From the Bible(Fcom):
EMERGENCY GENERATION AFTER LOSS OF ALL MAIN GENERATORS
If both AC BUS 1 and AC BUS 2 are lost and the aircraft speed is above 100 knots, the Ram Air Turbine (RAT) extends automatically. This powers the blue hydraulic system, which drives the emergency generator by means of a hydraulic motor. This generator supplies the AC ESS BUS, and the DC ESS BUS via the ESS TR.
If the RAT stalls, or if the aircraft is on the ground with a speed below 100 knots, the emergency generator has nothing to drive it. The emergency generation network automatically transfers to the batteries and static inverter, and the system automatically sheds the AC SHED ESS and DC SHED ESS buses.
When the aircraft is on ground :
Below 100 knots, DC BAT BUS is automatically connected to the batteries.
Below 50 knots, AC ESS BUS is automatically shed, leading to the loss of all CRTs.
Note : During RAT extension and emergency generator coupling (about 8 seconds), the batteries power the emergency generation network.
Note (my personal one): Captain87 with Rat out the two Ac1/2 bus are not powered by the Rat, in fact the emm gen only powers the Ac ess and Dc ess just as stated above..
Max Reverse, what about an AC2 bus fault or even a sub bus failure?
The bus itself could have a problem even if Gen2 is running?
Speevy