Hot tub - TN-C-S

[snipersmith87]

Im after some advice regarding the earthing of a 32Amp hottub. I've yet to install one and am dubious about the risks associated.
The supply is T-N-C-S.
My plan is to put a seperate supply direct from origin via henleys to a 50amp MCB. 10mm 3c SWA approx 25m to a small wooden workshop. Within the workshop I'm locating a seperate DB with 30mA RCBO to supply small power / lighting within and the 32Amp hot tub sited on patio.
6mm 3c swa to rotary isolator. At this point I'm going to lose the supply armour via stuffing gland and internal cpc via wago. I was then going to install an earth rod connected to the load swa cpc and armouring upto hottub.
Can anybody see any issues with my scenario or further advice?

 

[pc1966]

First thing is for the sub-main feed you will get better selectivity with a 50A fused-switch from the Henley blocks (instead of a MCB).

Second thing, if you are planning to TT the hot tub supply then make sure the RCBO supplying it is a DP switching one. Most for typical single slot CU are only phase-switching, though Wylex and Crabtree RCBOs are DP switching.

 

[Megawatt]

Im after some advice regarding the earthing of a 32Amp hottub. I've yet to install one and am dubious about the risks associated.
The supply is T-N-C-S.
My plan is to put a seperate supply direct from origin via henleys to a 50amp MCB. 10mm 3c SWA approx 25m to a small wooden workshop. Within the workshop I'm locating a seperate DB with 30mA RCBO to supply small power / lighting within and the 32Amp hot tub sited on patio.
6mm 3c swa to rotary isolator. At this point I'm going to lose the supply armour via stuffing gland and internal cpc via wago. I was then going to install an earth rod connected to the load swa cpc and armouring upto hottub.
Can anybody see any issues with my scenario or further advice?

Just make sure it’s protected by a RCD and ground all water piping if it’s metal

 

[snipersmith87]

First thing is for the sub-main feed you will get better selectivity with a 50A fused-switch from the Henley blocks (instead of a MCB).

Second thing, if you are planning to TT the hot tub supply then make sure the RCBO supplying it is a DP switching one. Most for typical single slot CU are only phase-switching, though Wylex and Crabtree RCBOs are DP switching.

Yes regarding the double pole RCBO what is the purpose of this on a TT? Potential difference on the neutral to True Earth during maintenance? Surely the rotary isolator would suffice?

 

[pc1966]

Yes regarding the double pole RCBO what is the purpose of this on a TT? Potential difference on the neutral to True Earth during maintenance? Surely the rotary isolator would suffice?

The problem is if you are very unlucky and have a fault from power to water then a SP RCD will isolate the live and so you think you are safe, but if you have a PME fault at that time then your neutral that is not isolated in this case is still connected to the water, and could be at tens or even almost hundred or so volts!

Typically incomer style RCD are always all-pole (so DP single phase, 4P three-phase) but the CU style of RCBO are often single pole switching only.

 

[westward10]

There is no requirement for an rcbo to be double-pole on a TT earthing system.

 

[snipersmith87]

There is no requirement for an rcbo to be double-pole on a TT earthing system.

So you see nothing wrong with the arrangement I've suggested westward?

Im struggling with the mixed opinion on the double pole RCBOs regarding TT. My understanding is that it was for maintenance purposes in regards to isolating the neutral as it could have a potential difference to true earth presenting a possible shock. Therefor spurs or in my case the rotary isolator had to be double pole in order work on equipment.

 

[westward10]

Isolation on TT systems must be double-pole, circuit protection does not.

 

[pc1966]

Isolation on TT systems must be double-pole, circuit protection does not.

I had not realised that, as most TT incomers are all-pole and I presumed if you were doing it via RCBO instead they would be the same.

Certainly for caravan outlets they have to be all-pole disconnection (708.415.1) which was the sort of case I was thinking when water & PME was discussed that often forces TT operations.

 

[davesparks]

I think it is a terrible design, a rotary isolator with a CPC connected to one earthing system flapping around in a wago and another different earthing system brought in to the same enclosure!

 

[westward10]

Why do you plan to run a three core swa when the cpc (third core) would not be required.

