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Application of Advances in Lightning Research to lightning Protection INTRODUCTION MARTIN A. UMAN University of Florida Significant advances in lightning protection have been made during the last decade. These advances have been a result of progress in two general areas of lightning research: (1) lightning phenomenology, including the technology for determining real-time strike locations, and (2) lightning physics, particularly the characteris- tics of return stroke currents and electromagnetic fields (see Krider, Chapter 2, this volume, for a description of the return-stroke phase of a lightning flash, as well as of the other salient events that make up the flash). (1) By phenomenology, we mean those characteristics of thunderstorms that are associated with numbers of lightning events, as opposed to the physical properties of the individual events. A phenomenological parameter of particular interest is the average lightning flash den- sity, that is, the number of lightnings per square kilome- ter per year (other units are possible) as a function of location. This parameter represents the starting point for almost all lightning protection designs (for example, the lightning overvoltage protection of utility power lines) because the number of lightning failures per year for which a system is designed is directly proportional to the number of ground flashes per unit area per year. Real-time identification of phenomenological parame- ters such as the total number of lightning events per 61 storm and the lightning flashing rate is now possible with newly developed detection equipment. This equip- ment also makes possible real-time decisions on utility system repair and repair preparation, early warning and detection of lightning-caused forest fires, and a va- riety of other warning functions in situations that allow protective action to be taken, such as launches at the NASA Kennedy Space Center. (2) When an object (e.g., aircraft, building, power line, or person) is struck directly by lightning, or is ex- posed to the intense electromagnetic fields of a nearby flash, the potentially deleterious currents and voltages that appear in the object are determined by the physical characteristics of the lightning currents and fields and by the electric characteristics of the object that is struck. For example, it is thought that, to a first approximation, the voltages that are induced in electronics within an airborne metal aircraft that is struck by lightning are indirectly initiated by the fastest part of the current rate of rise. This fast change in current induces resonant os- cillations on the metallic exterior of the aircraft (like a pestle striking a bell) that are then coupled inside the aircraft via holes or apertures, such as windows, in the conducting metal skin. Lightning protection is cur- rently of considerable concern for the latest generation of military and commercial aircraft that operate with low-voltage computer circuits and have lightweight ep

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62 oxy surfaces (potential apertures) replacing the more- conventional conducting metal. For these types of air- craft and other similar advanced systems, the microelectronic components used are often more easily damaged by lightning-induced voltages and the shield- ing 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-detec- tion techniques for protection; those properties of light- ning that cause damage, the mechanisms of lightning damage, and new methods of protection; and some re- maining 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 bet- ter lightning protection design for power lines. The EPRI research is being carried out using lightning-locat- ing technology recently developed through basic re- search (Krider et al., 1980~. For the initial part of the study a network of automatic lightning direction find- ing (DF) stations called the East Coast Network and op- erated 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 addi- tional portions of the United States and perhaps Can- ada. 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 develop- ment that has occurred since 1976. In addition, light- ning-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 pe- riod. The primary user of lightning-location data in the United States at present is the Bureau of Land Manage- ment (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 Cana- dian 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 consider- able savings in natural resources and in the cost of fight- ing the fires. BLM data are also disseminated in real time to all National Weather Service Offices in the west- ern region via AFOS, to the National Severe Storms MARTIN A. UMAN 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 dis- played in real time at the FAA Washington Air Route Traffic Control Center (ARTCC) in Leesburg, Vir- ginia, the National Weather Service Forecast Office in Albany, New York, and Langley Air Force Base in Hampton, Virginia. In addition to applications-oriented research, opera- tional 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 com- panies (e. g., Tampa Electric Company, China Power of Hong Kong) to missile launches (e.g., Kennedy Space Center, Vandenburg AFB) to sensitive military installa- tion (e.g., Buckley Air National Guard Base, Colorado, Cudjoe Key AF Station, Florida). In addition, lightning maps from these lightning locating systems are becom- ing widely shown on TV weather shows, as they are of- ten 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 cur- rents 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 light- ning 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 cur- rent, (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 propor- tional to the peak current. For example, a 50,000-A cur- rent injected into a 400-Q power line produces a line voltage of 20,000,000 V. Such large voltages lead to

