PURPOSE: To sharply prolong the life of a relay and ensure its miniaturization and high reliability by preventing the relay from always continuing operating at normal time, i.e. at non-power failure.

CONSTITUTION: When power is normally supplied from a power supply in normal service, i.e. when a switch SW2 turns on at non-power failure, a transistor Tr1 is ON and a transistor Tr2 is OFF and a relay Ry is not operated. The power from the power supply 3 is supplied to load through a transistor PT, rectifier R, and smoothing capacitor C1. Even when the switch SW2 turns on at power failure, Tr1 turns OFF and Tr2 turns ON and Ry is operated. This permits a contact relay Rb to open and a contact Rya to close. The load section is separated from the power supply in normal service 3 and automtically connected to an emergency power supply 4