Pulse discharges preceding sparkover at low voltage gradients

Breakdown of gases between clean metallic electrodes has been studied extensively and has resulted in an acceptable explanation for sparkover where the gradient is of the order of 30 kv/cm (kv per centimeter) or more, but it does not explain the sparkovers which occasionally occur in high-voltage apparatus where the gradient appears to be nearer 5 than 30 kv/cm. A previous paper1 has shown that for negative corona, sparkover voltage is a function of the resistivity of any coating on the anode or passive electrode and can be especially low if the resistivity is high. This type of sparkover was found to be preceded by repetitive current pulses. Each pulse was found to have the same current-time characteristics and the same velocity of propagation as the single prebreakdown streamer described by Meek and Craggs2 and Parks and Cones,3 but which were produced with clean electrodes and at higher gradients.