PURPOSE: To protect a continuously-operated load by controlling a power unit so that no voltage difference is caused between the output voltage of a main power circuit and an auxiliary power circuit by a voltage difference limiting circuit to satisfy a latch-up condition to a continuously-operated load.

CONSTITUTION: The service voltage difference between a main power circuit 6 and an auxiliary power circuit 7 is limited to the voltage difference to inhibit the occurrence of a latch-up condition by a voltage difference limiting circuit 12 in which by use of the auxiliary power circuit 7 connected to a constant- supply battery power source 5 as the power source a there is provided with continuously-operated load 2 introducing overcurrent breakdown which is caused by the occurrence of a latch-up condition when the signal voltage receiving from a timely-operated load 3 connected to the main power circuit 6 becomes higher at a predetermined value than the output voltage of the auxiliary power circuit 7, between the main power circuit 6 and the auxiliary power circuit 7. Thus the device can protect the continuously-operated load 3 such as car- mounted control unit which must not have any failure and trouble from the stand point of safe traveling of trains.