I participated in (but didn't initiate) the thread back in June - from which
I learned quite a lot - but have a few more questions.
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
currently has a 6mm^2 T&E cable connected to it, which was originally
intended for an electric shower - but we used stored water and a shower pump
instead - so this way and cable (with a suitable MCB) can be re-deployed.
The 6mm^2 cable ends (with a couple of feet spare) just above the ceiling of
an en-suite bathroom - more or less vertically
above the point where the house end of the garage connection needs to be. I
intend to join some 6mm^2 SWA cable to this
cable, and take it through the wall and down the outside of the house, and
across a passage door frame into the garage.
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.
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.
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
* 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]
* 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
* 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
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? Do I need to earth the SWA's armour even if it isn't
being used as the protective earth?
Your expert advice will be much appreciated.
Oh, and in case anyone mentions Part P - I think that what I am doing is
probably covered by Part P. AIUI, extending an existing radial circuit
isn't, but when you start taking it outside, it is. The building itself is
exempt from building regs, being a detached garage of less than 30 M^2 floor
area, and meeting the other exemption conditions. My inclination therefore
is to forget about Part P and hope that it gets lost in the noise when the
property is eventually sold (hopefully far into the future). Nevertheless, I
want to ensure that I end up with a 'safe' electrical installation - hence
the questions.
--
Cheers,
Roger
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