The impulse waveshape produced by the stroke is the same as that of the stroke
current. Although the time to half value of this surge exceeds that of the standard
lightning impulse, and thus the CFO for this surge would be less than the standard
lightning impulse CFO, the CFO employed is usually assumed as the standard CFO,
negative polarity, which from Chapter 1 is 605 kV/m times the strike distance S.