is now recognized that sharp bends in a downconductor, such as arise at the edge of a roof, do not significantly impede the discharge of a lightning current, nor are the mechanical forces produced by a lightning current likely to endanger the conductor or its fixings. In contrast, re-entrant loops in a conductor can produce high inductive voltage drops so that the lightning discharge may jump across the open side of the loop. As a rough guide it can be stated that this risk may arise when the length of the conductor forming the loop exceeds 8 times the width of the open side of the loop. It follows from the above that there is no need to round the path of the downconductors at the edge of a roof and that an upturn within the limits stated is acceptable. Where large re-entrant loops as defined cannot be avoided, e.g. for some cornices or parapets, the conductor should be arranged in such a way that the distance across the open sides of the loop complies with the principles given above. Alternatively, such cornices or parapets should be provided with holes through which the conductor can pass freely. (See Figures 4.5 and 4.6). An exception to the above practice is necessary for a building cantilevered out from the first storey upwards. The downconductors in this case should be taken straight down to the ground since, by following the contour of the building, a hazard could be created to persons standing under the overhang formed by the cantilever. In such a case, the use of internal ducts for downconductors is recommended (see Figure 4.7). 4.10.3 Mechanical damage Where any part of a lightning protection system is exposed to mechanical damage it should be protected by covering it with moulding or tubing preferably of non-conductive material. If metal is used, the conductor should be electrically connected to both ends of the protective covering.

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4.11 TEST LINKS Where practicable, test links should be provided to enable the continuity of each individual parallel path of the lightning conductor system to be measured. Where a driven or buried earthing electrode is provided as part of the lightning protection system, test links should be provided to permit measurement of the resistance of the individual earth terminations, in such a position that, while not inviting unauthorized interference, is convenient for use when testing. Such resistance measurements are indicative only and provide the basis of comparison to determine whether any deterioration in the earthing system has occurred in service (see also Appendix B). 4.12 EARTH TERMINATIONS 4.12.1 General principles Each downconductor should be connected to an earth electrode or to the earth termination network. The design of earth terminations should be such that lightning currents are discharged into the earth in a manner that will minimize touch and step potentials and the risk of side flashing to metal in or around a structure. This can be achieved by ensuring that the potential with respect to the general mass of the earth at each earth termination is limited by a sufficiently low resistance to earth and that the discharged current flows uniformly in all directions away from the structure. Ionization of the soil near an earth electrode carrying lightning current tends to reduce the potential of the electrode relative to remote earth to a lower value than that which would be calculated using the earth resistance measured at low currents. Appendix B provides information on the effectiveness of various forms of earth electrode systems for lightning protection purposes and on the associated calculation/measurement procedures. 4.12.2 Resistance to earth 4.12.2.1 Basis for measurements The term earth resistance is used in this Clause and elsewhere in this Standard because the most common measuring instruments available are low frequency devices. A more appropriate measurement for lightning protection purposes is that of earth impedance and such measurements are preferred when suitable high frequency or impulse type instruments are available. 4.12.2.2 Recommended values In general, the whole of an interconnected lightning protection system should have a resistance to earth not exceeding 10 Ω before any bonding is effected to services which are not part of the lightning protection system. In addition, each earth electrode of an interconnected lightning protection system which is not interconnected at or below ground level should have a resistance to earth not exceeding the product obtained by multiplying 10 Ω by the number of downconductors. NOTE: Where the installation has two or more air termination networks not directly interconnected, such as a twin-towers building, then for the purpose of determining the required earthing resistance, it should be considered as consisting of separate lightning protection systems.

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FIGURE 4.5 GENERAL PRINCIPLES OF A RE-ENTRANT LOOP IN A CONDUCTOR TAKEN OVER A PARAPET WALL

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FIGURE 4.6 ACCEPTABLE METHOD OF TAKING A CONDUCTOR THROUGH A PARAPET WALL

FIGURE 4.7 ROUTES FOR DOWNCONDUCTORS IN A BUILDING WITH CANTILEVERED UPPER FLOORS

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A reduction of resistance to earth can be achieved by extending or adding to the electrodes or by interconnecting the individual earth terminations of downconductors. Notwithstanding the above recommendations, electrodes complying with either of the following, need not comply with the 10 Ω criterion: (a) For a substantial structure effectively encircled by a buried earthing electrode, a resistance to earth not exceeding 30 Ω should be satisfactory. A buried earthing electrode covering at least three sides of the structure may be regarded as effectively encircling the structure. (b) For any system incorporating two or more downconductors, it should not be necessary to install a total length of more than 50 m of widely separated horizontal or vertical electrodes per downconductor, regardless of the resistance to earth. Where reinforced concrete footings are used as earthing electrodes for a building, compliance with the recommended maximum resistance values should be determined by the measurement of resistance of typical footings which support the building structure. The measurements should be made at the stage of building construction when the footings are structurally isolated and may be treated as independent earthing electrodes. 4.12.3 Common earthing electrode and potential equalization 4.12.3.1 Common earthing electrode Where conditions permit potential equalization techniques to be used, a common earthing electrode may be installed to serve the lightning protection system and other appropriate services. The earth electrode should comply with the recommendations in this Standard and with any regulations which may govern the appropriate services (for communication services, see Clause 4.12.3.2). The resistance to earth should be the lowest required by any of the regulations for such services. Where isolation is required, a common earthing electrode should not be used but the separate earthing electrodes should be bonded via a spark gap or surge arrestor to minimize potential differences between the earthing systems in the event of a lightning strike. 4.12.3.2 Communications protective earths Where a communications protective earth is installed at a dwelling or similar small building, that earth should be connected to other earths present (see Paragraph A5.6, Appendix A). However, the protective earth of some older types of telex systems carries direct current and, for such systems, the protective earth should be bonded to other earths through a normally non-conducting protector or surge arrestor. 4.13 EARTHING ELECTRODES 4.13.1 General considerations An earthing electrode may be of any type provided— (a) it achieves a low resistance to the general body of earth, as recommended in this Standard; (b) it has adequate mechanical strength and corrosion resistance to ensure the desired service life will be achieved when installed in the environment concerned; and (c) it has adequate current-carrying capacity for the discharge of lightning surges without sustaining damage that might jeopardize its continued effectiveness.

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NOTES: 1 Electrode earth resistance may be measured by standard methods (see Paragraph B10, Appendix B). 2 If the soil resistivity is known, the electrode earth resistance may be calculated as shown in Paragraph B3, Appendix B. It should be noted, however, that such calculations are only approximate and it is important that the electrode earth resistance should in fact be determined by field test. 3 It is fairly easy to determine soil resistivity by test as set out in Paragraph B10.1, Appendix B.

The (i) (ii) (iii) (iv)

selection and design of the earthing system should therefore take account of the following: Soil resistivity. The corrosion aggressiveness of the soil. The physical structure of the soil (rocks, obstructions and other services). The corrosion compatibility of the electrode system with other structures to which it will be, or may become, bonded. (v) The options available for installation at the site (trenching, driving, drilling, land excavation or use of structural metalwork). (vi) The effects which it may have on other systems (electrical or communications). 4.13.2 Connections to electrodes 4.13.2.1 Mechanical protection Where conductors which are connected to electrodes are accessible to the public, such conductors should be protected against mechanical damage. Where conductors connecting driven electrodes in parallel are not kept above the ground, they should be buried not less than 500 mm below the surface. 4.13.2.2 Selection of materials Care should be exercised in the selection and application of materials for connections to electrodes to avoid the possibility of galvanic corrosion, e.g. because of differences between the materials of such connections and the electrodes.

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4.13.2.3 Joints Joints between earthing conductors and electrodes should be of adequate strength and current-carrying capacity, and be arranged so as to ensure that there will be no failure of the connection under conditions of use or exposure that can reasonably be expected. Clamps and similar mechanical connections should be designed and constructed so that the connection will not slacken off in use. 4.13.2.4 Test links If test links are inserted in earthing conductors connected to electrodes, they should be either bolted or sweated in the closed position and be arranged so that the opening of any one link does not interfere with earth connections other than the one under test. 4.13.3 Inspection and testing of electrodes The resistance of earthing electrodes should be determined by test both at the time of installation and regularly during the life of the installation. For details of inspection and testing, see Section 8. 4.14 METAL IN AND ON A STRUCTURE

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NOTE: The term ‘metal in or on a structure’ includes all metal such as reinforcement rods and bars, pipes, metal chimneys, corrugated iron and tubing containing electric wiring. Metal hidden from view should not be overlooked. Tubing containing electric conductors, or cable sheaths, is, for instance, often embedded in an external wall and may be quite close to the lightning protection system.

4.14.1 Use of metal in or on a structure as a part of the lightning protection system Where a structure contains electrically continuous metal, e.g. continuous metal frame, or metal within a roof, wall, floor or covering, this metal suitably bonded in accordance with Clause 4.14.2.3 may be used as part of the lightning protection system, provided that the amount and the arrangement of the metal render it suitable for use, as recommended in Clauses 4.9 to 4.13 inclusive. Where a structure is simply a continuous metal frame without external covering, it requires no air termination or downconductor; it is sufficient to ensure that the conducting path is electrically continuous and that the base is adequately earthed. A reinforced concrete structure or a reinforced concrete frame structure may have sufficiently low inherent resistance to earth to form part of the lightning protection system and, if connections are brought out from the reinforcement at the highest and lowest points during construction, a test may be made to verify this on completion of the structure (see Clause 4.12). If the resistance to earth of the steel frame of a structure or the reinforcement of a reinforced concrete structure is found to be satisfactory for the purpose, a horizontal air termination should be installed at the top and bonded to the steel frame or to the reinforcement. Where regular inspection is not possible, it is recommended that a corrosion-resistant material be used for bonding to the steel or to the reinforcement and that this be brought out for connection to the air termination. Downconductors and earth terminations will of course be necessary if the inherent resistance of the structure is found to be unsatisfactory when tested. 4.14.2 Prevention of side flashing 4.14.2.1 Methods of prevention When a lightning protection system is struck, its electrical potential with respect to earth is raised and, unless suitable precautions are taken, the discharge may seek alternative paths to earth by side flashing to other metal. Two methods exist to prevent side flashing: bonding and isolation. Bonding is the procedure whereby metal parts are positively connected to one another so as to prevent inadvertent electrical connection occurring due to side flash. Isolation is the separation or insulation of metal parts in such a way that electrical breakdown or side flash to them is prevented. Isolation may be achieved by separation of the lightning protection system from the structure protected or by separating metal parts and services in a non-conductive structure from the lightning protection system. Bonding effectively eliminates any local potential difference between the metal parts that are bonded together. However, it is possible to obtain large potential differences for very short times between adjacent metallic objects which are connected together at a remote location. These potential differences could be hazardous if the bonding system is inadequate. Many structures can be effectively bonded so as to eliminate any hazard, however care should be taken to prevent subsequent installation of a metallic service creating a hazard. It should be noted that any conductive element which is bonded into the lightning protection system can be expected to carry a proportion of the lightning current. With isolation, it is often difficult to obtain and to maintain the necessary safe clearances, and to prevent connection of an ‘isolated’ lightning protection system back to the structure via ground and buried metallic services. To achieve isolation it may be necessary to utilize a protection system that is completely separate from the protected structure, and is remotely earthed. If the structure is constructed with conductive materials such as reinforced concrete or steel frames, isolation of a protection system mounted on the structure requires the use of high impulse strength, high voltage insulation. In general isolation can be achieved at low cost, using a protection system mounted on the structure, for small structures only.

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4.14.2.2 Bonding The conditions under which bonding should be effected are as follows: (a) Where practicable, all structural steel and metallic reinforcement in a structure, if not used as a part of the lightning protection system, should be bonded to that system. As indicated in Clause 4.3.2.4(b), metal rods in in situ reinforced concrete may be considered to be electrically continuous. Consequently, bonding may be achieved with a reasonable number of connections to the rods, a bonding connection to each rod being unnecessary. Where prestressed concrete elements are involved it has been found the prestressing cables frequently remain electrically isolated from other structural metal at the completion of the stressing process. Such cables should be bonded at both ends to the lightning protection system (see Note) particularly where the structural element is exposed to the weather.

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NOTE: This bonding is not recommended out of concern for a side flash causing immediate structural damage, but rather to avoid the chance of the side flash causing cracking of the corrosion protecting concrete grout used around the cable. Prestressing cables under stress are highly susceptible to corrosion.

