out of morbid curiosity, since this question already came up, I wondered why exactly will the APU fire extinguisher bottle not work when BAT is OFF.
According to FCOM both APU fire detection loops as well as the single APU bottle squib is powered by DC BAT; NOT DC2, HOT1, HOT2 as the engine extinguisher squibs (or DC ESS for 1 of each engine detection loops).
So, I'm thinking out loud here....BAT OFF will close the contactor from TR1 which supplies DC BAT (?), therefore no fire protection is available (you should see "INOP SYS : APU FIRE DET") and that's why the logic will initiate an auto shutdown if you try to start the APU (?), not a "normal" shutdown, but quick and dirty, in the interest of safety, hence no de-oiling, cooldown etc.
I assume Mr Airbus has put some thought into this, it makes some sense that not everything can be connected to DC ESS etc.
Last edited by deptrai; 12th Dec 2014 at 12:55.