On 23 Apr, 21:37, John Rumm wrote:
ac1951 wrote:
I noticed on a recent thread about cable size to an outside detached
Garage, that there where some issues mentioned regarding whether or
not you where exporting a PME earth.

The points raised that interested me related to the size of earth
conductor and usage of the supply.
In my case I have, already in-place, a 2.5mm twin SWA cable connecting
my garage which converts to a 2.5mm T&E cable on the outside of my
house. This is connected to a 6A RCBO in the CU. (Total distance circa
40M)

Did you really mean 6A or was that typo for 16A?

What is the imbalance trip current rating for the RCBO?

In the garage is a 2 way CU with 2x5A mcb's, one feeding the lights
and the other feeding 2xDouble 13A sockets.
I use the sockets for basic power tools (drill, planer, sander etc)
and also electric lawn mower. All the tools are double insulated and
therefore do not have an earth.

That means you are not at risk of taking the PME earth into the garden
etc. Which is good. ;-)

Is this installation legal ?

What is the construction of the garage like? Any other services in there
like water or gas?

What would you do differently ?

Probably make the garage a TT install with its own RCD and local earth.

Where can I find a concise explaination of do's/don't regarding the
exporting of a PME.

Well you could say BS7671 is "concise" to the point of being terse at
times. I don't recall seeing any particularly details articles or
coverage in books on the subject - although there are plenty of items
that cover at least a part of the requirements.

This one is ok:

http://www.iee.org/Publish/WireRegs/...ents/Issue16/2...

Failing that, our very own:

http://wiki.diyfaq.org.uk/index.php?...ricity_outside

Touches on a number of salient points (even more so since I added some
today!)

--
Cheers,

John.

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A timely posting and I hope my questions do not get taken as a
highjack !

I'm about to deal with a similar situation. The garage is fed with a
6mm T&E from it's own mcb in the house CU - earthing is TN-S and I had
already considered that a TT earth for the garage would be a good idea
(30m plus run from the CU) with the earth to the house disconnedted.

Two questions - as the TT earth is relatively high resistance compared
to the supplier earth, and hence the fault current will not be high,
does the earth cable size have to be any bigger than 2.5mm ? If it is
to be bigger, what is the justification for this?

Secondly, although there is no water pipes, etc, the garage roof is
steel frames; should these be bonded ?

Thanks

Rob