Abstract: With wide application of overhead insulated lines, the disconnection fault has risen to be one of the common faults in distribution networks. In this paper, aiming at ungrounded distribution system, a single-phase disconnection fault model was established. The fault type, including ungrounded fault, disconnected fault and power side or load side grounding fault, as well as the ratio of the capacitance to ground of the fault line, disconnection position, load impedance, grounding resistance of the sinking point, are the factors which was discussed in this paper. By taking these factors into account, the ground potential and zero-sequence voltage offset law at neutral point were analyzed. The changing characteristics of phase sequence, as well as the amplitude and phase of the three-phase voltage and line voltage, before and after the fracture were further deduced. The electrical characteristics of the simple single-phase grounding fault in the ungrounded distribution system were used to make comparison with it. The accuracy of the analysis results was verified by digital simulation. The results of this paper can provide a theoretical basis for the research on the detection and location methods of disconnected faults using voltage signals for distribution network.