Study on the change of sparkover voltages of the standard sphere gaps by calculation of ionization index

Relation between the change of field utilization factor and the one of sparkover voltage

Abstract

The field utilization factor η (the mean electric field/the maximum electric field) of the standard sphere gaps was calculated by the charge simulation method, taking into account the ground plane and shanks. Sparkover voltages were also calculated by integration of ionization index, except the sparkover for positive impulse.
η and the sparkover voltage may change by the length and the diameter of shanks in the range permitted by the IEC standard 60052. Generally, the rate of the change of sparkover voltage is not larger than the one of η, because the sparkover voltage is averaged by integration of ionization index on the center axis between the sphere electrodes. In the range where the gap length is near the sphere radius, the change rate of η and the sparkover voltage is almost same.
The reason is that the streamer criterion is satisfied near the crest point of the upper sphere electrode.