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APPLICATION OF RESEARCH TO LIGHTNING PROTECTION _~2, W\ ~L o ~ 9 of it -~, 4-'~ ~ ~ ' irk ~ 1\ ~ at, Y!} ~ ~ _ 1~ ~ I've. MA C' FIGURE 5.1 A map showing the location (dots) of lightning direction finding (DF) stations in place in summer 1984. Connected circles around each DF indicate area of lightning coverage. The area of coverage along the East Coast is the SUNYA East Coast Network. 63

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64 ~5 425 325 _ 275 225 75 fl1lliill!lllll~lll!ll~IiIIit!Il c+~BllliliilllllllllIlllillillilT - _ 1 1 ! ~i 1 1- ! 1 1 11 ! i' ~ 1! ~ ! i I I ~ I I I I I ! ! i I I ~ I I I ! I ~ I I I . I I ! ! I I I I I . ! ! ! ! ! I I I ! I I ~ I j i I I I I | 1~1~ 50 100 ~ 50 200 250 300 350 RUGUST 8, 1979 13:00 TO 16 00 [EDST) 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 cir- cled. The two DF locations in the Tampa Electric Company's light- ning location system are identified. Map scale is in thousands of feet. 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 sys- tem 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 golf- course green marker is shown in Figure 5.3. The mag- netic forces produced by the peak lightning currents are large and can crush metal tubes and pull wires from walls. 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 = 10~ A/ see, 1000 V is generated across the wire. Voltages of this level often cause damage to solid-state electronic de- vices. 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 (aver- age current times time). Generally, large charge trans- fers are due to long-duration (tenths of a second to sec MARTIN A. UMAN onds) 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 infor- mation 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, fortu- nately, 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 in- sulators 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 im- pedance 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 posi- tive 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 light- ning is shown in Figure 5.7. Two properties of the electromagnetic fields are suffi- cient 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 ex- posed 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 re- cent findings on lightning current and field characteris- tics. 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 charac- terized. Positive lightning apparently produces very large peak currents, charge transfers, and action inte- grals, much larger than the usual negative lightning. The Japanese report that their power systems are dis

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APPLICATION OF RESEARCH TO LIGHTNING PROTECTION 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).

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66 900 ~ `,, 800 _ E E En a o s 700 _ 600 _ 500 _ ~ 400 _ o IL o 300 _ 200 100 MARTIN A. UMAN Al0.051cm(0.20in) rupted by a large fraction of the positive lightning / strike, whereas only a small fraction of negative light ' Dings have this effect (Nakahori et al., 1982~. / `' Ti 0.05 cm (0.02 0 in) / ,1~/{ .~? / / '' /AI 0. 1 60 cm,50.060 int Em.,) _ _ 1~/ ~ TO O. 1 60 ci0.060 in) 0 40 80 120 160 200 240 280 Ti 0. 1 0 2 cm (0.040 in) LIGHTNING FLASH CHARGE (COULOMBS) FIGURE 5.5 Area of holes melted through aluminum and titanium of various thicknesses by lightning charge (Fisher and Plumer, 1977~. 1 000 100 cot 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 in- side. 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 . .. l 1 1 11 I I I~ I I I1 I I I__1 I T I~ 10 1 I Cu\A`9~\Mg 0.5 1.0 1 000 iT2dt = 0.25 x 106 A2-S AREA (cm2) 100 10 II I ~, ~ II T - IT l I TI I . ~ ~ iI2dt=2.0x 106A2-S . . ~ 1: ~ ~- l l l l 1 1 1 T ~1 1 - ~1 T. 1 ~ \ 1 1s.s~. - 1 1 \ ' \ - ~9\ Al --~9 1.5 2.0 . ~ ~ . xS . ~S . ~ ~ id\ \\ \\ Cu\ 0.5 1.0 AREA (cm2) 2.5 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).

