What was found
(CAN) DURING CRUISE, THE CREW NOTED THAT THE LEFT ESSENTIAL BUS WAS ABNORMALLY HOT, SHORTLY AFTER THE NR 1 AUDIO CIRCUIT BREAKER TRIPPED. AIRCRAFT RETURNED TO BASE. INVESTIGATION FOUND THE HOT AIR DUCT IN THE LEFT ESSENTIAL BUS AREA HAD BECOME DISLODGED DIRECTING HOT AIR ONTO THE BUS. IN ADDITION, THE NR 1 AUDIO PANEL CIRCUIT BREAKER WAS FOUND CRACKED.