A B777 ON OVERWATER FLT DECLARED AN EMER AND DIVERTED DUE TO ELECTRICAL BURNING SMELL IN AFT CABIN. CABIN PURSER ADVISED CAPT AND THEN SWITCHED OFF ENTERTAINMENT SYSTEM.

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APPROX 2 AND HALF HOURS INTO THE FLT I GOT A CALL FROM THE FLT ATTENDANTS IN THE BACK GALLEY (I WAS THE PURSER) STATING THAT THEY SMELLED A STRONG ELECTRICAL BURNING SMELL IN THE 4R DOOR AREA. THEY FELT THEY RULED OUT THE SMELL COMING FROM THE GALLEY; BUT COULD NOT FIND THE SOURCE. I RPTED THE INFO TO THE CAPT; THEN TURNED OFF THE POWER PORT AND MAIN POWER SWITCH FOR THE ENTERTAINMENT SYSTEM. I WENT TO THE BACK TO CHK AND FELT THE SMELL WAS VERY STRONG AND THOUGHT THE CAPT SHOULD COME CHK ON IT; WHICH HE DID. FIRST; WE WENT THROUGH THE COCKPIT CHKLIST AND SHUT OFF READING LIGHTS; GALLEY POWER; ETC. MOST PAX WERE STILL ASLEEP THROUGHOUT THIS TIME. THE CAPT INFORMED ME WE WOULD HAVE TO LAND IN ZZZZ TO TRY AND FIX THE PROB BEFORE CONTINUING ON. I INFORMED THE REST OF THE CREW AND WE STARTED TO PREPARE THE GALLEYS; ETC; FOR LNDG. I THEN MET WITH THE PLTS IN THE COCKPIT AND DISCUSSED HOW MUCH TIME WE HAD TO LAND (1 HR 15 MIN); EVACUATION WOULD NOT BE NECESSARY UNLESS THE SIT WORSENED AND THEN WE DISCUSSED WHETHER TO RUN THE EMER CHKLIST IN THE CABIN. BECAUSE WE DIDN'T KNOW WHERE THE BURNING SMELL WAS COMING FROM AND WE STILL HAD OVER AN HOUR UNTIL LNDG; WE DECIDED TO ERR ON THE SIDE OF CAUTION AND PREPARE THE PAX IN CASE WE ENDED UP WITH FIRE/SMOKE IN THE CABIN PRIOR TO LNDG. WE LANDED WITHOUT INCIDENT; AND PAX DEPLANED AT 1L DOOR WITH EMER LIGHTS ON. PAX WERE CALM. COCKPIT AND CABIN DID A GREAT JOB COMMUNICATING AND WORKING TOGETHER. WE CONTINUED ON 4 AND A HALF HOURS LATER. SEAT XX POWER UNIT FOR ENTERTAINMENT SYSTEM WAS THE PROB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE PURSER RPTED THE AFT CABIN ATTENDANTS RPTED AN ELECTRICAL SMELL FROM THE AFT CABIN AND THIS WAS PASSED ON TO THE CAPT. THE PURSER IMMEDIATELY SWITCHED OFF THE POWER PORT AND PAX ENTERTAINMENT SYSTEM AND FOLLOWED THE CAPT TO THE AFT CABIN. THEY WERE UNABLE TO LOCATE THE SOURCE OF THE BURNING SMELL AND THE CAPT ELECTED TO DIVERT. AFTER THE DIVERSION; THE PROB WAS FOUND UNDER SEAT XX; A PAX ENTERTAINMENT UNIT.