Thus one shield wire may be adequate for areas of
low ground flash density, whereas two shield wires are required in areas of higher
lightning activity. Therefore, a design based on a nonzero value of SFFOR is suggested.
The actual design value of SFFOR must be the prerogative of the designer so
as to permit economical designs. For lines serving critical loads, a design value of
0.05/100 km-years may be suitable