Where metal exists in a structure, such as reinforcement in a precast concrete spandrel panel, which cannot be bonded into a continuous conducting network and which is not or cannot be equipped with external earthing connections, its presence should be disregarded. The danger inseparable from the presence of such metal can be minimized by keeping it entirely isolated from the lightning protection system. (b) Where the roof structure is wholly or partly covered by metal, care should be taken that such metal is provided with a continuous conducting path to earth. (c) Metal which is attached to the outer surface of a structure should preferably be bonded as directly as possible to the lightning protection system. Where bonding is difficult and where the consequences of side flashing to isolated metalwork is not considered serious, bonding may be omitted. Where such metal has considerable length, e.g. cables, pipes, gutters, stairways, and runs approximately parallel to a downconductor or column, it should be bonded at each end and at intervals of not more than 10 m. (d) In curtain wall construction, where the framework would otherwise be electrically isolated, the frame should be made electrically continuous and should be bonded to the lightning protection system at intervals not exceeding 10 m around the perimeter of the building. This should occur at the top and bottom of each curtain wall and at levels separated by not more than 20 m vertically, including those sections which are less than 45 m above ground. (e) Where there is insufficient clearance from the lightning protection system, metal entering or leaving a structure in the form of sheathing, armouring or piping for electric, gas, water, telephone, steam, compressed air or other services, should be bonded as directly as possible to the earth termination at the point of entry or exit outside the structure on the supply side of the service. In this operation, the appropriate Standards and any regulations which may apply to such services should be observed. (f) Masses of metal in a building, such as a bell-frame in a church tower, should be bonded to the nearest downconductor by the most direct route available. 4.14.2.3 Isolation The necessary separation distance from any point on the lightning protection system depends on the electric potential, or voltage, generated at that point by the lightning discharge. To achieve a sufficiently low probability of side-flash, the responses of the protection system to a range of severe stroke current waveshapes have to be considered. Because the time for a lightning stroke current waveshape to significantly change its steepness is similar to the time taken by the incident wave to travel from the point of strike to the earth termination, travelling wave techniques are used to calculate the voltage waveforms generated. However, an approximate voltage waveform sufficient to estimate the required separation distance can generally be calculated from the resistive and inductive voltage drops in the system; the calculation procedures are outlined in Appendix C. For conventional lightning protection systems using typical bare metal downconductors, the separation distance in air at a given point on the protection system is required to be not less than D, where D, in metres, is the greater of D 1 and D2 as defined below and shown in Figure 4.8. D1 is the required clearance associated with the discharge voltage of the design first stroke of a severe lightning flash and takes account of the design maximum lightning current. D 1 is defined only for H/n < 30. D2 is the required clearance associated with the discharge voltage of the design subsequent stroke of a severe lightning flash and takes account of the design maximum steepness of the current wavefront. To take account of systems with a common earthing electrode it is necessary to separate D 1 into two components as follows: D1 = Di + De ..... 4.14.2.3 where Di

=

De H n R

= = = =

, for

< 30

0.3R length of downconductor from the point considered to earth, in metres number of downconductors connected to a common air termination combined earthing resistance of lightning protection system, in ohms COPYRIGHT

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Di is the component of the first stroke separation distance associated with potential difference generated within the structure. De is the component of the first stroke separation distance associated with local earth potential rise, and is independent of the point on the lightning protection system considered. This term is applicable to any remotely earthed objects, such as services entering the building, which do not share a common earthing electrode with the lightning protection system, and to any long unearthed objects within a relatively non-conductive structure. Where a common earthing electrode in accordance with Clause 4.12.3 is used, the term D e may be neglected (R = 0 in Figure 4.8). Where it is applicable, the clearance De should be maintained throughout the structure and thus determines the minimum separation distance at the base of the structure. The required clearance for steep-fronted surges, D 2, may be read from the dotted curve given in Figure 4.8. As the separation distance D 2 varies with the length of downconductor from the point considered to earth, D 2 normally determines the required separation in the upper parts of tall structures. The shortest separation distance over the surface of non-conductive structural material should be 2D for protected dry surfaces and 3D for external surfaces. The separation distance through solid non-conductive structural material should exceed 0.5D.

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NOTES: 1 For a substantial reinforced or structural steel frame building which utilizes the structure as part of the lightning protection system, the separation distance may be obtained from Figure 4.8 by taking n to be 1.5 times the number of reinforced or steel columns. The term De may be neglected for these buildings by assessing D for R = 0 except when considering remotely earthed services entering the building. 2 The extent to which uninsulated services may be considered to be affected by local earth potential rise can be determined by a test in which a known current is injected into the lightning protection system and potential differences to the system earth are surveyed.

FIGURE 4.8 REQUIRED SEPARATION DISTANCES IN AIR

4.14.2.4 Effects of bonding on cathodically-protected metal In the bonding of adjacent metalwork to the lightning protection system, careful consideration should be given to the possible effects such bonding would have upon metalwork which may be cathodically-protected (for cathodic protection see the AS 2832 series of Standards). 4.14.2.5 Bonding of underground sevices In the ground, bonding between the earth termination network of any structure and buried metal service pipes is essential, unless the service can be effectively isolated. If this is not done, an electric breakdown can occur through the soil between these systems and the electric arc can cause structural damage or may puncture a service pipe (see also Clause 4.14.2.2 (d)).

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SECTION 5 PROTECTION OF PERSONS AND EQUIPMENT WITHIN BUILDINGS 5.1 SCOPE OF SECTION This Section sets out recommendations for the protection of persons and equipment within buildings from the effects of lightning. These recommendations may be applied irrespective of whether a lightning protection system for the building structure is provided in accordance with other sections of this Standard. 5.2 NEED FOR PROTECTION Whilst persons and equipment within buildings may be protected from a direct lightning strike, many circumstances arise where the effects of lightning are transmitted within the building, by various means as described below, placing persons and equipment at risk. Communications and electronic equipment are particularly susceptible to damage from lightning impulses and such damage may occur at energy levels well below those needed to cause injury to persons. In addition, there is a significant fire risk associated with impulse failure of many types of electrical and electronic equipment.

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5.3 MODES OF ENTRY OF LIGHTNING IMPULSES There are three principal modes of entry of lightning impulses into buildings, as described below, and these may occur separately or in combination: (a) Directly by the interception of lightning on exterior metalwork Lightning impulses may be transmitted to within the building as a consequence of a strike to exterior metal which has a direct conductive connection to the interior of the building, e.g. via communications antennas, plumbing fittings and the like. This mode of entry is characterized by a series path for the full impulse energy and is capable of conveying the full destructive effect of the lightning discharge. The waveshape of the lightning impulse is usually not significantly modified. (b) Indirectly by the interception of lightning on other structures or services A lightning strike to other structures or services which have conductive connection to the building, e.g. the low voltage electricity distribution system or other services, may result in an impulse being transmitted into the building. The impulse is characterized by a lower energy level compared to that involved in Item (a), being a shunt path to the interior of the building served by the low voltage mains. It is an earth potential rise (EPR) effect originated by the lightning impulse passing to ground through the neutral/earth conductor resulting in a increase in potential by ordinary ohmic means. The magnitude of the impulse at the structure is governed by the neutral/earth impedance at the interception point, the length of the service line, the number of earth features per unit length on the line adjacent to the interception point and, lastly, the electrical characteristics of the lightning discharge. Where a common multiple earthed neutral electricity supply system exists (high voltage earth bonded to low voltage earth), the regulatory authorities require a neutral/earth resistance of not more than 1 Ω. As might be expected this limits the EPR impulse voltage considerably. In addition, in urban areas the number of services with an earth (i.e. neutral/earth) connection is considerable, perhaps 100 per kilometre. Consequently the EPR lightning impulse is rapidly reduced, perhaps to insignificant levels in about 60 metres or so. On the other hand, sparsely settled areas with distribution systems other than the MEN type can give rise to high EPR values which may not reduce to safe levels for some hundreds of metres. The impulse wave is normally modified by the transmission path in the EPR mode by distributed electrostatic capacity and transmission line effects. This reduces the severity of the impulse but prolongs the conduction time of protection equipment. Although the energy levels involved in an EPR impulse are substantially less than those which apply for Item (a), they may still be of a high order. Based on sparks or arcs observed in incidents involving personal injury or equipment damage under EPR conditions, voltages of the order of 100 000 V are not uncommon and cases involving voltages of about 1 000 000 V have been observed. Impulse currents in this mode can range from a few amperes to several thousand amperes. It should be noted that EPR conditions can arise singly or as a combination of occurrences. In addition to lightning intercepting the low voltage overhead distribution, other lightning leaders may intercept trees, clothes lines, sheds or other nearby structures, giving rise to a quite complex overall EPR condition. (c) Inductively by electric and magnetic field coupling In general, this mode of entry involves low energy levels and is of limited incidence in comparison with the mode in Item (b). Induction occurs when a lightning strike to ground gives rise to electromagnetic and electrostatic fields. These fields induce an impulse in conductors that intercept them. The conductors which are most affected by this mode of entry are electricity reticulation and telecommunications lines. Commonly the former is not damaged but the impulse may be transmitted to customer terminals and appear as a lower level EPR type impulse. This may damage or disable some forms of communications equipment. These three modes of lightning impulse entry to a building are illustrated in the examples given in Figure 5.1. Mode (b) is by far the most common of the three entry modes. This is due to the relatively large geographic collection area provided by overhead electricity distribution systems. NOTE: Of the lightning incidents involving electrical equipment that have been investigated by Telecom Australia, some 80 percent can be attributed to entry mode (b).

Protection systems designed to counteract EPR impulses will normally provide adequate protection against impulses arising from entry modes (b) and (c).

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FIGURE 5.1 MODES OF ENTRY OF LIGHTNING IMPULSES

5.4 GENERAL CONSIDERATIONS FOR PROTECTION Because of the many variables involved, each building will require specific consideration of the protective measures which should be applied. Particular attention should be given to possible entry and exit points for lightning impulses which may include one or more of the following: (a) Roof top or external structures (e.g. TV antennas, communication antennas, metallic flues and ventilation outlets) or other exposed metal work not protected by the lightning protection system for the building structure (e.g. metallic guttering and down pipes, metallic fences). These features will invariably be possible entry points for a lightning discharge. (b) The electricity service entry. This will normally be an entry point for lightning if the service is aerial or overhead. It may be either an entry point or an exit point if the service is underground but it is more likely to be an exit point in such cases.

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(c) The communication services entry. This may be an entry point if the service is overhead using a dropwire or aerial cable. The service is more commonly underground and in such cases could be either an entry point or exit point. (d) Gas supply systems. These are almost always exit points for lightning but may occasionally present an EPR entry condition. (e) Water supply systems. These are almost always exit points for lightning but may occasionally present an EPR entry condition. (f) Other conductive services. These are almost always exit points for lightning but may occasionally present an EPR entry condition. (g) Earthing systems (often there are several). These are almost always exit points for lightning but may occasionally present an EPR entry condition. (h) The lightning protection system for the building (if provided). By design these systems provide both an entry and exit for a lightning discharge but, because of bonding, will present an EPR condition to other services. An illustration of the possible entry and exit points for a lightning discharge is provided in Figure 5.2.

NOTES: 1 See Clause 5.4 for further information and qualifications concerning entry and exit points. 2 Equipotential bonding which may be necessary for protection is not shown.

FIGURE 5.2 POSSIBLE ENTRY AND EXIT POINTS FOR A LIGHTNING DISCHARGE

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5.5 PROTECTION OF PERSONS WITHIN BUILDINGS 5.5.1 Objectives of protection The principal objective of measures for the protection of persons within buildings is to prevent hazardous potential differences between conductive parts with which the person(s) may be in contact. This is normally achieved by applying equipotential bonding between any conductive path into and out of the building, i.e. the entry points and exit points referred to in Clause 5.4. If such bonding has been installed it does not matter if a person is subject to an earth potential rise with respect to distant earth as all conductive materials in the vicinity will be at approximately the same potential. An important consideration in the installation equipotential bonding is how to install such bonding without adversely affecting the operation of the various services involved, particularly the protection systems associated with the respective systems. This is explained further in Clause 5.5.2. 5.5.2 Installation of equipotential bonding All possible points of entry and exit for the lightning discharge should be electrically bonded together in as direct a manner as practicable. The route taken by the bonding conductors is important. If incorrectly routed the bonding conductors themselves may damage other circuits or equipment from induction or side flashing as currents of the order of tens of kiloamperes and voltages of the order of several thousand volts with respect to distant earth may be involved. Consequently, bonding conductors should not be grouped with other cables which are sensitive to induction unless they are bonded to the lightning protection system. If the bonding conductor is long (some tens of metres) it must be considered as an impulse transmission line, in which mode the protection afforded by the bonding will be limited. Some specific recommendations applicable to bonding of the entry and exit points referred to in Clause 5.4 are given below: (a) Rooftop antennas and communications hardware The bonding conductor should be attached to the most substantial part of the structural metal supporting the equipment consistent with it fulfilling the requirements of an air termination for the lightning protection system of a building. The bonding conductor to the antenna or communications hardware should be insulated to at least the level required in AS 3191 or NZS 6402, if run within the building, but may be uninsulated if run externally. The cross-sectional area of the bonding conductor should be at least 16 mm2 of copper. (b) The electricity supply service entry There are two distinct considerations which apply. Firstly, the supply earth should be bonded to the lightning protection system earth with a conductor dimensioned in accordance with Section 5 of AS 3000, or to a copper conductor of 6 mm2 cross-sectional area, whichever is the greater. Secondly, surge arrestors should be installed for each active conductor. If the surge arrestor is mounted on, or in, the building its earthing system should be bonded to the lightning protection system by a conductor having a cross-sectional area of not less than that utilized for its own earthing conductor. Where the surge arrestor equipment is separated from the building (e.g. mounted on an electricity supply service pole), the surge arrestor earth should not be used as the earthing system for the building lightning protection system, however, the lightning protection system earth and the surge arrestor earth may be bonded together, if desired. (c) The telecommunication service entry This may be either aerial (overhead) or underground. If aerial, the service should be regarded as a potential entry point for lightning and a telephone protector should be fitted, subject to the requirements of the telecommunications regulatory authority*. The earthing conductor and earthing system should be bonded either to the electricity supply earthing system, or to the lightning protection system earth. The cross-sectional area of the bonding conductor need not exceed 4 mm 2 if of copper, as this is of higher current-carrying capacity than the telecommunications lines used in Australia. If the telecommunications service is underground, the service will act essentially as an exit for lightning. In this mode it may be necessary to fit a telephone protector to the service(s) to provide a bonding point for potential equalization. This bonding may also require a local earth, as determined by the telecommunications regulatory authority. The bonding conductor should have a cross-sectional area of not less than 4 mm 2 if of copper. (d) Water and gas supply systems Metallic water and gas supply systems should be bonded to the lightning protection system and connected to the electrical supply earth. However, some water and gas supply authorities fit insulating spacers or ferrules for corrosion control at customers’ installations. These may require bridging, particularly in the case of gas services, by a surge arrestor as determined in consultation with the gas supply authority. Bonding conductors to these services should have a cross-sectional area of not less than 4 mm2 of copper. If calculation or local experience indicates that the water service is of very low resistance to ground (e.g. less than 0.5 Ω), it may well be the principal exit for the lightning impulse. In such circumstances, consideration should be given to upgrading the current capacity of the bonding conductor between the lightning protection system earth and the water service to a cross-sectional area of at least 16 mm 2 of copper.

