A B727-200 ON RETURN TO THE GATE DECLARED AN EMER AND EVACED THE AIRPLANE DUE TO SMOKE AND FUMES IN THE CABIN. CAUSED BY APU ELECTRICAL FAILURE.

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IMMEDIATELY PRIOR TO TAKING THE RWY; WE HAD THE #1 FUEL TANK FORWARD BOOST PUMP LIGHT THAT WOULD NOT EXTINGUISH. CHKED CIRCUIT BREAKER; FOUND THAT IT HAD POPPED. ALL CIRCUIT BREAKERS WERE CHKED AT GATE DURING PREFLT. ELECTED TO RETURN TO GATE AFTER CONSULTING OPERATING MANUAL AND MEL. APU STARTED. PA MADE TO PREPARE FOR ARR. ENGS SHUT DOWN. APPROX 5 SECONDS LATER; APU GENERATOR BREAKER WOULD NOT CLOSE. AFTER A FEW MORE SECONDS; I PICKED UP THE INTERPHONE TO TELL THE FLT ATTENDANTS THAT THE CABIN WOULD BE DARK AND NOT TO WORRY. THERE WAS ALREADY TAKING PLACE THAT INDICATED THAT FLT ATTENDANTS HAD BEEN TRYING TO CALL COCKPIT AND SMOKE WAS HVY THROUGHOUT THE CABIN. WE ACCOMPLISHED THE CABIN FIRE/SMOKE CHKLIST. COCKPIT DOOR WAS OPENED AND I NOTICED HVY SMOKE IN THE CABIN. THE GND EVAC WAS CALLED FOR VIA THE CHKLIST. FLT ATTENDANTS EVACED PAX VIA FORWARD GALLEY SLIDE; BOTH AFT SLIDES; AFT AIRSTAIRS. THERE WERE INITIAL DIFFICULTIES OPENING FORWARD ENTRANCE DOOR DUE TO POSSIBLE JETBRIDGE PROBS; BUT PROB WAS RESOLVED AND PAX DEPLANED. FLT ATTENDANTS DID AN EXCELLENT JOB GETTING ALL PAX OFF AND LEAVING THEIR LUGGAGE BEHIND. NO INJURIES OCCURRED NOR WERE THERE ANY RPTS OF ANY PAX ASSEMBLED OUTSIDE ACFT AND STATION PERSONNEL AIDED THEM BACK INTO TERMINAL. FAA AVIATION SAFETY INSPECTOR SHOWED UP WITH ASSISTANT. I SPOKE WITH CHIEF PLT AND RECEIVED PERMISSION TO SPEAK WITH HIM AS WELL AS FLT ATTENDANTS TO SPEAK WITH HIM. FAULT ANNUNCIATION PANEL SHOWED PHASE IMBAL.