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

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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.
 
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.
 
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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.

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?
 
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?
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?
 
Oh for a deep garden pond........in Iceland. :wink_smile:
 
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?
 
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?

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.
 
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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.
 
Hi

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

Cheers

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.
 
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.

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
 
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
 
Correct me if I am wrong but BS7671 tells us that, in the case of a caravan hook up as an example, we must NOT extend the PME earthing due to the danger posed in the case of a broken PEN where the largely metallic caravan could develop a dangerous PD between it and the ground. So, how does the hot tub differ from this, assuming there are metallic parts of it that are likely to be able to be touched by a person wet through (immersed) and in contact with the ground? How can the MI's then suggest just extending LEN from the CU directly to the tub? Having said that, I have seen loads of tubs installed like this in domestic gardens.
 
To be honest I've never yet seen a metal hottub, all of the ones I've done have been fibreglass and a plastic or wood surround.
Presumably hot tubs are built to a product standard which bs7671 takes in to consideration with its recommendations.

As far as bs7671 goes there is only a recommendation that an earth electrode with an Ra less than 20ohm be connected to the equipotential bonding of the installation (ie a 10mm copper bond from electrode to MET)


Sent from my iPad using Tapatalk
 
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

Why not use a real supply voltage since we are considering the real world here.
Why 25V? I thought the usual limit was 50V?

As for quoting a current in Kw, I presume that is a mistake?
 
Why not use a real supply voltage since we are considering the real world here.
Why 25V? I thought the usual limit was 50V?

As for quoting a current in Kw, I presume that is a mistake?

Hi Dave

BS7671, does it say "a suitable" electrode? a higher Ra may well be okay, it depends upon the expected diverted neutral current, that's for the designer to assess.

Well at 50 Volts and over disconnection is required, at 25 Volts no disconnection is required, this would also give some leeway with seasonal variation effecting Ra. The kW i'snt a mistake, just a means of assessing the system impedance.

And id agree I've never seen a metal hot tub, but then the water is supplied via class1 heaters etc.

Cheers
 
Hi Dave

BS7671, does it say "a suitable" electrode? a higher Ra may well be okay, it depends upon the expected diverted neutral current, that's for the designer to assess.

Well at 50 Volts and over disconnection is required, at 25 Volts no disconnection is required, this would also give some leeway with seasonal variation effecting Ra. The kW i'snt a mistake, just a means of assessing the system impedance.

And id agree I've never seen a metal hot tub, but then the water is supplied via class1 heaters etc.

Cheers

I can't remember exactly what it says, but in my opinion it needs to be clarified and made more than just a minor note (but then that can be said of a lot of 7671)

All I can say is that you cannot quote a current in kW, current is measured in Amps and power in Watts.

The water may well be heated via class one heaters, but those heaters won't be exposed to touch if it is a class two construction and the water itself is not a very good conductor
 

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