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Effects of water immersion on electrical conductivity and permittivity as well as thermally stimulated depolarization (TSD) currents of two types of silicone rubbers are reported. The rubbers were room temperature and high temperature vulcanized and had different filler particle concentrations. The material with lower concentrations of filler particles (RTV) showed weaker sensitivity to the water immersion on the electrical properties than the highly filled material (HTV). It was also clearly visible on the variation of the TSD current spectra. By assuming a gradual formation of water layer on the filler particles the spectra were modelled and obtained agreement with the experimental results was good.

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Electrical properties of composite insulators, like specific flashover voltage and leakage current are determined by the surface wetting conditions. The paper deals with investigations concerning the influence of corona discharges and moisture on the hydrophobicity of silicone rubber surfaces. firther results due to the hydrophobicity transfer into an artificial pollution layer are presented.

Model‐insulations made of silicone‐rubber were tested with an ageing‐procedure according to the recommendations of IEC‐TC 36 in a fog‐chamber. The influences of the fog‐parameters (fog‐intensity, fog‐conductivity) on the flashover voltage as well as on the ageing process were investigated. The results enable the reproducibility to be improved. The problems combined with a continuous stress of the insulating surfaces in the fog‐chamber are discussed by means of measured values of the equivalent salt deposit density. To solve them an intermittent fog‐test is proposed and relevant criteria to evaluate the damage degree on the insulating surface are recommended.

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