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Insulator contamination is the decisive factor in making insulation design of most transmission lines in Japan. This paper describes the performance of contaminated insulators against a short-time ac overvoltage in the order of several cycles, taking into consideration comparatively higher overvoltage due to the one-phase-to-ground fault of recent long-distance bulk power transmission lines with series capacitors. It has been made clear by this study that, in the case of a fault clearance within 0.07 sec, e.g., the flashover voltage increases by about 15% more than in a continuous energization. This implies that the insulation of series-capacitor-compensated lines can be met by the conventional insulation design levels.