* In Australi a, these requir ements are set out in AUSTEL Technical Standard 009. COPYRIGHT

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(e) Other service lines Specific considerations may apply for some structures. For example, a radio telephone tower should be bonded to its associated equipment building; similarly, a pump station should be bonded to an elevated water tower. For both examples given, the bonding conductor is likely to carry the full lightning current and should therefore have a cross-sectional area of at least 16 mm2 copper. (f) The earthing systems Buildings frequently have several earthing systems that may be installed independently at different times. These include the electricity supply earth, telecommunications earth (sometimes more than one), lightning protection earths and other special purpose earths. It is generally desirable to bond all such earths but there may be specific reasons for not doing so. Direct-current-carrying earths, e.g. older telex systems, should usually be isolated to prevent corrosion damage to other services and earths. In such cases consideration should be given to bonding these earths through a polarization cell, to facilitate the protection of persons from lightning surges. This type of cell can be used where there is a corrosion-based objection to bonding, e.g. copper-based earths to galvanized iron earths or structures, of which the latter would suffer galvanic corrosion. If 50 Hz or audio frequency bonding is not needed, a gas discharge arrestor may serve the purpose. Bonding conductors between earthing systems should have a cross-sectional area not less than 4 mm2 copper. (g) The lightning protection system earth Where a lightning protection system is in place all of the above services should be bonded to the lightning protection system earth. 5.6 PROTECTION OF EQUIPMENT 5.6.1 General Lightning induces overvoltages in both power and telephone or data lines. Cables carrying these circuits usually have different points of entry to a structure and may have protective devices connected to different earths. Equipment overvoltages may be experienced in the following ways: (a) On telephone lines. Transients occur both line to line, and line to earth. (b) On power lines. Transients may occur active to earth, active to neutral, or active/neutral to earth. (c) Simultaneously on both power and telephone lines exposed to the same event. (d) On one or both of the protective earthing systems for the above services. The protection of equipment against overvoltages involves the provision of appropriate voltage limiting (clamping) devices either at the point of entry of the service(s) to which the equipment is connected (primary protection) or within the equipment (secondary protection), or both. Protective measures relevant to primary protection and secondary protection are described in Clauses 5.6.2 and 5.6.3 respectively. 5.6.2 Primary protection The primary protection of equipment sensitive to overvoltages may be achieved by use of protective devices connected to a common earthing system at the service point of entry. However, in many existing structures separate earthing systems may have been provided for power, telephone, computer and lightning protection. Significant earth potential rises can occur on only one of these earths which, for equipment using multiple services (e.g. facsimile machines), stresses electrical insulation and may result in breakdown to the protective earth which remains at normal potential. In such cases special protection devices at the equipment location are usually required. Equipotential bonding is primarily provided for the protection of persons by preventing breakdown of earthed systems to other systems. Such bonding will also contribute to the protection of equipment but may not of itself be sufficient to prevent equipment damage. Also, active to neutral voltage transients on the electricity supply can cause damage to or malfunction of some electronic equipment. Figure 5.3 shows a multistage protection system where a computer or signal earth cannot be commoned to other earths for electrical noise or other operational reasons. Primary protection is placed at the point of entry for power, telephone, signalling and data circuits. Secondary protection is placed adjacent to the main installation and is referenced to the special equipment earth. 5.6.3 Secondary protection 5.6.3.1 General Electrical equipment will break down at the point of lowest impulse dielectric strength. Telecommunications line circuits may present sufficient electrostatic capacity to local earth to allow an impulse discharge to them as a consequence of an EPR condition, i.e. the earth terminal of the equipment is ‘live’ and the line circuits are subjected to a fault discharge across conductor strips on a printed circuit board with a breakdown voltage lower than the EPR impulse. Protection of sensitive equipment utilizing both power and telephone/data lines is best carried out at or within the equipment. Specialized protection equipment may guard against common and transverse mode impulses on all lines entering equipment. This alone may not be sufficient to protect the equipment if their reference earths vary in potential. In this case, clamping devices which limit earth potential variation become essential for the security of the equipment. Classes of equipment prone to damage from earth potential rise include electronic PABX, modems and facsimile machines.

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FIGURE 5.3 ‘FLOATING’ COMPUTER COMMON IS REFERENCE EARTH FOR SECONDARY PROTECTION

5.6.3.2 Protective devices Protective devices usually fall into one of the following categories: (a) Gas discharge devices These devices usually consist of glass or ceramic tubes filled with an inert gas sealed at each end with a metal electrode. They have breakdown voltages in the range 70 V to 15 kV with surge current ratings up to 60 kA. The strike time and firing voltage of these devices is dependent on the rate of increase of voltage. Typical strike times are in the range 10 ns to 500 ns. Unlike most other devices, gas discharge devices conduct at a much lower voltage than their firing voltage. This conduction voltage is typically below 30 V. Gas discharge devices are available in both two electrode and three electrode configurations. The latter provide a means of clamping a pair of wires to earth regardless of which conductor was subjected to the overvoltage. (b) Varistors These devices are voltage-dependent resistors. The earlier forms of varistors were constructed from carbon or silicon carbide but most modern devices are made from metal oxide and are known as metal oxide varistors (MOVs). The resistance of varistors drops significantly when the voltage exceeds a limit thus clamping the voltage near the limit. Varistors are used on circuits operating at voltages between 10 V and 1 kV. They can handle surges up to several kiloamperes and respond in tens of nanoseconds. Because the performance of MOVs deteriorates with repeated operation, it is usual to allow a high safety margin in the selection of the device rating in lightning prone areas. Alternatively, facilities should be provided to give an indication of device failure. (c) Solid state devices These devices consist of special zener diodes which exhibit voltage limiting characteristics. The breakdown voltages of such devices are typically in the range 5 V to 200 V. They have current ratings up to several hundred amperes and response times of the order of 10 picoseconds. Information on waveshapes which may be used to specify the performance of these devices is given in Appendix D. 5.6.3.3 Application of protective devices With any signal or power transmission system employing two lines and a separate protection earth, two types of transients can occur. The first type appears as a difference between the two lines, independent of their potential differences to earth; this is known as a differential mode transient (also called transverse mode or normal mode). It is illustrated in Figure 5.4 where the transient voltage source is superimposed onto the normal signal carried by the lines.

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The second type appears as a transient between each line and the earth, and is known as a common mode transient (sometimes called a longitudinal transient). It is illustrated in Figure 5.5 where the transient voltage sources are superimposed onto the normal potentials between the lines and earth. This mode is that commonly experienced by twisted pair circuits as each wire is equally exposed to the transient voltage source. The use of two non-earthed lines is common. The a.c. mains use the active and neutral lines to supply power, with an accompanying earth line for protection. Telephone lines use two wires over which the signal is transmitted, with neither line tied to earth. RS-422 signalling for computer data uses two lines for each data channel, which is known as balanced-pair signalling. When protective equipment is connected to such lines, both differential and common mode transients must be suppressed. Placing a protective device across the two signalling lines alone is not sufficient. The high potentials to earth created by common mode transients can cause insulation breakdown and arc-over, and can damage electronic components. The use of opto-isolators for signalling lines does not necessarily eliminate this problem. Opto-isolators suitable for printed board mounting are rated as high as 5000 V isolation between input and output but transients caused by lightning can easily exceed this value resulting in breakdown of the isolator, with transients ‘punching through’ and damaging subsequent circuitry. However, special purpose fibre optic opto-isolators are available with significantly higher isolation ratings. Protection against transients is best achieved by the provision of voltage clamping or diversion devices between the lines, and between the lines and earth. These will shunt common mode transients to earth before they are allowed to reach breakdown potentials. When used to protect equipment the gas discharge devices will normally handle the largest amount of energy with the solid state devices handling the least amount of energy. Robust equipment such as electromechanical equipment is normally protected by the addition of only gas discharge devices while sensitive electronic equipment may require all three types of device in combination. The typical method of combining these devices in a signal line can be seen in Figure 5.6. Much modern equipment already has the varistor and solid state devices incorporated in its design and only the high energy gas discharge device and its isolation impedance needs to be used. It is therefore important to match the protection device to the equipment. Gas discharge devices are generally not suited to the protection of mains supplied a.c. equipment because of the fold back nature of their operation. Metal oxide varistors (MOVs) are normally used in mains protection circuits. When configured as shown in Figure 5.7 they provide essential clamping against both differential and common mode transients. These MOVs are usually specified to initiate clamping at an effective r.m.s. voltage of 275 V. However, high-current surges may still produce peak voltages exceeding 1200 V within the rating of the device. Equipment may be subject to rates of rise of thousands of volts per microsecond prior to the clamping device becoming effective. In Figure 5.8 a filter is added to condition the residual transient and to reduce the high rate of voltage rise observed immediately prior to clamping. It is important to note that radio frequency interference filters may not be suitable for power circuit protection. Transient current levels may cause inductor saturation which will degrade the filter action. Figure D3 in Appendix D shows a typical voltage/time tolerance curve for electronic equipment. Hybrid devices should be used to absorb the energy levels shown in Figure D1 and Table D1 according to their exposure. The residual transient after clamping and filtering should lie within the tolerance curve for the equipment being protected. A major cause of equipment breakdown has been traced to earth voltage differentials. The past practice of forced separation of power and telephone earths has allowed significant potentials to occur inside equipment. Figure 5.9 shows how use of an earth differential clamp limits potential difference. Devices of the type shown in Figure 5.9 must have both telecommunications and power authority approval.

FIGURE 5.4 DIFFERENTIAL MODE TRANSIENT

FIGURE 5.5 COMMON MODE TRANSIENT

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FIGURE 5.6 MULTI-STAGE PROTECTION FOR TELEPHONE AND SIGNALLING CIRCUITS

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FIGURE 5.7 SURGE DIVERTER PROTECTION FOR POWER CIRCUITS

NOTE: Most standard radio frequency interference filters are not suitable for this application.

FIGURE 5.8 LOW-PASS FILTER ACTS TO REDUCE RATE OF RISE OF VOLTAGE AFTER CLAMPING

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NOTE: Combination units are of particular value where power, telecommunication, computer and lightning protection earths may not be bonded.

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FIGURE 5.9 COMBINATION UNITS FITTED WITH EARTH DIFFERENTIAL CLAMP ARE SUITABLE FOR EQUIPMENT SUCH AS ELECTRONIC PABX, COMPUTER MODEMS AND FACSIMILE MACHINES

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SECTION 6 PROTECTION OF MISCELLANEOUS STRUCTURES AND PROPERTY 6.1 SCOPE OF SECTION This Section provides recommendations for the protection of a variety of structures and property against lightning where such protection is deemed necessary (see Section 2). The recommendations of Sections 4 and 5 should be observed except where otherwise indicated. 6.2 STRUCTURES WITH RADIO AND TELEVISION AERIALS 6.2.1 Indoor aerial system Structures protected against lightning in accordance with the recommendations of this Standard may be equipped with indoor radio and television receiving aerials without further precautions, provided that the clearance between the aerial system, including its down leads or feeders, and the external lightning protection system or any of its internal sections is in accordance with the values in Clause 4.14. 6.2.2 Outdoor aerials on protected structures Structures protected against lightning in accordance with the recommendations of this Standard may be equipped with outdoor radio and television aerials without further precautions, provided that every part of the aerial system, including any supporting metalwork, is within the zone of protection of the lightning protection system (see Clause 4.2). Where these conditions cannot be fulfilled, precautions should be taken to ensure that the lightning current can be discharged to earth without damage to the structure and its occupants as follows: (a) With an aerial system fitted directly onto a protected structure. This can be accomplished by connecting the aerial bracket structure to the lightning protection system at the nearest point accessible below the aerial installation. (b) With an aerial system fitted on a metallic support structure which projects above the lightning protection system. This can be accomplished by connecting the aerial support structure to the lightning protection system at the nearest point accessible below the aerial installation. Consideration should be given to the fitting of surge arrestors in the conductors connected to the aerial system. 6.2.3 Aerials on unprotected structures Before installing an aerial on an unprotected structure, the need to provide a lightning protection system should be assessed as described in Section 2. 6.2.4 Earthing of radio systems The earth electrode of the lightning protection system may also be used for the purpose of earthing a radio system.

