Site Considerations for Equipment Installation, Grounding, and Wiring Manual
D301452X012
October 2019
18 Lightning Arresters and Surge Protectors
Figure 4-1: Chance of Being Struck by Lightning
Thunderstorms are cloud formations that produce lightning strikes. Across the United States there
is an average of 30 thunderstorm days per year. Any given storm may produce from one to several
strikes. Data on the subject indicates that for an average area within the United States there can be
eight to eleven strikes to each square mile per year. The risk of strikes activity is increased for
various areas such central Florida where up to 38 strikes to each square mile per year are likely to
occur.
To determine the probability of a given structure (tower, building, etc.) within your location being
struck, perform the following computation:
1.
Using the map in Figure 4-1 (or a comparable meteorological map for your location), find the
isokeraunic level (I) for your area. Then using Chart 1 (located below), find A for your area.
2.
Refer to Figure 4-1 to find the latitude. Then using Chart 2, find B for your latitude (Lat.°).
3.
Multiply A x B to get C.
4.
To calculate the number of lightning strikes per year likely to strike a given object (tower,
mast, etc.), use the following equation (where C was calculated in step 3 and H is equal to the
height of the object):
Strikes Per Year = (C x H2) ÷ (0.57 x 106)
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