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Investigations have been made of the performance of composite insulators because of the repeated flashover accidents on the lightly iced insulators in winter in the north of China. Based on the field research, a new test method was developed by taking into account the combined influences of the light ice accumulation and the secondary contamination deposition on the ice surface, and a different view of reproducing the atmospheric and environmental conditions in laboratory was provided accordingly. The test results confirmed that the icing flashover voltages of composite insulators with secondary contamination deposition were considerably lower than the contamination flashover voltage when the insulators were both precontaminated with the same equivalent salt deposit density (ESDD). In these areas where similar accidents happened, the increase in the insulation strength should be considered.