UK 32a supply for a hot tub of a PME supply.

[Scottishlad123]

Hi there,

I have been asked to install a 32a supply for a hot tub for a client. I haven’t installed a supply for a hot tub before and having done a bit of research I am getting mixed answers about doing this on a PME system. I have a DB under the stairs with plenty of spare ways so I would be planning to take a 6mm armour from here outside to a rotary isolator. what I am unsure about is the earthing scenario, I have seen a lot of people saying this will require an earth rod. There is also the possibility of taking tails directly of the Henley blocks into a small garage unit and feeding the hot tub of its own DB. Any help is greatly appreciated thanks in advance.

 

[pc1966]

You will see quite a few opinions on the use of PME supplies for hot tubs and similar!

Some folk prefer to make it a TT supply but putting a dedicated RCD for the tub and putting separate earth rod for it. In that case simply feeding it from the main CU via a MCB is perfectly fine.

The argument for splitting the tails for a fused-switch feed is for selectivity with downstream MCBs, that makes sense for a garage with several circuits. But as the hot tub is the sole load you don't really care if it has to be reset in the house as you don't need further over-current protection at the tub.
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Also the closest thing in the reg is a swimming pool so 702.410.3.4.3 subsection (ii) offers the alternative of an earth rod/mat below 20 ohms, but that would also force the use of a CPC of 10mm copper equivalent.

So probably your original plan of house CU feeding cable, then rotary isolator, then RCD and local earth rod is a simple and reliable way.

 

[pc1966]

You don't need a metal enclosure (amendment 3 style) outdoors, so you can get an inexpensive plastic one to IP55 or similar for the RCD such as this:

4 Module Insulated IP55 Enclosure

Enclosures with top and bottom knockouts

www.cef.co.uk

 

[Lister1987]

There is also the matt-e option that has been used in hot tub feeds, this removes the need for an earth rod iirc.

 

[pc1966]

There is also the matt-e option that has been used in hot tub feeds, this removes the need for an earth rod iirc.

Probably, but it costs more than an RCD and rod!

 

[Scottishlad123]

You will see quite a few opinions on the use of PME supplies for hot tubs and similar!

Some folk prefer to make it a TT supply but putting a dedicated RCD for the tub and putting separate earth rod for it. In that case simply feeding it from the main CU via a MCB is perfectly fine.

The argument for splitting the tails for a fused-switch feed is for selectivity with downstream MCBs, that makes sense for a garage with several circuits. But as the hot tub is the sole load you don't really care if it has to be reset in the house as you don't need further over-current protection at the tub.
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Also the closest thing in the reg is a swimming pool so 702.410.3.4.3 subsection (ii) offers the alternative of an earth rod/mat below 20 ohms, but that would also force the use of a CPC of 10mm copper equivalent.

So probably your original plan of house CU feeding cable, then rotary isolator, then RCD and local earth rod is a simple and reliable way.

thanks for your help very much appreciated

 

[Mark.fardon]

Why do you need an earth rod ? Are there exposed conductive parts near to the tub ?

 

[Ian1981]

Why do you need an earth rod ? Are there exposed conductive parts near to the tub ?

Because every electrician on YouTube says you do

 

[radiohead]

Hi there,

I have been asked to install a 32a supply for a hot tub for a client. I haven’t installed a supply for a hot tub before and having done a bit of research I am getting mixed answers about doing this on a PME system. I have a DB under the stairs with plenty of spare ways so I would be planning to take a 6mm armour from here outside to a rotary isolator. what I am unsure about is the earthing scenario, I have seen a lot of people saying this will require an earth rod. There is also the possibility of taking tails directly of the Henley blocks into a small garage unit and feeding the hot tub of its own DB. Any help is greatly appreciated thanks in advance.

Not quite on topic I know but bear in mind you will need to reference section 7 as this is classed as a pool installation, there are strict regulations on the positioning of adjacent lighting and switchgear. I only point this out because more often than not it is overlooked and a non-compliant installation results.

 

[davesparks]

There is also the matt-e option that has been used in hot tub feeds, this removes the need for an earth rod iirc.

Which regulation would you use to support this statement?

 

[Ian1981]

What I don’t get is that if the hot tub comes with a plug, then people are happy to install a socket and plug it in, if it needs hard wiring and a dedicated supply however, then it needs an Earth electrode?
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Not quite on topic I know but bear in mind you will need to reference section 7 as this is classed as a pool installation, there are strict regulations on the positioning of adjacent lighting and switchgear. I only point this out because more often than not it is overlooked and a non-compliant installation results.

