1 INTRODUCTION
In the preceding chapter, the geometric model of the last step of the lightning stroke
was introduced and used to determine the number of flashes to the shield wires. The
purpose of these shield or overhead ground wires is to act as collectors of the flashes
and insofar as possible to prevent flashes from terminating on the phase conductors
and causing a flashover. However, in the practical case, flashes cannot be totally
prevented from reaching the conductor, unless the phase conductor is completely
surrounded by shield wires. In addition, it may be uneconomical to shield the conductor
so that no flashovers occur. Therefore, the goal should be to locate the shield
wires so that a specific number of flashes result in flashover. For example, the goal
could be to shield the line so that the shielding failure flashover rate, the SFFOR, is
0.05 flashes per 100 km-year.