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6.3 STRUCTURES NEAR TREES When a tree is struck by lightning, a voltage drop develops along its branches, trunk and roots. The side flash clearances between the tree and adjacent structures are set by taking 100 kV/m as the flash-over strength of unseasoned wet timber and 500 kV/m as the breakdown strength of air. If the tree does not exceed the height of the structure its presence can be disregarded. If the tree is taller than the structure, the following clearances between the structure and the tree may be considered as safe: (a) For normal structures; one-third of the height of the structure. (b) For structures with explosive or highly flammable contents; the height of the structure. If the clearances cannot be met then the structure should be fitted with lightning protection in such a manner that the side flash always terminates on the protection system. If the tree is fitted with a lightning protection system, no further protection will be necessary for the structure provided that the conditions for the zone of protection and separation are fulfilled. 6.4 PROTECTION OF TREES The protection of trees against the effects of lightning needs to be considered only where the preservation of the tree is desired for historical or other reasons. For such cases the following recommendations are made: (a) A main downconductor should be run from the topmost part of the main stem to the earth termination and should be protected from mechanical damage near ground level. (b) Large upper branches should be provided with branch conductors bonded to the main conductor. (c) In the fixing of the conductors, allowance should be made for swaying in the wind and the natural growth of the tree. (d) Test joints may be waived. (e) The earth termination should consist of two rods driven into the ground on opposite sides of, and close to, the trunk of the tree. A strip conductor should be buried to a depth of 0.3 m to encircle the roots of the tree at a minimum distance of 8 m radius from the centre of the tree or at a distance equal to 1 m beyond the spread of the foliage, whichever is the greater. This conductor should also be bonded to the rods by two radial conductors. The earth terminations and resistance should comply with Clause 4.13.

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(f) Where two or more trees are so close together that their encircling earth conductors would intersect, one conductor adequately connected to the earth rods should be buried so as to surround the roots of all the trees. NOTE: The recommended earth termination network is designed to protect the roots of the tree and to reduce the potential gradient, in the event of a lightning discharge to the tree, to a safe value within the area bounded by the outer buried strip conductor.

6.5 CHIMNEYS, METAL GUY-WIRES OR CABLES 6.5.1 General Metal guy-wires or cables attached to steel anchor rods set in earth may be considered as sufficiently earthed. Other guy wires should be earthed. For means of securing conductors to structures, see Clause 4.8. Metal chimneys or flues need no protection against lightning other than that afforded by their construction, except that they should be properly earthed. If the construction of the foundation does not provide ample electrical connection with the earth, ground connections should be provided similar to those recommended for chimneys made of materials other than metal (see Clauses 4.12 and 4.13). 6.5.2 Metal ladders and metal linings Where chimneys have a metal ladder or lining they should be connected to the lightning protection system at their upper and lower ends. 6.5.3 Reinforced concrete chimneys Chimneys consisting partly or wholly of reinforced concrete should comply with the recommendations of Clauses 4.3, 4.8, 4.10 and 4.14, and, in addition, the reinforcing metal should be electrically connected together and electrically connected to the downconductors at the top and bottom of the concrete.

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NOTE: In existing chimneys, the reinforcement of which may be electrically continuous, it is recommended that additional connections be made at points where the reinforcing rods are accessible.

6.6 PROTECTION OF MINES 6.6.1 Factors influencing need for protection In mining operations, electric shocks, possible premature detonation of explosives and ignition of flammable gases from the effects of lightning are recognized additional hazards. Because these hazards are associated with the effects of lightning at or below the ground surface, factors additional to the risk index values of Section 2 influence the need to provide lightning protection. These additional factors are associated with ground resistivity, depth of the mining operation, presence of personnel and the presence of flammable gas or explosives. The degree of hazard is regarded as greater the shallower the depth of the operation and the higher the resistivity of the ground involved. Generally, these additional factors will indicate that lightning protection should be provided or precautionary work procedures adopted. 6.6.2 Object of recommendations The following recommendations for lightning protection for mining operations are aimed at reducing the risk of electric shock and premature detonation of explosives. While the recommendations will also reduce the risk of ignition of flammable gases from the effects of lightning, flammable gas ignition is best prevented by ensuring that flammable concentrations of gases do not occur. The intent of the recommended lightning protection system is to reduce the possibility of substantial voltages appearing between conducting structures and between conducting structures and earth in their immediate vicinity. Absolute protection against the effects of lightning cannot, however, be guaranteed with the recommended protection system alone; consequently, recommendations are also given for operational procedures for the use of explosives when lightning occurs close to the mine site. In surface workings, premature detonation of explosives, both directly and through electric detonators, are considered possible, while in underground operations premature detonation of explosives is considered possible only through electric detonators. 6.6.3 Underground workings 6.6.3.1 General The following recommendations apply particularly to underground workings where electric detonators are used as the means of initiating explosives. 6.6.3.2 Electric detonators Detonators specially designed to reduce the risk of ignition by electrical discharge across the fuse head should be used. 6.6.3.3 Shot firing circuit Requirements for circuit equipment and procedures to be adopted for firing explosives electrically are set out in AS 2187.2 or NZS 4403. Additional to those requirements, where fixed wiring is used as part of the firing circuit, the conductors should be enclosed in metal screening, armouring or conduit. This metal screening, armouring or conduit should be connected to the electrical system earthing and bonded to other metallic structures as described in Clause 6.6.3.7. 6.6.3.4 Overhead power lines To minimize the magnitude of incoming lightning surges on overhead power lines, overhead earth wires should be provided on all overhead lines within 1.5 km of the mine. Additionally, surge arrestors should be installed at the termination of the overhead line for protection of connected cables or equipment.

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6.6.3.5 Surface structures Lightning protection should be provided on all structures above underground openings, such as winder head frames. With other structures and buildings the need to protect or not should be determined from Section 2. Lightning protection of surface structures should be carried out in accordance with Section 4 and, where these buildings have explosive or highly-flammable contents, the additional recommendations of Section 7. Various conductive structures such as metallic enclosures of air, water and electricity services, reinforcing steel concrete in foundations, are laid in or on the ground and advantage should be taken of these to reduce the earthing resistance of the lightning protection system by interconnecting and bonding these structures together and to the lightning protection earthing system. The sizes of bonding conductors are given in Table 4.4. 6.6.3.6 Bonding of surface metalwork All metal structures entering openings to underground workings of a mine should be bonded together at the point of entry to the opening and connected to the earthing system of structures above the opening. This includes any reinforcing steel in the shaft, concrete lining, shaft steel work, guides and ladders, armouring and sheathing of electrical cables, air, water and ventilating pipes, rails and bell rope attachments. The sizes of bonding conductors are given in Table 4.4. 6.6.3.7 Bonding of underground metalwork In addition to the bonding recommended in Clause 6.6.3.6, metal structures and services in underground access shafts should also be bonded together at intervals of not more than 75 m. Rock-bolted support structures are deemed to provide an adequate earth for this purpose. Winding ropes, guide ropes and balance ropes cannot be bonded to other structures except at fixing points and, possibly ineffectively, through conveyances. High voltages relative to their surrounds could occur during lightning activity.

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6.6.3.8 Further precautions The degree of hazard in any mine, both from electric shock and initiation of electric detonators, is related to the depth of the operations. This relationship is inadequately defined at present. Shaft sinking and drifting are particular operations where lightning is a recognized hazard. With these operations all work associated with electrical blasting should be suspended and personnel withdrawn to a safe distance when an electrical storm is approaching. A conservative approach would require that the precautions applied to shaft sinking and drifting be applied to all underground operations. 6.6.4 Surface workings 6.6.4.1 General The following recommendations apply to surface mining operations where any type of explosive is used in the mining operation. 6.6.4.2 Equipment For many surface workings involving blasting operations, action need only be taken in the immediate vicinity of the area where blasting takes place. This is because no interconnection by metallic structures, such as air/water/electricity services, exists with distant structures or ground. Where these services exist the recommendations apply to these services for underground working, i.e. Clauses 6.6.3.3 to 6.6.3.5 and, where practicable, the bonding recommendations of Clause 6.6.3.7 should also apply. Where electric detonators are used, electric detonators of the type described in Clause 6.6.3.2 should be used. 6.6.4.3 On-site precautions All work associated with blasting operations should be suspended and personnel should be withdrawn to a safe distance from explosives when an electric storm is approaching. High equipment such as drilling rigs, shovels and draglines which may increase lightning attraction should be moved to a safe distance from the area where blasting is to take place prior to explosives being brought to the site. 6.6.5 Lightning detector Specially-designed lightning detectors should be provided to warn of approaching electrical storms so that the precautions set out in Clauses 6.6.3.8 and 6.6.4.3 may be taken. 6.7 PROTECTION OF BOATS 6.7.1 General A boat should be considered to be at risk both because of its method of construction (except for metal-hulled boats) and because it forms a marked protrusion above the surrounding water surfaces. Overseas statistics show that in excess of 10 percent of fatalities occurring on cruising sailing boats are due to lightning. While the principles to be applied will not differ from those for land-based structures, the methods employed will depend on the form of construction and the type of boat to be protected. 6.7.2 Elements of the protection system 6.7.2.1 Air termination A metal mast or the metal fitting on a timber mast will act as an adequate air termination.

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6.7.2.2 Downconductors The mast, if metallic or if provided with a metal track, and stays will both act as downconductors and each should be connected to an earth termination. Stays as small as 3 mm diameter steel wire will serve as effective downconductors, but may be damaged under severe lightning discharges. 6.7.2.3 Earthing Any metal surface which is normally submerged in the water will provide adequate earthing. Propellers, metal rudder surfaces, metal keels, or the earth plate for the radio transmitter may be used. A metal or a ferro-cement hull constitutes an adequate earth. 6.7.2.4 Metallic objects Metallic objects which are permanent parts of the boat and whose function would not seriously be affected by earthing should be made part of the lightning protection system by interconnection with it. NOTE: The purpose of interconnecting the metal parts of a boat with the conductor is to prevent side flashes to metal objects which could form part of an alternative path to earth or which could bridge out a substantial length of the downconductor.

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A general rule is, that if the non-conducting part of the alternative path through such object is less than one-eighth the length of downconductor bridged out, then that object should be electrically interconnected with the downconductor. 6.7.2.5 Radio transceivers A whip antenna consisting of a fine wire embedded in a glass fibre tube cannot be considered a satisfactory lightning conductor and should be folded down during a lightning storm. All radio equipment or other navigational equipment with exposed transducers such as radar, wind speed/direction indicators, and the like, should be fitted with effectively-earthed spark gaps or surge arrestors. Alternatively, input cabling should be disconnected from the equipment if there appears to be imminent danger of the boat being struck by lightning. 6.7.2.6 Corrosion Care should be taken that the design of the lightning protection system does not promote the occurrence of electrolytic corrosion. Bonding of dissimilar metals and interconnection of the earth terminals of different pieces of electrical equipment should not be undertaken without expert knowledge of the possible problems involved (see also Clause 4.5.2). 6.7.3 Installation recommendations 6.7.3.1 Protection of boats with masts Sailing or power boats which have a mast or masts of sufficient height to give an adequate zone of protection in accordance with Clause 4.2 may be protected by earthing the lower ends of the standing rigging and the base of a metallic mast, or the lower end of a continuous metal sail track on a timber mast. Where the mast of a boat is stepped on deck, particular care should be taken to ensure that the conductor from the base of the mast follows a direct route if it passes through the accommodation section of the boat, otherwise a situation analogous to that shown in Figure 4.7 may occur. A typical small sailing boat with aluminium mast stepped on deck, glass fibre hull with the metal ballast encapsulated in the glass fibre (or unballasted and with a non-metallic centreboard) and with chainplates moulded into the hull provides something of a problem. In such cases it is suggested that some protection be sought when necessary by temporarily connecting the mast and stays together at deck level by a length of chain or other flexible conductor and allowing a short length of the conductor to hang in the water at each chainplate. 6.7.3.2 Protection of boats without masts Boats without masts do not constitute as high a risk as boats with masts. However, where the size of the boat is such as to cause a marked protrusion above the surrounding water surfaces, such boats should be fitted with air terminations which will give at least the protection recommended for land-based structures in Section 4. 6.7.4 Precautions for persons and maintenance suggestions To the extent consistent with safe handling and navigation of the boat during a lightning storm, persons should remain inside a closed boat and avoid contact with metallic items such as gear levers or spotlight control handles. Persons should stay as far as practicable from any parts of the standing rigging or other items forming part of a downconductor. No person should be in the water or dangle arms or legs in the water. If a boat has been struck by lightning, compasses and navigation instruments should be checked for calibration. Protective coatings on steel hulls and glass fibre sheathing over ballast keels should also be checked for damage. All standing and running rigging and associated fittings should be checked in detail. 6.8 FENCES If an extended metal fence is struck it is raised momentarily to a high potential relative to earth. Persons or livestock in close proximity to, or in contact with, such fencing at the time of a lightning discharge to the fencing may therefore be exposed to danger. Fences which give rise to the most risk are those constructed with posts of poor conducting material, such as wood or concrete. Fences built with metal posts set in earth are less hazardous, especially if the electrical continuity is broken. Breaking the electrical continuity prevents a lightning stroke from affecting the entire length of a fence, as it can if the stroke is direct and the fence continuous, even though earthed.

