Impulse Breakdown of Negative Rod-Plane Gaps in Hydrogen and Hydrogen-SF6 Mixtures

Impulse breakdown voltages have been measured for negative rod-plane gaps in hydrogen and in H2/SF6 mixtures for pressures in the range 0.5-3.0 bar and electrode spacings up to 0.25m. In hydrogen, the negative-impulse breakdown voltages are 60-100% higher than those previously reported for positive polarity: However the increase in breakdown voltage with the addition of small quantities (⩽1%) of SF6 is smaller than for positive-impulse conditions and is confined to pressures below~H2 bar, with the result that the positive and negative impulse strengths of rod-plane gaps in such low SF6-concentration mixtures are almost equal. The results are discussed with reference to observations of the effect of SF6 on the prebreakdown corona activity.