Site Considerations for Equipment Installation, Grounding, and Wiring Manual
D301452X012
October 2019
Lightning Arresters and Surge Protectors 17
Section 4: Lightning Arresters and Surge
Protectors
This section covers the following topics:
▪
Strike
▪
Use of Lightning Arresters and Surge Protector
4.1 Strike
Lightning takes the form of a pulse that typically has a 2 µS rise and a 10 µS to 40 µS decay to a 50%
level. The IEEE standard is 8 µS by 20 µS waveform. The peak current will average 18 KA for the first
impulse and about half of that for the second and third impulses. Three strokes (impulses) is the
average per lightning strike. The number of visible flashes that may be seen is not necessarily the
number of electrical strokes.
A lightning strike acts like a constant current source. Once ionization occurs, the air becomes
luminous conductive plasma reaching up to 60,000° F. The resistance of a struck object is of little
consequence except for the power dissipation on the object (I2 x R). Fifty percent of all lightning
strikes will have a first impulse of at least 18 KA, ten percent will exceed the 60 KA level, and only
about one percent will exceed 120 KA.
4.1.1 Chance of Being Struck by Lightning
The map in Figure 4-1 shows the average annual number of thunderstorm days (isokeraunic level)
for the various regions within the continental U.S.A. This map is not representative of the severity
of the storm or the number of lightning strikes since it does not take into account more than one
lightning strike in a thunderstorm day. The isokeraunic (or isoceraunic) number provides a
meteorological indication of the frequency of thunderstorm activity; the higher the Isokeraunic
number the greater the lightning strike activity for a given area. These levels vary across the world
from a low of 1 to a high of 300. Within the United States the Isokeraunic level varies from a low of
1 to a high of 100.
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