What I mentioned was that aircraft that rely on electronic flight controls or have standby instruments that are LCDs or CRTs have RATs. I know that some of the older aircraft don't, but I'm pretty sure that RATs are standard on the big FBW aircraft.
I believe the 747 also has a RAT, but it only provides hydraulic power... not sure about that. OF
course, 747 doesn't have the same power demands as a 777, which I do know has a generator attached to its RAT.
As for generator failure without engine failure, I guess I was awfully vague on that... I'm with that its possible, though from what I know, it seems very unlikely. I would guess you could have bus or other electrical problems that produce the same affect as well.