NZS/AS 1768—1991
Australian Standard
R
New Zealand Standard
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
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which subscribing members receive, and which give details of new publications, new editions and amendments, and of withdrawn Standards.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
NZS/AS 1768—1991
Australian Standard
R
New Zealand Standard
Lightning protection
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,
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.
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Page SECTION 1 SCOPE AND GENERAL
1.1 SCOPE . . . 5
1.2 APPLICATION . . . 5
1.3 REFERENCED DOCUMENTS . . . 5
1.4 DEFINITIONS . . . 5
SECTION 2 ANALYSIS OF NEED FOR PROTECTION 2.1 NEED FOR PERSONAL PROTECTION . . . 7
2.2 NEED FOR PROTECTION OF BUILDINGS AND CONTENTS . . . 7
2.3 NEED FOR PROTECTION OF PERSONS AND EQUIPMENT WITHIN BUILDINGS. . . 8
SECTION 3 BEHAVIOURAL PRECAUTIONS FOR PERSONAL SAFETY 3.1 SCOPE OF SECTION . . . 13
3.2 PERSONAL CONDUCT . . . 13
3.3 EFFECT ON PERSONS AND TREATMENT FOR INJURY BY LIGHTNING . . . 13
SECTION 4 PROTECTION OF BUILDINGS 4.1 SCOPE OF SECTION . . . 14
4.2 ZONES OF PROTECTION . . . 14
4.3 METHODS OF PROTECTION. . . 14
4.4 MATTERS TO BE CONSIDERED WHEN PLANNING PROTECTION . . . 18
4.5 MATERIALS . . . 21
4.6 FORM AND SIZE OF CONDUCTORS . . . 23
4.7 JOINTS. . . 24 4.8 FASTENERS . . . 24 4.9 AIR TERMINATIONS. . . 24 4.10 DOWNCONDUCTORS. . . 26 4.11 TEST LINKS . . . 27 4.12 EARTH TERMINATIONS . . . 27 4.13 EARTHING ELECTRODES . . . 29
4.14 METAL IN AND ON A STRUCTURE. . . 30
SECTION 5 PROTECTION OF PERSONS AND EQUIPMENT WITHIN BUILDINGS 5.1 SCOPE OF SECTION . . . 33
5.2 NEED FOR PROTECTION. . . 33
5.3 MODES OF ENTRY OF LIGHTNING IMPULSES . . . 33
5.4 GENERAL CONSIDERATIONS FOR PROTECTION . . . 34
5.5 PROTECTION OF PERSONS WITHIN BUILDINGS . . . 36
5.6 PROTECTION OF EQUIPMENT. . . 37
SECTION 6 PROTECTION OF MISCELLANEOUS STRUCTURES AND PROPERTY 6.1 SCOPE OF SECTION . . . 42
6.2 STRUCTURES WITH RADIO AND TELEVISION AERIALS . . . 42
6.3 STRUCTURES NEAR TREES . . . 42
6.4 PROTECTION OF TREES . . . 42
6.5 CHIMNEYS, METAL GUY–WIRES OR CABLES. . . 43
6.6 PROTECTION OF MINES . . . 43
6.7 PROTECTION OF BOATS . . . 44
6.8 FENCES . . . 45
SECTION 7 PROTECTION OF STRUCTURES WITH EXPLOSIVE OR HIGHLY–FLAMMABLE CONTENTS 7.1 SCOPE OF SECTION . . . 48 7.2 GENERAL CONSIDERATIONS . . . 48 7.3 AREAS OF APPLICATION . . . 48 7.4 EQUIPMENT APPLICATION . . . 48 7.5 SPECIFIC OCCUPANCIES. . . 49
SECTION 8 INSTALLATION AND MAINTENANCE PRACTICE 8.1 WORK ON SITE . . . 53 8.2 INSPECTION. . . 53 8.3 TESTING . . . 53 8.4 RECORDS . . . 53 8.5 MAINTENANCE. . . 53 APPENDICES A THE NATURE OF LIGHTNING AND THE PRINCIPLES OF LIGHTNING PROTECTION . . . 54
B NOTES ON EARTHING ELECTRODES AND MEASUREMENT OF EARTH IMPEDANCE . . . 66
C THE CALCULATION OF LIGHTNING DISCHARGE VOLTAGES AND REQUISITE SEPARATION DISTANCES FOR ISOLATION OF A LIGHTNING PROTECTION SYSTEM . . . 77
D WAVESHAPES FOR ASSESSING THE SUSCEPTIBILITY OF EQUIPMENT TO TRANSIENT OVERVOLTAGES DUE TO LIGHTNING . . . 84
E ALTERNATIVE DETERMINATION OF INDEX E BASED ON LIGHTNING FLASH DENSITY/ENERGY DATA . . . 88
F REFERENCED DOCUMENTS. . . 94
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.
