Three-phase four-wire power supply line null line disconnection detection system and method

The invention provides a three-phase four-wire power supply line null line disconnection detection system. The system comprises detection equipment, a null line voltage injection device and a three-phase phase voltage acquisition device. The null line voltage injection device injects a bias voltage to a null line in a three-phase four-wire power supply line according to a voltage injection instruction issued by the detection equipment. The three-phase phase voltage acquisition device acquires the three-phase phase voltage after the bias voltage is injected into the null line and the three-phase phase voltage before the bias voltage is not injected into the null line. The detection equipment determines an injection time period in which the bias voltage is injected into the null line, and issues a voltage injection instruction when the injection time period is up; and the detection equipment judges whether the null line is broken or not according to the three-phase phase voltage after the bias voltage is inject