PURPOSE: For eliminating spark gap for example and decreasing the quantity of current switching elements, to use a vacuum switch for turning off a tap bridge circuit on tap change as the overvoltage protective element of a change-over switch.

CONSTITUTION: The load current of a transformer for example is fed to an external circuit from an output terminal A through a vacuum switch C. If the voltage flowing between taps 2 and 3 becomes higher than the withstand voltage value of said switch C due to surge inflow through said terminal 4, the electrodes of said switch C is shorted. Next, when said tap 2 is changed over to said tap 3, said switch A opens to switch load current to the electric circuit of resistor R1-fixed contact a1-movable bridge arm(a)terminal 4. Next, said switch C closes so that circulating current flows between said taps 2 and 3. Next, said movable arm(a) shorts contacts a1 and a2 and engagement is made only with a contact a2, said switch 3 closes so that load current is fed only from said tap 3 through a resistor R2, said contact a1 and an arm. Next, switching procedure is completed when the connection of said switch A is changed as indicated as dotted line.