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Part of DC arcs may levitate from the insulators' surface to form air-gap arcs in the process of insulator's flashover, especially in the low air pressure. The air-gap arcs can cause the bridging between ribs (or sheds) of insulators easily, and the U-I characteristics of air-gap arcs and surface arcs are different. Thus, the two aspects should be taken into account when evaluating the DC flashover performance of insulators at low air pressure. This study analyses the effect of arc-levitating on the DC flashover voltage of polluted insulators and calculates the insulators' DC flashover voltages with and without considering the effect of air-gap arc. The comparisons between calculated and experimental results show that the DC flashover voltages of polluted insulators are overestimated when the air-gap arcs are not taken into account.