Roger Mills wrote:

My detached double garage is currently being built, and is about 1.1 metres
from the house. I intend to use a garage consumer unit - probably
http://tinyurl.com/zmmh4, which I intend to connect to a spare way in my
domestic CU. This

It looks rather like the RCD covers both ways on that. Depending on how
you plan to use the garage you may wish to opt for some arrangement that
will not lose the lights at the same time you lop through a power cord
with a still spinning circular saw! (i.e. switched two way CU with a
RCBO for the sockets, or perhaps the one pictured with a non maintained
emergency light)

Although the garage consumer unit can handle 40 amps, this is more than I
need (and the sum of the individual MCBs is a lot less than this anyway) so
I intend to use a 32 amp Type-B MCB at the house end of the connection -
which seems more
appropriate than a higher value if relying on the 2.5mm^2 earth within the
existing T&E cable.

On the same theme as above you may need to bear in mind is
descrimination of the protective devices. You want to try and ensure
that a trip on the power circuit in the garrage only takes out the local
MCB and not the upstream one in the main CU as well hence losing your
lights.

My questions concern earthing arrangements, and physically joining the
cables together.

From my description of my incoming mains back in June, Andy Wade thought
that I have a TN-S system, and David Hansen thought it was more likely
TN-C-S. In case it helps, I've posted a photo of the main fuse and meter
connection at
http://www.mills37.plus.com/Incoming_mains.JPG Just to remind you, I
estimate that I have about 10 metres of T&E, and will need about 6 metres of
SWA.

That is a different cable head end from any I have seen in use in this
area, so can't give you a definitive answer. However the connection of
the (undersized!) earth connector to the side of the main cutout like
that would suggest TN-C-S.

Where does that other earth wire go that drops down from the CU? What is
that little box at the bottom of the frame?

The options for earthing appear to be as follows:

* Use 3-core SWA cable, and use one core (6 mm^2) as the earth - and join it
onto the end of the T&E's 2.5mm^2 protective earth conductor - thus
exporting the earth from the domestic CU

This is slightly more reliable that the next option in that there is
less chance of the earth becoming poor due to corrosian etc at the gland
- but as you later say these are internal connections and a new garage
is unlikly to be damp so there is not much in it. If doing this I would
tend to use one core *and* the screen.

* Use 2-core SWA, and use the steel armour as an earth, again joined to the
T&E's earth conductor - again exporting the
earth. [There will be no external connections to be subjected to 'weather',
so corrosion shouldn't be a problem]

One slight advantage with this one is the cable will be a little thinner
and easier to handle.

* Use 2-core SWA and an earth spike at the garage end - thus providing the
garage with its own earth - independent of the house supply

If the house end is not already TT then the arguments for a separate TT
supply in an outbuilding usually come down two one of two reasons -
either it is a long distance (obviuosly not the case here), or, when
there will be potential difficulty extending the house's equipotetial
zone into the outbuilding (of most concern with a PME house).

* Some combination of the above. [Is there a down side to exporting the
earth *and* having an earth spike?]

I would welcome constructive comments on the relative merits of each of the
above options - plus identification of any viable options I may have missed.

Now to joining the cables . .

The most convenient solution would be to have the joint inside a deep
(metal) socket box high on the wall of the en-suite
bathroom - sunk into the blockwork, and covered by a flush blanking plate.
The T&E would come down from above, and be chased into the plaster and the

If coming from above, could you not have an adaptable metal box in the
loft space instead?

SWA would go straight out through the wall from the back of the box - with
its gland fitted to a knock-out in the box. Does anyone see any problems
with this? How should I physically join the cables? Is a large chocolate
block connector (inside the box) ok - or is there anything purpose-made
which would be better?

You could use crimps, but since it will remain accessable screw
connectors are ok. If you opt for using the screen as a CPC I would be
reluctant to have the gland connection burried in a wall behind a metal
box.

Do I need to earth the SWA's armour even if it isn't
being used as the protective earth?

It is good practice since it offers better protection to the cable
(especially since you don't currently appear to have a RCD at the head
end). If you were using TT in the garage then you can earth it at the
head end, but leave the other end disconnected (would require a
insualted CU in the garage)

Your expert advice will be much appreciated.

As usual it is worth what you paid! ;-)

--
Cheers,

John.

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