hi need to replace a fuse in my fuse box but can't get old one out .have switched off main switch  and pulled but it will not budge . please help is there a trick to doing this.

Sorry if it's a silly question but... Have you released the DIN rail catch? The photo Lectrician posted shows two of these in the unlatch position (I think). You will probably have to remove a bus bar as well.

My mistake.  I have a different CU and the MCB don't just pull out if that's what you are trying to do.

LEC,
Where can those MEM Fuse carriers be had from?

I have a MEMSHIELD 2 3 phase board which feeds 3 single phase consumer units in a local hotel via 40A type B mcbs. I am thinking that these do not provide proper fault discrimination with the individual consumer unit mcbs and would like to swap em out for Fuses instead.

I've looked and looked but have never found any available before.

TT

They are no longer available from new.  The 45amp ones take up two ways.  They would fit the TP boards (it was the newer AD MCB's that would not bit the commercial boards due to narrower busbars/terminals on the newer AD models).

A C type MCB is always preferable on sub-mains.  What size cable is it?  Could you go up from a 40?

The submains from the 3 ph board are run in what in looks like 16mm T&E so am thinking around 50A max. Might get away with 63a but without checking I can't say for certain.
Am thinking Zdb + Zs at C/U subccts might be exceeded if we go any higher.

Poor discrimination on magnetic (not thermal) trip side.  Get a short on the sockets, and you could trip the submain, not the final circuit.

See what you mean.

The Zdb at the sub board would have to meet the max Zs for the C type or HRC fuse being fitted to protect the submain, but the final circuits from the submain would not be affected, they have their own overcurrent devices, and their own max Zs figures.