EpsomDog, in general I agree with your comments on the "simplicity" of FCOM ELEC diagrams. They are not wrong, though. Please take care with the terms used, to avoid confusion.
Fire protection is powered from the hot battery bus, regardless of electrical system configuration. That way you have fire protection during the start sequence on a "dead" aircraft.
On a cold aircraft the fire detection loops and bottle #2 only become available after the BAT are selected ON so power is made available to the DC BAT bus.
dream747 I only have the FCOM for guidance, if we look at PRO-NOR-SOP-26-A:
BAT 1 pb-sw and BAT 2 pb-sw ..................................................OFF
Wait until the APU flap is fully closed (about 2 min after the APU AVAIL light goes out), before switching off the batteries. Switching the batteries off before the APU flap is closed may cause smoke in the cabin during the next flight.
If the batteries are off while the APU is running, APU fire extinguishing is not available.
Further up the procedure says EXT PWR as rqrd". The text of BAT1+2 procedure should be good for both cases.
My unerstanding of the italic note:
1) APU flap > this is only important when the EXT PWR had been selected OFF. CAUTION: vilas says otherwise!
2) Fire extingusihing > puzzled here. As per FCOM diagram: if the APU was running DC BAT bus is supplied via DC 1/2 and, together with HOT BAT 2, the fire protection is available... ??
hence my Q:
(i) Is the APU fire protection avilable with batteries OFF but DC BAT bus supplied and BAT 2 normally charged? The DSC and PROC chapters of FCOM seem to contradict. If NOT is the reason electrical (beyond FCOM pilot-digestable diagrams) or logical?
Moreover I wonder to what extent are the failure cases already considered. E.g. EXT PWR trip-off during APU start and shutdown. Or ops with APU GEN u/s.