Armoured Cable Query

[EFLImpudence]

I really don't see that the fact that someone might connect them with cables of an inadequate CSA is a credible reason for 'forbidding' (or regarding as 'potentially hazardous') the connection of an earth electrode to a TN-C-S earth (which is what we are talking about). By the same logic, you would 'forbid' sockets, cooker, shower circuits etc., on the grounds that someone might wire them with 1mm² cable.

It's not that someone might wire them by mistake with too small a cable.
It is usually the reason for TTing a location so that a large bonding conductor does not have to be purchased.

In any event, in what you say above, the connection to the earth electrode is not the issue. Very few domestic earth electrodes would have an impedance anything like as low as 20Ω, and even at 20Ω, a 1mm² conductor would be more than adequate (CCC-wise). Is the connection to other (possibly low impedance) extraneous-c-ps that could be of inadequate CSA, and hence potentially hazard, but that has got absolutely nothing to do with whether is is allowed/safe to connect an earth electrode to a TN-C-S earth.

...but you cannot connect the electrode without connecting the extraneous-c-ps - one of which may be a water pipe similar to yours.

 

[JohnW2]

I really don't see that the fact that someone might connect them with cables of an inadequate CSA is a credible reason for 'forbidding' (or regarding as 'potentially hazardous') the connection of an earth electrode to a TN-C-S earth (which is what we are talking about). By the same logic, you would 'forbid' sockets, cooker, shower circuits etc., on the grounds that someone might wire them with 1mm² cable.

It's not that someone might wire them by mistake with too small a cable. It is usually the reason for TTing a location so that a large bonding conductor does not have to be purchased.

Yes, but as I keep saying, you have turned this into a discussion about outhouses, whereas I was talking about the general concept of connecting an earth electrode to a TN-C-S earth, which so many people (in UK) seem to think that is potentially hazardous and/or 'forbidden'.

As I've said, if there is a problem such as you describe (with an outhouse), it is going to be because of other extraneous-c-pc, not because of the earth electrode. Whether there are such 'other' extraneous-c-ps or not, connecting an earth electrode (with any credible cable ≥1mm) to the TN-C-S earth will not create any new hazard.

Kind Regards, John

 

[EFLImpudence]

Yes, but as I keep saying, you have turned this into a discussion about outhouses, whereas I was talking about the general concept of connecting an earth electrode to a TN-C-S earth, which so many people (in UK) seem to think that is potentially hazardous and/or 'forbidden'.

It is an example where the electrode must not be connected to the TN-C-S earth.
There is no 'general concept'; it depends on the installation.

As I've said, if there is a problem such as you describe (with an outhouse), it is going to be because of other extraneous-c-pc, not because of the earth electrode.

Precisely.
So - there is no reason not to connect an electrode to a TN-C-S earth unless it is connected to other parts which may not be so connected (inadequate bonding back to source).
If the electrode is alone and so able to be connected what is its purpose? (Why is it there?).

Whether there are such 'other' extraneous-c-ps or not, connecting an earth electrode (with any credible cable ≥1mm) to the TN-C-S earth will not create any new hazard.

It will if that path is via a distribution circuit having a 1.5mm² CPC to which is also connected a water pipe whose path to earth gives rise to a Zs of 0.2Ω.

 

[JohnW2]

Yes, but as I keep saying, you have turned this into a discussion about outhouses, whereas I was talking about the general concept of connecting an earth electrode to a TN-C-S earth, which so many people (in UK) seem to think that is potentially hazardous and/or 'forbidden'.

It is an example where the electrode must not be connected to the TN-C-S earth.

As I've said, I don't understand that. AFAICS, whether other extraneous-p-s are adequately bonded or not, adding an earth electrode will not result in any additional hazard. What am I missing?

So - there is no reason not to connect an electrode to a TN-C-S earth unless it is connected to other parts which may not be so connected (inadequate bonding back to source).

Again, I find it hard to believe that in any real world situation, it would be possible for the path back to the 'source' (MET) not to be adequate for the earth electrode. The path may be inadequate for other extraneous-c-ps, but that's a totally different matter (which exists whether or not there is an earth electrode)

If the electrode is alone and so able to be connected what is its purpose? (Why is it there?).

That's a good question, but totally different from the belief that it is hazardous and/or 'not allowed'. You could, of course, ask that same "Why is it there?" question in relation to the earth electrode connected to TN-C-S earths which is apparently compulsory throughout (or virtually throughout) the US. I suppose the simplest answer is that it adds another 'M' to 'PME'.

