The effects of back corona on the sparkover of a point-plane electrode system subjected to applied negative direct voltage and negative pulsed voltage has been studied. The reduction in sparkover voltage is caused by the surface potential gradient existing in the vicinity of the back discharge channel. This potential gradient facilitates the formation of a stable discharge in the contaminant and, at higher applied voltage levels, helps the back discharge to develop into space streamers. The reduction in sparkover voltage is dependent on the contaminant thickness in the range 125-375 μm. This is due to the conservation of energy at the streamer's tips. The average field strength in the air space at which positive streamers can propagate is approximately 5 kV/cm. The sparkover voltage measured under conditions of negative pulsed voltage is higher than that which is measured under direct voltage, this is concurrent with the average number of charge carriers being injected into the air space between cathode and the contaminant. © 1979.
