As an
illustration of the use of Figs. 17 and 18, assume a design value of SFFOR of 0.051
100 km-years in an area having a ground flash density of 10. For an average phase
conductor and shield wire height of 30meters and a critical current of 5 kA, a
shielding angle of about 8' is obtained from either figure. For these same conditions
but for a ground flash density of 1, a shielding of 17' to 18O results.