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Thus it is desirable to limit the length of fencing so affected by the provision of gaps, and also to provide several earth electrodes in each section so as to facilitate the discharge to earth of the lightning current. In addition, persons or livestock can be endangered by potential differences in the ground in the proximity of fences (see Figure 6.1). The risk is greatest on rocky ground. No value can be given for the earth termination resistance, since this must be largely governed by the physical conditions encountered, but the lower the resistance to earth the less risk will result to persons and livestock. In this connection, it should be borne in mind that because of large body spans and bare contact areas many types of livestock are more susceptible to electric shock than humans.

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FIGURE 6.1 EQUIPOTENTIAL LINES NEAR METAL FENCE CAUSED BY LIGHTNING DISCHARGE TO FENCE

6.9 MISCELLANEOUS STRUCTURES 6.9.1 Large tents and marquees Where large temporary structures of this type are used for such purposes as exhibitions and entertainments involving large numbers of people, consideration should be given to their protection against lightning. In general such structures are manufactured from non-metallic materials and the simplest form of protection will usually consist of one or more horizontal air terminations suspended above them and connected solidly to earth. A non-metallic extension of the vertical supports provided for such structures may, if convenient and practicable, be used for supporting a system of horizontal air terminations but a clearance of not less than 1.5 m should be maintained between the conductor and the fabric of the enclosure. Downconductors should be arranged outside the structure away from exits and entrances and be connected to earthing rods which in turn should be connected to a ring conductor in such a manner as to be inaccessible to the general public. Those types of tented structure which have metal frameworks should have these efficiently bonded to earth at intervals of not more than 30 m of perimeter. 6.9.2 Small tents For small tents, no specific recommendations can be given. 6.9.3 Metal scaffolding and similar structures, including overbridges Where metal scaffolding is readily accessible to the general public, particularly when it is erected over and on part of the common highway or may be used in the construction of public seating accommodation, it should be efficiently bonded to earth. A simple method of bonding such structures consists of running a strip of metal other than aluminium, 20 mm × 3 mm size, underneath and in contact with the base plates carrying the vertical members of the scaffolding and earthing it at intervals not exceeding 30 m. With public seating accommodation only the peripheral members of the structure need bonding to earth. Other steel structures, such as those used for pedestrian bridges over main trunk roads, are frequently sited in isolated situations where they may be prone to lightning strikes and should therefore be bonded to earth, particularly at the approach points. 6.9.4 Tall metal masts, towers, cranes and revolving and travelling structures Masts and their guy wires, floodlighting towers and other similar structures of metallic construction, particularly those to which the general public have access, should be earthed in accordance with this Standard. Cranes and other tall lifting appliances used for building construction purposes, shipyards and port installations also require bonding to earth. For cranes or revolving structures mounted on rails, efficient earthing of the rails, preferably at more than one point, will usually provide adequate lightning protection. In special cases, where concern is felt regarding possible damage by lightning to bearings, additional measures may be justified.

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Mobile towers, portable cranes and similar structures mounted on vehicles with pneumatic tyres can be given a limited degree of protection against lightning damage by drag chains or tyres of conducting rubber such as are provided for dissipating static electricity. 6.10 PROTECTION OF HOUSES AND SMALL BUILDINGS 6.10.1 General considerations The application of this Clause is intended to be restricted to relatively small buildings, such as houses or similar buildings, of a smaller size than those envisaged in Section 4 of this Standard. Lightning protection for a house or small building in complete accordance with the recommendations of Section 4 may be difficult to justify on economic grounds. However, there may be a need to provide some degree of protection against lightning damage. Houses and small buildings vary greatly in the degree to which their construction provides inherent lightning protection. Small buildings with mainly non-metallic materials offer little or no inherent protection against lightning, whereas a building with a metallic roof, metallic gutters, and metallic downpipes leading into the ground have a high degree of inherent protection, since the main elements of a lightning protection system are already present. If lightning strikes a house with little or no inherent lightning protection, the lightning is likely to penetrate through the roof and attach to electrical wiring in the roof area. This will usually result in damage to electrical equipment in the house, and in extreme cases, may result in a fire, or in hazard to persons within the house. The objective in protecting small buildings should be to provide conductors to intercept the lightning, to provide a low-resistance path to ground, and to provide at least two earth stakes or equivalent earthing electrodes for conveying the lightning current into the earth. 6.10.2 Air termination for the building If the building roof consists mainly of metallic materials, then it will serve as the air termination. It is necessary to ensure that there is electrical continuity between the various parts of the roof. Adequate continuity will often be provided by the way in which the metallic parts are overlapped and fastened. If the building roof consists mainly of non-metallic materials, then separate air termination conductors will have to be provided. Suitable materials are listed in Clause 4.5. Copper wire and copper strip are recommended for their durability. At least one conductor should be run along the highest parts of the roof, for example, the highest ridge of the building. If the roof has a complicated shape, it may be necessary to run additional conductors along the highest parts of each section of the roof. All conductors should be joined together. To be in accordance with this Standard, the cross-sectional area of the conductors should be at least 35 mm 2, achieved, for example, by copper strip 25 mm × 1.5 mm. However, it should be realised that much thinner conductors are able to carry most lightning currents without damage, and that almost any conductor would be better than none. Even if the conductor were to melt, it would have carried out its function for that one strike, as the lightning current would flow through the path of the molten metal, rather than penetrate below the roof of the house. For a large, more-or-less flat roof of non-conducting material, the simplest form of air termination may be a series of vertical metallic rods above the roof level, all connected together. The zone of protection provided by a vertical rod may be estimated using the information in Clause 4.2. Metallic gutters may become a strike attachment point. If there are metallic gutters around the roof, these should be connected to the air termination conductors. With metallic roofs, these connections may already exist in the fastenings of the guttering to the roof. With non-metallic roofs, the guttering should be connected to the air termination conductor at no less than two points. 6.10.3 Provision of downconductors for the building There should be at least two low-resistance paths to convey towards the ground the current from any lightning strike to the roof. Metallic downpipes from metallic gutters may be used for this purpose, provided that they afford a direct low-resistance path for the lightning current. In the absence of any low-resistance path from roof to ground, at least two conductors should be provided to serve as downconductors. These may be continuations of the conductors forming part of the air termination, and the same recommendations apply as in Clause 6.10.2. 6.10.4 Provision of earthing electrodes A path to earth for the lightning current should be provided at no less than two well-separated points, for example, at opposite ends of the house. Preference should be given to areas that are usually damp, such as gardens. A metallic water pipe buried in the ground would be a satisfactory earthing electrode. Each downconductor should be connected to an earthing electrode by the shortest possible route, with the proviso that downconductors and earthing electrodes should not be placed close to entry doors, or places where persons are likely to stay for long periods. For example, earthing electrodes should not be placed close to swimming pools. Earthing electrodes and their connected conductors should be examined periodically to ensure that they are intact, and not suffering corrosion or mechanical damage.

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SECTION 7 PROTECTION OF STRUCTURES WITH EXPLOSIVE OR HIGHLY-FLAMMABLE CONTENTS 7.1 SCOPE OF SECTION This Section provides a guide to the protection of structures containing explosives, or highly-flammable solids, liquids, gases, vapours or dusts, from lightning or induced discharges, and indicates ways of protecting those structures that are not inherently self-protecting. Reference should be made to the AS 2430 or NZS 6101 series for information on areas that are likely to have an explosive atmosphere.

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7.2 GENERAL CONSIDERATIONS 7.2.1 Acceptable risks An acceptable risk may be present when the quantity of dangerous material is strictly limited, as in a laboratory or small store, or where the structure is specifically designed to restrict the effects of a catastrophe, or is sited in an isolated position. Circumstances may also arise in which the dangerous materials are not exposed but are completely encased in metal of an adequate thickness, and under these conditions lightning protection may not be necessary. In other situations the risk to life and property may be so obvious that the provision of every means possible for protection from the consequences of a lightning discharge is essential. 7.2.2 Protection required The presence of explosives or highly flammable material in a structure may increase the risk to persons and to the structure in the event of a lightning discharge. For this reason, except in the circumstances described in Clause 7.2.1, the recommendations in this Section should be followed for structures in which explosives or highly-flammable solids, liquids, gases, vapours or dust are manufactured, stored or used, or in which highly flammable or explosive gases, vapours or dusts may accumulate, i.e. in those areas which may be classified as hazardous. Because of the increased risk, a rolling sphere of 20 m radius should be used when determining zones of protection in accordance with Clause 4.2, in lieu of the sphere of 45 m radius recommended for general application. 7.2.3 Electrostatic shielding The electrostatic induced voltage on isolated objects in the field of a storm cloud may cause sparks to ground when a lightning discharge occurs to some adjacent object. Isolated objects within a structure that is adequately shielded will themselves be electrostatically shielded. If the structure is not shielded or is only partly shielded, then the isolated objects should be earthed to prevent electrostatic sparks. For further discussion on the earthing of isolated internal objects, see Section 5. 7.3 AREAS OF APPLICATION Protection should, in all cases, be provided for the following structures: (a) Tanks and vessels containing flammable solids, liquids, vapours or gases, or highly-flammable or explosive dusts. (b) All metallic pipes and power and communication service lines at the point where they enter or leave a hazardous area. Piping which is not in electrical contact with its associated tank or vessel, such as an open discharge line into a water tank, should be bonded to the tank or vessel by a flexible conductor, and earthed. Cathodic protection may justify the insertion of an insulating flange which will interrupt the electrical continuity of the total length of line. Where flexible connections between pipelines and tanks do not incorporate an earth-continuity conductor, a separate conductor for earthing should be provided. No pipeline should be used for earth-continuity purposes as a substitute for the recommended earthing conductor. (c) Buildings in explosive areas which may contain explosive or large quantities of highly flammable materials, or nominated buildings which may, in emergency, be taken into use for the storage of explosives. (d) Buildings in explosive areas which may contain small quantities of highly flammable material or a large quantity of combustible material if sited within 50 m of a building specified in Item (c). (e) Any structure sited within 30 m of a building containing explosives, which thus constitutes a projectile hazard to this building in the event of dislodgment of masonry and the like by lightning. (f) Any structure sited within 30 m of a building containing explosives which if struck by lightning might constitute a subsequent fire hazard. 7.4 EQUIPMENT APPLICATION 7.4.1 Earth bosses Earth bosses should be made from low carbon steel, tapped to receive a bolt or stud. Pressure vessels should be provided by the manufacturer with a suitable boss to take the earth connection. Welding of bosses in excess of 40 mm in diameter on site may necessitate stress-relieving of the weld.

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Earth bosses should be about 50 mm long, but extended types, from 100 mm to 500 mm long, are used for fire-proofed steelwork and lagged vessels. In order to avoid corrosion, earth bosses should be installed at not less than 500 mm above ground level. 7.4.2 Bonding conductors Where various items of process plant or a number of vessels are mounted on an extensive concrete plinth which elevates the equipment above ground level, bonding conductors should be provided to form a common earth connection for all the downconductors from the plant. Copper tape should be installed along two opposite sides of the plinth, fastened to the walls not less than 500 mm above ground level to avoid corrosion. Tee-joints may be used between down and bonding conductor. Diagonally opposite ends of the base conductor should be provided with a test link from which connection is made to the earth termination network, preferably to earth busbars which provide alternative earth connections. Where one bonding conductor only is installed, test links and earth connections should be provided at each end. 7.4.3 Sizes of tapes Sizes of tapes should be in accordance with Table 4.4. For common earthing systems, larger sizes may be needed depending on the fault current. These should be selected in accordance with AS 3000 or the New Zealand Electrical Wiring Regulations. 7.4.4 Downconductors (see Clause 4.10) All high salient structures within a process area should be provided with at least two downconductors unless they are of welded construction or electrically continuous down to base level. Wherever possible, downconductors should be installed remote from stairs and operational walkways and ladders. Downconductors should preferably be installed at diagonally opposite corners of the structure which provide the shortest possible path for connection to the earth termination network; they should be installed on the outside of the structure and should not pass through it. Earth tape should be used for downconductors and while, wherever possible, it should be in a continuous length, test links may be attached for connection of down or base conductors at various levels. Where structural steelwork or columns do not require the installation of an air terminal, the downconductor should extend from above the highest point of the structure. Provision should be made for thermal expansion of the earthing conductor and associated structure. A test link should be installed in the downconductor in accordance with Clause 4.11, not less than 500 mm above ground level. Each downconductor from the highest point or points within the process area should take the shortest possible path direct to earth and should be equipped with its own set of earth electrodes to provide a path of minimum impedance for a lightning discharge. The earth electrodes should be interconnected below ground level with the bonding conductor belonging to the earthing system. 7.4.5 Air terminations (see Clause 4.9) All high salient structures which are not electrically continuous and which are not within the zone of protection of an adjacent protected structure should be equipped with air terminations in accordance with the recommendations of this Standard. Where two or more air terminations are employed they should be interconnected by roof conductors for connection to at least two downconductors as follows: (a) Roof conductors Copper tapes should take the shortest salient route between the various air terminations and with fasteners spaced as for downconductors. (b) Air termination network Buildings which are protected by an air termination network should be provided with at least two downconductors, which should be directly connected to the most widely-spaced parts of the air termination network. 7.5 SPECIFIC OCCUPANCIES 7.5.1 Protection of steel tanks 7.5.1.1 General precautions The following precautions should be taken to minimize the effects of lightning discharge on tanks containing petroleum products, including tanks with fixed roofs and tanks with floating roofs: (a) The shells of all tanks intended for the storage of highly flammable liquids which can produce an explosive gas atmosphere should be permanently and effectively earthed. Other tanks, such as water tanks, if located in a hazardous area should also be permanently and effectively earthed. The combined earth resistance of permanent earth connections to the tank should not exceed 10 Ω. The recommended method of earthing is by means of earth electrodes as detailed in Clause 4.13, but in some installations soil conditions and the earth resistance of the tank when isolated from associated pipelines may in themselves constitute permanent and effective earthing. In such cases the necessity for tank earth electrodes should be considered with particular reference to site measurements of earth resistance.

