A B747-400 ELECTRICAL DISTRIBUTION PANEL NEAR DOOR 4R CAUGHT FIRE FORCING AN EMER RETURN TO A SUITABLE ARPT.

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I RECEIVED A RPT FROM A FLT ATTENDANT IN THE AFT CREW REST AREA THAT HE AND TWO OTHERS HAD BEEN AWAKENED BY FUMES. HE SAID THE FUMES HAD AN ELECTRICAL AND OR SULPHUR TYPE SMELL. I SECURED THE UPPER DECK WITH ADDITIONAL FLT ATTENDANT GUARDS; SENT THE PNF TO INSPECT AND WOKE ONE OF THE PLTS IN THE PLT CREW REST AREA. THE PNF RPTED THAT THE FUMES WERE STRONGEST NEAR DOOR 4R AND CONTINUED FOR A FEW SEAT ROWS AFT OF 4R. WE ESTABLISHED COM WITH DISPATCH AND MAINT. AFTER ACCOMPLISHING THE APPROPRIATE IRREGULAR AND EMER PROCS IN THE FLT HANDBOOK AND FOM; WE WERE UNABLE TO DETERMINE THE SOURCE OF THE FUMES. WE WERE ALSO UNABLE TO ELIMINATE THE FUMES FROM THE CABIN. IN ACCORDANCE WITH THE FOM GUIDANCE AND WITH THE CONCURRENCE OF ALL THE FLT CREW MEMBERS; DISPATCH; AND MAINT; ZZZZ WAS DETERMINED TO BE THE NEAREST SUITABLE ARPT. AN EMER WAS DECLARED AND A TIMELY DIVERSION TO ZZZZ WAS ACCOMPLISHED. THE FUMES WERE STILL QUITE STRONG NEAR DOOR 4R AFTER LNDG. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT AN ELECTRICAL DISTRIBUTION PANEL IN THE 4R AREA ACTUALLY BURNED BUT PROVIDED NO INDICATION TO THE CREW THAT IT WAS THE FIRE'S SOURCE. THE RPTR SENT ONE CREW MEMBER TO THE CABIN TO HELP THE FLT ATTENDANTS AFTER PLACING GUARDS NEAR THE FLT STATION DOOR CONSIDERING THAT THE SMOKE MAY HAVE BEEN A TERRORIST DIVERSION. HIS COMPANY REQUESTED THAT THEY TURN BACK AND PROCEED TO ZZZZ1 INSTEAD OF A ZZZZ DIVERSION POINT. HE ELECTED TO IGNORE THAT SUGGESTION AND RETURNED TO ZZZZ ARPT AFTER CONSIDERING THE EQUIP AVAILABLE AT ZZZZ VERSUS UNKNOWN ASSISTANCE AT ZZZZ1 FACILITIES. RPTR STATED THAT HE FELT TOTALLY UNPREPARED BY HIS COMPANY'S TRAINING OR THE ACFT MANUFACTURER'S CHKLIST GUIDANCE TO ADDRESS A FIRE OF UNKNOWN ORIGIN.