 

[snipersmith87]

Why do you plan to run a three core swa when the cpc (third core) would not be required.

Yes i could run a 2core. I just find 3core more readily available and will use 3 core on the load side. No particular reason other that tbh.
I'll just earth the armouring at the workshop DB and not connect the 3c cpc at either end. I was just trying to stick with the TN earthing arrangement which i would consider highly more reliable than TT upto the furthest point.
[automerge]1589319410[/automerge]

I think it is a terrible design, a rotary isolator with a CPC connected to one earthing system flapping around in a wago and another different earthing system brought in to the same enclosure!

I think thats slightly harsh and you can see the principle of what I'm trying to achieve. We connect conductors with 230v / 400v potential within the same enclosure.

 

[davesparks]

I think thats slightly harsh and you can see the principle of what I'm trying to achieve. We connect conductors with 230v / 400v potential within the same enclosure.

Why harsh? You posted on a public forum to get opinions so I shared my opinion.

Yes we connect conductors in the same enclosure, but that is very different to having two different earthing systems present in the same enclosure.

 

[pc1966]

Yes i could run a 2core. I just find 3core more readily available and will use 3 core on the load side. No particular reason other that tbh.

If you have loads of 3 core already to hand it makes sense, otherwise it is easy and cheaper to buy 2 core SWA by the meter, etc, from any of the usual suppliers on-line or at trade outlets.

The usual reason for going 3-core on single phase are one or more of:

• Not meet the Zs for disconnection times for sub-main OCPD due to high R2
• Armour not meeting adiabatic limit for fault clearing
• Not meeting 10mm copper equivalent bonding for extraneous conductive parts on TN-C-S

There probably are other things I have missed and other folk will point out tomorrow...

I'll just earth the armouring at the workshop DB and not connect the 3c cpc at either end.

Seems a waste of copper! If you have a spare core with no other use then put it in for parallel earth on the SWA.

I was just trying to stick with the TN earthing arrangement which i would consider highly more reliable than TT upto the furthest point.

Where do you plan on putting the earth rod connection, in to the isolator switch box?

You should be thinking of the poor sod who follows you years later trying to sort something out - make the design clear and properly label things so anyone else needing to do work can see exactly what is being done (and ideally why) from a simple inspection of it.

 

[radiohead]

There is no requirement for an rcbo to be double-pole on a TT earthing system.

Not entirely correct, in practice there may be an indirect requirement.
The reason DP RCBO's are sometimes required on TT systems is because an S type time delayed RCD is often used as a main switch. This is a preferred arrangement because it provides back up earth fault protection where there is total reliance on the operation of an electronic RCD device. If a single pole RCBO is used it will not disconnect the neutral in the event of an earth fault on a final circuit, which could then result in the RCD main switch operating disconnecting the entire installation. If a DP RCBO is protecting the final circuit that wont happen, the affected final circuit will be completely disconnected. This would be a breach of the regulation requiring a single fault not to result in complete power failure.
The same scenario would not be the case if a dual RCD board is in use with SP MCB's and an up-front S type main switch, as the dual RCD's would both be DP.
Regarding the OP , the use of a TNCS system is permitted for a hot tub arrangement and I would have no issue using it. As already stated, the presence of two separate earthing systems may create more of a hazard.
Also a regulation often overlooked is the presence of external lights adjacent to the tub, there are strict regulations on where lighting can be installed which need to be considered and often aren't with domestic installations.

 

[westward10]

Not entirely correct, in practice there may be an indirect requirement.
The reason DP RCBO's are sometimes required on TT systems is because an S type time delayed RCD is often used as a main switch. This is a preferred arrangement because it provides back up earth fault protection where there is total reliance on the operation of an electronic RCD device. If a single pole RCBO is used it will not disconnect the neutral in the event of an earth fault on a final circuit, which could then result in the RCD main switch operating disconnecting the entire installation. If a DP RCBO is protecting the final circuit that wont happen, the affected final circuit will be completely disconnected. This would be a breach of the regulation requiring a single fault not to result in complete power failure.
The same scenario would not be the case if a dual RCD board is in use with SP MCB's and an up-front S type main switch, as the dual RCD's would both be DP.
Regarding the OP , the use of a TNCS system is permitted for a hot tub arrangement and I would have no issue using it. As already stated, the presence of two separate earthing systems may create more of a hazard.
Also a regulation often overlooked is the presence of external lights adjacent to the tub, there are strict regulations on where lighting can be installed which need to be considered and often aren't with domestic installations.

