Lightning strikes affects many businesses and individuals every each whether it be through damage to electrical and sesitive equipment or to even at worst, death. The latest edition of AS1768 was published in January 2007. The new Australian standards were published in January 2007 and included provisions for the protection of persons and property from hazards arising from exposure to lightning.
The recommendations specifically cover the following applications: 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. The protection of a variety of buildings or structures, including those with explosive or highly-flammable contents, and mines. The protection of sensitive electronic equipment (e.g. facsimile machines, modems, computers) from over-voltages resulting from a lightning strike to the building or its associated services.
Structural protection Buildings with structural steel framing may be protected by the installation of metal air terminals at the high parts of the building – the air terminals being connected to the steel framing and the framing earthed in the vicinity of the foundation. Structures containing continuous metal, for example, metal within a roof, wall, floor or covering may – under certain circumstances – utilise such metal as part of the LPS. Lightning tends to strike the ridges, corners, parapets and edges of building roofs.
Generally these attachment points coincide with the metal roof sheeting, capping or guttering being fixed by multiple screws, rivets or clips directly to the supporting steel purlins, beams and trusses below. While a direct strike can puncture a hole in thin metal sheeting the instances of such damage are rare. For many buildings that are roofed, or roofed and clad, with metal, it may be possible to dispense with some or all air terminals provided the supporting roof steelwork is directly connected to a down-conductor network or the earthing system.
Where a structure contains electrically continuous metal, for example, continuous steel frame, or metal within a roof, wall, floor or covering, this metal, suitably bonded, may be used as part of the LPS, provided that the amount and the arrangement of the metal render it suitable for use. Where a structure is simply a continuous metal frame without external metal covering, for example, a tower pole, it requires no air terminal or down-conductor; it is sufficient to ensure that the conducting path is electrically continuous and that the base is adequately earthed.
A steel frame structure or reinforced concrete structure may have foundations with sufficiently low inherent earthing resistance and, if connections are brought out from the reinforcement, a test may be made to verify its suitability for use as part of the LPS. Earthing the lightning protection system –whether or not it is the structure – is important and it is essential to ensure that structures are properly earthed. If a policy is adopted of electrically bonding structural members to foundation steel reinforcement, in many instances, this will prove to provide adequate earthing for lightning protection. A simple earth test can quickly verify this.
A hazard to persons exists during a thunderstorm. Between six and 10 people are killed by lightning in Australia each year. Lightning strikes 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 and touch potentials. The threat to personal safety is greatest if a person is out of doors when the thunderstorm is local. Personnel who work outdoors should become “lightning aware”. AS1768-2007 describes a simple rule, known as the 30/30 rule. This is not arbitrary as it might sound but is based upon scientific research.
Essentially an approaching thunderstorm is treated as local when the time interval between seeing lightning and hearing thunder is 30 seconds. Appropriate safety measures should then be taken to seek shelter in a substantial building, preferably with lightning protection or inside a metal bodied vehicle. The storm is considered no longer a threat when more than 30 minutes have elapsed after the last thunder is heard.
Personnel who work outdoors in lightning prone areas should be made aware of the hazards of lightning and under no circumstances should be working in exposed locations on the tops of tanks, gantries etc, even if these are earthed and have lightning protection fitted. Lightning warning systems have some place to play but many devices are reputed to be unreliable. Weather radar, available from the Bureau of Meteorology website is probably the most reliable indicator of storm activity.
Protection against indirect lightning
The need to provide protection against the indirect effects of a lightning strike is largely overlooked. Many consider that lightning protection comprises structural protection only. Nothing could be further from the truth. A lightning strike to a protected structure or in the vicinity of that structure, for example, to the ground, a tree or a nearby structure will produce a local earth potential rise and associated with the strike will be rapidly changing electromagnetic field. The effect at an industrial plant can be damage to equipment, loss of operation and the consequential loss of revenue.
Yet many may wonder why their equipment is damaged when lightning protection is supposedly already fitted. The answer is simple. Protection against the indirect effects of a lightning strike has probably been totally overlooked. Worse still, many consider that the installation of a UPS on the power supply will solve everything and wonder why their PLC I/O cards and instruments have still failed.
A lightning strike up to 1km away can be sufficient to induce voltages into field cabling causing the consequential loss of sensitive I/O cards in PLC and SCADA installations and in many cases damage field instruments as well. If surge protection is deemed necessary, then the standard dictates that every electrical service entering the structure under consideration requires protection. This includes power lines, telecommunications cables, field wiring and even antenna cables.
All must be protected if we are to adequately protect against the indirect effects of a lightning strike. AS1768-2007 section 126.96.36.199 provides an example of the principles involved.