In article ,
mike writes:
I need to swap an unused 40A Wylex MCB in a Wylex consumer it for a
32A MCB for a new ring main.

I'd assumed that different brands of MCB were interchangeable until I
saw how many different brands at similar prices Toolstation stocked -
and that made me think there must be a market for specific makes.

Toolstation don't do Wylex though -- so will other brands do?

In general, they aren't. Even if they mechanically fit, you're
going to be running outside the design and test criteria.
MCB's have issues with heat and magnetic fields straying from
one to another. One manufacturer will ensure their own work
correctly when used as specified, but they aren't going to go
to the trouble of making theirs work with alien stray fields
leaking in from someone else's.

There are some which are well known to be interchangable, such
as MEM, BILL, Eaton, because they're really all the same company
and production line.

Note that individual manufacturers often have several ranges,
and they often aren't even interchangable.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]