Experience from insulators with RTV silicon rubber sheds and shed coatings

Long-rod composite insulators, having weather sheds made of room-temperature vulcanizing silicon rubber compounds (RTV) were exposed for many years to HVAC (high-voltage AC) and HVDC (high-voltage DC) under realistic conditions and natural pollution. It was found that the shed material, contrary to the experience gained from insulators with sheds of other organic materials (e.g., ethylene polypropylenediene monomer rubber) undergoes a slow degradation which improves the already superior water-repelling properties of the silicon rubber compounds. The improvement seems to be due to a low molecular layer which is produced on the surface of the insulator sheds. This layer improves the hydrophobicity of the surface while protecting the surface from further degradation. Weather sheds of porcelain housings coated with a thin layer of RTV give similar results to those obtained with long-rod silicon rubber insulators. The RTV coating, although it led to increased salt-deposit density, reduces the leak currents and the withstand of the insulator under the same pollution conditions