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.
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.
APPENDIX A
THE NATURE OF LIGHTNING AND THE PRINCIPLES OF LIGHTNING PROTECTION
(Informative)
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).
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.
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.
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 Lightning characteristic
Percentage of events having value of characteristic greater than value shown below (see Note 1) Unit 99 90 75 50 25 10 1
1 Number of
common strokes 1 1 2 3 5 7 12 —
2 Time interval
between strokes 10 25 35 55 90 150 400 ms
3 First stroke peak
current Imax. 5 12 20 30 50 80 130 kA
4 Subsequent stroke
peak current Imax. 3 6 10 15 20 30 40 kA
5 First stroke
(di/dt)max. 6 10 15 25 30 40 70 GA/s
6 Subsequent stroke
(di/dt)max. 6 15 25 45 80 100 200 GA/s
7 Total charge 1 3 6 15 40 70 200 C 8 Continuing current charge 6 10 20 30 40 70 100 C 9 Continuing current Imax. 30 50 80 100 150 200 400 A 10 Overall duration of flash 50 100 250 400 600 900 1 500 ms 11 Action integral (see Note 2) 102 3×102 103 5×103 3×104 105 5×105 A2 .s 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 = the component of the magnetic flux density at right angles to the conductor, in teslas (T) l = the length of the conductor, in metres (m)
i = 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).
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