A300 ACFT ON APCH FOR CAT 3; APU STARTED FOR BACKUP ELECTRICAL PWR AND WHEN APU WAS UP TO SPD AC BUS #1 AND AC ESSENTIAL BUS TRIPPED OFF. FLC DECLARED EMER AND DIVERTED TO VFR ARPT.

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ON ILS RWY 4R APCH INTO JFK; LEVEL AT 1500 FT; CAPT STARTED APU PRIOR TO CAT III APCH. WHEN APU CAME UP TO SPD; AC BUS #1 AND AC ESS BUS TRIPPED OFF; AUTOPLT AND AUTOTHROTTLE DISENGAGED AND CAPT'S INST PANEL GOT VERY DARK. ABANDONED APCH; DECLARED AN EMER DUE TO THE FAILING ELECTRICAL SYS AND DIVERTED TO A VFR FIELD (PHL). ATC HANDLING WAS GOOD; PHL EMER RESPONSE WAS EXCELLENT AND NO PROBS WERE ENCOUNTERED. ACFT WAS A FERRY FLT FROM KIN; AFTER HAVING AN ELECTRICAL PROB REPAIRED. WE WERE NOT SURE HOW MUCH THE ELECTRICAL SYS WAS GOING TO DEGRADE; AND THEREFORE DECLARED AN EMER. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR FO HAD NOT HAD FEEDBACK ON WHAT THE SPECIFIC CAUSE OF THE BUS FAILURES WAS; BUT HAD HEARD THAT A THRUST REVERSER UNIT HAD BURNED UP. RPTR BELIEVES THAT; THOUGH IT'S NOT SUPPOSED TO DO IT; THE APU KICKED OFF THE BUSSES. APPARENTLY IT AUTOMATICALLY CONNECTED TO THE TIE BUS RESULTING IN AN OVERLOAD OR OTHER FAULT. BECAUSE THE WX WAS POOR WITH LOW VISIBILITY AT JFK; THE CAPT STARTED THE APU FOR AN ELECTRICAL BACKUP; BUT WHEN THE AC BUS #1 DROPPED OFF THE CAPT NO LONGER HAD HIS PRIMARY FLT INSTS AND WITH THE ELECTRICAL PROB THE FLC DECIDED TO DIVERT TO A VFR ARPT.