French term (edited): alimentés en secouru transitoire

I'm afraid none of your proposals will really do; I don't believe this has anything to do with 'inrush current etc.'; note the syntax 'en secouru transitoire'

'secouru' does indeed mean a standby or backup UPS — though in many institutional contexts, this is referred to as a 'maintained' supply; for example, in hospitals etc; you may see a number of e.g. red power sockets: these are for priority equipment, and if the main supply goes over, only these circuits have power 'maintained' to them.

I'm not entirely clear what they mean here by 'transitoire' — the tiny snippet of source text is not explicit, it remains to be seen if any more details are given elsewhere in your text. In the case of e.g. a hospital, the backup supply will be provided by typically a diesel generator, which is required to start within say 30 sec. Even this short outage may be unacceptable for certain particularly crucial or sensitive equipment, in which case they might have to resort to battery-powered solid-state UPSs, posisbly even through-feed ones, or at least, they will be able to take over within like 1 half-cycle of the mains!
The trouble is, running much power on a s-s UPS of course demands huge battery resources; so their use might be limited to only the very short 'transient' moment before (say) the generator gets going.

I am wondering if that is what is going on here?
Alternatively, it could simply be that they are being used to avoid even tiny mains 'glitches' or 'micro-outages' — the sort of things that are just enough to make your PC crash!
There are other possibilities for 'en transitoire', but it's impossible to be sure without more context.