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APPLICATION OF ADVANCES IN LIGHTNING RESEARCH TO LIGHTNING PROTECTION 62 oxy surfaces (potential apertures) replacing the moreconventional conducting metal. For these types of aircraft and other similar advanced systems, the microelectronic components used are often more easily damaged by lightning-induced voltages and the shielding against the intrusion of those voltages is often less adequate than is the case for more conventional systems. In the following three sections, we examine in more detail the recent and widespread use of lightning-detection techniques for protection; those properties of lightning that cause damage, the mechanisms of lightning damage, and new methods of protection; and some remaining questions that research can answer to facilitate additional improvements in lightning protection. APPLICATIONS OF NEW LIGHTNING-DETECTION TECHNIQUES TO PROTECTION In 1983 the Electric Power Research Institute (EPRI), the research arm of United States power utilities, funded a long-term study of lightning flash density in the United States for the purpose of making possible better lightning protection design for power lines. The EPRI research is being carried out using lightning-locating technology recently developed through basic research (Krider et al., 1980). For the initial part of the study a network of automatic lightning direction finding (DF) stations called the East Coast Network and operated by the State University of New York at Albany (SUNYA) (Orville et al., 1983) is being used. Future flash density studies can be expected to involve additional portions of the United States and perhaps Canada. As is clear from Figure 5.1 in which the East Coast Network is evident, over three quarters of the area of the United States and Canada is covered by DFs, a development that has occurred since 1976. In addition, lightning-locating systems of the DF type developed in the United States have been installed in Australia, Norway, Sweden, Mexico, South Africa, Japan, Hong Kong, and the People's Republic of China during the same time period. The primary user of lightning-location data in the United States at present is the Bureau of Land Management (BLM), which is responsible for the majority of the DFs in the western United States and Alaska (Figure 5.1). The BLM and the Forest Services of most Canadian provinces utilize the time and location of lightning storms to determine when and where to look for forest fires. Early detection of these fires results in considerable savings in natural resources and in the cost of fighting the fires. BLM data are also disseminated in real time to all National Weather Service Offices in the western region via AFOS, to the National Severe Storms Forecast Center in Kansas City, to Vandenburg Air Force Base, and to Nellis Air Force Base. Data from the SUNYA East Coast Network are currently being displayed in real time at the FAA Washington Air Route Traffic Control Center (ARTCC) in Leesburg, Virginia, the National Weather Service Forecast Office in Albany, New York, and Langley Air Force Base in Hampton, Virginia. In addition to applications-oriented research, operational forest fire management, and Weather Service storm warning, the newly developed lightning-locating equipment is used to warn of the approach of storms in a variety of practical applications where protective action can be taken. Examples range from power utility companies (e.g., Tampa Electric Company, China Power of Hong Kong) to missile launches (e.g., Kennedy Space Center, Vandenburg AFB) to sensitive military installation (e.g., Buckley Air National Guard Base, Colorado, Cudjoe Key AF Station, Florida). In addition, lightning maps from these lightning locating systems are becoming widely shown on TV weather shows, as they are often more meaningful to the typical viewer than the more conventional radar displays. An example of a 1- day lightning map from the Tampa Electric Company (Peckham et al., 1984) is shown in Figure 5.2. AMELIORATION OF LIGHTNING DAMAGE Mechanisms of Lightning Damage The amount and type of lightning damage an object suffers is due to both the characteristics of the lightning discharge and the properties of the object. The physical characteristics of lightning of most interest are the currents and electromagnetic fields, particularly those from the return stroke since these are usually the largest; hence protection against the return stroke will usually protect against the currents and fields from other lightning processes. Four properties of the return stroke current can be considered important in producing damage: (1) the peak current, (2) the maximum rate of change of current, (3) the integral of the current over time (i.e., the charge transferred), and (4) the integral of the current squared over time, the so-called action integral. Let us examine each of these properties and the type of damage that it can produce. For objects that present a resistive impedance, such as a ground rod driven into the Earth, a long power line, or a tree, the peak voltage on the object will be proportional to the peak current. For example, a 50,000-A current injected into a 400-â¦ power line produces a line voltage of 20,000,000 V. Such large voltages lead to
APPLICATION OF ADVANCES IN LIGHTNING RESEARCH TO LIGHTNING PROTECTION 63 Figure 5.1 A map showing the location (dots) of lightning direction finding (DF) stations in place in summer 1984. is Connected circles around each DF indicate area of lightning coverage. The area of coverage along the East Coast is the SUNYA East Coast Network. electric discharges from the struck object to the ground through the air or through insulating materials. Such flashovers can, for example, short-circuit a power system or kill people that are standing close to the object that is struck. An example of discharges across the ground caused by the high voltage on a struck golfcourse green marker shown in Figure 5.3. The magnetic forces produced by the peak lightning currents are large and can crush metal tubes and pull wires from walls.