There is no requirement in BS7671 for double-pole circuit protection on a TT earthing system. I understand your argument for the single fault scenario but nevertheless it is not required.

 

[snipersmith87]

If you have loads of 3 core already to hand it makes sense, otherwise it is easy and cheaper to buy 2 core SWA by the meter, etc, from any of the usual suppliers on-line or at trade outlets.

The usual reason for going 3-core on single phase are one or more of:

• Not meet the Zs for disconnection times for sub-main OCPD due to high R2
• Armour not meeting adiabatic limit for fault clearing
• Not meeting 10mm copper equivalent bonding for extraneous conductive parts on TN-C-S

There probably are other things I have missed and other folk will point out tomorrow...


Seems a waste of copper! If you have a spare core with no other use then put it in for parallel earth on the SWA.


Where do you plan on putting the earth rod connection, in to the isolator switch box?

You should be thinking of the poor sod who follows you years later trying to sort something out - make the design clear and properly label things so anyone else needing to do work can see exactly what is being done (and ideally why) from a simple inspection of it.

I understand the difference between a 2c and a 3c cable and the various reasons stated. Its not a 50mm its a 6mm. The cable run is local and cable readily available. I dont consider wastage or cost an issue tbh for such a short run.
I was simply reffering to the comment of 2 x earth sources within the same enclosure. By losing the cpc at DB it would eliminate that.

The earth rod will be located adajcent to the isolator.

As for inspection and testing I'd approach that isolator 2m from a hottub and clearly see the reason they've converted to a TT arrangement. Possibly applaud the effort made as they've understood the risks exporting a TNCS arrangement.

Its all very well criticising but not one person has stated a different approach regarding the TT. (I would rather not TT the workshop DB.)

 

[westward10]

You are not exporting TN-C-S, you are using a cpc which is derived from a TN-C-S source.

 

[radiohead]

There is no requirement in BS7671 for double-pole circuit protection on a TT earthing system. I understand your argument for the single fault scenario but nevertheless it is not required.

There is not a direct requirement I agree. But if a T/D RCD main switch is fitted with SP RCBO's it will not comply because a single fault on a final circuit may still take out the main switch. That does not comply. So where a T/D main switch is fitted with RCBO devices DP RCBO's are required to comply with the regulations. SP RCBO's would be a departure.

 

[westward10]

Single pole rcbos would not be a departure as there is no requirement which specifies them.

 

[Ian1981]

Regulation 531.2.2.201
Regulation 531.3.1.201
No requirement to disconnect Or switch the neutral under fault conditions or for protection against electric shock for TT or TN systems unless additional measures are required by part 7 such as caravan installations etc

 

[davesparks]

As for inspection and testing I'd approach that isolator 2m from a hottub and clearly see the reason they've converted to a TT arrangement. Possibly applaud the effort made as they've understood the risks exporting a TNCS arrangement.

Its all very well criticising but not one person has stated a different approach regarding the TT. (I would rather not TT the workshop DB.)

I wouldnt be applauding anything about the proposed plan of disconnecting the CPC at the isolator and connecting an earth rod.

I would not be converting any part of the installation to TT if possible, the TNCS earth will always be a better earth, I would prefer to install earth electrodes connected to the MET to guard against the possibility of a lost PEN.
They will eventually, hopefully, actually bring this in to our regulations.

 

[snipersmith87]

You are not exporting TN-C-S, you are using a cpc which is derived from a TN-C-S source.

Yes my error

I wouldnt be applauding anything about the proposed plan of disconnecting the CPC at the isolator and connecting an earth rod.

I would not be converting any part of the installation to TT if possible, the TNCS earth will always be a better earth, I would prefer to install earth electrodes connected to the MET to guard against the possibility of a lost PEN.
They will eventually, hopefully, actually bring this in to our regulations.

