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The impulse and switching surge flashover characteristics of a horizontal double-break disconnecting switch have been investigated. Starting with a single-pole arrangement the open-gap dimension was coordinated with the flashover of 1050-and 1300-kV BIL supporting post insulators for positive-polarity switching surge waves under dry conditions. In a similar manner the open-gap and pole-to-pole dimensions of a 3-pole arrangement were coordinated to the flashover of the insulators. With the terminal-to-terminal and pole-to-pole dimensions so determined, probability of flashover data was obtained for wet and dry conditions with a 150 X 5000-Â¿s wave to represent switching surges and a 1.3 X 50-Â¿s impulse wave. Critical flashover values were obtained during the probability tests. With the pole spacing of a closed switch coordinated with the insulator flashover a 50 X 3000-Â¿s bias voltage of opposite polarity was applied to the pole adjacent to the one being tested. Presented are derived mean slopes of the probability curves that may be used with confidence to select a withstand value based on a 50-percent flashover level.