Abstract:Once, an abnormal condition of 110V DC control power source caused the switch of the standby power source of PC bus of emergency power supply to trip. The reason of the bus voltage failure was analyzed, and it was found that there are many problems in the system design and device selection of the system. The solution was put forward, and the problem, which is that the DC power supply failure causes the switch in lower level to trip incorrectly, was solved thoroughly.