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Isolating PME from TT addition

Discussion in 'Electrics UK' started by equitum, 22 Jun 2021.

  1. equitum

    equitum

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    Well trodden debate in terms of say hot tubs or outbuildings where the origin is a TN-C-S, the general consensus rightly or wrongly is to not “export” the PME to the load side and instead “TT” it. However when the supply is via an SWA cable this raises the issue of the armour/glands becoming an extraneous conductive part and potentially introducing touch voltage risk outdoors.

    it seems to be commonplace to earth the armour at the supply end and then use stuffing glands or other means at the “TT” end to keep the PME earth isolated from the TT electrode earth and final circuits/load.

    Is it not logical to earth the armour from the TT earth and isolate at the supply end in similar fashion with stuffing gland etc therefore there is no risk from either touch voltage or lost PEN scenario causing the armour to become live ?

    I can see there is a problem in protecting the SWA cable by ADS in this manner but the use of an RCD providing fault protection for the SWA would be no different than a TT supply is from the DNO ?
     
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  3. AndyPRK

    AndyPRK

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    I think both methods get used.
    Dave savoury covered it in video a few days ago

    he used the earth at the supply end(for armour), to avoid having an RCD at the supply (which was far away)
     
    Last edited: 22 Jun 2021
  4. EFLImpudence

    EFLImpudence

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    Firstly, I disagree with that - although a lot do say it.
    Call it extending the equipotential zone instead of exporting the earth.

    It is not really an issue as it is not difficult to isolate the armour.

    Yes, that would be the normal way.

    Ok.

    That is not the main risk.
    Ensuring ADS on an earth fault because of penetration of the cable is more important.

    There you go. You are solving a problem you have created unnecessarily.

    The main reason for TTing an outbuilding is to avoid the necessary 10mm² (or equivalent) bonding cable back to the source which might be necessary.
     
  5. JohnW2

    JohnW2

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    I'm not sure I would personally say that was the ';main reason' (and it often wouldn't be necessary, anyway) - I personally would be more comfortable not having a TN-C-S earth which was 'touchable' at a hot tub or outhouse that was surrounded by other things at close to true earth potential.

    Kind Regards, John
     
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  6. EFLImpudence

    EFLImpudence

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    Ok. I did say outbuilding - a tub is different.

    All buildings are surrounded by other things at close to true earth potential.
     
  7. equitum

    equitum

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    Taking the outbuilding as an example, Most garden rooms sheds garages and the like kind do not have gas or water supplies or other extraneous conductive parts that need bonding. So is it really better to have a PME earth outside of the equipotential zone than have it isolated to within it - I’m assuming you are happy with a 30ma RCD providing protective measures for the final circuits /load
     
  8. EFLImpudence

    EFLImpudence

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    It wouldn't be outside the equipotential zone then.

    I note some people are, but -
    RCDs are additional protection - obviously unavoidable with a TT installation - would deliberately omitting other possible methods be acceptable?


    If the house was larger and reached to where the outbuilding was, would TTing the back room even be a thought?
     
  9. JohnW2

    JohnW2

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    It is a bit different, but the OPO did talk about "hot tubs or outbuildings".
    That's literally true, but it's a 'matter of degree'. Consider my house and outbuildings which, in this respect must be similar to many/most others - I have two or three outbuildings in relation to which I could theoretically be standing with bare feet on wet soil just outside the door, whilst reaching through to one door to operate (i.e. touch) the (metalclad) light switch just inside the door (and, in the case of the greenhouse, I could be standing on wet ground inside it).

    In contrast, whilst there are light switches just inside some of the external doors of my house, there is nothing approaching 'soil' (rather, stone, paving or concrete) that I could be standing on just outside of the door - not 'foolproof', sure, but likely to have a very much higher impedance to 'true earth' than soil.

    In the case of a hot tub, the potential issue is fairly obvious.

    Kind Regards, John
     
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  11. equitum

    equitum

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    The foundation of the argument to separate the hot tub or summerhouse from the PME earth is fundamentally around the likelihood of being stood in less clothes /footwear in closer proximity to terra firma and the risk of a lost PEN conductor along with absence of bonded services supply metalwork providing earthing in the aforementioned loss of PEN supply, otherwise wouldn’t we always just rely on the DNO earth on a TNCS /pseudo TNS TNCS system ?
     
  12. JohnW2

    JohnW2

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    Quite so - that's almost exactly what I wrote to EFLI.

    However, as he said (he called it 'the main reason', but I certainly don't agree with that), if there are extraneous-c-ps in an outbuilding that need bonding, that might be another reason for not wanting to use the TN-C-S earth.

    Kind Regards, John
     
    Last edited: 22 Jun 2021
  13. EFLImpudence

    EFLImpudence

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    I did concentrate on the outbuilding as the hot tub will nearly always need TTing.

    I don't think many electrical installations are carried out primarily with the thought of lost PENs.
    I am sure it has been said quite often that that is relatively unlikely and so it is not the most important element in the compromise of an electrical installation.

    If that is what you are considering plus the rest of your argument then probably you should TT every installation regardless.
     
  14. equitum

    equitum

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    I personally would rather use the DNO earth and “export” it in almost all cases domestically.l (including hot tubs) If there were parts to be bonded I’d just use 10mm or more 3 core swa. In a recent instance a garden room supplier insisted on the provision of a TT/rod and a 100ma time delay RCD upstream before they would do their connections to it and internals . Which raised the question around earthing the armour from which end etc
     
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  15. bernardgreen

    bernardgreen

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    They should be ! If the PME "Earth" in the property's equipotential zone is derived from a Neutral that has gone open circuit then the "Earth" inside equipotential zone will not be at Ground potential. Any conductive item, including human bodies, that bridge from the "Earth" in the equipotential zone to Ground will have a current flowing through it. There will be no fuses or RCDs in the wires carrying that current so it will flow unchecked (*).

    If the Neutral to the house is open circuit somewhere in the local network then the potential on the "Earth" in the equipotential zone will be the that of the star point of multiple single phase supplies to several properties and as such may not even be noticeable if the loads on the three phases are balanced.

    It is a very different matter if the break in the Neutral is between the street cable and the house. In this case then any load in the house will pull the disconnected Neutral up to full Live potential 230 V above ground and along with it the "Earth" in the equipotential zone.

    If there is no conductive path from "Earth " to Ground then the house is safe and appears to be suffering from a power cut as nothing is working but all wiring Live, Neutral and "Earth" is at 230 V above Ground, As long as no one creates a path between "Earth" in the equipotential zone and Ground outside the equipotential zone it is "safe" inside the equipotential zone.

    (*) It is because this current could in worse situations be tens if not hundreds of Amps that the regulations require a bonding conductor capable of safely carrying that high current.
     
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  16. bernardgreen

    bernardgreen

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    I tend to agree with that as a correctly installed and maintained TT installation remains safe in almost every fault situation where the fault is outside the property ( such as a failed Neutral in the local network )
     
  17. JohnW2

    JohnW2

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    Whilst I am obviously aware of your personal attitude to risk, I think it's a bit far-fetched to suggest that the (very rare) possibility of a 'lost PEN' should be the "primary" consideration when designing an electrical installation. I can think of countless things (including many things relating to risks much higher than that of a 'lost PEN') which should (must?) be given a higher priority as 'considerations'.

    In terms of the magnitude of risk, one has to remember than not only are 'lost PENs' very rare in the first place but that, even when they happen, they very often will not result in any major 'problems' ('dangers').

    Kind Regards, John
     
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