ALSO......looking at the Power distribution schematic of this a/c everything essential is always powered (thats why its called essential)......
Caveat: About the A330 I know nothing.
OK, in general terms aircraft electrical systems are designed to fail in such a manner that the core of the system is maintained. i.e. battery for essential DC and an inverter for essential AC. Pretty much every aircraft I am familiar with has something like that sitting in the middle of the system, to give you half an hour or so to either get a gen going or land.
Then we add some emergency power generation, maybe a RAT, or a hydraulic driven Gen, to allow us to fly indefinitely, albeit substantially degraded.
Then we add an APU gen, which, depending on the flight envelope and type will be able to run a good chunk of the aircraft.
Then we have the main engine gens that run the whole shooting match when all is well.
Obviously theres lots of variations on the theme, but when you get right down to it most modern electrical systems are conceptually identical.
As such we tend to get into the habit of assuming the system will fail in a manner its designed to cope with. Like in the Sim. Ho hum... an engines failed. Better start that APU. Oh dear... the other main gen has failed... what an annoying coincidence. Good job we started the APU. except... bummer, thats failed as well. Now we're down to some scrawny Gen spinning off a hydraulic pump or something. And wouldn't you know... its failed as well, leaving us at battery power. Time to land ASAP etc etc..
But real failures aren't always as cooperative. In 25 years of flying, apart from the occaisional gen failing, the only time I've seen a really nasty electrical problem, the bit that failed was the core of the system.
The essential buses fell over, taking with them either directly, or indirectly (because those core buses supply the relays and indications for large chunks of the rest of the systems) pretty much everything on the aircraft.
I forget exactly what we were left with, but it wasn't much. The first officer's instruments were ok, and a couple of conventional navaids. But we had no hydraulics, no other flight instuments (both mine and the standbys were dead), no intercom, we had a radio each but both audio management panels were dead. Pretty much every other gauge or indicator in the flight deck was dead.
The only saving grace of the whole situation was - thank god, because it was a grobbly day and the F/O had hardly any time on type - we hadn't taken off yet. We got the tug to stick us back on stand and left it for the Engineers!
The situation we'd seen had not been covered in grounschool, the sim, and there wasn't even a checklist for it.
The point of this story? Electrical systems can fail from the inside outwards instead of the other way round. The fact that one of the main buses is still powered is no gauruntee that the really crucial stuff is healthy.
pb