Hi John

Many thanks for the reply.

There are a few aspects of taking power outside like this that can get a
bit complex. You may find having a read through this:

http://wiki.diyfaq.org.uk/index.php?...ricity_outside

I was there earlier today and found it very helpful. I'll be looking
back again in the light of some of your comments.

So far there is a 20m run of 2.5mm SWA cable from the CU to an
external socket. The electrician put a 32A RCB on the circuit, which

If this run is inside the house, why is it in SWA?

A good question indeed given the relative cost and inflexibility
compared to the obvious alternative. I had the place rewired as part
of a major refurb, and at the time it wouldn't have been so hard to
sling the cable in, but with hindsight and now that I've audited and
looked more closely at the job that the electricians did, that would
be one question that I'd ask along with querying a number of
discrepancies between the schedule of test results and reality. They
weren't cowboys and were helpful with some unusual things that I
needed done at the time for the lighting, but now that I've had a
closer look, some of the wiring on the lighting circuits was sloppy
and careless.

I assume by RCB you mean MCB...

My typo.

Circuits need two types of protection: overcurrent and fault current.
The former is designed to clear a sustained overload that results in too
much current being drawn, and the latter is designed to operate when you
have an outright fault like a short circuit that results in a massive
fault current flowing. Normally both types of protection are provided by
the MCB at the head of the cable. However this does not appear to be the
case in your existing setup. I would guess the designed was relying on
the 32A MCB to provide only the fault current protection for the SWA
cable, and was relying on the fuse in the plug connected to the external
socket to limit the maximum current since the 32A MCB is of a too high a
rating to do this itself.

Right. A 32A breaker seems very high given what the circuit is for;
I'm not planning to run a few kilns!

is also covered by the RCCD that some other house circuits are on. I

This is generally a poor design choice. A fault in the garden will trip
power in the house, and also make nuisance trips more likely since
garden circuits are more likely to give rise to problems.

Good point. As we're not having a chest freezer or anything that you
don't want tripped this didn't seem too bad, but if there were trips
from the garden then that could become much more than just a nuisance.
I feel that it should be possible to have a garden install where a
trip was a rare, but if it didn't turn out that way then...

I would suggest a SWA cable submain to the outbuilding, and a consumer
unit installed there to supply any other circuits required.

Thanks.

You may want to consider running a larger cable through the house and
then converting to SWA for the external run.

I'd definitely do this if it were feasible but due to how the wiring
needs to route in the building it would mean ripping up a large floor
area in several rooms and cost ££££. I could route a new SWA out the
front of the building and then either around the house on the wall,
which would be unsightly, or bury it for some of the way, but really
neither would be good.

house supply is TN-C-S and I'd assumed that extending the earth from
the house would be ok

No - absolutely not! This is just the situation you really don't want to
extend a TN-C-S earth outside. If it were just going into the
outbuilding then it may have been a acceptable solution if implemented
correctly. However since you also need feeds into the garden itself for
lights and water features itself, using the TN-C-S earth is technically
not feasible, and could pose a serious safety hazard if you did it.

This is what you need to do. The outbuilding wants to be a TT install
with its own earth spike and RCD protection.

Thanks! I do need to get a cable laid to the outbuilding soon as the
concrete base is going down (probably tomorrow), and planned to take
the cable up through the concrete base so that it'll be there to feed
into the building when that's built. I'll go for a beefy SWA cable to
minimise losses on the run into the outbuilding, and would 2 core SWA
be fine for that or should it be 3 core?

Nick