I agree that i don't particularly want to use a TT with a less reliable earth. However i presumed thats standard practice regarding a hottub outside the equipotential zone. Thats why I've started the post for advice.
Could i use theTNCS upto the hot tub and just connect an earth rod at the workshop DB as backup? Is this what your immplying?

 

[davesparks]

Thats why I've started the post for advice.
Could i use theTNCS upto the hot tub and just connect an earth rod at the workshop DB as backup? Is this what your immplying?

Have you checked the regulations for outdoor swimming pools as they are the closest thing to an outdoor hottub?

I'm not implying anything, I'm sharing my opinion that earth electrodes connected to the MET are the best solution.
I'm certainly not suggesting 'just connect an earth rod' because a single earth rod is never enough to provide a reliably low Ra.

 

[pc1966]

I agree that i don't particularly want to use a TT with a less reliable earth.

I guess the real issue is not so much the reliability of the TT's earth as such (you could have two rods in different areas and two wires so no single point of failure) but making sure it will reliably disconnect on a fault.

With a TN-S / TN-C-S you have RCD, magnetic, and thermal actions that will all ultimately trip the breaker, but on TT only the RCD will realistically do it as you simply won't have a low enough Ze.

But if you go for the more traditional TT set up for the workshop with a 100mA S-type RCD incomer and then 30mA RCBO for each circuit then you are not relying on a single RCD to disconnect on a fault.

However i presumed thats standard practice regarding a hottub outside the equipotential zone. Thats why I've started the post for advice.

The subject of TN-C-S and outdoor stuff comes up regularly and always prompts a bit of a heated debate as there are sound points and strongly held beliefs on many fronts! So far hot tubs and similar have not really been a comon subject, but the risk of EV changers for cars had mandated something must be done, but not clear yet if a cheap and easy solution outside of EV kit is going to be widespread.

For now it is "to TT, or not to TT, that is the question"

Could i use the TNCS up to the hot tub and just connect an earth rod at the workshop DB as backup? Is this what your immplying?

A single standard rod will never get a low enough impedance to pull down a PME fault voltage far enough. If you had a couple of rods around the hot tub zone (or some buried mesh, etc) you might be able to keep the step/touch potential gradient down but that would be a major design effort (needing the sort of modelling software they use for lightning protection and earth validation for major installations) to establish its effectivness.

 

[davesparks]

What we need is a change of attitude towards protection against a PEN fault/earthing/taking responsibility.

Foundation trenches are a great place for a horizontal electrode, an earth connection to the steel reinforcing is also a great electrode. Its already normal for the reinforcing to be connected to the elctrical earth on big buildings, how much of a change would it be to make this common practice for small buildings?

If building regs changed to require 2 connection points to the rebar to be made available for earthing when any foundation or slab is cast then I think we could easily achieve good earth connections for all installations.
Obviously this wouldn't immediately affect existing buildings, but that's not a reason not to do it for new buildings, and slowly as extensions are built etc the overall effect would be to make everyone safer.

 

[radiohead]

Single pole rcbos would not be a departure as there is no requirement which specifies them.

Regulation 531.2.2.201
Regulation 531.3.1.201
No requirement to disconnect Or switch the neutral under fault conditions or for protection against electric shock for TT or TN systems unless additional measures are required by part 7 such as caravan installations etc

You are both missing the point. SP RCBO's would only be a departure where a T/D RCD main switch was installed, not because the regs state SP RCBO's are not permitted, but because a single fault would probably still trip the main RCD. If someone said they were protecting the entire installation with a single RCD and MCB's you would say that is non compliant, the effect of a fault on a final circuit would be exactly the same as a T/D RCD with SP RCBO's.....total loss of power, therefore that arrangement is non compliant as well for the same reason.

 

[pc1966]

The usual reason for going 3-core on single phase are one or more of:

• Not meet the Zs for disconnection times for sub-main OCPD due to high R2
• Armour not meeting adiabatic limit for fault clearing
• Not meeting 10mm copper equivalent bonding for extraneous conductive parts on TN-C-S

There probably are other things I have missed and other folk will point out tomorrow...

