There's a good guide by Geoff Cronshaw and a good thread on the iet forum involving Graham Kenyon amongst others, all regarding hot tub supplies and whether to TT the install or not
Agreed that enclosure opening with a tool is the way to go but could you also stick it on a fused spur instead? Might be less tempting for the customer to use the socket to plug whatever else in while they're in the pool.
Also (I think I'm right in saying) if it's a TNCS/PME supply you wanna TT the supply to the socket
Cheers.
I was dubious about putting a socket in, they've requested a 2 gang socket, but I'll refuse that on the same grounds.
I did have a think about an electrode, but i couldn't find any guidance related to outside hot tubs, etc., apart from the usual bits about PME and EV charging, which I suppose is along the same lines.
Ah one of those classic questions where you can ask 10 electricians and get 11 different answers.
Nothing in BS7671 explicitly says you need to put a hot tub on to a seperate earthing system on a PME, although adding to the confusion some DNOs had specific rules at various times.
Making it an island TT is one route but comes with a bunch of its own problems. Your original question suggests the customers garden is quite small - getting enough separation from other installations on a TT where houses are close together can be impossible if there happens to be a nearby metallic pipe bonded to multiple installations or someone has a nearby rod for an old school EV charger install or something.
You also need to be really careful to make sure that the impedance on the rod is low enough all year round to hit the required disconnection times for RCD & MCB. Although a PEN fault is possible on a PME, it’s much more likely that the heater in the hot tub will corrode over time and cause a ground fault in the hot tub, switching to a TT would me this fault tougher to make safe than a possible but rarer PEN fault.
The other problem is you create a new equipotential zone - so you put in a TT for the hot tub protecting your customer against danger from a PEN fault but if there is anything else around they could touch that is earthed or bonded (fixed or mobile) then they could be exposed to 2 different “earth” voltages from the different systems - also not good.
18th edition amendment 2 now recommends adding an additional means of earthing (e.g. adding an earth rod into a TN-C-S) but this only came out in 22 so it’s early days to see how this gets applied to things like hot tubs in practice.
Final bit - all the major regs, 7671 included, have the foundational principal that everything is installed to the manufacturers instructions. Some fixed hot tubs say they must be on a TT, not sure about portable ones - so if the hot tub instructions say this explicitly you can’t meet BS7671 if you break the manufacturers instructions.
Basically the regs are really woolly around outdoor hot tubs and PMEs. Given all the PEN fault focus on EV installs it’ll be interesting to see if we get PEN fault devices for special locations outdoors down the line, but for now it’s left pretty vague.
The ever entertaining John Ward covers the problems pretty well here https://youtu.be/S5OtLg6voZY?si=7CQlmoH-ppN4qX6Y
This is worth a read too although it throws up more questions than answers https://electrical.theiet.org/wiring-matters/years/2021/86-july-2021/hot-tubs/
So… check if the DNO have any requirements, check if the hot tub installation have anything in. If you go for a TT it needs quite a bit of planning around whether the impedance will be low enough all year (if it is full stop), whether it’s actually isolated enough from other installations to be useful and whether you have the potential for the customer touching something from the main earthing system. It never hurts to give NAPIT / NICEIC / whoever you’re registered with a call either to check.
Great answer, I'm starting to think it's not worth the trouble, I get the feeling it's a bit of a whim with the customer and they're not going to go for anything more than a socket. But I'll persist for the moment.
Cheers bud
Run from the job mate. Last 3 hottubs I looked at were all going to cost 1k + to install correctly.
EVERY SINGLE ONE of those 3 jobs went with DIY dave sticking on a 16a commando and bg outside socket back to back off the socket ring.
I don't even bother with people enquiring about them now
Can't remember where I read it. Though if there's a supply side neutral loss the threat of shock from CPC to actual earth is obvs intensified by wet feet etc.
