I think "float engagement" works in the context.
Hope it helps :)

It depends on the wider context — i.e. exactly WHAT is it the float does? From your other question, I think it might simply be actuating an electrical contact; but if that is not the case, then the verb required may change depending on the thing being 'done'.
Note that we would almost certainly use an active verb in FR rather than a noun — though in some circumstances, we might say "tripping of the circuit occurs when..." etc.. But here, it seems unlikely to be any kind of tripping. Note also that we might use a passive verb in EN: "The circuit is tripped when..."

So you can see just how important it is here to know more of the surrounding context.