Hot tubs and PME

vantech: 
 

I understand the issue of touch/step voltages, but if a rod is installed next to the consumer unit and the tub is perhaps 10 metres away, would this be sufficient? 
 

• PME to the tub, with a local rod as described in point 2 above, but with a Matt:e device that also protects for open PEN faults.

BS 7671 only recognises open-PEN detection and disconnection devices for EV charging installations.

One issue is that, downstream of the device, if you have supplementary bonding or accidental contact with anything connected to PME, the device is rendered ineffective.

Secondly, the voltages etc. in Section 722 are not suitable for someone in a bath or shower.

vantech: 
 

Hi, 

I am planning a hot tub install, and we circle back to the question of earthing arrangements! 
 

I am thinking of 3 different approaches to this as follows: 

 

• PME to the isolator next to the tub, gland off and convert the output to TT creating an island.

 

Q: by doing this, how far away from the tub does the rod need to go? 

The position of the earth electrode relative to the tub will not necessarily help local touch voltages. The voltage gradient can be as little as 3 m or so, which means someone over the other side of the tub to the electrode might not see the same “earth” as someone next to the electrode.

 

• PME to the tub, and install a rod that is then supplementary to the PME.

 

Q: can this rod be next to the consumer unit or cut out, or does it need to go next to the tub? 

Section 702 and GN 7 talk about a “mat” - G12/4 talks about a grid around “poolside areas” - but those are swimming pools, and it depends on where your determining it's a “section 701” or a “section 702” installation.

I understand the issue of touch/step voltages, but if a rod is installed next to the consumer unit and the tub is perhaps 10 metres away, would this be sufficient? 
 

If we are talking about an electrode next to the consumer unit, then, for a maximum demand of 100 A, in a single phase installation you'd need an electrode with a resistance of less than 1 ohm to make any difference.

gkenyon: 
 

vantech: 
 

Hi, 

I am planning a hot tub install, and we circle back to the question of earthing arrangements! 
 

I am thinking of 3 different approaches to this as follows: 

 

• PME to the isolator next to the tub, gland off and convert the output to TT creating an island.

 

Q: by doing this, how far away from the tub does the rod need to go? 

The position of the earth electrode relative to the tub will not necessarily help local touch voltages. The voltage gradient can be as little as 3 m or so, which means someone over the other side of the tub to the electrode might not see the same “earth” as someone next to the electrode.

 

• PME to the tub, and install a rod that is then supplementary to the PME.

 

Q: can this rod be next to the consumer unit or cut out, or does it need to go next to the tub? 

Section 702 and GN 7 talk about a “mat” - G12/4 talks about a grid around “poolside areas” - but those are swimming pools, and it depends on where your determining it's a “section 701” or a “section 702” installation.

I understand the issue of touch/step voltages, but if a rod is installed next to the consumer unit and the tub is perhaps 10 metres away, would this be sufficient? 
 

If we are talking about an electrode next to the consumer unit, then, for a maximum demand of 100 A, in a single phase installation you'd need an electrode with a resistance of less than 1 ohm to make any difference.

Thanks. 

So here the ideal (well only) solution would be to TT the hot tub entirely? 
 

How does this impact touch/step voltages? 

vantech: 
Thanks. 

So here the ideal (well only) solution would be to TT the hot tub entirely? 
 

How does this impact touch/step voltages? 

To be honest, this is partly outside the scope of BS 7671.

Some of the issues are to do with the supply conditions, the nature of extraneous-conductive-parts (and, if they are service cables or pipes, where they run).

There is also the situation that BS 7671 requires that simultaneously accessible exposed-conductive-parts are connected to the same earthing system (Regulation 411.3.1.1) - so yo must be very careful if there are two earthing systems in the same installation.

So, it's all down to the designer's assessment.

Even without an earth terminal the matt device detects fault conditions such as L-N voltage wildly out of whack which is often a secondary symptom of PEN issues. It may not be in ‘7671, but if you are worried then it is no bad thing to have such a detector, it is common in ’power of opportunity' installations.. (however unless you get your PME supply by overhead singles,  or you know that the street mains in your area are consac and failing,  the risk is actually very low, so  do not lose sleep over it. There are no piles of bodies at the bases of all the PME street lamps every morning, and if there were it would soon be fixed.)

When a current flows into an earth terminal then on the surface around it there is a voltage gradient in a sort of fried egg pattern - the top of the rod may be as high as 230V, but within a rod length or so the voltage rise has fallen to ~ 50 or so relative to the earth far away.

But this sort of gradient is only present in the instant that the RCD is operating, and anyone with one foot on the rod and one far away will indeed get a very nasty shock, but a non lethal time limited one.

Putting the rod in a pit by 10  cm  or so, such that the top is not accessible, and/or insulating the top foot or so means that the surface voltage directly over it is significantly reduced, and the resistance of soil above acts as a current limit, and by the time you throw in the odd paving slab reduces the step voltage risk to almost nil, even if the RCD does not fire.

For best electrode resistance any paving should be water permeable (there is a reason substations are on gravel like railway ballast - firstly it hurts if you are not in decent footwear, so folk will not linger,  and it does not support an unbroken surface layer of water) Pf put it somewhere no-one will stand - like a flower bed of roses..

Put the rod anywhere convenient, concerns about damage to the building, paving or nearby drains, water or gas services may well be dominant concerns.  protect the SWA armour with the house CPC, and but use an insulating stuffing gland where it goes into the first out door box, From there, run some G/y to your electrode.

