The back-up generators are rated at a maximum of 20kva they are variable speed variable frequency generators with their output fed (via the converter to make it satisfactory a.c. power) to their respective transfer bus during non normal conditions and during autoland.
They come on-line during engine start for 15 secs as a form of confidence test.
I think that if one IDG is inop. dispatch is with the remaining IDG feeding it's respective bus, and the APU Generator feeding the inop. IDG bus. (ETOPS 120mins??)
Failure of the APU or the 'good' IDG will result in the BUG coming on-line feeding it's transfer bus ie. flt. instruments etc. as already stated.
Land at the nearest suitable airfield!!
Hope that helps!
[This message has been edited by gas path (edited 20 March 2001).]