g.f.:
Essential power came off which generator the FE selected, except for the TWA config where, guess any pilot could select it. But no auto-switching, if STBY wasn't selected and 3 gens out--no flight instruments! I was always taught never to go with #3 gen inop--if you lost #1 and #2 engines there would be no way to power the standby hyd pump and no rudder.
The essential power selector was on the F/E panel in the standard Boeing configuration. Only the F/E could select essential power. It was the standby power switch that TWA paid Boeing to place on the overhead panel so any of the three could turn it on.