Emerson Process Management ControlWave XFC - Manual (Page 173)

Section 4 - Lightning & Surge Page 4-1 S1400CW
Section 4 - Lightning Arresters & Surge Protectors
4.1 STROKES & STRIKES
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 an 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 a 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 (I
2 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 of 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.
Figure 4-1 - Average Thunderstorm Days of the Year (for Continental USA)