B757-200 LOST ELECTRICAL PWR TO BOTH MAIN BUSSES; AND ACFT REVERTED TO STANDBY PWR. ONE GENERATOR RESET AND THE APU WAS STARTED; RESTORING PWR.

Narrative

BOTH AC BUSES SUDDENLY FELL OFF LINE LEAVING ACFT ON STANDBY PWR. STANDBY ENG INSTS INDICATED 0.0 ON BOTH EPR GAUGES WHICH LED US INITIALLY TO BELIEVE WE HAD DUAL ENG FAILURE. THE CAPT TOOK CTL OF ACFT AND STARTED THE APU WHILE I STARTED TO RUN LOSS DUAL ENG FAILURE QRC. MOVING THE THROTTLES TOWARD IDLE PROVIDED US WITH GOOD ENG INDICATIONS EXCEPT FOR EPR AND I ABANDONED THE QRC AND HEADED FOR LOSS OF AC BUS(ES) CHKLIST IN FM. ABOUT THIS TIME THE APU GENERATOR CAME ON LINE AND PWRED JUST THE L ELECTRICAL SYS. WE HAD AN ISOL LIGHT ON THE R ISOL BUS AND THE R GENERATOR WAS STILL OFF LINE. I RESET THE R GENERATOR AND THE BUS ISOL SWITCH AND BOTH RESET. AT THIS TIME; ALL ELECTRICAL PWR WAS RESTORED EXCEPT FOR THE L GENERATOR WHICH WOULD NOT RESET. WE ALSO HAD A L GENERATOR DRIVE LIGHT AND I ATTEMPTED TO DISCONNECT PER THE CHKLIST. AFTER COMPLETING GENERATOR DRIVE LIGHT CHKLIST AND LOSS OF AC PWR CHKLIST WE CONFIRMED WITH MAINT OVER COMMERCIAL RADIO. WE ALL DECIDED TO PRESS ON TO ZZZ SINCE WE HAD 2 AC PWR SOURCES AVAILABLE AND ZZZ NOW WAS THE CLOSEST FIELD. APCH AND LNDG WERE UNEVENTFUL.