Abstract: At almost the same time interval of the past two years, the unit No.4 of a power plant tripped while there was no protection device acted. Aiming at the phenomenon, the fault report was checked; the trip circuit was inspected; the design paper was analyzed; and the simulated fault experiment was carried out. The reason of unit trip was determined finally that there was defect in the emergency lighting circuit, which caused the AC power source to intrude into the DC loop during switchover operation in fixed period and made a grounding fault of DC system, and the grounding fault caused the unit outlet switch to trip. The emergency lighting circuit had been improved; and the hidden danger of the AC power source intruding into DC loop was eliminated.

Key words: protection, trip, emergency lighting, ground fault of DC system