kite
There is a complex reconfiguration of electrical supply during a CAT III DUAL approach. JETZ Tech explains the nub of it. The autopilots and other essential systems for the approach must be completely independent and isolated to provide redundency. The bus ties powering these systems must be kept open. If there is a failure of one generator the autopilot and other essential systems for the approach on the failed side would be dead. The reason the failed half is not completely electrically dead is because of the unique bus configuration in CAT III Dual.
JETZtech's nub..
The reason for this is during the approach mode (only) the busses split into three and there is no closing of the bus ties. So in order to have Cat3 Dual you need two separate power supplies Gen1 and Gen2 powering each side to get Cat3 dual.The a/c maintains this condition until on the ground.
There seems to be a fair bit of confusion here. The electrical system does not behave differently just because CAT 3 DUAL is displayed on your FMA. In normal operation the AC BUS ties are both open; so is the DC TIE between DC BAT BUS and DC BUS 2. None of this changes on the approach, there is no "complex reconfiguration", no "unique bus configuration". The normal configuration ensures an electrical split (AC and DC) between FMGC 1 and 2.
In the event of a GEN failure the AC BUS ties close and the remaining GEN powers the whole system. You hear a triple click and see CAT 3 SNGL annunciated, because the FMGC's are no longer electrically split. You will also get an ECAM warning above 800 ft.
It is that simple, and the important reaction is operational rather than technical. Why delve deeply when you don't even understand the basics?
As to why the APU GEN is not considered an independent electrical source .. I haven't a clue! Airbus have decided, and because their decision works in a safe direction I am not going to question it. Not sticking head in sand, just practical aviation.
Hope that helps - but if not, print out your posts and mine and take them along to your friendly neighbourhood TRE and ask his/her opinion.
TP