Hot tub - 2 core or not 2 core

[Brownerg]

You guys don't miss a thing .. many thanks for all help.

 

[JohnW2]

Whilst everything that has been said may be correct, surely if the armour is adequate for ADS and protection of the cable then it is adequate for ADS to protect the hot tub and occupants.

Indeed - but surely the question is whether, if the armour is used as the sole CPC, the Zs of the circuit will be low enough for ADS (to protect 'everything'). That is what the on-site electrician needs to ascertain.

Kind Regards, John

 

[EFLImpudence]

Indeed - but surely the question is whether, if the armour is used as the sole CPC, the Zs of the circuit will be low enough for ADS (to protect 'everything').

I don't think you have worded that very well.
The other way round, surely.

That is what the on-site electrician needs to ascertain.

Yes, agreed.

I got the impression people were saying it would NOT be - yet good enough for protection of the cable.

 

[JohnW2]

I don't think you have worded that very well. The other way round, surely.

. I don't understand. Missing out the middle clause, my sentence would read "... the question is whether .... the Zs of the circuit will be low enough for ADS". Do you disagree with that?

I got the impression people were saying it would NOT be - yet good enough for protection of the cable.

Fair enough, but that's not how I read any of the comments.

TTC might have confused things a bit my mentioning adiabatic calculations. I very much doubt that such a calculation would indicate that the armour was at any risk of melting under fault conditions (particularly if the Zs of the circuit was 'too high'), BUT it remains possible that the Zs of the circuit would be too high for ADS.

Kind Regards, John.

 

[EFLImpudence]

. I don't understand. Missing out the middle clause, my sentence would read "... the question is whether .... the Zs of the circuit will be low enough for ADS". Do you disagree with that?

You said:
"but surely the question is whether, if the armour is used as the sole CPC, the Zs of the circuit will be low enough for ADS (to protect 'everything')."
Yes, but it is determined the other way round.
Zs first and then whether the armour can be so used.

Fair enough, but that's not how I read any of the comments.
TTC might have confused things a bit my mentioning adiabatic calculations. I very much doubt that such a calculation would indicate that the armour was at any risk of melting under fault conditions (particularly if the Zs of the circuit was 'too high'),

Exactly.

BUT it remains possible that the Zs of the circuit would be too high for ADS.

So, it may not be suitable for ADS protection of the cable, either.

I think it extremely unlikely the armour may not be used as the CPC but some posts, I thought, stated the opposite.

 

[JohnW2]

You said: "but surely the question is whether, if the armour is used as the sole CPC, the Zs of the circuit will be low enough for ADS (to protect 'everything')." Yes, but it is determined the other way round. Zs first and then whether the armour can be so used.

Perhaps, but that doesn't alter the correctness of my statement!

If one wanted to use the armour as CPC, one could do it the 'other other' way around (at design stage, not after the cable had been laid!) - determine/estimate what the Zs would be with the size of SWA being considered and, if that Zs is too high (for ADS) then try again with a larger size of SWA.

So, it may not be suitable for ADS protection of the cable, either.

Indeed. I think some people make the mistake of thinking that just because it has 'earthed armour', SWA is always 'adequately protected', regardless of all else. However, if the armour is being used as CPC, then the impedance/resistance of that armour forms part of the Zs - so if the cable gets long enough, the Zs will be too high for ADS in the event of an L-armour fault at the end of that cable.

I think it extremely unlikely the armour may not be used as the CPC ...

You're probably right. The more common discussion is whether the armour is adequate as a bonding conductor, and the answer to that is far less of a foregone conclusion.

Kind Regards, John

 

[EFLImpudence]

The more common discussion is whether the armour is adequate as a bonding conductor,

Yes, that's what I said in my first reply because I thought it was being confused with the CPC.

 

[JohnW2]

Yes, that's what I said in my first reply because I thought it was being confused with the CPC.

Fair enough.

Of course, there are factors other than the measurements (when the cable is installed) which make some people reluctant to rely upon armour as the only CPC. Although rare, it can happen that a breach in the outer sheath allows water in - which, in turn, may eventually lead to corrosion and the complete or partial disintegration of the armour at the point in question. Unless the whole cable gets 'chopped', it's all-but-impossible that a core of SWA being used as a CPC could suffer isolated damage.

I must say that I've always been in the "don't rely on armour" camp, but I realise that opinions vary, sometimes strongly.

Kind Regards, John

 

[EFLImpudence]

Of course, there are factors other than the measurements (when the cable is installed) which make some people reluctant to rely upon armour as the only CPC.

There are.

Although rare, it can happen that a breach in the outer sheath allows water in - which, in turn, may eventually lead to corrosion and the complete or partial disintegration of the armour at the point in question.

Anything is possible.

Unless the whole cable gets 'chopped', it's all-but-impossible that a core of SWA being used as a CPC could suffer isolated damage.

It is.

