Abstract: In this paper,a ground fault caused by lightning counterattack on C phase of a double-loop line in the same tower is introduced,and an accident in which a subsequent lightning overvoltage causes a breakdown in the C-phase fracture of a switch where one of the lines has been separated is also introduced. Based on an analysis of the action report,the failure sequence and the fault recorder waveform record of the protection device,the fact of the internal breakdown of the switch is accurately determined and the occurrence of major secondary accidents is avoided.It is proposed that multi-source information should be fused and judged after the fault occurs,the factors that cause failure must be studied in depth combined with fault waveform analysis,so as to make a reasonable and correct judgment on the cause and possible consequence of the fault more quickly.