Traditionally, the withstand voltage of an insulator string to lightning impacts is checked using the standard impulse 1.2/50 μs. First, the authors show that when a lightning stroke hits the tower or a ground wire, the resulting overvoltages are characterised by much shorter tail durations than the tail of incident lightning currents. The analysis of wave shapes, in particular tail durations of the overvoltages affecting the insulator strings, is carried out in the form of a detailed parametric study. Secondly, the authors describe the routine tests carried out in a HV laboratory to evaluate the withstand voltage of various insulator strings as a function of tail duration of the applied impulses. Five strings of different lengths were tested, with impulse tail durations between 50 μs and 4 μs. In conclusion, it clearly appears, when considering the mechanisms resulting in back flashover, that the withstand voltage of insulator strings is much higher than their conventional value for the 1.2/50 μs impulse.