On Sun, 04 Jun 2006 20:50:52 +0100 someone who may be F wrote this:-
Ah. That would be another reason for having the 45A fuse separate.
With the main switch off unscrew the bit of green coloured plastic that is presumably under the carrier and through which the pins go. Does a cable go out from either of the terminals, probably the top ones which are the only ones with screws in? If not then it is spare and can be used for a new circuit.
The white, blue and red bits of plastic that the fuse or MCB fits into are usually interchangeable. With the main switch off unscrew some of them (make a note of which one goes where) and you will see that the terminals they plug into are wide enough to take a carrier of any size. The plastic bits just present different sized holes as a means of guarding against the wrong size protective device being inserted.
Under the green plastic of the 45A fuse you should find terminals of similar size. If there are then you can get a smaller fuse or MCB, screw in the bit of plastic that comes with it and that's that. If you want to replace the 45A fuse with a 45A MCB then that should just plug in to the green plastic bit.
However, I don't recommend replacing a cartridge fuse with an MCB unless you are able to measure or calculate whether the MCB is able to break a fault on the circuit. The plug in type of MCB are particularly unable to break fault currents. If they are unable to break a fault the results can be spectacular, though usually the "electricity board" fuse will act before the damage is too bad. Replacing rewirable fuses with MCBs is always a good idea, but cartridge fuses are a different matter.