Electric breakdown of SF6 in nonuniform fields

SEVERAL INVESTIGATORS have shown that in a nonuniform field the positive direct breakdown voltage versus pressure characteristic of sulfur hexa-fluoride (SF6) exhibits a very pronounced maximum. Pure nitrogen (N2) does not show such a maximum. This unexpected performance is best illustrated by Fig. 1. Although the positive direct breakdown voltage goes through a maximum, the positive impulse breakdown voltage is much lower over a considerable pressure range. This produces an impulse ratio as low as one half. At the same time, the positive direct voltage required for breakdown may be as high as six times that for nitrogen. In the region of this maximum, corona occurs at a voltage much lower than breakdown. The 60-cycle crest breakdown voltage practically coincides with the positive direct breakdown voltage.