Working Principle When measured piece Cx has partial discharge under testing voltage the circuit will have pulse current which get by the input unit. The pulse signal is amplified by front amplifier and the frequency band is chose by filter amplifier then been amplified to needed amplitude by the main amplifier. The pulse signal will be displayed on oscillograph screen. The pulse value is also sent to the pulse peak meter and displayed immediately. The time window unit chooses a working period of the pulse peak meter during each cycle of testing voltage and it increases the brightness of the relevant displaying area in this time. In this way it can avoid the disturbing of fixed phase. Input discharge volume to the measured piece Cx through JZF series PD external calibrator adjust the fine adjusting knob till the displayed value is equal to the discharge volume the discharge can be read directly and conveniently on the meter without counting. The testing voltmeter will have an over zero mark signal of testing voltage through the voltmeter resistance R and the zero mark pulse will be displayed on oscillograph screen. The testing voltage can be displayed on KV meter.