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(b) The minimum number of individual earth electrodes on storage tanks will depend upon the diameter and soil condition, and should be in accordance with the following schedule for single tanks: Diameter of tank m ≤ 30 > 30

Minimum number of independent electrodes 2 3

For a group of small tanks, earth electrodes common to the group may be installed, provided that each tank has two independent paths to earth. One of these paths may be through the pipeline earthing system. NOTE: The reason for the minimum of two earth electrodes is that during testing of one electrode the tank will remain earthed by the other electrode.

Earth electrodes for a tank may be interconnected around the periphery of the tank, and where two or more connections are used they should be spaced symmetrically round the tank. (c) Each earthing conductor should terminate in an approved design of cable lug and be attached to a steel boss welded to the tank body and tapped to receive a bolt or stud, preferably 10 mm diameter. Lock washers should be used on the connecting assembly. Soldered connections should be avoided. It is suggested that the boss be welded on the tank at a minimum height of 500 mm above the bottom of the tank. (d) When a pipe or rod earth electrode is driven into the ground, mechanical protection should be given to the head of the electrode. NOTE: It is the practice of some organizations to enclose all earth stake heads in a pit, where they are associated with ‘special’ earthing, such as lightning protection or static earthing.

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(e) Steel tanks with floating roofs according to their location should be protected by one of the measures described below: (i) Multiple shunt connections between the floating roof and the tank shell, in particular those designs incorporating mechanical linkage in the seal assembly. This is the most effective method of discharging induced static charges on the floating roof caused by atmospheric conditions; under this arrangement it is not necessary to bond across internal drainpipe joints or external moving stairway joints. (ii) Overhead earth wires or other suitable forms of interception protection in accordance with Clause 4.2 (see also Clause 7.2.2). This may be appropriate in areas where there is a known high thunderday level. 7.5.1.2 Above-ground steel tanks containing flammable liquids at atmospheric pressure The contents of steel tanks with steel roofs or riveted, bolted, or welded construction, with or without supporting members, used for the storage of flammable liquids, are considered to be reasonably well protected against lightning if the tanks comply with the following recommendations: (a) All joints between steel plates should be riveted, bolted, or welded. (b) All pipes entering the tank should be metallically bonded to the tank at the point of entrance. (c) All vapour or gas openings should be closed. (d) The metal tank and roof should have adequate thickness so that holes will not be burned through by lightning discharges (5 mm sheet steel roofs on tanks are considered adequate for this purpose*). (e) The roof should be continuously welded to the shell, or bolted, or riveted and caulked, to provide a gastight seam and electrical continuity. 7.5.1.3 Steel tanks with non-metallic roofs Steel tanks with wooden or other non-metallic roofs are not considered to be self-protecting, even if the roof is essentially gastight and sheathed with thin metal and with all gas openings closed or flameproofed. Such tanks should be provided with air terminals of sufficient height and number to receive all discharges and keep them away from the roof. The air terminals should be thoroughly bonded to each other, to the metallic sheathing, if any, and to the tank. Isolated metal parts should be avoided, or else bonded to the tank. In lieu of air terminals any of the following may be used: (a) Conducting masts suitably spaced around the tank. (b) Overhead earth wires. (c) A combination of masts and overhead earth wires.

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7.5.2 Installations handling crude oil and products 7.5.2.1 Jetties for marine tankers and barges The following recommendations should be observed as applicable: (a) General All pipelines to jetties and structural steelwork plant and bollards on jetties together with associated dolphins, walkways, and shore bollards should be connected to the earthing system. Electrical equipment on a jetty should be connected to an earthing system as specified in AS 3004 or NS ECP 29. Dependent upon site and operating conditions, it may be possible to obtain overall protection by using one earthing system. Where it is considered that one common earthing system may be adapted to comply with all the requirements, it is necessary to ensure that the value of earth resistance does not exceed 4 Ω. Where steel or steel box piles are not employed, an earthing conductor should be installed to enter the water below low water mark to provide a direct path for lightning discharge. (b) Jetties with cathodic protection It is recommended that the following precautions be taken where jetties are protected by either sacrificial anodes or power-impressed systems to prevent sparking at the tanker manifold when loading lines are being connected or disconnected: (i) Install an insulating flange at the jetty end of each loading line between jetty and vessel whereby all flanges shore-side of the insulating flange are earthed to the jetty earthing system and all flanges to the seaward side are earthed via the vessel. (ii) Ensure that the insulating flange cannot inadvertently be short-circuited by the electrical connection of exposed metallic flanges on the seaward side of the insulating flange to the jetty structure either by direct contact or by hose-handling equipment. (iii) Where sacrificial anodes are installed, it may be necessary to use manilla mooring ropes or straps to extend the life of the anodes and minimize current flow between jetty and vessel.

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(c) Ship/shore bonding cables An independent cable bonding connection between ship and jetty, with or without cathodic protection, is not considered as serving any useful purpose in— (i) the dispersal of static electricity; or (ii) minimizing possible current flow in conductive type loading hoses. 7.5.2.2 Bulk rail car loading and discharging For the bulk loading and discharging of rail cars reference should be made to Institute of Petroleum, Model Code of Safe Practice in the Petroleum Industry – Part I: Electrical. 7.5.3 Aircraft fuelling and de-fuelling Aircraft fuelling and de-fuelling should be suspended when electrical storms are in the vicinity. 7.5.4 Structures with explosive or highly flammable contents 7.5.4.1 Methods of protection Structures with explosive or highly flammable contents should be protected in one or more of the ways detailed in Clauses 7.5.4.2 to 7.5.4.5 and in accordance with the recommendations of Clauses 7.5.4.6 to 7.5.4.15, as appropriate. 7.5.4.2 Air termination network An air termination network should be suspended at an adequate height above the area to be protected (see Clause 4.2). Where a suspended conductor crosses chimneys or vents which emit explosive dusts or gases under forced draught, the suspended conductors should be at least 5 m above the chimney or vent. 7.5.4.3 Network of horizontal conductors Where the expense of the method described in Clause 7.5.4.2 cannot be justified, and where no risk is involved in discharging the lightning current over the surface of the structure to be protected, a network of horizontal conductors with a mesh between 3 m and 8 m according to the risk, should be fixed to the roof of the structure. Each separate structure protected as above should be equipped with twice the number of downconductors recommended in Clause 4.10. 7.5.4.4 Vertical conductors A structure or a group of structures of small horizontal dimensions may be protected by one or more vertical lightning conductors (see Clause 4.2). 7.5.4.5 Below-ground structures A structure which is wholly below ground and which is not connected to any services above ground can be protected by an air termination network as described in Clause 7.5.4.2 by virtue of the fact that soil has an impulse breakdown strength which can be taken into account when the risk of flashover from the protection system to the structure to be protected, including its services, is being determined. Where the depth of burying is adequate, the air termination network may be replaced by a network of earthing strips arranged on the surface in accordance with expert advice. Where this method is adopted, the recommendations on bonding between metal in, or metal conductors entering, the structure, given in Clauses 7.5.4.7 to 7.5.4.11 should be ignored. Where the underground structure has a reinforced concrete roof at or immediately below soil level, the reinforcement may be used as a protection system provided that the reinforcement is welded so that rectangular electrical conducting paths are formed with sides not exceeding 2 m in length.

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Where the underground structure has a roof which is not reinforced or where the reinforcement is not electrically continuous, a buried conductor network located above the structure and buried not less than 500 mm below the soil level may be used. Where the structure is such that protection cannot be provided by use of the reinforcement and the depth of soil above the roof is less than 500 mm, air terminations may be mounted on suitable bases above the structure at soil level and interconnected by a roof conduction network of closed mesh of between 3 m and 8 m. In structures containing nitroglycerine, the combined use of the systems described in Clauses 7.5.4.3 and 7.5.4.4 is recommended. 7.5.4.6 Interconnection of earth terminations The earth terminations of the earth protective system should be interconnected by a ring conductor. This ring conductor should preferably be buried to a depth of at least 500 mm and be at least 2 m from the walls of the structure unless other considerations, such as the need for bonding other objects to it, testing or risks of corrosion, make it desirable to leave it exposed. The resistance value of the earth termination network should be maintained permanently at 10 Ω or less. If this value proves to be unobtainable, the methods recommended in Clause 4.12 should be adopted, or the ring conductor should be connected to the ring conductor of one or more neighbouring structures until the above value is obtained. 7.5.4.7 Bonding of structural metal All major metal forming part of the structure, including continuous metal reinforcement and services should be bonded together and connected to the lightning protection system. Such connections should be made in at least two places and should, as far as is possible, be equally spaced round the perimeter of the structure at intervals not exceeding 15 m. 7.5.4.8 Bonding of internal metal Major metalwork inside the structure should be bonded to the lightning protection system.

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7.5.4.9 Electrical conductors entering structure Electrical conductors entering the structure should be metal-cased. The metal casing should be electrically continuous within the structure; it should be earthed at the point of entry outside the structure on the supply side of the service and bonded directly to the lightning protection system. 7.5.4.10 Electrical conductors connected to overhead supply line Where the electrical conductors are connected to an overhead electricity supply line, a length of buried cable with metal sheath or armouring should be inserted between the overhead line and the point of entry to the structure, and a surge protective device, e.g. of the type containing voltage-dependent resistors, should be provided at the termination of the overhead line. The earth terminal of this protective device should be bonded direct to the cable sheath or armouring. The sparkover voltage of the lightning protective device should not exceed half the break-down withstand voltage of the electrical equipment in the structure. In this operation, the appropriate Standard and any regulations which may apply should be observed. 7.5.4.11 Metal not continuously earthed Metallic pipes, electrical conductor sheaths, steel ropes, rails or guides not in continuous electrical contact with the earth, which enter the structure, should be bonded to the lightning protection system. They should be earthed at the point of entry outside the structure and at two points, one about 75 m away and one a further 75 m away. 7.5.4.12 Adit or shaft For a buried structure or underground excavation to which access is obtained by an adit or shaft, the recommendations in Clause 7.5.4.11 as regards extra earthing should be followed for the adit or shaft at intervals not exceeding 75 m as well as outside the structure. 7.5.4.13 Fences and retaining walls The metal uprights, components and wires of all fences, and of retaining walls in close proximity to the structure, should be connected in such a way as to provide continuous metallic connection between themselves and the lightning protection system. Discontinuous metal wire fencing on non-conducting supports or wire coated with insulating material should not be employed. 7.5.4.14 Avoidance of tall components Structures with explosive or highly flammable contents should not be equipped with tall components such as spires and flagstaffs or radio aerials on the structure or within 15 m of the structure. This clearance applies also to the planting of new trees, but structures near existing trees should be treated in accordance with Clause 6.3. 7.5.4.15 Tests of system Tests should be carried out in accordance with Clause 8.3 at intervals of not more than 2 years. The test equipment used should be certified for use in the particular hazardous area. In some cases, non-certified testing equipment may be used provided that the location where the tests are to be conducted has been proven to be free of combustible gases or vapours by competent persons.

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SECTION 8 INSTALLATION AND MAINTENANCE PRACTICE 8.1 WORK ON SITE Throughout the period of erection of a structure, all large and prominent masses of metalwork, such as steel frameworks, scaffolding and cranes, should be effectively connected to earth. Once work has commenced on the installation of a lightning protection system, an earth connection should be maintained at all times. During the construction of overhead power lines, overhead equipment for railway electrification and the like, the danger to persons can be minimized by ensuring that an earthing system is installed and properly connected before any conductors other than earth wires are run out. Once the conductors are run out and insulation installed, they should not be left ‘floating’ while men are working on them, but should be connected to earth in the same manner as when maintenance is being carried out after the line is commissioned. 8.2 INSPECTION All lightning protection systems should be inspected by a competent person after completion, alteration or extension, in order to verify that they are in accordance with this Standard. A routine inspection should be made at least once a year. 8.3 TESTING On the completion of the installation or of any modification to it, the resistance to earth of the whole installation and of each earth termination should be measured, and the electrical continuity of all conductors, bonds and joints and their mechanical condition verified. The testing should be carried out in accordance with Appendix B. If the resistance to earth of a lightning protection system, when so determined, exceeds the specified value for the particular applications the value should be reduced to be in accordance with the recommendations of this Standard. If the resistance is less than the recommended value but significantly higher than the previous reading, the cause should be investigated in accordance with Appendix B. The condition of the soil, the procedure adopted, details of salting or other soil treatment, and the results obtained should all be recorded as listed in Clause 8.4. 8.4 RECORDS The following records should be kept on site, or by the persons responsible for the upkeep of the installation: (a) Scale drawings showing the nature and position of all component parts of the lightning protection system. (b) The nature of the soil and any special earthing arrangements. (c) Date and particulars of salting, if used. (d) Test conditions and results in accordance with Clause 8.3. (e) Alterations, additions or repairs to the system. (f) The name of the persons responsible for the installation or its upkeep.