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APPLICATION OF RESEARCH TO LIGHTNING PROTECTION istics and how the properties of the system under consid- eration change these characteristics. Once such a deter- mination 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 cir- cuit 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 re- flect 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 67 FIGURE 5.7 Typical damage to a tree due to a direct lightning strike (Uman, 1971). coordinated way. Examples of some of these protective devices are shown in Figure 5.9. In recent years, a systematic approach has been de- veloped that allows an optimal lightning protection sys- tem to be designed for most structures. This new tech- nique is called "topological shielding," and it uses both diversion and shielding and the limiting of currents and voltages discussed above. The technique consists of nest- ing shields and "grounding" each shield to the one en- closing it. All incoming wires are connected to the out- side 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

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68 Air Terminal ~\t _-: (~r~nel'~r~tr~r ~7/ ~v er , ~r; ~-V 1~ FIGURE 5.8 A standard lightning protection system for a small structure. FIGURE 5.9 Examples of typical lightning protective devices: a and b are sealed spark gaps (crowbar devices); c is a solid-state metal oxide varistor; and d is a solid-state Zener diode. The diameter of c is about 1 inch. circuit is shown for the grounding of a building associ- ated with a communications tower. Figure 5. job shows an external view of the building after topological shield- ing, and Figure 5. lOc shows a schematic of the topologi- cal shielding technique. FUTURE RESEARCH NEEDED FOR IMPROVEMENTS IN PROTECTION The detailed physics of how lightning strikes a struc- ture, power line, or aircraft is still not well understood. The approaching lightning leader is not influenced by MARTIN A. UMAN Tower ; - Building l l ~ ~Lightnin9 _ | Grd. i__ a Metal Sheet Waveguide ~~> bonded Transient~ ~ / to sheet Protection ~1 / 13 JO ~1 | Metal hIeld [or underground service b Zone O (external environment) ~ MOOR 1 Shield 1 (building's Sleet metal) ( internal environment) ~ 1 ~ ~\ Transient Protection 04 \ Zone2 ( binet in de) =?~(cibinet, ~_< ~metal outside) Zone O | \ Zone 1 ad, \' ~/ Ground I Ground/ ~~\ ~ Shield 3 Zone 3 C FIGURE 5.10 A diagram illustrating the principles of topological shielding. a, Equivalent electric circuit for a grounded building served by power lines and a communications tower. b, External view of building after topological shielding. c, Schematic of the topological shielding. the object to be struck until it is perhaps a few tens to hundreds of meters away. At that time, an upward- moving spark leaves the object to be struck eventually and similar sparks may also leave nearby objects. The upward-moving spark connects to the downward-mov- ing leader attaching the leader to ground. (See Krider, Chapter 2, this volume, for a discussion of the attach- ment process.) When this process is better understood through basic research, we should be able to determine with higher probability what will and what will not be struck and to provide better lightning protection ac- cordingly. For example, the positioning of overhead

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APPL1CA T1ON OF RESEARCH TO LlGHTNlNG PROTECTION ground wires above transmission lines should be able to be optimized. More information is needed about the character of lightning currents, particularly those in processes other than return strokes. Is there, for example, an upper limit on the maximum rate of change of current? We need more data on positive lightning to be able to character- ize all aspects of it in a statistical way. Only then can it be taken account of properly in protection design. Much work needs to be done on the interaction of lightning currents and fields with objects like aircraft. For example, how are aircraft resonances affected by channel attachment? Computer models are now being developed with which to study these problems even in the presence of nonlinear discharge properties. CONCLUSIONS Basic research over the last decade has made possible impressive improvements in lightning protection. Those related to lightning detection and to the specification of current and electromagnetic-field wave shapes have 69 been discussed in this chapter. As with all research, new discoveries raise new questions. With the present inter- est In ~gntn~ng among scientists, due partly to recent successes and partly to the important unsolved prob- lems, we can expect continued progress in lightning pro- tection during the next decade. REFERENCES Fisher, F. A., and J. A. Plumer (1977). Lightning protection of air- craft, NASA Reference Publication 1008. Krider, E. P., B. C. Noggle, A. E. Pifer, and D. L. Vance (1980). Lightning direction-finding systems for forest fire detection, Bull. Am. Meteorol. Soc. 61, 980-986. Nakahori, K., T. Ogawa, and H. Mitani (1982~. Characteristics of winter lightning currents in Hokuriku District, IEEE Trans. PAS- 101, 4407-4412. Orville, R. E., R. W. Henderson, and L. F. Bosart (1983). An east coast lightning detection network, Bull. Am. Meteorol. Soc. 64, 1029-1037. Peckham, D. W., M. A. Uman, C. E. Wilcox, Jr. (1984). Lightning phenomenology in the Tampa Bay area, J. Geophys. Res. 89, 11789- 11805. Uman, M. A. (1971j. Understanding Lightning, BEK, Pittsburgh, Pa.