Whether there are such 'other' extraneous-c-ps or not, connecting an earth electrode (with any credible cable ≥1mm) to the TN-C-S earth will not create any new hazard.

It will if that path is via a distribution circuit having a 1.5mm² CPC to which is also connected a water pipe whose path to earth gives rise to a Zs of 0.2Ω.

What is this perceived additional hazard created by connecting an earth electrode to it? As I see it, no additional hazard would arise whether the Zs were 0.001Ω or 1000Ω.

Kind Regards, John

 

[EFLImpudence]

As I've said, I don't understand that. AFAICS, whether other extraneous-p-s are adequately bonded or not, adding an earth electrode will not result in any additional hazard. What am I missing?

That the electrode and extraneous-c-ps are connected.

Again, I find it hard to believe that in any real world situation, it would be possible for the path back to the 'source' (MET) not to be adequate for the earth electrode. The path may be inadequate for other extraneous-c-ps, but that's a totally different matter (which exists whether or not there is an earth electrode)

They cannot be separated.

That's a good question, but totally different from the belief that it is hazardous and/or 'not allowed'. You could, of course, ask that same "Why is it there?" question in relation to the earth electrode connected to TN-C-S earths which is apparently compulsory throughout (or virtually throughout) the US. I suppose the simplest answer is that it adds another 'M' to 'PME'.

Yes. but PBC says it must be alone.
It is not remote from the source and a TT in its own right with extraneous-c-ps connected.

What is this perceived additional hazard created by connecting an earth electrode to it? As I see it, no additional hazard would arise whether the Zs were 0.001Ω or 1000Ω.

That the 0.001Ω impedance e-c-p may be connected to the source earth by a very small conductor which in a fault situation may carry the full installation neutral current.

 

[JohnW2]

As I've said, I don't understand that. AFAICS, whether other extraneous-p-s are adequately bonded or not, adding an earth electrode will not result in any additional hazard. What am I missing?

That the electrode and extraneous-c-ps are connected.

The current which could flow through the conductor to the earth electrode will still be limited (to a low value) by the impedance of that electrode.

That's a good question, but totally different from the belief that it is hazardous and/or 'not allowed'. You could, of course, ask that same "Why is it there?" question in relation to the earth electrode connected to TN-C-S earths which is apparently compulsory throughout (or virtually throughout) the US. I suppose the simplest answer is that it adds another 'M' to 'PME'.

Yes. but PBC says it must be alone. It is not remote from the source and a TT in its own right with extraneous-c-ps connected.

It is you who has introduced this "remote from the source" issue - although, frankly I don't think what I'm saying is any different whether we are talking about 'at the source' or 'half a mile' from the source.

What is this perceived additional hazard created by connecting an earth electrode to it? As I see it, no additional hazard would arise whether the Zs were 0.001Ω or 1000Ω.

That the 0.001Ω impedance e-c-p may be connected to the source earth by a very small conductor which in a fault situation may carry the full installation neutral current.

Mr Ohm might be turning in his grave! If (very optimistically, domestically) one assumes that the earth electrode has an impedance of 20Ω, the greatest current that could flow along the conductor to it would be 11.5A (and that only if the conductor were connected directly to a 230V source) - and, as I've said, a 1mm² conductor would be OK for that. More commonly, the electrode would probably be at least 50Ω, in which case the maximum current which could flow would be 4.6A (so 0.5mm² cable would probably be adequate ). Again, what is this perceived 'hazard'?

Kind Regards, John

 

[EFLImpudence]

Mr Ohm might be turning in his grave! If (very optimistically, domestically) one assumes that the earth electrode has an impedance of 20Ω, the greatest current that could flow along the conductor to it would be 11.5A (and that only if the conductor were connected directly to a 230V source) - and, as I've said, a 1mm² conductor would be OK for that.

...but that current would also find its way to the very low impedance extraneous-c-p after flowing through the 1.5mm² CPC to the outbuilding.

If not, what is the need for 10mm² bonding conductor?

 

[JohnW2]

Mr Ohm might be turning in his grave! If (very optimistically, domestically) one assumes that the earth electrode has an impedance of 20Ω, the greatest current that could flow along the conductor to it would be 11.5A (and that only if the conductor were connected directly to a 230V source) - and, as I've said, a 1mm² conductor would be OK for that.

...but that current would also find its way to the very low impedance extraneous-c-p after flowing through the 1.5mm² CPC to the outbuilding. If not, what is the need for 10mm² bonding conductor?

