Nick wrote:

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!

In fact my bad possibly...

Just looking at table 4E4A in BS7671, the current rating for two core
SWA if it has a thermosetting insulation[1] capable of withstanding a 90
degree temperature (i.e. XLPE) is actually 36A (and 31A for three core).
So it would be adequately protected by the breaker you have (although
you may want to change that for a 32A HRC fuse to better discriminate
with downstream MCBs).

[1] Most SWAs are like this, however PVC insulated versions also exist
and the max current ratings for these are lower at 28A/25A (2/3 core).
To be sure check the markings on the sheath of the cable.

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...

It should be possible - certainly in the outbuilding anyway. However
there is greatly enhanced scope for problems, and as you say, if you are
unlucky it might prove to be a PITA. If you have a split load CU in the
house and there is space, I would be inclined to move the existing SWA
to the non RCD protected side. As a minimum I would include a double
pole switch such that you can completely isolate the garden feed.

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.

Your limitation with the existing cable is going to be one of voltage
drop. At full load it is already out of spec at over 12V drop at the far
end (19mV/A/m). (obviously if it only has a socket on it at the moment
you can't get it to full load and it is ok for its current use)

No scope for using the existing cable to pull through a new one?

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?

2 Core is fine. You would need to connect the armour to earth in the
house end, but then isolate it from the CU in the outbuilding. There was
a recent thread that covered the issues here quite well:

http://groups.google.com/group/uk.d-...d959cb137922ec
http://tinyurl.com/3k4py3


--
Cheers,

John.

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