over voltages due to switching or lightning may
cause damage for the transmission line insulators, transformers
and switchgear. The flow of overvoltage to earth through
transmission system towers causes an increase in the potential of
metal structure and the earth potential and may create back
flashover voltage causing failure to transmission system
insulators in which transmission line outages occur. In this paper
the effect of tower grounding surge impedance on the back
flashover is investigated. The coupling factors between the
shielding wires and the healthy conductors are considered.