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OCR for page 61
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
OCR for page 62
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
OCR for page 63
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
OCR for page 64
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
OCR for page 65
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).
OCR for page 66
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).
OCR for page 67
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
OCR for page 68
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
OCR for page 69
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.
Representative terms from entire chapter:
lightning currents
