Abstract: The disconnector of GIS is sealed in the shell,so it can′t directly observe the contact position.It can only judge the opening and closing position with the help of opening and closing indication and voltage and current signal.If the opening and closing of the disconnector is not in place due to the jam of the transmission mechanism and other reasons,the opening and closing indication and voltage and current signals can not reflect the real contact condition of the dynamic and static contacts,so it is very likely to cause further tripping accidents. A GIS discharge fault caused by the disconnector closing not in place was analyzed. Through field test and disassembly analysis,it was judged that the fault was caused by the serious corrosion of the disconnector mechanism,which led to the virtual connection,melting and dripping of the moving and static contacts after closing,and finally developed into the shielding cover to discharge the shell. Furthermore,the theoretical analysis and simulation calculation of the contact resistance and heating degree of the contact finger when the switch is not in place were carried out to verify the correctness of the fault cause analysis.Finally,the corresponding preventive measures were put forward,which provide references for the treatment of similar faults in the future.