As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHEN POWER WAS TRANSFERRED FROM EXTERNAL POWER UNIT TO APU, A LOUD ARCING NOISE WAS HEARD FROM BEHIND CAPTAINS SEAT. CABIN LIGHTS WENT OUT AND GROUND SENSE BUS CONTROL CIRCUIT BREAKER POPPED AND SMOKE APPEARED FROM CIRCUIT BREAKER PANEL. REMOVED AND REPLACED GROUND SERVICE EXTERNAL POWER RELAY IAW MM SECTION 24-40. PERFORMED A SYSTEM OPERATION CHECK AND SYSTEM CHECKED GOOD. (M)