I can't be expressing myself clearly enough ...

Yes, of course, if the bonding conductor to a "very low-impedance extraneous-c-p" were of inadequate CSA then that would obviously be a problem in itself, which would need to be remedied in order to avoid danger to that conductor and potentially dangerous 'touch voltages'.

However, my point is that connecting an additional earth electrode would make absolutely no difference, whether or not the bonding conductor to the "very low-impedance extraneous-c-p" was of adequate CSA. If, as you postulated, that extraneous-c-p has an impedance of 0.2Ω to earth, what material difference could it possibly make to add a 'parallel path' of 20Ω or 50Ω to earth?

Kind Regards, John

 

[EFLImpudence]

I can't be expressing myself clearly enough ...

Likewise.

Yes, of course, if the bonding conductor to a "very low-impedance extraneous-c-p" were of inadequate CSA then that would obviously be a problem in itself, which would need to be remedied in order to avoid danger to that conductor and potentially dangerous 'touch voltages'.

The small CPC to a TT outbuilding would become the bonding conductor if you connect the TT earth (the electrode) to the TN-C-S supply earth.

A thought - what are you going to use to connect the electrode to the supply earth?

 

[JohnW2]

Yes, of course, if the bonding conductor to a "very low-impedance extraneous-c-p" were of inadequate CSA then that would obviously be a problem in itself, which would need to be remedied in order to avoid danger to that conductor and potentially dangerous 'touch voltages'.

The small CPC to a TT outbuilding would become the bonding conductor if you connect the TT earth (the electrode) to the TN-C-S supply earth.

I can see that we're getting abit into semantics here. If, sticking with your 'remote location' scenario, one has a conductor which is connected to a TN-C-S earth at one end and to a 'TT' earth electrode at the other end, it's really fairly arbitrary as to what one calls that conductor.

A thought - what are you going to use to connect the electrode to the supply earth?

That (and only that) is what I have been talking about - and, as I said, I see no reason why, electrically speaking, it would ever need to be bigger than about 1mm² - whether all the way back to the MET in that CSA (if there are no other extraneous-c-ps) or just as far as a larger-CSA conductor bonding other extraneous-c-ps.

I am ignoring the question of why one would want two different earths connected together, since that's a different matter! The only point I'm making is that, AFAICS, connecting an earth electrode to a TN-C-S earth (anywhere) is never going to do any harm - although I'm not sure what 'good' (other than "adding an M") it would do. As for your outbuilding scenario, the real choice is to EITHER just use to TN-C-S earth (with adequate bonding for any "very low imoedance extraneous-c-ps") OR to insulate the outbuilding from the TN-C-S earth and just TT the outbuilding (including the bonding of any other extraneous-c-ps) - I see virtually no merit (albeit 'no harm') in 'both'.

Kind Regards, John[/QUOTE]

 

[EFLImpudence]

 

[ban-all-sheds]

That the 0.001Ω impedance e-c-p may be connected to the source earth by a very small conductor which in a fault situation may carry the full installation neutral current.

Possibly the current from multiple installations.

 

[ban-all-sheds]

I'm not sure what 'good' (other than "adding an M") it would do.

Given what you said about DNOs equating "multiple" and "two", I think that everybody adding an "M" has a great deal of merit.

 

[JohnW2]

That the 0.001Ω impedance e-c-p may be connected to the source earth by a very small conductor which in a fault situation may carry the full installation neutral current.

Possibly the current from multiple installations.

Indeed - but when we are talking about the conductor connecting a domesic earth rod to that TN-C-S earth (directly or indirectly), how are you ever going to get more than, say, 11.5A (and even hat assuming a very optimistic 20Ω rod impedance) flowing through the conductor?

EFLI simply seems to be talking about possible inadequacy of the bonding conductor to 'conventional' (potentially "very low impedance") extraneous-c-ps. That's obviously a legitimate concern, but, IMO, adding a local earth rod to the equation would not make one measurable iota of difference.

Kind Regards, John

 

[JohnW2]

I'm not sure what 'good' (other than "adding an M") it would do.

Given what you said about DNOs equating "multiple" and "two", I think that everybody adding an "M" has a great deal of merit.

Well, it's what westie told me. However, I have no reason to not believe him, in which case I totally agree with you.

From what we've been told, the Americans appear to have worked this out already - by demanding (rather than 'forbidding') an earth electrode to be connected to the incoming TN-C-S earth (albeit they seem to have much shorter LV network runs, hence much less scope for 'M's of their own)!

Kind Regards, John