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APPLICATION OF ADVANCES IN LIGHTNING RESEARCH TO LIGHTNING PROTECTION 64 Figure 5.2 A map of the cloud-to-ground lightning strikes in the Tampa Bay area for August 8, 1979. Individual storms have been circled. The two DF locations in the Tampa Electric Company's lightning location system are identified. Map scale is in thousands of feet. For objects that have an inductive impedance, such as wires in an electronic system, the peak voltage will be proportional to the maximum rate of change of the lightning current (V = L di/dt). For example, if 1 m of wire has an inductance L of 10-7 H and di/dt = 1010 A/sec, 1000 V is generated across the wire. Voltages of this level often cause damage to solid-state electronic devices. The heating or burn through of metal sheets such as airplane wings or metal roofs is, to first approximation, proportional to the lightning charge transferred (average current times time). Generally, large charge transfers are due to long-duration (tenths of a second to seconds) lightning currents in the 100- to 1000-A range rather than to peak currents that have a relatively short duration. An example of a hole burned in an aircraft skin by lightning is shown in Figure 5.4, and some information on hole size versus charge transferred is given in Figure 5.5. A typical lightning transfers 20 to 30 C and extreme lightnings hundreds of coulombs, but, fortunately, the lightning does not often stay attached to one place on an aircraft in flight for the duration of that transfer. The heating of many objects and the explosion of insulators is, to first approximation, due to the value of the action integral. In the case of wires, the action integral represents the heat that is generated by the resistive impedance of the wire. Some data on wire temperature rise for typical lightning action integrals is given in Figure 5.6. About 1 percent of negative strokes to ground have action integrals exceeding 106. About 5 percent of positive strokes is thought to exceed 107. In the case of a tree, this heat vaporizes the internal moisture of the wood, and the resultant steam pressure causes an explosive fracture. An example of typical tree damage from lightning is shown in Figure 5.7. Two properties of the electromagnetic fields are sufficient to describe most of the important damage effects: the peak value of the field and the maximum rate of rise to this peak. For certain types of antennas or metal exposed to the lightning field, the peak voltage on the metal is proportional to the peak field. These antennas are commonly referred to as capacitively coupled. For other antennas, such as a loop of wire in an electronic circuit or an underground communication cable, the peak voltage is proportional to the maximum rate of change of the field. New Results on Lightning Characteristics Chapter 2 (this volume) by Krider describes the recent findings on lightning current and field characteristics. Much of this work has application to protection. A major step forward has been made in identifying the maximum rates of change of currents and fields, and it should be noted that these are now thought to be at least 10 times larger than was believed to be the case a decade ago. These recent results have important implications for the design of protection against damage that is caused by fast rates of change of currents and fields. Chapter 3 (this volume) by Rust discusses positive lightning, recently identified and only partially characterized. Positive lightning apparently produces very large peak currents, charge transfers, and action integrals, much larger than the usual negative lightning. The Japanese report that their power systems are dis
APPLICATION OF ADVANCES IN LIGHTNING RESEARCH TO LIGHTNING PROTECTION 65 3 Figure 5.3 Lightning damage caused by a direct strike to a golf course green (photo courtesy of Weatherwise). Figure 5.4 A lightning hole burned in the wing tip of a Boeing 707 (Uman, 1971).