Hot tubs and PME

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. 

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.