B777 FLT CREW HAS SMOKE/FUMES IN COCKPIT DURING PUSHBACK/ENG START; ACFT EVACUATED.

Narrative

AFTER ENGINE START THE EICAS MESSAGE; 'ELEC AC BUS' APPEARED. WHILE WE PERFORMED THE ABNORMAL CHKLIST; THE ODOR SIMILAR TO THAT OF BURNING ELECTRICAL WIRES WAS SLIGHTLY NOTICEABLE. IN ADDITION; A SOUND BEST DESCRIBED AS THE OPENING AND CLOSING OF A RELAY WAS COMING FROM THE LOWER ELECTRICAL COMPARTMENT BELOW THE FO SEAT. THIS SOUND CONTINUED AT A RAPID PACE. I'M NOT CERTAIN WHEN IT CEASED. WE COMPLETED THE ABNORMAL CHKLIST. THE BUS REMAINED UN-POWERED. THE SMELL OF ELECTRICAL BURNING WORSENED. AT THAT TIME IT WAS ONLY A NOTICEABLE SMELL. THERE WAS NO SMOKE IN THE FLT DECK OR CABIN. WE SHUT DOWN THE R ENGINE IN HOPES OF FURTHER ISOLATING THE ELECTRICAL PROB. I NOTIFIED LHR GND CTL WE NEEDED TO RETURN TO A GATE. I BELIEVE FO NOTIFIED OPS AND THE FLIGHT ATTENDANTS OF OUR SITUATION. CAPT ADDRESSED THE PAX. LHR GND CTL ALSO INFORMED US OF SMOKE COMING FROM THE ACFT. THEY CLRED US TO GATE Y; WHICH WAS A SHORT DISTANCE FROM OUR PUSHBACK GATE X. OUR PLAN WAS TO EVACUATE THE PAX IN A QUICK BUT ORDERLY FASHION VIA THE GND STAIRS AS LONG AS THE SITUATION DID NOT DETERIORATE TO THE POINT OF EXCESSIVE SMOKE IN THE CABIN AND/OR A FIRE. THE PAX WERE TOLD TO LEAVE THEIR PERSONAL POSSESSIONS ONBOARD. SHORTLY AFTER BLOCKING INTO GATE Y BOTH ENGINES AND THE APU WERE SHUT DOWN. SMOKE BEGAN ENTERING THE FLT DECK. AFTER ALL REQUIRED PA ANNOUNCEMENTS WERE MADE; THE ADIRU AND ACFT BATTERY SWITCH WERE SHUT OFF. I PROCEEDED TO THE CABIN TO ASSIST IN THE EVACUATION; WHICH WAS PROCEEDING IN AN ORDERLY FASHION. FIRE FIGHTING PERSONNEL ENTERED THE COCKPIT AREA. CAPT ASSISTED THE FIREMAN BY RETRIEVING THE TOOL KIT FROM THE COCKPIT TO OPEN THE ACCESS PANEL DOOR TO THE LOWER ELECTRICAL COMPARTMENT. PRIOR TO EXITING THE ACFT; I MADE A QUICK CHECK OF THE CABIN TO ASSURE EVERYONE HAD VACATED THE ACFT; AND TO ASSURE ALL DOORS HAD BEEN DISARMED. NO PAX WERE INJURED.SUPPLEMENTAL INFO FROM ACN 729403: E & E COMPARTMENT INSPECTION AFTERWARDS SHOWED WHAT APPEARED TO BE SEVERE DAMAGE FROM ELECTRICAL FIRE AND ARCING. THIS COULD HAVE BEEN A VERY SERIOUS EMER HAD WE BEEN AIRBORNE. SOMETHING IN THE ACFT ELMS SYSTEM SHOULD HAVE REMOVED ELECTRICITY FROM WHATEVER BAD EQUIP STARTED THIS INCIDENT AND PREVENTED THE FIRE AND ARCING FROM EVER HAPPENING.