| 摘要 |
PROBLEM TO BE SOLVED: To avoid increase in the ground fault current enough to generate heat, even if the ground fault is left standing for a long time by disconnecting a ground circuit, when the unbalance of phase voltages to ground continues for over a fixed time. SOLUTION: A voltage sensor 40 always measures the voltage in a phase S to ground at a voltage measuring part 42. In the event of the ground fault, e.g. at the R phase side, the ground fault current flows in large amount through a fixed resistor 34 but a little through a fixed resistor 32 and hence the voltage at the S phase to the ground is larger than the voltage at the R phase to the ground. This actuates an overvoltage relay switch 60s to set on its normally closed contact 60a, causing a timer relay 54 to be turned on. After a preset time has lapsed, delay contacts 54a, 54b function, an indication lamp 50 and a buzzer 52 function and a normally closed contact 56b of a ground fault cancel relay 56 turns off to disconnect a variable resistor 36 from a ground fault line 16, resulting in the stoppage of a flowing ground current.
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