Lightning impulse flashover characteristics of a typical 24 kV pin-type porcelain insulator are experimentally investigated aiming at a better understanding of the flashover mechanism as affected by the amplitude and polarity of the applied voltage. Flashover probability distributions under standard lightning impulses of both polarities were obtained and salient characteristics of the discharge inception, growth and flashover were measured. The insulator was overstressed to study the effects of the applied impulse voltage peak on discharge characteristics. Flashover occurs through a streamer-dominated mechanism involving however, necessarily, the inception of a small leader. Flashover characteristics are closely related to those of leader inception and growth; the latter is facilitated when overstressing the insulator. At relatively high applied voltages flashover occurs sooner and at lower voltages under negative than positive impulses.

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