Exporting tt earth

bernardgreen · 26 Sep 2016

JohnW2 said:
it is still advisable not to 'join' the two TT earths (which is fairly obvious)

There are a few, very few circumstances, when bonding the two TT earths together is necessary. These are those situations where there can be high ground current from an external source giving rise to a steep voltage gradient across the site. Sites in close proximity to railway tracks with 25kV overhead supplies used to create steep voltage potential due the return current not all going along the rails. A return conductor is now used to return the current to the substation with current transformers that ensure teh return curren in the conductor is eaual to the current in the over head cable.

EFLImpudence · 26 Sep 2016

JohnW2 said:
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.

I was referring to cases where the distance may be quite small.

JohnW2 · 26 Sep 2016

EFLImpudence said:
I was referring to cases where the distance may be quite small.

Oh, I see. No matter how short the distance, the potential issues I mentioned remain - i.e. if it is possible to touch exposed-c-ps within the outhouse (commonly a light switch just inside the door) whilst standing on 'true earth' outside, or if there are potential paths to truth earth within the outhouse (e.g. the floor) which are essentially 'unbondable'. As bernard often reminds us, the same potential risk even exists if one 'exports' the house's earth to a tap attached to the outside of the house's wall.

You might regard those risks as too small to worry about (particularly given that faults within the house should be rapidly cleared by a protective device) but, if you don't, then exporting the house's earth of any sort (TT or otherwise) is probably not desirable.

Kind Regards, John

JohnW2 · 26 Sep 2016

bernardgreen said:
There are a few, very few circumstances, when bonding the two TT earths together is necessary.

Indeed, and I wasn't suggesting that it was 'necessary' from the earthing point-of-view. I was merely pointing out was that joining/bonding them in the outhouse might be perceived as a means of avoiding the theoretical need to isolate the house's earth from the outhouse - which can require a bit of thought if, as will be often the case, the supply enters the outhouse in SWA.

Kind Regards, John

EFLImpudence · 26 Sep 2016

The OP is asking for a rule book for connecting outbuildings but there isn't one; it depends.

My point is that in a house with TT, the outbuilding could be nearer the electrode and MET than other exterior parts of the house yet little consideration is given to this.
Should outside taps at the far end of a house have an electrode? What is the difference between them and an outbuilding not far away? The TN-C-S connection to true earth will be quite a long way away.

Some, in other places, state that they would always TT an outbuilding and treat extending the TN equipotential zone (not exporting the earth) as a no-no in all circumstances. If there are extraneous-conductive-parts then, effectively, there is (may be) already an electrode in place (I know it may not be allowed as such but it is) and if this extraneous-c-p is a metal water pipe from the house there is an electrode which is connected to the TN earth so why is connecting a true electrode to the MET so undesirable?
Should the house be a satisfactory TN-S, this could actually run under the outbuilding.

JohnW2 · 26 Sep 2016

EFLImpudence said:
The OP is asking for a rule book for connecting outbuildings but there isn't one; it depends.

Agreed.

EFLImpudence said:
My point is that in a house with TT, the outbuilding could be nearer the electrode and MET than other exterior parts of the house yet little consideration is given to this.

It could, but I'm not sure what sort of 'consideration' you feel should be give. No matter how near or far the outbuilding (or the house) is from the TT electrode, the potential difference (between house and outbuilding) I was talking about was the possibility of, say, an 'unbondable floor' in the outhouse which offered a path to earth, whereas the house should be constituted as an equipotential zone.

[In case someone quibbles, I would agree that an exposed-c-p within a house which can be touched by someone standing outside the house (e.g., again, a metal light switch just inside the door) presents the same potential risk - so, for the risk-averse, probably should be avoided.]

EFLImpudence said:
Should outside taps at the far end of a house have an electrode?

No. Taps are not 'electrical' (i.e. not exposed-c-ps) and therefore do not need to be earthed. As I've said (and bernard has said more often!), the ideal would be for them to be electrically isolated from the house's earthing system (e.g. by plastic plumbing).

EFLImpudence said:
What is the difference between them and an outbuilding not far away?

Nothing, IF, like the tap, the outbuilding has no exposed-c-ps, associated with electrical items.

EFLImpudence said:
If there are extraneous-conductive-parts then, effectively, there is (may be) already an electrode in place (I know it may not be allowed as such but it is) and if this extraneous-c-p is a metal water pipe from the house there is an electrode which is connected to the TN earth so why is connecting a true electrode to the MET so undesirable?

IMO, it clearly is not 'undesirable' or, at least, even if some people regard it as 'undesirable', it is inevitable. As you say, there is absolutely no difference between something called a TT electrode and a metal service pipe entering the property. If it is not only permitted, but very much required, that the latter be connected to the MET (in any installation, not just TN), then it is totally irrational to suggest that connecting a "TT electrode" to the MET is 'not allowed'. As we've been told, in a good few countries (inclkuding the US, IIRC) a local TT electrode is actually required with TN installations.

