A WDB ACR HAD TO DUMP FUEL; RETURN; AND LAND BECAUSE THEY LOST PART OF THEIR ELECTRICAL SYS ON TKOF.

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ON PREFLT THE FO; SO AND I DISCUSSED THE PREVIOUS WRITE UPS IN THE MAINT LOGBOOK; ESPECIALLY THE LOSS OF #3 BUS ON TKOF ON A PREVIOUS FLT. ON TKOF ON RWY 24 THE WX WAS VFR AND THE WINDS WERE 250 DEG AT 08 KTS. THE FO WAS FLYING. JUST AFTER LIFTOFF; WE EXPERIENCED A #3 FLT STATION BUS FAILURE; ELECTRICAL DIFFERENTIAL FAULT WHICH INDICATES A SHORT CIRCUIT BTWN THE GENERATOR BREAKER AND THE AC TIE BUS. THE FO HAD BASIC INSTS AND CONTINUED FLYING. I CONFERRED WITH SO AND WE DECIDED TO RETURN TO MANCHESTER. I TOLD THE TWR WE HAD AN ELECTRICAL PROBLEM; NEEDED TO DUMP FUEL AND RETURN FOR LNDG. DEP CTL VECTORED US TO A FUEL DUMPING AREA. WE COMPLETED ALL THE NORMAL; ABNORMAL CHKLISTS AND DUMPED ENOUGH FUEL FOR A LNDG WT OF 418000 POUNDS. I DECIDED TO MAKE THE APCH AND LNDG. I HAD FLT DIRECTION AND THE FO DID NOT. THE WX CONTINUED TO BE VFR; WITH THE WIND 250 DEG AND 08 KTS. THE VISIBILITY WAS 10 PLUS MI. THE LNDG AND TAXI WERE UNEVENTFUL. IN REVIEW; I AM GLAD WE REVIEWED AND DISCUSSED PRIOR PROBLEM FROM THE ACFT MAINT LOGBOOK; SO WE WERE NOT TAKEN BY SURPRISE BY THE ELECTRICAL FAILURE. THE NEXT DAY THE MECH TOLD US THAT THERE WAS A WATER LEAK IN THE BUSINESS CLASS LAV; THAT LEAKS INTO THE TOP OF A CARGO CONTAINER AND POOLED AND THAT PROBABLY ON LIFTOFF; THIS WATER DRIFTED ONTO THE #3 GENERATOR CTL UNIT; CAUSING THE ELECTRICAL SHORT.