Is it classed as a swimming pool tho or perhaps other basins?
Gn7 suggests maybe you should take into account section 702 but the regulations do not as does gn7 specifically say, a hot tub shall comply with section 702.
Taken from the IET

Persons involved in the design of the electrical installation for a hot tub should consult the product standard BS EN 60335-2-60: Specification for safety of household and similar appliances: Particular requirements for whirlpool baths and whirlpool spas.

 

[davesparks]

What I don’t get is that if the hot tub comes with a plug, then people are happy to install a socket and plug it in, if it needs hard wiring and a dedicated supply however, then it needs an Earth electrode?

What I don't get is that although the regulations clearly suggest that an earth electrode connected to the equipotential bonding system may be appropriate, people always want to make the hot tub supply seperate and a TT system all on its own?
That is not in any way connecting an earth electrode to the equipotential bonding system of the installation.

As I have said many times before I am of the opinion that we are well behind the rest of the world on this one and should be installing an earth electrode connected to the MET of every PME fed installation as a matter of course.
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Not quite on topic I know but bear in mind you will need to reference section 7 as this is classed as a pool installation, there are strict regulations on the positioning of adjacent lighting and switchgear. I only point this out because more often than not it is overlooked and a non-compliant installation results.

That is a matter of opinion.

There is a suggestion that outdoor hottubs should be treated similar to pools and indoor hottubs similar to baths, but that is from a guidance note not the regulations.

 

[Ian1981]

What I don't get is that although the regulations clearly suggest that an earth electrode connected to the equipotential bonding system may be appropriate, people always want to make the hot tub supply seperate and a TT system all on its own?
That is not in any way connecting an earth electrode to the equipotential bonding system of the installation.

As I have said many times before I am of the opinion that we are well behind the rest of the world on this one and should be installing an earth electrode connected to the MET of every PME fed installation as a matter of course.

I’d imagine that the addition of an electrode to a PME supply will come into effect in the not to distant future Dave.
It’s perfectly feasible on new builds ( domestic and commercial buildings ) which have a big metal mesh in the ground, a perfect electrode.

 

[davesparks]

I’d imagine that the addition of an electrode to a PME supply will come into effect in the not to distant future Dave.
It’s perfectly feasible on new builds ( domestic and commercial buildings ) which have a big metal mesh in the ground, a perfect electrode.

Yes, new builds and extensions would be easy, but I think it would need a requirement in the building regs to be put in place to ensure that all builders get the message that an earth connection to the reinforcing needs to be made.

A lot of the time we don't get to see the job until after the foundations have been poured and even if we do the builders are unlikely to listen/understand when we tell them we need to connect to the reinforcing mesh.

 

[Lister1987]

Which regulation would you use to support this statement?

Being brutely honest, I have no clue, whichever references exporting PME.

A quick Google search brings up;

Domestic EV Installations

use the matt:e EV connection units to make EV charge point installs simpler

matt-e.co.uk

The single-phase units from matt:e provide a simple single connection point that allows for the connection of 1 x,32amp single phase charge point to the existing PME earthing facility.

In line with BS 7671: 2018: Amendment 1: 2020 Regulation 722.411.4.1

(iv) Protection against electric shock in a single phase installation is provided by a device which electrically disconnects the vehicle from the live conductors of the supply and from protective earth in accordance with regulation 543.3.3.101 (ii) within 5 s in the event of the utilisation voltage at the charging point, between the line and neutral conductors, being greater than 253V rms or less than 207V rms. The device shall provide isolation and be selected in accordance with Table 537.4. Equivalent means of functionality could be included within the charging equipment. Closing or resetting of the device shall be possible only if the voltage between line and neutral conductors is in the range 207 to 253V rms.

Please note, these units are for use on buildings with a single-phase electricity supply only.

BS 7671 does not permit a protective device as described in indent (iv) of Regulation

722.411.4.1 to be used to protect single-phase charging equipment in three-phase installations.

Equally, BS 7671 does not permit charging equipment containing equivalent functionality to that described in indent (iv) of Regulation 722.411.4.1 to be installed in installations with three-phase supplies.

 

[davesparks]

Being brutely honest, I have no clue, whichever references exporting PME.

A quick Google search brings up;

Domestic EV Installations

use the matt:e EV connection units to make EV charge point installs simpler

matt-e.co.uk

There is no regulation which references exporting PME because there is no such thing as exporting PME.

You won't find a regulation to support your statement because none exists.

As per your link those units are intended to protect EV charging points and the regulations do specifically allow such protection to be used for EV charging points.

Nothing in the regulations allows these devices to be used in any other situation where PME is not permitted to be used.

 

[Lister1987]

There is no regulation which references exporting PME because there is no such thing as exporting PME.

You won't find a regulation to support your statement because none exists.

