In article ,
(John Rumm) wrote:

wrote:

Remote brick built shed, originally wired up by a previous owner. Fed
via

How remote?

The shed is about 25 feet from the house. The total length of SWA/T&E
from CU to CU is probably around 20m or less


Proposed new set up:
3core 4mm SWA - earth is taken through the yellow wire - with the
armour sleeving bonded to the yellow wire at both ends.

What type of earthing do you have at the house?

Sorry, forgot to mention that - it's the standard TN-S with the earth
supplied by the electric co.

You may not want to export the house earth if it is either a TT setup,
or if the shed is a long way from the house. If this were the case then
you could connect the earth to the shield at the source end, but
terminate in a plastic CU at the shed end and provide a local earth via
a rod.

Two way CU in the shed with mcbs and protected by integral 30mA RCD.
Sockets (probably three) in the shed, either as a ring main or as a
radial circuit, plus an external socket mounted externally on the
shed wall for garden use.
- 2.5mm T&E fed from a 16A mcb

Shed lighting plus external light
- 1.25mm T&E fed from a 6A mcb

1mm sq will be more than adequate for the lights on a 6A MCB.

What sort of work are you planning to do in the shed - how critical is
the loss of light on a RCD trip going to be?

Not a lot - it's the odd bit of drilling or (electrical) soldering and
that's about it.

If it is an issue you could fit a battery backed emergency light (about
30 quid from TLC).

OK

In-house connection is to a 32A mcb on a split CU and is protected by
the integral 30mA RCD in the CU (there are two split CUs in the
house, this CU only feeds shower and outside lights, so if it trips
it's not a problem).

Since it is protecting the shower, changing it for a higher trip rating
or a time delay version is not really an option. So that kind of
negates any benefit of the one in the shed.

SWA terminated in the shed and indoors in a metal knockout box using
suitable glands. Metal box is also earthed (I assume that a plastic
box could be used instead if required?)

If you are not going to export the house earth then plastic somewhere
(i.e. either the box or the CU) will be needed to isolate the shield.
You can usually take a SWA straight into a CU - it should have a 20mm
knock out that will take a gland)

I realise that this means that the shed light will be lost if the RCD
trips in the shed. I could solve this by using a separate CU in the
shed for lighting only (I have a spare CU) or is this not permissible
as the light would still be lost if the 32A mcb tripped in the house
CU?

It is not the house MCB that is likely to trip, but the house RCD. A
imbalance fault at the shed could trip either the shed, the house or
both RCDs

An alternative would be to use the existing supply from the fused
spur on the ring main to feed a separate CU in the shed just for the
lighting, using a 5A fuse in the spur instead of the current 13A one.
However, I would have liked to have got rid of that cable altogether.

Battery backed light in the shed sounds like a whole lot less fuss.

--
Cheers,

John.

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Thanks for the advice

Roger