As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING APU T/S WITH APU MASTER SWITCH SELECTED ON AND APU START SWICH SELECTED ON, AVIONICS COMPT FILLED WITH SMOKE AND FOUND ESS BUS AND ESS T/R SUPPLY 10XH CIRCUIT BREAKER TRIPPED OPEN. INSPECTED CIRCUIT'S & COMPONENTS ASSOCIATED TO C/B 10XH FOR EVIDENCE OF FAILURE - OVERHEAT DAMAGE IAW AWM 24-52-01. REPLACED ESS TR 1PC IAW AMM 24-32-11-401, TR FANS OPS CK OK FOR ESS AND TR-2 PER AMM 24-32-00-501.