Breakdown voltage characteristics of SF/sub 6//CF/sub 4/ mixtures in uniform and non-uniform field gaps

Binary gas mixtures consisting of sulfur hexafluoride (SF/sub 6/) and carbon tetrafluoride (CF/sub 4/) have been used for several years in the design of circuit breakers and electrical switchgear manufactured for operation in geographic regions of extremely low temperature. CF/sub 4/, which has a liquifaction temperature comparable to that of nitrogen (N/sub 2/) greatly reduces the risk of liquifaction and the loss of insulation of electrical equipment when mixed with SF/sub 6/ at practical pressures (200-400 kPa). This paper presents experimental results on breakdown characteristics for various mixtures of SF/sub 6//CF/sub 4/ at practical pressures. Data for power frequency alternating and positive polarity direct voltages, as well as standard lightning impulse voltages are presented for uniform, quasi-uniform and highly non-uniform electrode configurations and for various gap lengths.