The unit was operating at nominal power. At 01:07:15 a single-phase ground short-circuit of phase "C" occurred in the terminals box of electric motor of one out of six service water pumps. Due to overvoltage in the 6 kV auxiliary power lines caused by the above short-circuit a short-circuit occurred in the current duct of the service power feeder in the area of protections of the auxiliary power transformer (APT). Then the continuing short-circuit in the current duct extended onto the 3BB bus, which has led to bus disconnection causing deenergiztion of the 3 BW essential bus and start of the diesel-generator.
As a result of the short-circuit a fire occurred in the 6 kV metal-clad switchgear room. Fire extinguishing effort started after completion of necessary switchovers to assure safe conditions for the firemen. The fire was extinguished 1 hr 40 min after its start.
Deenergization of the 3BB bus has led to the tripping of 3RCP-4, which was followed by the tripping of 3RCP-1 due to failure of automatic changeover activation logic of the oil pumps. Reactor emergency protection actuated on the tripping of three RCPs. The reactor was brought to subcritical condition.
Deviations from limiting conditions of operation did not occur. Radioactivity releases and personnel exposures were not observed. Safety function availability was full.
Recovery repair is under way on the auxiliary power bus and on the current duct.
Event date: Wed, 04-03-1992