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.