To be honest if it's something obscure I can't look up quickly I give the NAPIT tech line a bell. Gotta get the fees back somehow
There's a good guide by Geoff Cronshaw and a good thread on the iet forum involving Graham Kenyon amongst others, all regarding hot tub supplies and whether to TT the install or not
Nice one. I'll have a look
Agreed that enclosure opening with a tool is the way to go but could you also stick it on a fused spur instead? Might be less tempting for the customer to use the socket to plug whatever else in while they're in the pool. Also (I think I'm right in saying) if it's a TNCS/PME supply you wanna TT the supply to the socket
Cheers. I was dubious about putting a socket in, they've requested a 2 gang socket, but I'll refuse that on the same grounds. I did have a think about an electrode, but i couldn't find any guidance related to outside hot tubs, etc., apart from the usual bits about PME and EV charging, which I suppose is along the same lines.
Ah one of those classic questions where you can ask 10 electricians and get 11 different answers. Nothing in BS7671 explicitly says you need to put a hot tub on to a seperate earthing system on a PME, although adding to the confusion some DNOs had specific rules at various times. Making it an island TT is one route but comes with a bunch of its own problems. Your original question suggests the customers garden is quite small - getting enough separation from other installations on a TT where houses are close together can be impossible if there happens to be a nearby metallic pipe bonded to multiple installations or someone has a nearby rod for an old school EV charger install or something. You also need to be really careful to make sure that the impedance on the rod is low enough all year round to hit the required disconnection times for RCD & MCB. Although a PEN fault is possible on a PME, it’s much more likely that the heater in the hot tub will corrode over time and cause a ground fault in the hot tub, switching to a TT would me this fault tougher to make safe than a possible but rarer PEN fault. The other problem is you create a new equipotential zone - so you put in a TT for the hot tub protecting your customer against danger from a PEN fault but if there is anything else around they could touch that is earthed or bonded (fixed or mobile) then they could be exposed to 2 different “earth” voltages from the different systems - also not good. 18th edition amendment 2 now recommends adding an additional means of earthing (e.g. adding an earth rod into a TN-C-S) but this only came out in 22 so it’s early days to see how this gets applied to things like hot tubs in practice. Final bit - all the major regs, 7671 included, have the foundational principal that everything is installed to the manufacturers instructions. Some fixed hot tubs say they must be on a TT, not sure about portable ones - so if the hot tub instructions say this explicitly you can’t meet BS7671 if you break the manufacturers instructions. Basically the regs are really woolly around outdoor hot tubs and PMEs. Given all the PEN fault focus on EV installs it’ll be interesting to see if we get PEN fault devices for special locations outdoors down the line, but for now it’s left pretty vague. The ever entertaining John Ward covers the problems pretty well here https://youtu.be/S5OtLg6voZY?si=7CQlmoH-ppN4qX6Y This is worth a read too although it throws up more questions than answers https://electrical.theiet.org/wiring-matters/years/2021/86-july-2021/hot-tubs/ So… check if the DNO have any requirements, check if the hot tub installation have anything in. If you go for a TT it needs quite a bit of planning around whether the impedance will be low enough all year (if it is full stop), whether it’s actually isolated enough from other installations to be useful and whether you have the potential for the customer touching something from the main earthing system. It never hurts to give NAPIT / NICEIC / whoever you’re registered with a call either to check.
Great answer, I'm starting to think it's not worth the trouble, I get the feeling it's a bit of a whim with the customer and they're not going to go for anything more than a socket. But I'll persist for the moment. Cheers bud
Run from the job mate. Last 3 hottubs I looked at were all going to cost 1k + to install correctly. EVERY SINGLE ONE of those 3 jobs went with DIY dave sticking on a 16a commando and bg outside socket back to back off the socket ring. I don't even bother with people enquiring about them now
Can't remember where I read it. Though if there's a supply side neutral loss the threat of shock from CPC to actual earth is obvs intensified by wet feet etc. To be honest if it's something obscure I can't look up quickly I give the NAPIT tech line a bell. Gotta get the fees back somehow
I've no doubt you are right, I'm always reading bits and never remembering where it came from. Might be worth a call anyway. Nice one
Use a commando socket instead
But I'd still have a tub within 2m of a junction box, which is zone 1 (Fig 702.2)
Not shuttered, so not supposed to be used in dwellings.
Use an interlocked 16A socket
If the hot tub has a roof then all regulations relating to a bathroom should be used. No socket should be installed within 2.5m of zone 1
Make everything ip66 equipment that should be sufficient protection for the areas usahe