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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-
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.
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.