The capacitance current to ground in a non-earthing system is increased due to cables, which results in the low-frequency nonlinear oscillation caused by the single-phase earthing fault and hence leads to the fuse of potential transformer (PT), seriously affect the safe operation. The principle of PT fuse caused by low-frequency nonlinear oscillation is analyzed, and a 10 kV neutral point ungrounded system is simulated with ATP-EMTP software. Calculations show that the maximum overvoltage of low-frequency nonlinear oscillation is less than 2.0 p.u.; after 0.1 s of fault occurse, the capacitance relative to the earth increases, the zero-sequence resistance decreases, and the fuse risk increases. Simulation results show the effectiveness of suppression measures for PT with four wires and the neutral point at its high voltage side in series with a varistor. Measurement errors of the zero-sequence voltage are also calculated. Finally, the platform of a 10 kV system for low-frequency nonlinear oscillation tests is built to verify the validity. It is valuable to improve the safety of PT operation in distribution network.