Garden Hot Tub safety !

[Spike1947]

Hi
I am thinking of putting in a Hot Tubat the end of our garden, fed from the a garden shed, more or less next to it, now this might be a silly question ( Ive only just got up) but how do I make sure that the hot tub is safe , ie garden, person,bathtub yes it will be earthed from the shed supply, but it won't be within the equipotential bonding of the house earthing !, would I have to put in a earth rod ( not one for the shed, should there of been one !).
Excuse me if this is a stupid question, not finished my first coffee of the day yet.

cheers
Spike

​

 

[Intoelectrics]

Hi
I am thinking of putting in a Hot Tubat the end of our garden, fed from the a garden shed, more or less next to it, now this might be a silly question ( Ive only just got up) but how do I make sure that the hot tub is safe , ie garden, person,bathtub yes it will be earthed from the shed supply, but it won't be within the equipotential bonding of the house earthing !, would I have to put in a earth rod ( not one for the shed, should there of been one !).
Excuse me if this is a stupid question, not finished my first coffee of the day yet.

cheers
Spike

​

It's not a stupid question! The fact that you are considering safety shows that you are conscientious and concerned about people's wellbeing. Some would just go-ahead and install the tub without any thought whatsoever.

I'm just drinking my morning coffee whilst going through my emails so still a bit blurry eyed too.

 

[timhoward]

Certainly a good question.
There are likely to be differing opinions about the earthing.
My view (without seeing it) and making some assumptions about your supply, is that the safest would be to connect it to the supply earth (from the shed).
There is an issue that in many situations earth and neutral are joined not too far away, and that neutral currents can cause a small voltage on the supply earth and this in turn creates a small difference between real earth and things that are earthed.
Some have found that if standing with wet feet on grass and touching the metal frame of hot tub a small tingle is felt as the body bridges this difference. It can un-nerve people when getting in and out. My view is that it’s best to address this with rubber matting or decking.
I prefer to design such that a live-earth fault in the hot tub has a decent path to the supply earth, plenty of current can flow, and the protective device will operate.
The other design with no supply earth and an earth rod relies on an RCD and these are great devices but have a higher failure rate especially if untested for years.

The latest revision to the regs is starting to suggest considering additional earth rods in some circumstances. In this situation it might be worth thinking about as that would provide a lower resistance path to real earth than the human body for stray neutral currents and solve the ‘tingle’ problem.

 

[pc1966]

Hot Tubs make a regular appearance on these forums for various reasons, some due to needing large dedicated circuits, others from needing that but coming with 13A plugs that overheat...

But the biggest issues are the one you raise here - the safe earthing arrangements for it. Normally you just make sure your CPC is sound and able to do its job, but with hot tubs you may also end up having to consider the "open PEN" danger where a fault on the electrical supply network itself causes your house earth terminal to be at an elevated voltage, potentially very dangerous when folks are wet and half way in the water and feet or hands on the ground (true Earth potential) outside the tub.

So it can be a more complicated situation and there are a couple of approaches to this problem:

• Earth rods / mats. If you can provide your own earthing arrangement you may be able to reduce the CPC to true Earth voltage in that region. It is (18th AM1) mentioned for swimming pools under reg 702.410.3.4.3 note (ii) that is below 20 ohms. However, this is often impractical.
• Make the pool supply TT. So you have a local earth rod that is adequate for RCD disconnection (below 200 ohms, much easier to meet) and then a dedicated double-pole RCD for the pool supply with the pool CPC to the rod.

Of course if your house is on a TT supply no need to worry, but that is rare in the UK, most are (or will become) TN-C-S where the open PEN risk is present.

 

[timhoward]

I was fairly sure “open pen” would come up in this thread! Every time this question is asked, I wrestle afresh with whether a hot tub fault or an open pen fault is more likely.
There are loads of factors at play here, the quality of hot tub design, manufacture, and assembly, the reliability of RCDs, ground conditions, the increase in TNCS supplies, the condition of the DNO cables etc.

Last time I checked there are a few hundred reported open pen cases in the UK each year. Considering how many supplies there must be I’ve arrived at the personal opinion that it shouldn’t influence my circuit design decisions too much.
(I’d like to see open pen protection devices drop in price as I’d see more value in fitting these than SPDs and AFDDs!)

There been at least one hot tub recall I the last year and in the global race to the bottom
of price and quality I’m afraid I consider it more likely that poor design or poor assembly would result in a hot tub fault.

This is all subjective stuff and I don’t think there’s a ‘right’ answer! The OP has the full spectrum of views now!
Both solutions meet the regs and both should be safe enough.

 

[pc1966]

This is all subjective stuff and I don’t think there’s a ‘right’ answer! The OP has the full spectrum of views now!
Both solutions meet the regs and both should be safe enough.

Exactly, there is no "right" answer.

The risk trade-off of TT RCD failing versus open-PEN at dangerous levels is unknown, and while an earth mat beneath the tub and immediate surrounding area would negate both risks, it is often impractical to implement (unless you have friendly builder/farmer with a small digger to bury the wire/mat a half meter down, etc).

As mentioned, having the tub on decking can reduce the risk and odd experience of even a few volts MET-to-Earth potential when TN earthing used, but won't always be practical either.

 

[pc1966]

Oh final risk to consider - how much will it cost to run the tub?

 

[Spike1947]

Hi
Thanks for all your replies, tbh I did think of a local Tera Tera to the outhouse CU, it all ready has a RCB, so I think i will go for that, allthough I am still a bit concerened for the kids, I will have to have a good think on this, to see if I can come up with something that will put my mind at rest, while watching the kids getting in and out of the Tub !.

Cheers and thanks for all the feedback .
Spike

 

[pc1966]

Thanks for all your replies, tbh I did think of a local Tera Tera to the outhouse CU, it all ready has a RCB, so I think i will go for that

Put in a separate RCD for the tub.

Yes, you won't get fault selectivity between it and the local CU's RCD, but here it hardly matters. What you do gain is no single point of failure for protection if an RCD goes bad.

 

[Alan-N]

Always found it odd that the regs don't specifically mention outdoor hot tub installations, and the advice given is for swimming pools. I done one a couple of weeks back, it was a new supply to an outbuilding on a 10mm SWA, a new CU in outbuilding which has RCBO's to lights, sockets and a dedicated RCBO to an external socket for the hot tub, it was one of those blow up easy-spa things on a 13A plug, sat on top of a newly slabbed patio. IF it had been bigger I would have divorced the supply earthing and used an earth rod at the outbuilding BUT in this case wasn't so sure, had a chat with tech support at NAPIT and advice was to go with the existing supply earth (The outbuilding had no extraneous metal or pipework, it was basically a concrete & wood shed)

 

[buzzlightyear]

dedicated RCBO to an external socket for the hot tub, it was one of those blow up easy-spa dolls

I need to get one of those. Which store is that from or is it a secret.

 

[telectrix]

and with the energy price increases, make sure you have access to a mortgage/bank loan.

 

[Alan-N]

I need to get one of those. Which store is that from or is it a secret.

Screw fix obviously

 

[marcuswareham]

Put in a separate RCD for the tub.

Yes, you won't get fault selectivity between it and the local CU's RCD, but here it hardly matters. What you do gain is no single point of failure for protection if an RCD goes bad.

Exactly I always put 2 RCDs in line on any TT even if its just for a lamp post , then you have back up for RCD failure

 

[marcuswareham]

Personally for sure I would have made the tub TT (and possibly the shed just for easy of installation), I would have the tub protected by a 10mA rcd and a 30mA rcd in line with each other

But then again I do have my outside lights TTed