This particular aircraft was having problems with the "#3 A/C Bus" connecting to external power upon engine shutdown. Interrogation of the EPCU (Electrical Power Control Unit) had 3 repeating faults of "#3 Bus Sense", "C Phase of External Power" and "#3 BTR (Bus Tie Relay)" failures.
A Bus Sense issue, if it was a continuous or active fault would most likely be caused by an open "Sense Fuse" that is used to feed a "hot bus" signal to both the EPCU and the GCU (Generator Control Unit) for that particular bus. In this case, the #3 Bus Sense signal feeds the #3 GCU. I did not investigate the "C Phase External Power" because the aircraft did take power properly (all three buses) and that particular fault might have resulted from the #3 BTR not closing. A relay was in stock to be used for replacement.
The #3 BTR (R2-6) is located in the right A/C power bay. It is mounted upside-down on the lower shelf. (One note: These bus transfer relays are really contactors
as they are carrying large amperage loads.)
Two of the "Sense Fuse's" can be seen on the left.
This is a poor image, but the "#3 Bus Tie" is showing open as the APU generator is powering the aircraft. The bus tie would be closed on external power. The bus tie is also closed with the #3 generator online and paralleled with the other two engine generators. (Paralleling is a matching of sine-wave outputs of the three generators. Pretty amazing engineering of the IDG's or Integrated Drive Unit's if you ask me.)
Another poor image but, one thing I did find on the installed unit was the wrong placement of the flat washer. It was installed under the terminal. This is not correct. The proper order is terminal, flat washer, lock washer, nut.