PURPOSE: To discriminate sparking and short circuiting from the output levels of comparators in accordance with the output currents of photocouplers and to improve safety, by inserting the photocouplers in the electrical circuit in a dust precipitation part.

CONSTITUTION: A photocoupler 20 is inserted between buses 7 and 8, and a photocoupler 21 between buses 7 and 9, respectively. The emitters of the output transistors of the photocouplers 20, 21 are grounded with resistors 22, 35, and the counter ground side terminals of the resistors are connected respectively to comparators 23, 24 and comparators 25, 26. The output terminals of the comparators 23, 24, 25, 26 are all connected to any of the input terminals of a microcomputer chi 27. One of the output terminals of the chip 27 is connected with a buffer inverter 28 to the base of a transistor 30, and the collector of the TR 30 is connected with a relay 32 to a DC power source.