PROBLEM TO BE SOLVED: To protect a load an a power unit by stopping the power supply to the load upon detecting disconnection (including unconnected states) of feeders and voltage detecting lines. SOLUTION: A voltage detecting section 9 outputs an overvoltage detecting signal 9c, upon detecting disconnection of a positive feeder 4a and a negative feeder 4b from the voltage, between positive feeder terminal 2a and a negative voltage detecting line connecting terminal 2c and between a negative feeder terminal 2b and a negative voltage detecting line connecting terminal 2d, when the voltages exceed a disconnection detecting voltage. The section 9 also outputs a voltage drop detecting signal 9b upon detecting disconnection of voltage detecting lines 5a and 5b from the both-terminal voltage of a resistor R0 for detecting load when the voltage has become nearly zero. A control section 8 stops the output of a switching control signal 8a, based on the overvoltage detecting signal 9c and voltage drop detecting signal 9b. Consequently, the operation of a power converting section 6 is stopped, and the power supply to a load 3 is stopped.
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