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In article ,
(Patrick Olner) writes:
I've just run 6mm SWA from the consumer unit in the house to a
workshop some 25m away. The workshop has 4 circuits - 2 lighting and 2
smallish ringmains, run in 2.5mm. The SWA from the house will feed a 4
way CU in the workshop via a separate RCD - my questions a

1. What size MCB needs to be fitted to the 6mm SWA in the house CU -
32a or 40a?

You can use 40A with 6mm cable. As you also have an RCD at
the house end, you can ignore earth fault loop impedance.
At 40A design load, your 25m of 6mm cable has used up 80%
of the permitted voltage drop -- this probably doesn't
matter as the length of circuits in the workshop won't be
large, and your design load probably isn't really anywhere
near 40A (nearly 10kW).

2. What size MCB needs to be fitted in the workshop's CU - 32a for the
ring and 6a for the lights?

Yes.

3. How do I avoid the lack of discrimination caused by running 2 MCB's
in line - ideas include i, using type C's in the workshop or ii,
discrimination will be provided by having 40a MCB in the house and 32a
in the workshop or iii fitting traditional fuses in the workshop,
rather than MCB's

You can't avoid the lack of discrimination caused by running 2 MCB's
in line, except by not doing so. You could use a 40A BS1361 cartridge
fuse at the house end, or you could ignore the problem -- how often
have you had a ring circuit short out live to neutral (which is what
will cause this problem) verses having an earth fault in an appliance
(which will have much the same effect, tripping the RCD in the house)?
I would suggest to you that the RCD represents a much more likely
reason for losing all the power to the workshop than a live-neutral
short circuit on a ring circuit, so I wouldn't worry about lack of
discrimination.

I would ask if you really need two ring circuits -- that's more than
7kW load, and a damn hot workshop;-) I would have thought one would do
or if you want more than one circuit, what about two 20A radial ones?
Still won't help discrimination issue much though.

You might also want to make the workshop a TT system with its own
earth rod, rather than exporting the house earth. If you do this,
you still earth the SWA armour at the house end, but you make sure
it isn't connected to anything at the workshop end, indeed make sure
it's insulated and not exposed. You have to have an RCD if you do
this.

If the workshop contains machinary which might cause a danger if all
the power is suddenly cut and you lost lighting whilst machinary is
still in motion under its momentum, I would look at schemes which
didn't compromise the lighting in the event of an earth fault. Please
say if this is the case, and I (or someone else) will suggest some
suitable alternative schemes.

--
Andrew Gabriel