Sparkover in the rod-plane gap under composite slow front positive impulsealternating voltages

Problems are discussed which relate to the measurement of the sparkover vol tage of an airgap under a composite voltage comprising an impulse superimpo sed on an alternating voltage. The rod-plane gap has been taken as an examp le. Sparkover values have been measured with the impulse generator fired at angles 0 degrees, 90 degrees, 180 degrees and 270 degrees in the alternati ng cycle. Tests have also been made with firing at random angles. Sparkover voltages do not differ significantly fi om those for simple impulse alone, except for the case of random firing: when a small apparent increase occur s. It is shown that the 'up and down' method of measurement is satisfactory where specific firing angles are used, but is unsatisfactory and misleadin g when firing is at random angles.