| 摘要 |
PROBLEM TO BE SOLVED: To detect the leakage and ground fault of a load apparatus securely and cut off a line by a method wherein, when the width of a pulse signal outputted from a level discriminator exceeds a predetermined value, a leakage alarm signal and/or an output signal for cutting off an AC line are/is generated. SOLUTION: The output waveform of a low-pass filter 6 having a resonance frequency is so shaped as to make a level discriminator 7 discriminate a ground fault easily even if the polarity of the peak side of the output of the low-pass filter 6 is opposite to the discrimination polarity of the level discriminator 7. A phase control waveform which is obtained when a continuity angle is changed within a range about 45-180 degrees by a power converter, etc., using thyristors, etc., includes harmonics which have maximum amplitudes in a frequency range of 2-4 times of an input frequency. If the resonance frequency is set within a range of 100-240Hz, the low-pass filter 6 resonates the harmonics and hence the phase control waveform has opposite polarity parts, so that the level discriminator 7 can discriminate the ground fault current regardless of the polarity of the ground. |
