In the three-phase four wires system, the neutral line is uncharged under normal circumstances, because any point of the neutral line and the earth are of the same potential. However, when the zero line is disconnected, the neutral point on the load side will generate displacement voltage. At this time, if people touch the broken zero line, the current will form a path from the phase line load zero line human body zero point phase line, which is extremely dangerous and the rise of zero line voltage will also cause damage to electrical equipment. In order to solve the harm of zero line fault, this paper puts forward a zero line fault early warning device, which includes voltage acquisition, logic judgment ,attention device and telecommunication equipment, This device can effectively detect the fault points with poor zero line contact, alarm the fault, prompt the staff to check, reduce economic losses and casualties, and improve the safety of power use.