SPARKOVER IN THE ROD-PLANE GAP UNDER COMBINED DIRECT AND IMPULSE VOLTAGES

Experiments are described in which composite voltages, produced by sup erimposing positive and negative switching impulse voltages on pre-str essing positive or negative direct voltages, are applied to a rod-plan e gap of lengths up to Im. Changes in 50% sparkover voltages are descr ibed over a range of positive and negative pre-stressing voltages; it is shown that negative pre-stressing always reduces the composite spar kover voltage, but that positive pre-stress increases the positive com posite sparkover voltage and reduces the negative: one. Recordings of photomultiplier observations and 'still' photography of pre-breakdown corona have been made at the 50% sparkover level and the results relat ed to the sparkover characteristics. It is shown that the latter depen d on the way in which leader initiation is affected by the preceding D C corona.