To provide an insulating strength control structure wherein insulation between main circuits of different phases is coordinated, in a switch equipped with a stationary electrode and a movable electrode housed in an insulating case for every one phase.

Intermediate electrodes 7 are horizontally fixed to the electrodes respectively by screws 8 on the upper side of the projecting wall 3b of the outer surface of an insulating case 3 so that their tip parts 7a face the outside. Discharge gaps are formed between the tip parts of the respective electrodes, the tip part of the screw 8 protrudes to the inside of the insulating case 3, and a discharge gap G2 is formed between the tip part and the head part of a stationary spring 10 in an arc distinguishing chamber 9. Thereby, insulating strength can be controlled so as to satisfy the following; insulating strength between the main circuit and the ground is less than insulating strength between main circuits of different phases, which is less than insulating strength between main circuits.

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