Abstract: Focusing on a event that the voltage switching device had incorrect operation caused by the inverse connection of the auxiliary contact of the disconnector.Conbined with the state table of auxiliary contacts under different operation mode, the signals from the device and the node poten- tial ,this essay carries out a defect analysis on the signal in terms of normal connection and incor- rect connection.Ultimately ,it verifies the concealment of the defect and portraies the process of solving problem and the measures to evade processing risks.