I know there was another reason:

• High integrity earthing (high leakage currents) with two CPC

Now can I turn this in to a Spanish Inquisition sketch?

 

[Ian1981]

You are both missing the point. SP RCBO's would only be a departure where a T/D RCD main switch was installed, not because the regs state SP RCBO's are not permitted, but because a single fault would probably still trip the main RCD. If someone said they were protecting the entire installation with a single RCD and MCB's you would say that is non compliant, the effect of a fault on a final circuit would be exactly the same as a T/D RCD with SP RCBO's.....total loss of power, therefore that arrangement is non compliant as well for the same reason.

I do get your point, mearly pointing out what the regulations say about the neutral on TT and TN systems.
However there’s a difference between a departure and a non compliance, your example is not a departure as to depart is to afford the same degree of safety etc etc.
Completely agree that you would want selectively between rcd’s

 

[radiohead]

I do get your point, mearly pointing out what the regulations say about the neutral on TT and TN systems.
However there’s a difference between a departure and a non compliance, your example is not a departure as to depart is to afford the same degree of safety etc etc.
Completely agree that you would want selectively between rcd’s

Agreed I used the wrong terminology....not a departure but definitely a non compliance IMO, been too many weeks not working!

 

[suffolkspark]

I'm about to install a new supply for one on a job, after enquiring the hot tub manufacturer has said it does not require it's own earth electrode and is fine on tn-c-s...

Sparky mate of mine is installing one and the manufacturer for his one have said it must be on TT.

As much of a minefield as sodding car chargers

 

[Midwest]

I'm about to install a new supply for one on a job, after enquiring the hot tub manufacturer has said it does not require it's own earth electrode and is fine on tn-c-s...

Sparky mate of mine is installing one and the manufacturer for his one have said it must be on TT.

As much of a minefield as sodding car chargers

Never done myself, and I’m aware of the various issues, but isn’t one of the main concerns is what the bathers are stepping onto when they get out of the tub. That is grass or floor of wooden shed for example?

 

[Lister1987]

I've seen Matt:e units used to resolve earthing on TN-C-S arrangements, a few YouTubers using them;

View: https://youtu.be/l5DjBRiwCDc

View: https://youtu.be/usXBONOMAuQ

 

[suffolkspark]

I wonder if the tn-c-s safe ones simply have double insulated pumps? I'm going with manufacturer info on the one I'm doing, price is in now ?

 

[pc1966]

I suspect water heating is the risk, not the pump.

The Matt:e box is quite a reasonable approach but it relies on the volts being out of range and I did see a demo where you might not get enough L-N anomaly to detect a PME fault that has raised N+E to 70V or so above true Earth.

Still, nothing is ever perfect and it is about managing risk levels to a reasonable degree.

 

[suffolkspark]

True, every one I've done in the past I've staked, but this manufacturer saying it's ok has thrown me, I'll check it when it comes, they do final connections

 

[ChrisElectrical88]

I have done 3, always put a TT rod in for them. Would be interested to know why they say PME is ok with their product.

 

[davesparks]

I've seen Matt:e units used to resolve earthing on TN-C-S arrangements, a few YouTubers using them;

Of course the youtubers are using them, matt:e will be paying them to do it or giving them the units for free as its great marketing.

 

[snipersmith87]

I'm about to install a new supply for one on a job, after enquiring the hot tub manufacturer has said it does not require it's own earth electrode and is fine on tn-c-s...

Sparky mate of mine is installing one and the manufacturer for his one have said it must be on TT.

As much of a minefield as sodding car chargers

I went for the option of TT at the hot tub. Just sat better with me. I think you have to look at the external influences and siting of the tub. This was not near any other exposed conductive parts and located near retaining walls with soil etc. To me it should be treated as a swimming pool. If smashing a rod in will create a form of equipotential zone around the tub I'm doing it., albeit relying on a RCD for fault protection.

 

[Lister1987]

Of course the youtubers are using them, matt:e will be paying them to do it or giving them the units for free as its great marketing.

That is true but you would hope they have the professional integrity not to back rubbish.