ABSTRACT
For the backflash, the highest voltage is on the tower rather than on the conductor and flashover appears to occur from the tower or ground to the conductor. The flashover is backwards from that in the laboratory, thus the term backflash. This chapter aims to present simplified equations to estimate the impulse or high-current resistance of concentrated grounds and to attempt to examine the impulse resistance of counterpoises. High magnitudes of lightning current, flowing through the ground resistance, decrease the resistance significantly below the measured low-current values. Soil resistivity and thus footing resistance depends largely on the water content of the soil and the resistivity of the water; these values will vary considerably with weather conditions. The chapter discusses ground-rod and counterpoise resistances that are assuming a constant value of soil resistivity. However, rarely is the soil resistivity constant with depth or location.
