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APPLICATION OF ADVANCES IN LIGHTNING RESEARCH TO LIGHTNING PROTECTION 66 rupted by a large fraction of the positive lightning strike, whereas only a small fraction of negative lightnings have this effect (Nakahori et al., 1982). Figure 5.5 Area of holes melted through aluminum and titanium of various thicknesses by lightning charge (Fisher and Plumber, 1977). Protection Techniques There are two general types of lightning protection: (1) diversion and shielding and (2) limiting of currents and voltages. On a residential or commercial building, for example, the diversion of lightning currents to ground via a standard system of lightning rods, down leads, and grounds is sufficient to protect the building structure itself and to decrease by imperfect shielding potentially harmful effects to electronic equipment inside. An example of a diversion and shielding system is shown in Figure 5.8. More complete protection of electronic equipment must include limiting of currents and voltages induced by the direct strike to the structure or by traveling waves into the structure on electric power, communication, or other wires connected to the outside world. The design of the current-and voltage-limiting system is obviously dependent on an understanding of the wave shapes of the deleterious signals that are to be controlled; and this in turn requires a knowledge of the lightning character Figure 5.6 Temperature rise in various types of wires of various cross-sectional areas for two values of action integral (Fisher and Plumer, 1977).
APPLICATION OF ADVANCES IN LIGHTNING RESEARCH TO LIGHTNING PROTECTION 67 istics and how the properties of the system under consideration change these characteristics. Once such a determination is made, three general types of current-and voltage-limiting devices can be used for electronic or power systems: (1) voltage crowbar devices that reduce the voltage difference effectively to zero and short circuit the current to ground (the carbon block and gas tube arrestors used by the telephone company are good examples of crowbar devices); (2) voltage clamps such as recently developed solid-state metal oxide varistors (MOVs) or Zener diodes, which do not allow the voltage to exceed a given value; and (3) electric filters that reflect or absorb the higher and generally more damaging frequencies in the lightning transient. Frequently, all three of these forms of protection are used together in a coordinated way. Examples of some of these protective devices are shown in Figure 5.9. Figure 5.7 Typical damage to a tree due to a direct lightning strike (Uman, 1971). In recent years, a systematic approach has been developed that allows an optimal lightning protection system to be designed for most structures. This new technique is called ''topological shielding," and it uses both diversion and shielding and the limiting of currents and voltages discussed above. The technique consists of nesting shields and "grounding" each shield to the one enclosing it. All incoming wires are connected to the outside of each successive shield by a transient protective device, and therefore, at each successively inner shield, the voltage and power levels to be protected against are reduced. In Figure 5.10, we illustrate the principles of topological shielding. In Figure 5.10a, the equivalent