Sparkover in the rod-plane gap under composite slow front positive impulse/alternating voltages

Problems are discussed which relate to the measurement of the sparkover voltage of an airgap under a composite voltage comprising an impulse superimposed on an alternating voltage. The rod-plane gap has been taken as an example. Sparkover values have been measured with the impulse generator fired at angles 0°, 90°, 180° and 270° in the alternating cycle. Tests have also been made with firing at random angles. Sparkover voltages do not differ significantly from those for simple impulse alone, except for the case of random firing, where a small apparent increase occurs. It is shown that the `up and down´ method of measurement is satisfactory where specific firing angles are used, but is unsatisfactory and misleading where firing is at random angles