Posting a message to the forum will remove the above advertisement
  1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.
  2. angel While you're here, would you mind checking out our Electrician's Insurance section to see if we could save you a few quid? - Specifically our Van Insurance Deals and Public Liability Insurance Deals. Thanks for supporting the forum! angel

    5% Discount from Electrical2Go.co.uk for ALL members! - Click Here

Discuss External Hot Tub Circuit on TN-C-S supply help advice please. in the Electrical Forum area at ElectrciansForums.co.uk.

Thread Status:
Not open for further replies.
Please make sure you checkout our forum sponsors, many do discounts for members and, they keep the forum free to use.
  1. flyingsparc
    Offline

    flyingsparc Active EF Member

    Hello everyone,



    I know this has come upquite often, I have gone back through the many threads relating tothis topic. The reason for wanting to start my own thread is afterhaving read those many threads, in the end, a definitive answereither way, is hard to find.



    I have just installed anew circuit designed for a hot tub. The hot tub hasn't arrived yet,but it requires a 32 Amp circuit.



    There are not going tobe any pipes (copper or otherwise) running to the tub, just a drain.



    Starting at the house,



    TN-C-S supply (notlabelled on cut out) but now verified by identical PFC and PSCC of2.8kA measurements, and confirmed neutral-earth connection at cutout.



    Ze at origin = .08 ohms(this is in London, and so guessing very close to sub station).


    New 10mm tails fromservice connector block to dedicated hot tub consumer unit populatedwith 1 x 63A dp 30mA rcd and 1 x 32A type C mcb.



    4mm 3 core XPLE SWAleaving dedicated cu via SWA gland, running through house into backgarden to rotary isolator, again using SWA glands. Same SWA out toIP65 box coiled up on wooden base that the hot tub is to sit on,whilst awaiting hot tub people to arrive and install this weekend.



    Calculated Zs = .23ohms.

    Measured Zs (using notrip 3 wire) .45 ohms.

    RCD x 1 = 27.2ms. &x 5 =12.4ms.



    Now, when I designedthis circuit, I hadn't realised that the supply was TN-C-S, only whenI turned up to start install I realised.



    My dilemma now is, whatis the safest way to continue with this installation before puttingit into service. As mentioned above, having read many threads on thissubject on various forums, opinion seems quite divided.



    I can either leave itas it is using the earth provided by the incoming supply. By doingthis the risks as I see it are – If a broken neutral on the supplyhappened then dangerous voltages could appear on any extraneousconductive parts (if there are any) of the hot tub. Also there couldbe problems caused by the difference of potential between the supplyearth and and true earth (wet bodies). It will be possible to stepout of hot tub onto patio slabs.



    The manufactures(believed USA) guidance for the electrical requirements amongst otherthings say this-



    “A bonding lug for bonding copper wire has been provided on theelectrical pack to allow connection to local ground points. Theground wire must be at least 8AWG (8.36mm2) copper wire and must beconnected to a grounded metal structure such as a cold water pipe”.
    I know the last bitabout cold pipes it not permitted in BS7671 any more.



    If correctlyinterpreting correctly what the manufacturer/supplier is saying, thenI must TT the load end?





    BYB says (702.410.3.4.3– ii, page 212) “NOTE - Where a PME earthing facility is used asthe means of earthing for the electrical installation of a swimmingpool or other basin, it is recommended that an earth mat or earthelectrode of suitably low resistance, e.g. 20 ohms or less, beinstalled and connected to the protective equipotential bonding”.



    So, lets say I need toput in some rods, then connect the rod/s (20 ohms or less isn't goingto be easy), to to the tub lug and a bond running back to theMET?....really?



    Cant use the armour for bond, I believe its equivalent is 6.43csa.



    How do you test/record?either ELFI (difficult as the supply is via an rcd) or with Electrodetester? (will have to sub someone in as I don't have an electrodetester).




    Other things I shouldconsider -

    PME boundary/overlap poss. not the correct term/s, how will this change things if at all?



    Splitting the TN-C-Searth at the external rotary isolator, blank off armour of SWA withheat shrink and use nylon gland for out going side of isolator. Asuitable label, something like “This installation/circuit uses twodifferent types of earthing arrangement TN-C-S at origin, TT atload”.



    Not really a TT is it?




    My head really hurts,spent far to long thinking about what's right/correct way ahead.



    I would reallyappreciate some guidance here, please go easy, I just want it to beas safe as possible.



    Thanks in advance forlooking at this, and for any responses.



    Darren.
     
  2. Leesparkykent
    Offline

    Leesparkykent Super Moderator Staff Member Trusted Advisor

    Location:
    Kent
    The rod doesn't have to be at the hot tub it can be connected to the MET and you can use the suppliers earth for the hot tub.
     
  3. darkwood
    Offline

    darkwood Super Moderator Staff Member Trusted Advisor

    Location:
    West Yorkshire
    Its not just a network N fault you need to consider, your supply earth and true ground could be at different potentials, this may increase with distance from property but other facors can effect this too.

    Is your TNCS a PME or not?


    Like Lee says, Rod locally to the Hot Tub back to met or interlink them all if you want to do the belt and brace approach.
     
    Last edited: May 4, 2016
  4. flyingsparc
    Offline

    flyingsparc Active EF Member

    Hello Lee, thanks for your reply, please could you clarify, I can put the rod anywhere in the garden (other services permitting) and then connected to the MET in house, and then I can use suppliers earth as is?
     
