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Polymer insulators are widely used aU over the world, owing to their excellent contamination performance, light weight, anti-vandalism characteristics, etc. Ageing deterioration is, however, inevitable with these insulators due to organic materials used for the weather shed and core, and mechanical andlor electrical breakdown ofthese insulators are a point of ongoing concern. Diagnostic methods to detect deteriorated polymer insulators are not established yet, although ClGRE SC 22 reported various techniques. Some serious erosions and shed punctures were observed on some of the polymer insulators removed from a 110-kV transmission tine in China. Diagnostic tests were made on these insulators in an outdoor test station and the results were compared with those obtained in an artificial fog chamber.