Adapting the statistics of soil properties into existing and future lightning protection standards and guides

Risk estimates for lightning faults make use of log-normal distributions for lightning characteristics such as peak current. Simplifications for highly correlated parameters, notably peak current and rate of current rise, justify use of equivalent front time of 2 μs in backflashover calculations for first negative return strokes. The transmission line backflashover rate is also affected by uncorrelated and broad statistical variations in soil resistivity, tower footing impedance Z
_f
and resistance R
_f
. Statistical properties of R
_f
from transmission lines in Tennessee USA and Portugal are compared. Modeling of R
_f
variation using a ten-step distribution from the IEEE Standard 1243 FLASH program is compared with estimates using finer probability step intervals, log-normal and log-logistic models.