This work chiefly presents the results of a laboratory investigation of ac leakage current and ac flashover on a short insulator string. The string was partially iced with supercooled droplets at -1.5°C simulating a rime fog; this being one of the factors responsible for a large number of flashovers on Ontario Hydro's 500 kV system in March 1986. It was found that the leakage current of insulators in the presence of short icicles was very small and consequently no flashover occurred under normal line-to-ground voltage applied to the insulators. Flashover was produced only under an applied voltage about 80% higher than normal voltage. It was concluded that the presence of fog was associated with the flashovers occurring in March 86, but that other parameters such as insulator surface contamination and water condensation were also active factors contributing to these flashovers.
Ice accretion, together with superimposed contamination often causes a decrease in the flashover voltage of outdoor insulators, resulting in occasional outages. Insulation failures, due to the ice-bridging of insulators are reported from the USA (Kawai, 1970), Canada (Melo et aI., 1988), Japan (Matsuda et aI., 1991), Norway (Fikke et al., 1992), China (Shu, 1991) and a few other countries. Ice is generally accreted during periods of sub-zero °C temperatures, by the impact of supercooled droplets on the surface of the insulators. Depending on the atmospheric conditions and the heat transfer between the impinging water droplets and the insulators, ice is basically formed in a dry or a wet regime. In a dry regime, all the droplets impinging on the surface of the insulators are completely frozen and there is no run-off. The temperature of ice deposition in dry growth is below O°C and ice formed is porous and opaque (Macklin and Payne, 1967).