On 27 Jul 2003 12:38:25 -0700, Patrick Olner wrote:

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

You also need and RCD at this point.

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

That is what I would fit.

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

Type C are the "slower" ones so that really needs to be at the house
end so the faster type B has a (slim) chance to operate before the
type C.

ii, discrimination will be provided by having 40a MCB in the house
and 32a in the workshop or

TBH there is not that much difference between a 40 and 32A trip.
Remember they are designed to carry that current indefinitely and will
take a significant amount of time to trip at double their rating.

iii fitting traditional fuses in the workshop, rather than MCB's

Fuses are even slower. I was sort of thinking that with an RCD in the
house you don't need overload protection at that point but I guess
that it still is conceivable that a LN fault could occur causing
overload of the SWA. So fuse it at the house end and have MCBs in the
workshop CU. It may even be useful to fit another RCD in the workshop
CU feeding the rings. Indeed any socket that may have an appliance
plugged in that is used outside must have RCD protection. Be aware of
the problems of cascading RCDs...

--
Cheers
Dave. pam is missing e-mail