EMER ACFT EVAC WHEN ELECTRICAL FIRE STARTS FROM SHORTED ELECTRICAL SYS ASSOCIATED WITH 'B' HYD PUMP. B737-400. CIRCUIT BREAKER TO HYD PUMP 'B' HAD BEEN RESET.

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DURING PREFLT FO SELECTED HYD ELECTRIC B PUMP ON TO PWR CTLS DUE TO HIGH WINDS. THE PUMP WAS FOUND TO BE INOP. MAINT NOTIFIED; MECH RESET INTERNAL SWITCH (INSIDE CIRCUIT BREAKER PANEL) THEN CLOSED CIRCUIT BREAKER PANEL. A SHORT CIRCUIT OCCURRED; LATER FOUND TO BE A SCREW HEAD CONTACTING A CHAFED AND BARE WIRE; CAUSING AN ELECTRICAL FIRE. CAPT ORDERED ACFT EVACED AT GATE. ALL AC PWR WAS REMOVED FROM ACFT; APU SHUT DOWN AND FIRE EXTINGUISHER USED. BUT FIRE WOULD NOT GO OUT UNTIL THE BATTERY WAS TURNED OFF. EVAC SUCCESSFUL; NO INJURIES. MAINT TOWED ACFT TO HANGAR FOR REPAIRS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR CALLED BACK TO STATE THAT THE ACFT WAS EVACED USING THE PAX DOORS ONLY. SOME PAX STOPPED TO GET THEIR PERSONAL BELONGINGS BUT THE CABIN ATTENDANTS STOPPED THAT RIGHT AWAY. LATER THE PAX WERE ALLOWED ON; 2 OR 3 AT A TIME; TO GET THEIR BELONGINGS. THE FIRE HAD STARTED RIGHT AFTER THE CIRCUIT BREAKER WAS RESET AND THE MECH 'SLAMMED' THE PANEL DOOR SHUT. THE WIRE ARCED AT THAT POINT WITH A GREEN FLAME SHOOTING OUT TOWARDS THE FO'S HEAD. (THE PANEL IS BEHIND THE FO'S HEAD; ON THE STARBOARD SIDE OF THE ACFT.) THE SUBJECT SCREW HEAD WAS INSIDE THAT PANEL AND HAD WORKED ITSELF LOOSE; FRAYING THE WIRE. THE CIRCUIT BREAKER DIDN'T HAVE AN EFFECT ON OPENING THE CIRCUIT AGAIN AS POSTFLT INSPECTION SHOWED THAT 26 WIRES WERE FUSED INSIDE THE PANEL. PRIOR TO THE ACFT BEING TOWED TO THE HANGAR THE MECHS TURNED THE BATTERY BACK ON; WITH ANOTHER ARCING SIT OCCURRING. THE ACFT WAS OTS FOR OVER A WK.