Lightning is still the main cause of transmission and distribution lines outages and represents an important problem for insulation design of power systems through the back-flashover phenomenon. This paper discusses the back flashover phenomenon on the Egyptian 500kV single circuit tower and 220 kV double circuit tower lines. The induced voltages across the insulator strings of the two lines are calculated. The effects of the lightning stroke magnitude and shape, the striking distances and the tower footing resistance on the induced voltage across the insulator strings are discussed.