Abstract: When the operating personnel of a certain power plant were testing the DC accident oil pump of the steam driven feedwater pump, the main steam valve of the steam pump automatically closed. The analysis shows that this fault is caused by a two point grounding in the DC system, which leads to the closure control circuit of the main steam valve of the steam pump. In response to the weak insulation points in the DC system, improvement measures such as replacing DC contactors, replacing control circuit cables, sorting out grounding wires, improving protection logic, etc., to eliminate potential faults and ensure safe operation of the equipment without any abnormalities.