Impulse flashover of particle-contaminated spacers in compressed sulphur hexafluoride

The impulse flashover stresses of coaxial-disc spacers, both clean and when contaminated with conducting particles, have been measured for pressures of the ambient SF6 gas between 0.47 and 1.5 MN/m2. Clean spacers had a higher flashover stress on positive than on negative polarity, but, for contaminated spacers, the positive stress was reduced much more than the negative stress. The effect of particles increased with their size over the 35¿210 ¿m range used in these experiments, and was greater at the higher gas pressures. Insulating particles of similar sizes had little effect. Experimental results followed a Gaussian distribution, and could be extrapolated to give stresses at which the flashover probability was very low.