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.
1.4.10 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.
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 (ds)—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.
SECTION 2 ANALYSIS OF NEED FOR PROTECTION
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.
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.
TABLE 2.1
INDEX FIGURE A (TYPE OF STRUCTURE)
Usage and contents Value of index A
Protecti on not justi fi ed having regard to nature of building, occupancy and
contents −10
Structure and contents inert , occupati on infrequent, e.g. domesti c outbuil ding,
farm shed, roadside hoarding, metal chimney or mast 0
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
1 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 2 Cinema, church, school, boat, historical monument of medium importance,
densely populated marquee 3
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
4
Petr ol and gas installati on, hospital 5
Explosives building 15
TABLE 2.2
INDEX FIGURE B (CONSTRUCTION)
Constructi on Value of index B
Full y metalli c str ucture, electr ically continuous 0
Reinforced concrete or steel frame with metall ic roof 1 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 2 Large area building of timber or masonry with metall ic roof
Small buil ding of timber or masonry wit h non-metall ic roof 3 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
4
TABLE 2.3
INDEX FIGURE C (HEIGHT)
Height of structure, m
Value of index C Exceeding Not exceeding
0 6 12 6 12 17 0 2 3 17 25 35 25 35 50 4 5 6 50 70 100 70 100 140 7 8 9 140 200 200 10 11 TABLE 2.4
INDEX FIGURE D (SITUATION)
Situation Value of index D
On the flat, at any elevation 0
Hill side up to three-quarters of the way up, or mountainous country up to
1000 m 1
Mountain top above 1000 m 2
TABLE 2.5
INDEX FIGURE E (LIGHTNING PREVALENCE)
Average thunderdays per year*
Value of index E Exceeding Not exceeding
0 2 4 2 4 8 0 1 2 8 16 32 16 32 64 3 4 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) Assessment of risk Need for protection
<11 Negligible Not needed
11 Small Not needed
12 Fair Mi ght be advisable
13 Medium Advisable
14 Great Strongly advisable
>14 Very great Essential
TABLE 2.7
EXAMPLES OF THE CALCULATIONS FOR EVALUATING THE NEED FOR PROTECTION
Example Index values Assessment of risk Protecti on A B C D E R = A + B + C + D + E Type of
structure Constructi on Height Situation Prevalence (Table 2.1) (Table 2.2) (Table 2.3) (Table 2.4) (Table 2.5)
10 m high domestic dwelli ng, brick wall s, non-metall ic roof located on hill side—15 thunderdays
1 3 2 1 3 10 Negligible Not needed
15 m high domestic dwelli ng, brick wall s, non-metall ic roof located on hill side—30 thunderdays
1 3 3 1 4 12 Fair Mi ght be
advisable
20 m high historic
tr ee on fl at—60 thunderdays
3 3 4 0 5 15 Very great Essential
15 m high aircraft hangar, steel frame with metall ic roof, located in hill y countr y at 1000 m —15 thunderdays
4 1 3 2 3 13 Medium Advisable
10 m high church, bri ck wall s, metal roof, located on hill side—30 thunderdays
3 2 2 1 4 12 Fair Mi ght be
advisable 24 m high offi ce building,
reinforced concrete, located on fl at—15 thunderdays
2 2 4 0 3 11 Small Not needed
40 m high offi ce building, reinforced concrete, located of fl at—30 thunderdays
2 2 6 0 4 14 Great Strongly
advisable 16 m high wooden masted
yacht on open sea—10 thunderdays
3 3 3 0 3 12 Fair Mi ght be
advisable 20 m high brick chimney
located on fl at— 30 thunderdays 2 3 4 0 4 13 Medium Advisable 10 m high marquee located on fl at— 40 thunderdays 3 4 2 0 5 14 Great Strongly advisable
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 MA P OF AU STRA LIA
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 MA P OF NE W ZEA LAND
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.
