NOTE: If CRL Is < 1000, protection against transients is required
DESCRIPTION
FENV = environmental factor from table 443.1
Lp = risk assessment length
Ng = lightning ground flash density fron figures 44.2
Lp = 2 Lpal + 0.4 Lpah + 0.2 Lpch
Lpal = length in KM of lv overhaed supply