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Earth on Garage Consumer Unit

Postby Chalkie » Wed Jul 02, 2008 2:15 pm

Just had an outbuilding converted to an office and am putting in a new CU for the 3 scokets and lights. Also linking to this is an amoured cable to take power to a garage some 30m away.

I have two questions please:

1) The amoured cable is four core and the way it was wired before two were used for the live and two for the neutral - does it not have an earth?

2) Do I need to earth the new CU? Obviously all the earths from the ring mains and lights are connected in the unit but do I then need to run an earth cable from this? If so can I attach this to a copper oil tank pipe which is partially submerged in concrete or do I need an earth rod?
The CU in the garage has no earth running to it so same question applies here please.
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Simply Build It

Postby TheDoctor4 » Wed Jul 02, 2008 2:37 pm


Have you had a look at the DIY Doctor projects section yet? If not, have a look on the following URL: You may find the answer in one of the electics projects

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Postby ericmark » Wed Jul 02, 2008 6:58 pm

You don’t give any figures on cable size or feed current but you may be able to use the steel armouring as an earth cable. There is a paper by Ian Reeve on shed wiring and one by John Peckham on use of steel wire armouring as an earth on IET web site and the latter gives the equivalent sizes of steel wire armouring to copper for use as an earth. But at 30 meters distance it may be as well to use a TT earthing system. As to the use of oil tank pipe 542.2.4 rules this out. If you are unable to use the steel wire armouring you will have to fit an earth rod. The earthing arrangements are complex with measurements being required. Not really a DIY job. And at 30 meters the feed cable my be very limited in what power it can handle.
All best Eric

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