I see that vilas has done a much better job of addressing Natstrackalpha's question than I did!
What I meant to go on to say was that, if you use the ECAM ELEC page to estimate the peak 28VDC load demanded by the APU starter motor during a routine sea-level start, it may be in excess of 800A (a bit like the engine starter motor of a very large family car, but at a higher voltage). I would suggest that the AC power involved to produce that would be beyond the capability of the RAT emergency generator - hence the Emergency TR is not given that task.
In any case, that noisey little RAT has the far more important task of pressurising the Blue hydraulic system, which sustains the primary flying controls in the double-engine failure case. (Any Green and/or Yellow hydraulic pressure from windmilling engines is simply an unreliable bonus.)
Last edited by Chris Scott; 21st Oct 2013 at 19:53.
Reason: Minor improvements...