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This study, based on AC and DC flashover tests of porcelain, glass and composite insulators under the conditions of low air pressure and pollution, discusses the differences between AC and DC flashover performance and discharge process of ice-covered insulators. Results of the study indicate that the influence of air pressure on ice-covered insulator string's AC flashover voltage is greater than that on DC flashover voltage, whereas the influence of pollution on both AC and DC flashover voltage is almost the same. Good agreement is achieved between experimental results and the equivalent electrical circuit model proposed previously. Moreover, n (characteristic index of the influence of air pressure on ice-covered insulators' flashover voltage) differs slightly when calculated with the icing-water-conductivity method and solid-layer method, respectively, and the duration from the occurrence of local arc to flashover is longer under the condition of AC than DC. Although arc floats are more seriously under the condition of DC.