In theory problems may arise if there are exposed metal things on the house CPC,like outside lights or perhaps pipework that may be reached by a tub user, or simultaneously touched as any metalwork on your TT earth. However, this is not much worse than holding the said outside light or whatever while standing on the pre-existing ground, so you have not really added a significant new risk.

Mike.

mapj1: 
 

Even without an earth terminal the matt device detects fault conditions such as L-N voltage wildly out of whack which is often a secondary symptom of PEN issues. It may not be in ‘7671, but if you are worried then it is no bad thing to have such a detector, it is common in ’power of opportunity' installations.. (however unless you get your PME supply by overhead singles,  or you know that the street mains in your area are consac and failing,  the risk is actually very low, so  do not lose sleep over it. There are no piles of bodies at the bases of all the PME street lamps every morning, and if there were it would soon be fixed.)

When a current flows into an earth terminal then on the surface around it there is a voltage gradient in a sort of fried egg pattern - the top of the rod may be as high as 230V, but within a rod length or so the voltage rise has fallen to ~ 50 or so relative to the earth far away.

But this sort of gradient is only present in the instant that the RCD is operating, and anyone with one foot on the rod and one far away will indeed get a very nasty shock, but a non lethal time limited one.

Putting the rod in a pit by 10  cm  or so, such that the top is not accessible, and/or insulating the top foot or so means that the surface voltage directly over it is significantly reduced, and the resistance of soil above acts as a current limit, and by the time you throw in the odd paving slab reduces the step voltage risk to almost nil, even if the RCD does not fire.

For best electrode resistance any paving should be water permeable (there is a reason substations are on gravel like railway ballast - firstly it hurts if you are not in decent footwear, so folk will not linger,  and it does not support an unbroken surface layer of water) Pf put it somewhere no-one will stand - like a flower bed of roses..

Put the rod anywhere convenient, concerns about damage to the building, paving or nearby drains, water or gas services may well be dominant concerns.  protect the SWA armour with the house CPC, and but use an insulating stuffing gland where it goes into the first out door box, From there, run some G/y to your electrode.

In theory problems may arise if there are exposed metal things on the house CPC,like outside lights or perhaps pipework that may be reached by a tub user, or simultaneously touched as any metalwork on your TT earth. However, this is not much worse than holding the said outside light or whatever while standing on the pre-existing ground, so you have not really added a significant new risk.

Mike.

 

 

Thanks for this Mike. 

I take from this that actually PME is OK if you have underground supply cables only (reduced risk of PEN fault), however does it make sense to supplement the PME next to the hot tub with an earth rod? If so, is anything up to 200ohm acceptable or does it need to be below 20ohm? 

TT makes perfect sense, no questions here. Max 200ohm with a 30mA double pole RCD and ensure it’s away from anything connected to the PME as well as pitted away from the tub itself. 

I have seen an article by the NIC and others that suggest PME is perfectly OK. 

Here: 

https://www.askthetrades.co.uk/downloads/hottubs.pdf

It seems this subject is down to the presence of the installer and a risk assessment. 

Thanks. 

As I more or less said above , if PME was as dangerous as some folk make out we'd have safety fence around all the lamp posts, and dogs would explode every time they cocked a leg against one . This does not happen very often, if at all (well OK, occasionally ). The chance of a PME fault and someone being half in and half out of the tub at that very instant  is small, they are much more likely to get hurt slipping on a wet surface.

So just to plug the thing in on PME is unlikely to be dangerous. However in normal operation the PME CPC may be a volt or ten above terra-firma, so there may be nasty tingles on climbing in or out. For this a TT island is the fix.

Mike.

mapj1: 
 

For this a TT island is the fix.

 

Provided that you're not going to convert the tingles into possibly more serous touch-potential with simultaneously-accessible exposed-conductive-parts (or extraneous-conductive-parts) connected to the PME part of the installation.

mapj1: 
 

As I more or less said above , if PME was as dangerous as some folk make out we'd have safety fence around all the lamp posts, and dogs would explode every time they cocked a leg against one . This does not happen very often, if at all (well OK, occasionally ). The chance of a PME fault and someone being half in and half out of the tub at that very instant  is small, they are much more likely to get hurt slipping on a wet surface.

So just to plug the thing in on PME is unlikely to be dangerous. However in normal operation the PME CPC may be a volt or ten above terra-firma, so there may be nasty tingles on climbing in or out. For this a TT island is the fix.

Mike.

 

If decking is installed around the tub, this presumably would then reduce the tingles on entry and exit. 

What happens if there is a PEN fault? The heater on the tub would become live, as would the water. If you make contact with the earth or someone dips a finger in, they’d receive a shock that would not disconnect by means of an RCD. 

Surely this risk alone would warrant TT on all installations like this, albeit highly unlikely to happen. I suppose the Matt:e would resolve this issue. 

accessible exposed-conductive-parts (or extraneous-conductive-parts) connected to the PME part of the installation.

Be aware that you already had potential for tingles  if you touch those parts while stood on the ground in bare wet feet, before you added the TT island,  but it is in effect making your wet feet bigger or more likely,  so yes try and keep exposed things on the 2 earthing systems more than arms length apart. Its another good reason for a rod to be tidily in a pit beneath a plastic or concrete cover rather than exposed.

versus