To clarify some of the above . The A320 Family comes with 2 standards of RAT and Emergency Electrical System .
For both standards , the RAT just pressurises Blue , which drives the EG which supplies AC ESS and DC ESS ( and AC ESS SHED and DC ESS SHED ) , It does not supply anything else , including the Batteries , and the only other Busbars available are the 2 Hot Bat Busses and the Static Inverter Bus. There is no published limit for Flight in this configuration , but ECAM will display LAND ASAP in Red . The aircraft is in Alternate law ; APs , FDs and A/THR are unavailable .
For the 'old' standard , a minimum airspeed of 140 kts is required for RAT operation ; but if the Nose Landing Gear is extended - at any speed - the EG is disconnected ; Batteries supply AC and DC Ess (and Static Inverter Bus) but both 'Shed' busses are lost ( leading to loss of FMGC1 , MCDU1 and ND1 , amongst other things ) . Battery Endurance is quoted as 22 mins , but may be extended to 'up to 30 mins' through application of the Paper 'Flight On Batteries Only' procedure , which involves de-powering ADR1 and the subsequent loss of PFD1 Air data ( use Standby Alt & ASI )
After the GA , and following NLG Retraction , the EG can be recovered through the 'RAT and EG MAN ON' p/b ( min 140 kts) .
For the 'new' standard , RAT stall speed is 125 kts ; and there is no EG disconnect with NLG extension . The EG should remain supplying until touchdown and so the aircraft should never become 'Batteries Only' .
This explains the difference between the Procedures for the Old and New standards : in the former , there is a concern both with preserving Battery Power , and preparing for the consequences of loss of FMGC1 with Gear Extension ; for the New standard these are not considered important .
For instance , for the Old standard , the Procedure advises against attempting an APU start ( for 'simultaneous' loss of Gens/Busses 1 & 2 : even if the APU is started , the Electrical System Fault which has in all probability caused the original loss of both AC busses will also prevent APU use ; but the cost of the attempt is 3.5 mins worth of Battery ) .
For the New Standard , the advice is to try and start the APU . It might recover AC1 or AC2 ( or both) , but if it doesn't , any Battery power loss is not significant to the overall situation.
