Oh it being a fuse makes sense, although the way it is wired makes even less sense, then. The middle device (the 'contacter'?) is not used by any load at the moment. But I wanted to start using it again.

Bottom view:

Don't mind the loose wire. It's french wiring. It's not even connected at other end.

Global view:

I have two boilers that are now both connected to the same switch (and this is time dependent somehow). This switch is shown left. The black wires are from boiler 1, and the blue and beige wires are from boiler 2. The orange pair are neither input nor the load, so they must be 'commanding' the switch operation.

I want to start using the middle contacter for boiler 2 so that each boiler has its own switch.

The bottom of the fuse (so far right) is connected to A1 of contacter (so thing in middle) (via gray cable) as well as to A2 of switch (far left in global pic) (via orange cable).

I'm not sure whether to call it a switch or contacter again.

There is 230V between A1 and neutral (at the moment. This may change, depending on the time of day.)