  5. Leesparkykent
    Offline

    Leesparkykent Super Moderator Staff Member Trusted Advisor

    Location:
    Kent
    Yes but the rod still has to have a low impedance to be effective. As Darkwood points out in post 3 it wouldn't be unusual to notice "tingles" if the suppliers earth is used due to the potential difference between true earth and a that of the suppliers. This would obviously be more noticeable when wet and barefooted.... Is the Hot tub sat on decking?
     
  6. ipf
    Offline

    ipf Trusted Advisor

    Oh for a deep garden pond........in Iceland. :wink_smile:
     
  7. flyingsparc
    Offline

    flyingsparc Active EF Member

    Lee - The tub will be sat on a raised wooden plinth, it will be possible to step out of tub onto patio slabs/true earth

    Darkwood - Thanks for your reply, I do not know if TN-C-S is PME, I could try asking DNO? Also please could you expand on "or interlink them all for belt and brases please?
     
  8. darkwood
    Offline

    darkwood Super Moderator Staff Member Trusted Advisor

    Location:
    West Yorkshire
    Just meant link MET-ROD-Earth lug on hot-tub with 10mm... it covers all bases and just bring your 4mm in as normal as supply, this way you are just extending your equipotential zone and meeting the earth requirements of the tub.

    The TNCS/PME comment shouldn't really matter too much domestic, I was just curious with the lack of info on the cutout why you called it PME if you could only visually identify as a TNCS which could run direct to the Sub-TX with no multi-tapped earth points.
     
    Last edited: May 4, 2016
  9. flyingsparc
    Offline

    flyingsparc Active EF Member

    Thanks for theresponses so far, which are helpful, but I am still left a littleconfused as to the best way forward.


    Based on the replies Iwould like to summarise the advice.


    I should install anelectrode in the garden somewhere either way, but far enough apart sothat each earthing arrangement can not be simultaneously touched?


    Then either of thefollowing options is compliant with BS7671? -


    Disconnect/isolate/insulateTN-C-S earth at isolator, but continue with Live Neutral to tub, andconnect electrode to tub only (less than 200 ohms will suffice)?


    OR


    Run a 10mm fromelectrode to tub earth lug and all the way back through house tohouse MET.
    But the electrode inthis option must be at/or lower than 20 ohms?


    Have I understood thiscorrectly, and that above are my only options?


    To add to my confusion,my customer emailed me, concerned about the extra and increasedexpense, and tells me he called the supplier of the hot tub, who havetold him that “the hot tub does not require any additionalearthing, we just need to have a live neutral and earth running fromthe main switchboard, So providing it does then we are ok”.


    So now my customerseems to wants to take the suppliers advice, given over the phone,against my advice, which I am still preparing, with the help of thispost.


    Hummmmm.
     
  10. GEOFF S
    Offline

    GEOFF S Electrician's Arms

    Location:
    MARS
    Hi

    Drop the PME, TT it, belt and braces two rcds in series.

    Cheers
     
    Last edited: May 5, 2016
  11. darkwood
    Offline

    darkwood Super Moderator Staff Member Trusted Advisor

    Location:
    West Yorkshire
    I never understand the reasoning of puting in a somewhat less reliable earthing arrangement which can be subject to fluctuations dependent on everything from weather to ground conditions when you have an excellent reliable earth provided for you, by combining the two as I mentioned ealier you then achieve your belts and braces set-up IMO, as for putting multiple RCD's in series, I see your trying to create redundancy in the circuit but when your trying to win a Quote it won't do you any favours to get the job.
     
    • Like Like x 1
  12. GEOFF S
    Offline

    GEOFF S Electrician's Arms

    Location:
    MARS
    Hi

    The electrode will only serve a purpose under a broken pen condition, what touch voltage would you suggest as the upper limit for someone submersed in water?

    Cheers
     
  13. Leesparkykent
    Offline

    Leesparkykent Super Moderator Staff Member Trusted Advisor

    Location:
    Kent
    And will help negate the potential difference.
     
  14. GEOFF S
    Offline

    GEOFF S Electrician's Arms

    Location:
    MARS
    Hi

    Okay, firstly lets start with your upper limit of touch voltage, then what diverted neutral current are we to consider in kW, 10 kW maybe?

    Cheers
     
  15. GEOFF S
    Offline

    GEOFF S Electrician's Arms

    Location:
    MARS
    Hi

    Well just as an example, assuming a supply voltage of 230 V, a touch voltage limit of 25 V and a diverted neutral current of 10 kW, you will need a combined Ra and Rb of 0.62 ohms, good luck with that!

    Connect an electrode at the terminal of the hot tub and run to CPD(isolated from PME), its normally no problem having rcds in series, just an additional one at the isolator, this is usually backed up with one at the main CU, ensure you drive well away from any services to avoid picking up on PME currents, so no real extra costs.


    For darkwood and Lee, how does your set up deal with transferred voltages, lets say a 5 second fault on a distribution circuit, this could well be 150 Volts transferred to your hot tub?

    Cheers
     
Loading...
Similar Threads - External Circuit supply Forum Date
How to earth SWA to external EV charge point Electrical Forum Sep 17, 2018
Enclosure advice for external cables Electrical Forum Jul 10, 2018
Thread Status:
Not open for further replies.

Share This Page

Users found this page by searching for:

  1. hot tub shock if neutral is broken on tncs

Electricians Directory Post a Domestic Job Post a Commercial Job