Similarity law and discharge mechanisms in pure, natural and moist SF/sub 6/ for positive polarity

The breakdown mechanisms in a quasiuniform field in SF/sub 6/ is considered for positive polarity. Deviations from the similarity law at high pressures are commonly ascribed to the presence of protrusions on the surfaces of electrodes. However, the gas composition effects must be pinpointed to explain the dielectric strength loss in the high-pressure range. Such deviations are caused by the addition of the usual contaminants (O/sub 2/, H/sub 2/O) to pure SF/sub 6/. The pure SF/sub 6/ experimental breakdown characteristics have been found to lie above the predicted curve deduced from the usual critical avalanche criteria. Both effects are explained on the basis of mechanisms of discharge development and recent knowledge about streamer propagation in SF/sub 6/.<>