For general design, it is recommended that the design value of SFFOR be set at
0.05 flashovers per 100 km-year, that either the Brown-Whitehead or the IEEE-1992
equations be employed, and that only vertical strokes be assumed. These equations
provide a reasonable conservative limit to shielding angle. Again, these angles should
be considered as the average shielding angle along the span. That is, they are the
shielding angles at the tower height minus 213 of the sag. If more conservatism is
desired, the shielding angle may be considered as that at the tower.