Sparkover in the rod-plane gap under combined direct and impulse voltages

Experiments are described in which composite voltages, produced by superimposing positive and negative switching impulse voltages on pre-stressing positive or negative direct voltages, are applied to a rod-plane gap of lengths up to 1 m. Changes in 50% sparkover voltages are described over a range of positive and negative pre-stressing voltages; it is shown that negative pre-stressing always reduces the composite sparkover voltage, but that positive pre-stress increases the positive composite 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 related to the sparkover characteristics. It is shown that the latter depend on the way in which leader initiation is affected by the preceding DC corona