PURPOSE: A protecting device of a constant current regulator, an insulation resistor, and a single core detecting device for an air field lighting are provided to protect the constant current regulator and a serial lighting device by bypassing an overcurrent through a high voltage switch. CONSTITUTION: A voltage sensor(11) is connected in parallel to an output terminal of a constant current regulator and senses a voltage. A current sensor(12) senses a current value flowing in one terminal of an output terminal of the constant current regulator. A high voltage switch(13) shorts the output terminal of the constant current regulator. An operation circuit(14) calculates a root mean square value of the sensed current. A microprocessor(16) controls the short of the output terminal of the constant current regulator according to the comparison result of the root mean square value. An interface circuit(17) blocks the constant current regulator and outputs a signal to a control tower.
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