Abstract: The insulation level of DC system is often damaged due to aging of power cable insulation, damaged construction, damp accumulation and faulty wiring, which affects the safe and stable operation of units. In the light of the phenomenon that two sets of DC insulation monitoring devices are given an alarm at the same time but both sets of DC bus voltage are normal during the operation of unit 2 of a power plant, it is found that there is a loop network fault in the DC system, and this phenomenon is theoretically analyzed, the fault rules of DC system loop network are summarized, and some suggestions for reliable operation of DC system are put forward.

Key words: DC power supply system, loop net fault, cable insulation