| 摘要 |
PROBLEM TO BE SOLVED: To prevent a circuit from being burnt by setting a circuit-shutting relay between an injection transformer and a voltage impression circuit and a control circuit for shutting the relay when a wiring current is not smaller than a ground fault current. SOLUTION: A circuit-shutting relay 4 which is always closed is interposed between an amplifier AMP 2 and an injection transformer OT. The relay 4 is connected to a relay control circuit 7 consisting of a time constant circuit 5 and a comparator 6. The time constant circuit 5 stores for a fixed time a signal level of a leak current detected by a zero phase rectifier ZCT. The comparator 6 detects when a voltage of a capacitor C1 is not smaller than a ground fault current level. When a ground fault takes place at a wiring line 2, the time constant circuit 5 instantaneously detects a high level signal corresponding to a current of the fault. If the level exceeds a comparison reference of the comparator 6, an output of the comparator 6 is inverted and the relay 4 is urged into an OFF state. In consequence, the relay 4 shuts between the amplifier AMP 2 and injection transformer OT instantaneously thereby protecting the circuit. |
