PURPOSE: To improve the reliability and to stabilize the characteristic by providing a switching transistor at one side of a shutdown relay and a relay off circuit against an abnormal signal at the other side thereby forming a double protection circuit.

CONSTITUTION: A shutdown relay 9 is inserted to the collector side of a transistor 8 wherein the base of the transistor 8 is communicated through a diode 7 to a delivery temperature relay 1 and a thermal relay 2 for detecting the overcurrent. A hold circuit 3 connected with the sensor relays 1, 2 is connected to the shutdown relay 9 at the opposite side from the transistor 8. Consequently, as the thermal relay 2 for detecting the overcurrent or the delivery temperature relay 1 functions, one side of the shutdown relay 9 is held at high potential and the current circuit at the other side is interrupted by means of the transistor 8, thereby the turn-off operation of the shutdown relay 9 is performed reliably even if a circuit at the transistor 8 side or the hold circuit has failed.