744 Electrical System Puzzle

[NSEU]

I was depowering a 744 (admittedly the wrong way, in hindsight).

Both EXT's were plugged in and selected ON and the Battery Switch was OFF.
I deselected EXT 2 on the overhead panel ... and half the ship went dead (SSB didn't close automatically).

I assumed that BCU 2 had become unpowered, causing the SSB not to close, but, looking at the schematics, BCU 2 should still have been powered through the A, B and C pins of Ext 2 power plug and CB C861. Ref SSM 24-41-02.(Note that this was an older aircraft which doesn't have the option of powering BCU 2 via the Ground Handling Bus, but there should still have been power from the #2 Ext lead). I know the onside(adjacent) GCU's trip the respective BTB's if they lose contact with the BCU, but the GCU's are unpowered with the Bat switch OFF and engines off (leaving the BTB's magnetically latched closed).

I would have spent a little time analysing the situation, but the EEC's had automatically powered up and I was getting fuel valve messages (not a good sign.. because it probably meant the ignitors were armed*).

Avionics gurus??? A fault.. or normal ops?

Thanks.
NSEU.

*armed, but not necessarily firing (depends on ignition switch selections).

[NSEU]

I'd imagine it would have something to do with the high electrical loads imposed during cargo loading. When all 4 engine generators are running, the load is distributed evenly among the 4 synched generators. When the APU is running or External Power is connected, the synch busses are split(SSB open if there are two sources). If the cargo handling busses were _not_ inhibited when #2 APU/EXT are selected, then #2APU/EXT might have to bear their normal heavy pre-start electrical loads AND the main deck cargo handling loads.

Make sense?
Cheers,
NSEU

[Tinwacker]

Ray Cosmic

APU2 avail gives ground handling for loaders to do their tasks, cargo door opening, with out coming into the cockpit and touching switches.
EXT2 avail does the same.

TW