Kind Regards, John

Risteard · 26 Sep 2016

JohnW2 said:
As we've been told, in a good few countries (inclkuding the US, IIRC) a local TT electrode is actually required with TN installations

You needn't look as far away as that. In the south of Ireland very few installations are TT (Direct Earthing) now. Almost all are TN-C-S (Neutralised), and an earth electrode is required to be connected to the MET of every installation.

A Main Protective Conductor (or Neutralising Conductor) connects the MET to the PEN conductor, and an Earthing Conductor connects the MET to the earth electrode.

Where this is not present it is considered a serious defect.

EFLImpudence · 26 Sep 2016

JohnW2 said:
No. Taps are not 'electrical' (i.e. not exposed-c-ps) and therefore do not need to be earthed.

No, but they are bonded and may become live in a fault condition and a person may be stood on wet ground at a different potential.
Should (or would it be better if) the pipe be locally bonded to the ground?

Agree with everything else. I thought I was missing something.

EFLImpudence · 26 Sep 2016

Risteard said:
You needn't look as far away as that. In the south of Ireland very few installations are TT (Direct Earthing) now. Almost all are TN-C-S (Neutralised), and an earth electrode is required to be connected to the MET of every installation.

Is there a stated reason or purpose for this?

A Main Protective Conductor (or Neutralising Conductor) connects the MET to the PEN conductor, and an Earthing Conductor connects the MET to the earth electrode.

Surely the MPC (or NC) is THE earthing conductor.

Risteard · 26 Sep 2016

EFLImpudence said:
Is there a stated reason or purpose for this?

Basically the PEN will be spiked at the ESB transformer, at each mini-pillar etc. and at each installation. A better question might be why does BS7671 not require us to fit an earth electrode within our installation to ensure the PEN conductor is kept at Earth potential. (Obviously a fault could still cause quite a voltage rise on the neutral with respect to Earth, but it is still another point at which the neutral is earthed.)

Surely the MPC (or NC) is THE earthing conductor.

No. You can't apply BS 7671 terminology to ET 101.

The Earthing Conductor is what is ensuring that the PEN conductor is kept at Earth potential. The Main Protective Conductor is obviously what will be more influential in ensuring that disconnection times are met.

JohnW2 · 26 Sep 2016

Risteard said:
You needn't look as far away as that. In the south of Ireland very few installations are TT (Direct Earthing) now. Almost all are TN-C-S (Neutralised), and an earth electrode is required to be connected to the MET of every installation. A Main Protective Conductor (or Neutralising Conductor) connects the MET to the PEN conductor, and an Earthing Conductor connects the MET to the earth electrode. Where this is not present it is considered a serious defect.

Thanks. As I've said, it would make no sense for connection of a TT electrode to the MET of a TN system to be 'prohibited', given that it's no different from an incoming metal service pipe (which must be connected to the MET).

What is actually achieved by requiring a TT electrode (in RoI, US or wherever) with a TN system, I'm not so clear about. Whilst, at first sight, it sounds like a ('belt and braces') 'good idea', in reality the impedance to earth of a domestic TT electrode is usually going to be so (relatively) high as to afford minimal protection against the consequences of the much-feared (but undoubtedly very rare) 'lost neutral' with a TN-C-S supply - and, so long as the supply neutral is intact (and connected to the MET), I cannot see that it achieves anything (other than affording some protection in the case that the connection between Neutral and the MET is removed). I guess I must be missing something?

Kind Regards, John

Risteard · 26 Sep 2016

Have a look at this PDF for more information:

http://local.ecollege.ie/Content/APPRENTICE/liu/electrical_notes/LL226.pdf

Page 6 deals with the Earthing Conductor & Main Protective Conductor.

Page 9 deals with the Earth electrode.

JohnW2 · 26 Sep 2016

EFLImpudence said:
No, but they are bonded and may become live in a fault condition and a person may be stood on wet ground at a different potential.

Precisely, that's the theoretical danger - which is why the safest (and 'foolproof') thing is for the outside tap not to be 'bonded' (i.e. isolated from bonded metal pipework within the house by a plastic interruption).

EFLImpudence said:
Should (or would it be better if) the pipe be locally bonded to the ground?

If it were a risk that concerned me in the first place (which it isn't), I'd be happier with the tap simply to be electrically isolated from the house's 'equipotential zone'. However, if it were connected to bonded pipework within the house and you did "bond it locally to ground", the local TT electrode would have to be very close to the tap - just a few feet away and you could 'be in trouble', particularly if the ground were dry!

Kind Regards, John

EFLImpudence · 26 Sep 2016

I agree about isolating the tap but "nobody" does that and yet "the same people" insist on TTing outbuildings which may be no farther away from the MET.

EFLImpudence · 26 Sep 2016

Risteard said:
Page 6 deals with the Earthing Conductor & Main Protective Conductor.

Where it says that your EC is dependent on the size of the largest phase conductor, what is the relationship between the two? Please.

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Exporting tt earth

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