737NG External Power Application
I've been told that you don't need the Battery Switch to be ON on an NG to keep external power hooked up to the main busses on the 737NG (albeit without some of the Battery Busses being powered). Can you also apply external power without having the main battery selected ON?
If not, I'm having trouble understanding the Boeing Wiring Schematics (I suspect a diode has been missed from a diagram). Diagram 24-41-11 shows that the solenoid of the external power contactor can get power from the section of wiring between the external power socket and the contactor contacts A1, B1, C1 (The diagram shows power from this section going through a 3 phase CB "EXT PWR BPCU" to the BPCU, through a Transformer Rectifier and back out of the BPCU (pin 1 of D10898A) to pin 1 of the Ext Power Contactor and on to the solenoid, energising the contactor if the Ground Power Switch is switched on (and the external power source has been deemed to be OK)).
I see only transitting aircraft (APU kept running), so I would rarely have the opportunity to try this out for myself.
Thanks.
Regards.
Q.