PURPOSE:To detect only one-line ground fault reliably by blocking output from a current differential element employing zero-phase current based on the output from at least one of means for detecting the magnitude of zero-phase current in respective feeders. CONSTITUTION:A neutral point is grounded through a resistor 11 for limiting ground current. Outputs from CTs 4-1-4-3 on feeders 3-1-3-3 connected with a bus 2 are sampled and held 6 in a digital bus protective relay 1 through auxiliary current transformers 5-1-5-3, then they are multiplexed 7 and A/D converted 8 and fed to a CPU 9. The CPU 9 executes logic operation through a current differential element 87S employing phase current, a current differential element 87G employing zero-phase current, and a ground overcurrent judging element 51G for judging the magnitude of zero-phase current in each feeder. Operation of current operating element employing zero-phase current is blocked based on the detection results of the magnitude of zero-phase current in each feeder. By such arrangement, protective operation is carried out reliably only for one-line ground fault.
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