As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
GROUND POWER FAILED WHILE PARKED AT GATE, TR UNIT LIGHT ILLUMNIATED AFTER POWER WAS RESTORED AFTER STARTING APU AND PUTTING IT IN THE BUSS. ACFT GROUNDED. FOUND CIRCUIT BREAKERS FOR TRU 1 ON P91 PANEL OUT, TRU 2 AND TRU 3 ON P92 PANEL OUT. DETERMINED CIRCUIT BREAKERS COULD BE RESET IAW MM 20-60-01. TR INSTALLATION TEST NORMAL IAW MM 24-32-11. SUSPECT LOST PHASE ON GROUND POWER, GROUND TESTS SUCCESSFUL.