I must say that I've always been in the "don't rely on armour" camp, but I realise that opinions vary, sometimes strongly.

I'm not surprised.


H o w e v e r , the OP said he had 10mm² two-core and I thought people were suggesting that this was going to be unsuitable when, in fact, it may not be.

The OP should and must employ an electrician as outside power is not, in my opinion, a DIY job.



As a matter of interest, as you are extra safety-conscious, would you, as a personal preference, TT a hot tub in the garden regardless?

 

[JohnW2]

H o w e v e r , the OP said he had 10mm² two-core and I thought people were suggesting that this was going to be unsuitable when, in fact, it may not be.

I didn't notice anyone saying that it was 'unsuitable', but I did see several people say that an electrician needs to check that it is suitable. As you go o n gto say ....

The OP should and must employ an electrician as outside power is not, in my opinion, a DIY job.

I'm not surprised ...... As a matter of interest, as you are extra safety-conscious, would you, as a personal preference, TT a hot tub in the garden regardless?

I'm not bernard, you know - and if you hadn't worked it out, what I 'preach' and what I practice are often too very different things What I often do do is "play Devil's Advocate", in order to make sure that a balanced picture of a range of opinions is presented to readers.

As for your question, in my house the question would not really arise, since it's hard to think of any credible scenarios in which 'exporting' a TT earth would do any harm. However, if I had TN-C-S, I would give at least some thought to bernard's view. In the very rare scenario of the TN-C-S earth being at a high potential relative to true earth, I would probably not want to be the person (or to care about the person) whose nearly-naked wet body had one foot on surrounding ground at roughly true earth potential and the other in a hot tub whose water was at a high potential relative to true earth!

Kind Regards, John

 

[EFLImpudence]

What I often do do is "play Devil's Advocate", in order to make sure that a balanced picture of a range of opinions is presented to readers.

Ah. No wonder you seem contradictory at times and we get confused.

As for your question, in my house the question would not really arise, since it's hard to think of any credible scenarios in which 'exporting' a TT earth would do any harm. However, if I had TN-C-S, I would give at least some thought to bernard's view.

... but you do, don't you because of the water pipe?

So, you are already up with the proposal for electrodes to be installed.


I asked because I am frequently puzzled by people, not necessarily on here, who, when asked about outside power to sheds etc., state that they would TT it out of preference as a matter of course - even if extending the equipotential zone was not a problem.

 

[JohnW2]

Ah. No wonder you seem contradictory at times and we get confused.

My view has always been that, when there is a range of opinions (or interpretations of the regs), then it is important that DIYers are presented with the more 'cautious' ('safety-conscious') ones, as well as the others. If no-one else has done it, I will therefore often presnt those 'cautious' opinions (usually preference by something like "Some people feel/believe...."), even if they are 'far more cautious' than I would ever contemplate doing myself, in my home.

... but you do, don't you because of the water pipe?

Ah - are you talking about my 'quasi TN-C-S earth? If so, I suppose you have a point, so maybe I should have said "if I had a 'pure' TT system"!

I asked because I am frequently puzzled by people, not necessarily on here, who, when asked about outside power to sheds etc., state that they would TT it out of preference as a matter of course - even if extending the equipotential zone was not a problem.

I can understand that viewpoint, which I suppose one could probably call "bernard's view". In those very rare 'neutral fault' condition, the problem with an exported TN-C-S (as with a metal-plumbed outside tap) arises if it is possible to stand (maybe with bare wet feet) on ground at roughly true earth potential outside the outhouse (hence outside of the equipotential zone) whilst simultaneously touching something within the equipotential zone - a metalclad light switch just inside the door of the outhouse would be an example.

Kind Regards, John

 

[EFLImpudence]

Yes, I realise that, BUT -

if no one ever does anything about the tap on the outside of the house, why do they feel it is different with a shed?

 

[JohnW2]

Yes, I realise that, BUT - if no one ever does anything about the tap on the outside of the house, why do they feel it is different with a shed?

Maybe at least some of them do "do something about the tap on the wall"? After all, it only needs a couple of inches of plastic pipe to eliminate the potential 'problem'.

I suppose this is an example of the "Devil's Advocate" thing. I understand the views of those who feel that it can be 'dangerous' to export a TN-C-S earth, and I feel that DIYers should be made aware of the fact that some people hold that view. However, as far as I am concerned the magnitude of the risk is so small (I would talk about 'extremely/vanishingly low probability' if I was talking to anyone but you!) that" I'm personally really not concerned about it. "Lost TN-C-S neutrals" are rare enough in the first place, and the chances of anyone touching a tap or shed light switch whilst standing with bare feet on the ground, at the very right moment, are ..... !!!!

Of course it can happen - but people also get struck by lightning!

Kind Regards, John

 

[333rocky333]

I always thought the IET or someone like that had calculated that the armour of 2 core is always suitable unless you delv into larger size swa, thats unlikely to be used for a domestic hot tub