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NOTE: Detection of the occurrence of lightning flashes to the structure and the magnitude of the discharge current may be estimated by magnetic links, magnetic tape strips or other current monitoring devices.

8.5 MAINTENANCE If the general recommendations of this Standard have been duly observed, little maintenance should be needed. The periodic inspection and tests described in Clauses 8.2 and 8.3 will indicate what maintenance, if any, should be undertaken. Particular attention should be paid to earthing, to any evidence of corrosion and to any alterations or additions to the structure which may affect the lightning protection system. Examples of such alterations or additions are as follows: (a) Changes in the use of a building. (b) Installation of fuel oil storage tanks. (c) The erection of radio and television receiving aerials.

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APPENDIX A

THE NATURE OF LIGHTNING AND THE PRINCIPLES OF LIGHTNING PROTECTION (Informative)

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A1 SCOPE OF APPENDIX This Appendix deals with the nature of the phenomena involved in a study of lightning protection and the basic principles of designing such protection. A brief description of various elements of a lightning protection system and their function is also provided. Recommendations for systems to protect against the direct or indirect effect of lighting are given in the body of this Standard. A2 THE NATURE OF LIGHTNING A2.1 Nature of lightning Thunderstorms occur under particular meteorological conditions, and partial separation of electrical charges within the thundercloud usually results in regions with net negative charge mainly in the lower parts of the thundercloud, and regions with net positive charge mainly in the upper part. Lightning is an electrical discharge between differently charged regions within the cloud (cloud flash) or between a charged region, nearly always the lower negatively charged region, and earth (ground flash). A complete ground flash consists of a sequence of one or more high amplitude short duration current impulses, or strokes. In some ground flashes low amplitude long duration currents (sometimes termed continuing currents) flow between the strokes or after a sequence of strokes. The currents are unidirectional and usually negative, i.e. a negative charge is injected into the object struck. For all practical purposes the stroke can be considered to be generated by a current source whose waveshape and magnitude are unaffected by the characteristics of the ground termination. A2.2 The lightning attachment process The first stroke of a ground flash is normally preceded by a downward-progressing low-current leader discharge which commences in the negatively charged region and progresses towards the earth, depositing negative charge in the air surrounding the channel. When the lower end of the leader is roughly 100 m from the earth, electrical discharges (streamers) are likely to be initiated at protruding earthed objects, and to propagate towards the leader channel. Several streamers may start, but usually only one is successful in reaching the downcoming leader. The high current phase (return stroke) commences at the moment the upward moving streamer meets the downcoming leader. The position in space of the lower portion of the lightning channel is therefore determined by the path of the successful streamer, i.e. the one which succeeded in reaching the downcoming leader. The primary task in protecting a structure is therefore to ensure a high probability that the successful streamer originates from the lightning protection conductors, and not from a part of the structure that would be adversely affected by the lightning current that flows subsequently. As the path of the successful streamer may have a large horizontal component, e.g. many tens of metres, as well as a vertical component, an elevated earthed conductor will provide protection for objects spread out below it. It is therefore possible to provide protection for a large volume with a relatively small number of correctly positioned conductors. This is the basis for the concept of a zone of protection provided by an elevated earthed conductor, and provides the basic principle underlying interception lightning protection. Thus the basic protection system consists of air termination electrodes to provide launching points for streamers, and downconductors and earth electrodes to deliver the lightning current into the earth. A2.3 Thunderstorm and lightning occurrence Thunderstorm occurrence at a particular location is usually expressed in terms of the number of calendar days in a year when thunder was heard at the location, averaged over several years. The resulting information is given as an Average Annual Thunderday Map (see Figures 2.1 and 2.2). The rate of occurrence of ground flashes at a particular location can be roughly estimated from the annual thunderdays using the information given in Figure A1, where the unit of rate of occurrence (ground flash density) is given per square kilometre per year. Local topographical features may cause variations in the occurrence of ground flashes. The occurrence will be higher than the average on high ground, e.g. ridges, and lower than average on nearby low ground. In some cases, a large topographical feature such as a high mountain may interact with prevailing meteorological conditions to cause a concentration of thunderstorms and ground flashes. Such effects may be identified by enquiry of local telephone and electricity supply engineers or meteorological stations, and of local residents. On a smaller scale, tall objects, e.g. roof of a building, tree top or overhead conductor, tend to divert lightning flashes to themselves, as explained in Paragraph A2.2, thus shielding a certain surrounding area from direct strikes. Lightning detection systems have been in use in some areas of Australia which enable the direct determination of ground flash density and, in some cases, the peak current of ground flashes within a given region. Such data, where available, provide a more meaningful indication of lightning activity than data based on thunderdays per year (see Clause 2.2.5 and Appendix E).

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Solid line: relationship estimated from Australian thunderday and lightning flash counter records and lower limit of world-wide estimates of relationship. Dash line: upper limit of several world-wide estimates of relationship between ground flash density and thunderdays per year.

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FIGURE A1 APPROXIMATE RELATIONSHIP BETWEEN GROUND FLASH DENSITY AND THUNDERDAYS PER YEAR

A3 EFFECTS OF LIGHTNING The principal effects of a lightning discharge to an object are electrical, thermal and mechanical. These effects are determined by the magnitude and waveshape of the current discharged into the object. Statistical distributions of some characteristics of ground flashes are given in Table A1. When the lightning current flows through the building or its lightning protection system, the electrical potential of the building may rise to a high value with respect to remote earth (this terminology is usually adopted despite the fact that the potential is usually negative with respect to remote earth). It may also produce around the earthing electrodes a high potential gradient which can be dangerous to persons and to livestock. The rate of rise of current in conjunction with inductance of the discharge path produces a voltage drop that will vary in time depending upon the current waveshape. As the point of strike on the lightning protection system may be raised to a high potential, there is also the risk of a flashover from the lightning protection system to nearby metal objects. This is called a side-flash. The risk of side-flash is increased at any deeply re-entrant bend or loop in a downconductor due to the local increase in inductance. If such a flashover occurred, part of the lightning current would be discharged through internal installations with consequent risk to the occupants and the fabric of the building. The amount of energy deposited in any object carrying lightning current may be calculated by multiplying the action integral by the electrical resistance of the object. From this, the temperature rise may be calculated. It should be noted however that the resistance of most objects other than metallic conductors, e.g. wood, masonry or earth, is very non-linear for the large currents associated with lightning. It should also be noted that the passage of lightning current through moist resistive materials such as masonry or wood can convert the moisture to high-pressure steam, causing the material to explode or shatter.

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The thermal effect of a lightning discharge is confined to the temperature rise of the conductor through which the lightning current is discharged. Although the amplitude of a lightning current may be high, its duration is so short that the thermal effect on a lightning protection system, or on the metallic parts of a structure where this is included in the lightning protection system, is usually negligible. This ignores the fusing or welding effects which occur locally consequent upon the rupture of a conductor which was previously damaged or was of inadequate cross-section. In practice the cross-sectional area of a normal lightning conductor is determined primarily by mechanical and secondarily by thermal considerations. At the point of attachment of a lightning discharge channel to a thin metal surface, a hole may be melted in the surface. In this case, some thermal energy will be deposited directly in the metal from the hot plasma of the discharge channel, as well as the thermal energy caused by the passage of current through the metal. The size of the hole melted in the sheet depends on the material, the thickness of the sheet, and the charge delivered. For example, a moderately severe lightning flash delivering a charge of 70 C would melt a hole about 100 mm2 in area in a sheet of galvanized iron 0.38 mm thick. TABLE A1 SUMMARY OF THE FREQUENCY DISTRIBUTIONS OF THE MAIN CHARACTERISTICS OF THE LIGHTNING FLASH TO GROUND Item No 1 2 3 4 5 6 7 8 9 10 11

Lightning characteristic Number of common strokes Time interval between strokes First stroke peak current Imax. Subsequent stroke peak current Imax. First stroke (di/dt)max. Subsequent stroke (di/dt)max. Total charge Continuing current charge Continuing current Imax. Overall duration of flash Action integral (see Note 2)

Percentage of events having value of characteristic greater than value shown below (see Note 1) 99 90 75 50 25 10 1

Unit

1

1

2

3

5

7

12

—

10

25

35

55

90

150

400

ms

5

12

20

30

50

80

130

kA

3

6

10

15

20

30

40

kA

6

10

15

25

30

40

70

GA/s

6 1

15 3

25 6

45 15

80 40

100 70

200 200

GA/s C

6

10

20

30

40 150

70

100

C

30

50

80

100

200

400

A

50

100

250

400

600

900

1 500

ms

102

3 × 102

5 × 103

3 × 104

103

105

5 × 105

A2.s

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NOTES: 1 The values shown in this Table have been derived from a number of sources, and have been rounded in accordance with the accuracy with which these data are known. Values at the 1 percent and 99 percent levels are very uncertain, and are given only to indicate an order of magnitude. 2 The action integral, defined as ∫i2dt for the whole flash, is equivalent to the energy deposited in a one-ohm resistor by the passage of the entire current for the duration of the flash.

The passage of lightning current through a conductor causes a force on the conductor given by the equation: F = B×l×i ..... . . A3 where F = the force on the conductor, in newtons (N) B l i

= = =

the component of the magnetic flux density at right angles to the conductor, in teslas (T) the length of the conductor, in metres (m) the current through the conductor, in amperes (A)

A4 POTENTIAL DIFFERENCES CAUSED BY LIGHTNING A4.1 General A lightning flash to a building or structure, or a flash to ground near a building or structure will cause a potential rise in the vicinity of the strike attachment point, and may also cause a potential rise of objects remote from the point of strike. For example, a lightning strike to a service conductor (power or communications, or other metallic service) can cause current to be transmitted to the building, thus raising the potential of the building. A lightning flash to ground can also induce voltages and currents in remote conductors by electric and magnetic coupling (see also Section 5 and Appendix D).

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A4.2 Earth currents At the point where the lightning current enters the ground the current density is high. Hazardous earth potential gradients may be generated. Earth electrodes should be distributed more or less symmetrically, preferably outside and around the circumference of a structure, rather than be grouped on one side. This will help to minimize earth potential gradients near the building, and tend to cause the lightning current to flow away from the building rather than underneath it. In addition, with earth connections properly distributed, the current from a lightning flash to ground near the building will be collected at the outer extremities. Thus current flow underneath the building, as well as ground potential gradients, will be minimized. A4.3 Side-flash If a lightning conductor system is placed on a building and there are unbonded metal objects of considerable size nearby, there will be a tendency for side-flashing to occur between the conductors of the lightning protection system and the unbonded metal objects. To prevent damage from side-flash, interconnecting conductors should be provided at all places where side-flashes are likely to occur. This is referred to as equipotential bonding, although complete equalization of potential is never achieved. As the currents required to equalize potentials are considerably less than the full lightning current, conductors of relatively small cross-section are adequate for this purpose (see also Clause 4.14.2). A4.4 Potential (voltage) differences The impedance of the earth termination network to the rapidly changing lightning current influences the potential rise of the lightning protection system. This in turn affects the risk both of side-flashing within the structure to be protected, and of dangerous potential gradients in the ground adjacent to the earth termination network. The potential gradient around the earth termination network, on the other hand, depends on the physical arrangement of the electrodes and the soil resistivity. In Figure A2, a lightning flash is assumed to occur to the lightning protection system of a building. For the purposes of the illustration, no equipotential bonding is shown although such bonding is required in accordance with this Standard. As the lightning current is discharged through the downconductor and the earthing electrode, the conductor system and the surrounding soil are raised, for the duration of the discharge, to a potential with respect to the general mass of the earth. The resulting potential differences as shown by ‘step’, ‘touch’ and ‘transferred’ potentials in Figure A2 may be lethal; hence the importance of keeping the impedance of the earth termination network low, and of preventing large local potential gradients by equipotential bonding, and by the manner in which the earth electrodes are arranged. A5 PRINCIPLES OF LIGHTNING PROTECTION A5.1 Purpose of protection The purpose of lightning protection is to protect persons, buildings and their contents, or structures in general, from the effects of lightning, there being no evidence for believing that any form of protection can prevent lightning. A5.2 Interception of lightning The function of an air termination electrode in a lightning protection system is to divert to itself the lightning discharge which might otherwise strike a vulnerable part of the object to be protected. It is generally accepted that the range over which an air termination electrode can attract a lightning discharge is not constant, but increases with the severity of the discharge. The path of a lightning discharge near a structure is determined by the path of the successful streamer (see Paragraph A2.2) which will usually be initiated from a conducting part of the structure nearest to the downcoming leader. The initiation of streamers is also influenced by the local electric field. The upper outer edges and corners of buildings or structures, and especially protruding parts, are likely to have higher local electric fields than elsewhere, and are therefore likely places for the initiation of streamers. When the downcoming leader is within about 200 m of the building, the electric field at these protruding parts and corners will exceed the breakdown field strength of air, resulting in corona currents that cause these parts to be surrounded by ionized air. The resulting space charges influence the electric field in such a manner that the field is limited to the breakdown strength of air. However, these complicating factors do not alter the fact that the most probable strike attachment point on a building is the edge, corner, or other protruding part closest to the downcoming leader. This is the basic reason why the rolling sphere method gives a reliable guide to the most probable strike attachment points. Hence, if air terminations are placed at all locations where high electric fields and streamer initiation are likely, there will be a high probability that the discharge will terminate on some portion of the lightning protection system. A5.3 Determination of lightning strike attachment points to buildings A5.3.1 The rolling sphere method The procedure for determining lightning strike attachment points is based on the rolling sphere method whereby a sphere of specified radius (45 m for standard level of protection, see Paragraph A7) is imagined to be rolled across the ground towards the building, up the side, and over the top of the building, and down the other side to ground. This can be carried out in various orientations with respect to the building. Any point on the building touched by the sphere is a possible lightning strike attachment point.

