Exporting tt earth

Feel the power · 25 Sep 2016

Evening all, just a quick question, I've been looking at this subject quite a bit now can anybody give me some hard evidence requarding exporting a earth to a out building, I've heard so many diffrent things like you shouldn't connect a pme earth to a outbuilding earthing terminal and all sorts, but I can't find any evidence from the regs to say exactly what should be done and what can't be done it's confusing me tbh as every electrician seams to have a diffrent way of doing it that they think is correct, any help would be helpful thanks.

333rocky333 · 25 Sep 2016

It can vary, subject to if there is Extraneous conductive parts, like water pipes in the outbuilding

http://www.diynot.com/diy/threads/exporting-tt-earth-to-detached-garage.222818/

EFLImpudence · 25 Sep 2016

... and then it only really depends on whether you can afford (want to spend on) the necessary bonding conductor.

Feel the power · 25 Sep 2016

But we're can i find actual guidelines to follow for all the diffrent circumstances?

333rocky333 · 25 Sep 2016

Sorry I posted a link for the link to the IEE but it dont seem to work now.

Taylortwocities · 25 Sep 2016

333rocky333 said:
Sorry I posted a link for the link to the IEE but it dont seem to work now.

The IET have been what they laughingly call 'improving' their web site.
It is now located here
http://electrical.theiet.org/wiring-matters/16/elect-inst-outdoors.cfm?type=pdf

Note, this is from 2005, but the basic principles still apply.

EFLImpudence · 25 Sep 2016

Feel the power said:
Evening all, just a quick question,

Ha ha.

I've been looking at this subject quite a bit now can anybody give me some hard evidence requarding exporting a earth to a out building,

There aren't any.
The same rules apply that do anywhere else - earthing, bonding, volt drop, EFLI.

I've heard so many diffrent things like you shouldn't connect a pme earth to a outbuilding earthing terminal

That's not true.

but I can't find any evidence from the regs to say exactly what should be done and what can't be done

Yes you can - BS7671.

it's confusing me tbh

Not surprising.

as every electrician seams to have a diffrent way of doing it that they think is correct,

but may not be.

Just treat the outbuilding like a kitchen however many metres away it may be.
If it has extraneous-conductive-parts they will need main bonding; this will today be 10mm² minimum back to the MET so if it is 100m. it may be cheaper to TT it.
If your supply cable is 10mm² anyway, you can use that.

There is no standard plan. It's up to you.

JohnW2 · 26 Sep 2016

Feel the power said:
Evening all, just a quick question,....

Just to be clear (I can be slow at times!) ... are you saying (as the title seems to imply) that it is a TT earth that you wish to export?

Kind Regards, John

Feel the power · 26 Sep 2016

Yes it is John, but if possible I would like to know we're I can find information about powering outbuildings with diffrent earthing systems at the dwelling as I can't seam to find any concrete answers requarding this subject

EFLImpudence · 26 Sep 2016

That's still not clear. Is the main building TT earthed?

Feel the power · 26 Sep 2016

Yes the main building is tt earthed

Taylortwocities · 26 Sep 2016

Then it's a no brainer. You should not export the existing earth. Just bang in a new rod at the outbuilding.

JohnW2 · 26 Sep 2016

Taylortwocities said:
Then it's a no brainer. You should not export the existing earth. Just bang in a new rod at the outbuilding.

Indeed, which is why I asked the question.

Since the OP was interested in knowing about the general considerations when earthing an outbuilding ..... As with any locally TT'd outbuilding, one does not want a situation in which one can simultaneously touch the local TT earth and the imported earth (whether it be TT or TN), if it is not 'bonded', since there is a risk of the imported house earth being a a high potential (relative to true earth) due to a fault. One could simply connect ('bond') together the two TT earths in the outbuilding, which would prevent any dangerous pds existing within the building, provided that any extraneous-conductive parts within it were bonded to the local earth. However, if one did that, an LE fault in the house could still result in any exposed-c-ps within the outbuilding acquiring a fairly high potential relative to truth earth - which would represent a potential hazard if one could touch one of those exposed-c-ps whilst standing outside the outbuilding, or if there were 'extraneous-c-ps' (like a floor) which were effectively unbondable.

I would therefore think that, just as when the house earth is TN, it is still advisable not to 'join' the two TT earths (which is fairly obvious) and, more importantly, not to allow the house's TT earth to be touchable within the outbuilding - for example, by ensuring that the metal parts of any SWA gland on an incoming cable are not touchable.

Kind Regards, John

EFLImpudence · 26 Sep 2016

It would surely depend on the distance to the outbuilding and between the two rods.

JohnW2 · 26 Sep 2016

EFLImpudence said:
It would surely depend on the distance to the outbuilding and between the two rods.

I don't think it would make much difference unless the distance was so great (and the cable of such a size) that the resistance/impedance of the cable was appreciable in relation to the resistance/impedance of the TT electrode, which I doubt would often be the case.

Kind Regards, John