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.)
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).
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:
r = (90h−h2
) . . . . 4.2
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).
TABLE 4.1
HEIGHT AND SPACING OF AIR TERMINATIONS TO PROTECT A FLAT ROOF
metres Height of air termination Horizontal distance for which roofis protected Maximum spacing distance for array
h r d 0.5 1.0 2.0 6.7 9.4 13.3 9.5 13.3 18.8 4.0 8.0 18.5 25.6 26.2 36.2
FIGURE 4.1 ZONE OF PROTECTION ESTABLISHED BY AIR TERMINATIONS ON THE HIGHER PARTS OF A STRUCTURE
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
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.
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.
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.
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.
FIGU RE 4.4 TYPICA L SYS TEM EM PLOY ING HO RIZONTAL AN D VE RTICAL AIR TER MINA TION NETWORK
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.
TABLE 4.2
TYPICAL MATERIALS FOR CURRENT-CARRYING COMPONENTS
Material Standard Grade or type
Castings Leaded gunmetal Aluminium alloy AS 1565 AS 1874 C92410 EA401 or AA607 Bars and rods
Copper, hard-drawn or annealed Phosphor-bronze Naval brass Aluminium bronze Aluminium Aluminium alloy Galvanized steel Stainless steel AS 1567 AS 1567 AS 1567 AS 1567 AS 1866 AS 1866 — AS 2837 110 518 464 627 1050 6063 or 6463A — — Tubes Copper Galvanized steel AS 1432 or NZS 3501 AS 1074 — — Strip Copper, annealed Aluminium Galvanized steel Stainless steel AS 1566 AS 1866 AS 1397 or NZS 3441 AS 1449 110 1200 — — Stranded conductors Copper, hard-drawn Aluminium Galvanized steel Stainless steel AS 1746 AS 1531.1 AS 1222.1 — — — — — Fixing bolts and screws for copper
Phosphor-bronze Naval brass Aluminium bronze Common brass Stainless steel AS 1567 AS 1567 AS 1567 AS 2738.2 AS 2837 518 464 627 272 303 Fixing bolts and screws for aluminium
and aluminium alloys Aluminium alloy Galvanized iron or steel
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.
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 30Ω.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
Deleterious effect of this metal/alloy on other bonded underground
ferrous metals
Deleterious effect on this metal/alloy from bonding to MEN
(copper-based) systems
Galvanized iron or steel Nil Damaging
Solid copper Damaging Nil
Copper-clad steel Damaging Can be damaging—
may be acceptable Solid stainless steel or nickel iron alloy Generally acceptable Can be damaging—
may be acceptable
Stainless-steel-clad steel Generally acceptable Can be damaging
Bronze Generally damaging May be acceptable
Brass Can be damaging May be acceptable—
can be dezincified
Zinc Nil Damaging
Aluminium Nil Extremely damaging
Magnesium Nil 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 mm2
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.
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.
TABLE 4.4
TYPICAL SECTION DIMENSIONS OF MAIN CURRENT-CARRYING COMPONENTS
Component Typical section dimensions (see Note)
Air terminations Strip Rods Stranded conductors 20 mm×3 mm 10 mm dia. 35 mm2 Downconductors Strip Rods Stranded conductors Galvanized materials 20 mm×3 mm 10 mm dia. 35 mm2 35 mm2 Earthing electrodes and base conductors
Hard-drawn copper rods for direct driving into soft ground
Hard-drawn or annealed copper rods for indirect driving or laying in ground Galvanized star stakes
Stainless steel
Galvanized steel water pipe Galvanized steel or copper strip:
Base conductors Earth electrodes 12 mm dia. 10 mm dia. 25 mm×19 mm×19 mm 10 mm dia. 12 mm dia. 30 mm×5 mm 30 mm×3 mm Fixed connections (bonds)
External: Strip Rods Internal: Strip Rods 20 mm×3 mm 10 mm dia. 20 mm×1.5 mm 6.5 mm dia. Standard flexible connections (bonds)
External Internal
70 mm2 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.
