All correct above by my reasoning. The BUGs are BackUp Gennies only. 3 Full A/C sources (1 per engine + APU) required for 180+ ETOPs (plus the BUGs). 2 A/C sources for 120 minutes (BUG requirements seem to vary depending upon local tastes). Down to 1 A/C source (regardless of BUG status) - land ASAP.
There is another (small) DC generator on the RAT also (B777) for those really desperate moments.
MG
[This message has been edited by MasterGreen (edited 25 March 2001).]