I just looked through my very old training manual (2006 FSI) and, of course, it does not describe the electrical circuits for the floats! It does show, however, that for the liferaft jettison there are both DC Primary and Battery Bus circuits, implying you can jettison the liferafts with just the battery, so it seems odd that the floats are not the same - unless the logic was that the rafts might be deployed after shutdown, with the AC Generators offline. Similarly, there is no figure given for the lowest Nr when the AC Generators will drop offline (and hence the DC Primary Busses), other than the fact that the frequency under-protection is not active when airborne, so the Nr will be much lower than the 95% used with ground logic. ( PS - for non-S92 drivers, the AC Generators are driven by the MGB, and are not engine starter/generators.)
Igor13 - it would be good to have some references to your facts, if possible. Regardless, if the 80% Nr figure you quote for the generators dropping off is correct, it is hard to think of a scenario where that could happen. Certainly not OEI, and double, simultaneous, flame-out is almost impossible to imagine. Plus, if the aircraft hit the water with less than 80% Nr, there is no way it would be that undamaged.