Fault message on front of BPCU (Bus Power Control Unit).
This message is associated with the BPCU seeing a valid interlock circuit or a external power contactor coil problem. The interlock deals with pins E&F of the external power GPU plug. The BPCU must see continuity between these pins before allowing power to the aircraft (Reference: https://rotate.aero/forum/safety-aware/265-external-power-plug#452
. As far as the contactor coil, if the BPCU supplies a signal to close the contactor, but it does not close, it will log the fault shown above.
Pins E&F can be checked directly at the plug.
In the case of a 767 (most likely other types of Boeing aircraft), two fuses are placed inline from pins E&F of the external power receptacle to the BPCU. I found a broken solder joint on one of the fuses.
The broken wire was spotted almost immediately upon opening the P34 panel. I still wanted to verify operation of the external power contactor. The BPCU supplies approximately 28VDC to close the contactor. I tried substituting my "easy 28" https://rotate.aero/forum/test-equipment/246-easy-28#409
, but because the contactor is so large...... there wasn't enough power from the batteries to close it.
I brought a DC Power Supply out to the aircraft and activated the contactor with it. I did this directly at the contactor and just to verify the BPCU to contactor wiring was good, I fed this voltage from the BPCU rack plug also.
As a note..... the BPCU has a small transformer rectifier
that converts the 3 phase AC external power input to approximately 28VDC (I found 25.3 on the unit I was working with). It uses this DC voltage to control the contactor. The BPCU verifies voltage, phasing, and frequency of the supplied external power. If all of these are within tolerance, it will close the contactor and supply power to the aircraft.