JPH0535833B

An apparatus for detecting degradation of a grounded arrester, including a wave conversion circuit for producing an output signal corresponding to the total leakage current flowing through the arrester, and a phase shifter circuit coupled to the output signal for providing a capacitive-component leakage current signal, which is cancelled from the total leakage current signal by means of a differential amplifier to produce a resistive-component leakage current signal. According to one embodiment, this latter signal is applied to a synchronous rectifier circuit for synchronous rectification of the output signal from the differential amplifier circuit. A real resistive-component leakage current signal is then detected by adjusting the phase shift amount in the phase shifter circuit so that the output signal from the synchronous rectifier circuit may become zero, thereby bringing the phase of the cancelling wave signal in agreement with the phase of the capacitive-component leakage current signal. In another embodiment, the resistive-component current leakage signal is applied to a multiplier and a time averaging circuit, the output of which is likewise nulled by means of the adjustable phase shifter.