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NOTES: 1 Person X is in contact with the ground at a and b: person Y is in contact with the ground at c and the conductor at d; person Z is in contact with the conductor at e and a metallic handrail f shown grounded at g. 2 Person X is subject to ‘step’ potential. 3 Person Y is subject to ‘touch’ potential. 4 Person Z is subject to ‘transferred’ potential. 5 The potential depends on the current magnitude and the impedance of the path of the lightning discharge. 6 Step potential increases with the size of the step a-b in the radial direction from the conductor and decreases with the increase in the distance between person X and the conductor. 7 The transferred potential increases with increase in the radial distance between the downconductor and the ground g. 8 The diagram does not show equipotential bonding which may be necessary to protect persons from hazardous potential differences of the type described in this diagram (see Sections 4 and 5).

FIGURE A2 INSTANTANEOUS POTENTIAL DIFFERENCES DURING A LIGHTNING FLASH TO AN EARTHED CONDUCTOR

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The physical basis for this method is as follows. As the lightning leader stroke approaches the ground, the electrical field at various salient points, such as the upper corners of buildings, will help to launch electrical discharges, or streamers, which progress upwards toward the tip of the down-coming leader stroke. The position in space of the lightning discharge channel, and the location of the strike attachment point, is determined when the leader and one of the upward streamers join to complete the lightning discharge path. The upward streamer which determines the strike attachment point is generally that launched from the salient point or earthed conductor closest to the downcoming leader. The rolling sphere will tend to touch those salient points, and the method therefore provides a geometric means of identifying such points. A5.3.2 The striking distance The striking distance, ds, is the distance between the leader tip and the eventual strike attachment point at the moment when it has become inevitable that the gap, of dimension d s, will be bridged by the discharge channel. The rolling sphere method is closely related to the electrogeometric method developed for predicting lightning attachment to electric power lines, whereby the lightning leader is supposed to progress until it comes within the distance d s of an earthed object, when the final discharge path is determined to that object. There are theoretical and observational grounds for a relationship between ds and the i max., where imax. is the peak return stroke current. The following relationship has been proposed: 10 × imax.0.65 .....A5.3 ds = where ds = the striking distance, in metres (m) the peak current of the return stroke, in kiloamperes (kA) imax. = The advantage of the rolling sphere method is that it is relatively easy to apply, even to buildings of complicated shape. The limitation of the method is that no account is taken of the influence of electric fields in initiating return streamers, and the method therefore does not distinguish between likely and unlikely lightning strike attachment points. In particular, the enhancement of electric field at the upper outer corners of a building makes these corners the most probable strike attachment points, whereas return streamers are unlikely to be initiated from a flat surface away from a corner or edge, even if on the roof and touched by the sphere. Some qualitative indication of the probability of strike attachment to any particular point can be obtained if the sphere is supposed to be rolled over the building in such a manner that its centre moves at constant speed. Then the length of time that the sphere dwells on any point of the building gives a qualitative indication of the probability of that point being struck. Thus for a simple rectangular building with a flat roof, the dwell time would be large at the corners and edges, and small at any point on the flat part of the roof, correctly indicating a high probability of the corners or edges being struck, and a low probability that a point on the flat part of the roof will be struck. The rolling sphere method needs to be applied with electric field enhancement effects in mind, so that high priority is given to providing air termination electrodes at the more probable attachment points. For a building of more or less rectangular shape with a flat roof, this means giving top priority to providing air termination electrodes around the periphery of the roof. This could take the form, for example, of a metallic perimeter handrail. A5.4 Protection of the sides of tall buildings When the rolling sphere method is applied to a building of height greater than the assumed sphere radius, then the sphere touches the sides of the building at all points above a height equal to the sphere radius. This indicates the possibility of strikes to the sides of the building, and raises the question of the need for air termination conductors on the sides of the building. Practical experience indicates that strikes to the sides of tall buildings do occur but are uncommon. There are theoretical reasons for believing that only flashes with low i max. and consequently low ds values are likely to be able to penetrate below the level of the roof of the building and strike the sides. The consequences of a strike to the sides of a building are likely to be dislodgement of masonry, as the current penetrates to the building reinforcing steel. If it is decided that some protection for the sides of a building is justified, then conductors should be provided at the most probable lightning attachment points on the sides of the building. The most probable attachment points are at protruding corners and vertical edges of the sides of the building, including changes of direction that are determined as requiring protection in accordance with Clause 4.9.3.1. The conductors will generally serve both as air termination conductors and downconductors and will in general be connected to the roof air termination conductors at their upper ends, and to the earthing network at their lower ends. The conductors may be made flush with the surface, and should be placed as near as practicable to the vertical edge to be protected. Where the building construction includes extensive metal objects on the vertical outer surfaces, such as large metallic window frames, then such objects can form part of the interception protection system. It is necessary to provide electrical connections between adjacent metal objects both in the horizontal and vertical directions, and to provide periodic connections between the surface metalwork and the reinforcing steel, or the downconductors if separate from the reinforcing. This provides multiple paths for the lightning current from any point on the surface metalwork to earth, and local potential differences will be reduced to an acceptable level.

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Return streamers are more likely to develop from a good conductor on the surface than from a poor conductor in a similar position. Thus if a wall consists mainly of a poorly conducting material, and there are isolated objects with earth connections made of a highly conductive material distributed over the surface, then the return streamers will tend to originate preferentially from the highly conducting objects with earth connections. A wall of poorly conducting material can therefore be substantially protected by earthed metallic studs placed on a grid pattern flush with the surface of the wall. Alternatively, protection can be provided by a system of metallic strips flush with the surface. Even if the lightning happens to strike a point on the non-conducting surface away from an earthed conducting point, it is likely that it will track across the non-conducting surface and terminate on the earthed conducting point. A5.5 Safe discharge to earth A5.5.1 General If the air termination network is adequately connected to earth the current will pass to ground without damage to the structure. Metallic parts of a building or structure may usefully be made part of the lightning conductor system, provided that the passage of lightning current will not cause harm (for bonding of metal in or on a structure, see Clause 4.14.2.2). A5.5.2 Use of reinforcing steel as a downconductor It is sometimes suggested that overlapped and tied reinforcing rods do not provide good electrical connections, and are therefore not suitable for carrying lightning currents. However, the situation differs greatly from that in which a conducting path for power currents is required. Even if there are thin films of iron oxides and cement between the bars, the voltage required to cause breakdown of these films would be less than 1000 V. Once breakdown has occurred, there would be localized arcing between the steel bars, with a voltage drop of a few tens of volts. The initial breakdown across the oxide and cement films would occur during the first few microseconds of the first stroke when there is a large inductive voltage drop from top to bottom of the building; this voltage would be very much larger than the voltage required to break down the oxide and cement films between bars. Thus there are good reasons for relying on the reinforcing bars to act as downconductors, even when no special precautions have been taken (such as welding the bars together) to ensure electrical continuity. The localized arcing referred to above would produce relatively small amounts of energy in relation to the thermal capacity of typical reinforcing bars, so heating effects should be negligible. Where the structural steel reinforcement of the building is to be utilized as the downconductor system, it is important that there be an effective electrical connection between the air termination system and the steel reinforcement. Such connections should be made as close as practicable to the top of the building and preferably at a number of points around the building perimeter. Tall metal structures, such as chimneys, provide an adequate conducting path, but care must be taken to ensure that they are also suitably earthed. Special precautions are needed for the protection of structures containing explosives, highly flammable materials and gases. The principles involved in such protection systems are given in Section 7. A5.6 Potential equalization As explained in Paragraph A4, lightning strikes may give rise to harmful potential differences within a building. Of particular concern is the occurrence of potential differences which may exist between incoming conductors such as metallic water services, telecommunication systems, power systems, and local earth. Reduction of these potential differences may be achieved by a system of coordinated bonding of all affected conductors contained in the building. This includes all incoming metallic services, protection earths associated with power and communication systems, and the building lightning protection earthing system (if provided). Potential equalization (understood to imply approximate potential equalization) may be effected by including in the bonding scheme earthed building metalwork such as reinforcement metals and metal framework, if any. In cases where the presence of dissimilar metals may create corrosion problems or for other reasons, the commoning path may be effected by using suitably rated overvoltage protection devices. A6 ELEMENTS OF A PROTECTION SYSTEM The main parts of a typical lightning protection system for a building or structure may be summarized as follows, noting that not all parts will be present in all systems. (a) Air terminations are placed so as to achieve interception lightning protection, ensuring a high probability that lightning will attach to the air termination network, and not to parts of the protected object that could be damaged by lightning current. Existing metalwork should be used as far as possible, supplemented by carefully positioned air terminations giving priority to high probability attachment points. These are the upper outer corners and edges of the building and any salient or protruding objects on the roof. The form of air termination should be chosen for simplicity and low cost consistent with adequate mechanical strength, durability and aesthetic acceptability. (b) Downconductors are used to convey lightning current towards the earth. Existing building metalwork should be used as far as possible, especially steel frames and reinforcing steel in reinforced concrete columns, supplemented where necessary by external downconductors. If these downconductors are also to serve as part of the air termination network for the sides of a tall building, they should preferably follow the outer vertical corners of the building. Where the number of downconductors required exceeds the number of vertical corners, the remaining downconductors should be placed uniformly between the ones at the corners.

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AS 1768—1991/NZS/AS 1768—1991

(c) Test links may be required between the downconductors and the earth electrodes to facilitate the testing of the lightning protection system. (d) The earth termination network consisting of one or more earth electrodes, and any interconnecting conductors between earth electrodes, serves the purpose of delivering the lightning current into the general mass of the earth. The footings of large reinforced concrete buildings will generally provide a better earth connection than can be provided by driven electrodes around the periphery. Where the superficial layers of the earth have high resistivity, deep driven electrodes may be needed to reach low-resistivity regions, and achieve an acceptable earth resistance. (e) Equipotential bonding is used to reduce or prevent hazardous potential differences between any pair of extended conducting objects in the building or structure. Equipotential bonding becomes particularly important in buildings or structures having a high earth resistance. In the extreme situation where an acceptable connection to earth cannot be achieved, it would be necessary to rely entirely on equipotential bonding to protect persons and equipment against hazardous potential differences caused by lightning. Equipotential bonding may, in some situations, be achieved by means of overvoltage protection devices, where direct connection of the conducting parts results in an unwanted effect, for example, corrosion of metals. (f) Overvoltage protection is achieved by using various types of overvoltage protection devices (e.g. spark gaps, gas-filled surge arrestors or metal oxide varistors) to prevent hazardous potential differences being applied to persons or equipment, while allowing correct operating potentials to exist (see Section 5 and Appendix D). A7 VARIATION IN THE LEVEL OF LIGHTNING PROTECTION PROVIDED The protection recommended in Section 4 (see Clause 4.2) is referred to as the standard level of protection. This protection will be satisfactory for a wide range of buildings and structures. However, there are some situations in which a higher level of protection is desirable, for example, where there is vulnerable and expensive equipment in the building, or where the building contains explosive or flammable materials (see Clause 7.2.2). There are some situations in which economics may prevent the installation of standard lightning protection, but a reduced level of protection at a lower cost can be justified. These variations from the standard level of protection may be achieved as follows. The rolling sphere radius, specified as 45 m for standard protection, should be reduced to give enhanced protection, and increased to give reduced protection. For example, a sphere radius of 20 m will result in more closely spaced air termination conductors, giving improved protection against low-current discharges. Conversely, a sphere radius of 70 m will result in a protection system with a higher probability that low-current discharges will penetrate past the air termination conductors and strike an unprotected part of the structure. The consequences of such a protection failure need to be weighed against the economic saving made by choosing a reduced level of protection. The effect of varying the rolling sphere radius is indicated in Table A2. TABLE A2 EFFECT OF VARYING THE RADIUS OF THE ROLLING SPHERE Corresponding peak current (approximate) Approximate percentage of events with lower peak current m kA % 20 3