Abstract: The 10kV and 35kV systems are usually ungrounded systems. If the single-phase grounding fault is not eliminated for a long time, it not only threatens the safe operation of the equipment, but also the high voltage is easy to cause the expansion of equipment safety hazards and equipment shutdown. Therefore, the 10kV and 35kV ungrounded systems are usually equipped with small current grounding line selection devices. As the substation operator, the principle, operation principle and abnormal handling of the small current grounding line selection device should be accurately mastered during the switching operation. This paper mainly analyzes an accident case of the small current grounding line selection device misoperation, so as to prevent the occurrence of misoperation events and equipment damage caused by inadequate operation and maintenance.