| 摘要 |
PROBLEM TO BE SOLVED: To prevent the burning of an insulation monitoring device by the carrying of an excessive earth fault current to a low-voltage cable run in the event of an earth fault accident in the low-voltage cable run in the insulation monitoring device for inserting, in order to monitor the insulation resistance to the earth of the low-voltage cable run subjected to a B-class earthing work, a signal transformer to the earthing conductor of the B-class earthing work to apply an insulation monitoring voltage to the low-voltage cable run. SOLUTION: A protective means 10 comprises a differential amplifier 12a, and the signal of an oscillator 22 and the output signal of an amplifier 23 are generally inputted thereto and mutually compared, and regulated so that the difference is zero. In the event of the earth fault accident, a voltage of commercial frequency generated on the P side of the signal transformer Ts is applied to the differential amplifier 12a, and only the voltage according to the earth fault current appears from this amplifier. When this level exceeds a set value, a circuit breaking relay 11 is operated to break a connecting circuit 24.
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