Return stroke current magnitude and strike distance (length of the last stepped leader) are interrelated. A number of equations have been proposed for determining the striking distance.
The principal ones are as follows:
- S = 2 x I + 30(1−e − I / 6.8) – Darveniza (1975)
- S = 10 x I0.65 – Love (1987; 1993)
- S = 9.4 x I2/3 – Whitehead (1974)
- S = 8 x I0.65 – IEEE (1985)
- S = 3.3 x I0.78 – Suzuki (1981)
S is the strike distance in meters
I is the return stroke current in kiloamperes
It may be disconcerting to note that the above equations vary by as much as a factor of 2:1. However, lightning investigators now tend to favor the shorter strike distances given by Equation 4.
Anderson, for example, who adopted Equation 2 in the 1975 edition of the Transmission Line Reference Book (1987), now feels that Equation 4 is more accurate.
Mousa (1988) also supports this form of the equation. The equation may also be stated as follows:
I = 0.041 x S1.54