PURPOSE: To reliably prevent the ferroresonance phenomenon of an instrument transformer (PT) by disconnecting an inter-pole capacitor from a break section when the fixed time elapses after the break operation and connecting the inter- pole capacitor in parallel with the break section concurrently with the turn-on operation.

CONSTITUTION: When a breaker is turned on, (a) contact points 1a1, 1a2 of the breaker are turned on, the optimum current limited by a resistor 10 flows through the driving motor 5 of a disconnecter 2, which is positively rotated and closed, and an inter-capacitor 3 is connected in parallel with a break section 1. When the breaker is turned off, contact points of the breaker are reversed, a timer 7 is operated by the (b) contact point 1b of the breaker, an auxiliary relay 6 is operated by the (a) contact point of the timer 7 after the fixed time, when the (a) contact points 6a1, 6a2 of the auxiliary relay 6 are closed, the optimum current limited by the resistor 10 flows through the driving motor 5 of the disconnecter 2 in the opposite direction, the disconnecter 2 is reversely rotated, the disconnecter 2 is opened, and the inter-pole capacitor 3 is disconnected from the break section 1. The ferroresonance phenomenon of the PT can be reliably prevented accordingly.