"If you notice, the power source for the External Power Contactor is Switcehd 28VDC power (CB named Bus Power Control Unit). That contactor cannot be closed until 28VDC is applied."
My diagram also shows two other possible power sources for the contactor....
1. A rectified voltage from the hot side of the Ext Power Contactor (CB called EXT PWR BPCU) and...
2. The 28Vdc Bus 1 (CB called Bus Pwr Cont Unit).
The latter, of course, will not be available until power is on the main busses, but the first one seems to be available with the Ext Power plugged in (but not switched on).
The contactor control logic circuits, however, may need a particular power source(?)
Cheers.
Q.