As per your link those units are intended to protect EV charging points and the regulations do specifically allow such protection to be used for EV charging points.

Nothing in the regulations allows these devices to be used in any other situation where PME is not permitted to be used.

Rightly or wrongly I've seen a few YT sparks use the matt-e for hot tub installs a opposed to EV chargers, my impression was they were using them to avoid putting in an earth rod and making use of the existing PME arrangement, in a fashion, I base my loose and likely faulty understanding on that

 

[davesparks]

Rightly or wrongly I've seen a few YT sparks use the matt-e for hot tub installs a opposed to EV chargers, my impression was they were using them to avoid putting in an earth rod and making use of the existing PME arrangement, in a fashion, I base my loose and likely faulty understanding on that

I suspect they are using them due to a lack of knowledge, or due to the manufacturers of the device paying them to use them.
It could also be down to one of them doing it for these reasons and the rest just following like sheep.

Remember there is nothing regulating the technical accuracy of the content on YouTube,
There was recently a video published somewhere online where an American demonstrated making 'English tea' that used a microwave and came out looking like a mug of dishwater, it's on the Internet but that doesn't make it right!

You would do far better to get your information from trusted, provably reliable sources.

This subject kind of puts me in mind of D.W. Cockburn's book on earthing and bonding, it was published by a major publisher but completely and utterly wrong/dangerous

 

[marcuswareham]

Rightly or wrongly I've seen a few YT sparks use the matt-e for hot tub installs a opposed to EV chargers, my impression was they were using them to avoid putting in an earth rod and making use of the existing PME arrangement, in a fashion, I base my loose and likely faulty understanding on that

In my opinion using the Matt-e single phase verison for a hot tub gives you a false sense of security, as it only protects against some PEN conductor faults but not all, it is likely you can have a open PEN conductor and still have a voltage between 207v-253v between L - N depending how the phases are balanced and where the break is, Matt-e themselves even demonstrate this on there 3 phase o-pen video

Although this does mitigate do a degree and perhaps the likelihood of a PEN fault is small. The effects of a fault can be fatal

e.g. PME supply with plastic supply pipes and therefore no extraneous conductive parts water heating element bonded to CPC and in contact with the hot tub water, if the hot tub is placed on grass/soil and you step out and simultaneously in contact with true earth and the water (effectively connected to the PME earth during a fault) then there could be a dangerous voltage between the water and true earth (and you are also wet)

I don't know how using a matt-e would fit within the regs, but as others have suggested swimming pools is the closest/same thing

 

[Lister1987]

I suspect they are using them due to a lack of knowledge, or due to the manufacturers of the device paying them to use them.
It could also be down to one of them doing it for these reasons and the rest just following like sheep.

Remember there is nothing regulating the technical accuracy of the content on YouTube,
There was recently a video published somewhere online where an American demonstrated making 'English tea' that used a microwave and came out looking like a mug of dishwater, it's on the Internet but that doesn't make it right!

You would do far better to get your information from trusted, provably reliable sources.

This subject kind of puts me in mind of D.W. Cockburn's book on earthing and bonding, it was published by a major publisher but completely and utterly wrong/dangerous

I'm not replying on them solely but every day is a school day

 

[davesparks]

I'm not replying on them solely but every day is a school day

Indeed, and watching people work is a great way to learn practical methods and techniques, tips and tricks etc.

For technical information and knowledge you need to make sure that what is being presented by uncontrolled sources is actually correct and not just someone opinion or own misunderstanding being presented as fact.

Everyone makes mistakes, this includes people on YouTube.

 

[suffolkspark]

The way I read the regs is its fine on pme if you can supplement it with a rod etc under 20ohm. I have recently achieved this on a couple of rods but dont often, so I always opt to seperate it off the pme

 

[Mark.fardon]

Because every electrician on YouTube says you do

You may roll your eyes and maybe every electrician on you tube does suggest/do this.
But that doesn’t make it a good idea necessarily. The regs suggest as “recommendation” in a PME supply to provide an earth electrode < 20 ohm for a swimming pool supply. But do a risk assessment- are there any exposed conductive parts liable to be touched by the user of hot tub (likely to be class 2) ? Is it an overhead supply /rural installation ? What is the likelihood of a lost neutral in the supply ? In the event of the lost pen conductor is the bonding to water and gas in place and reliable ?

Weigh up pros/cons of using the DNO earth and bonding any extraneous conductive parts (Using 3 core SWA, marshalling terminal if required) and having a Ze of 0.2 Ohm, having protection by ADS fault protection and additional protection via an RCD ...or isolating this excellent earth , having a variable earth rod Ra of 100 ohms plus in most cases and relying solely on an RCD to achieve fault protection disconnection time in 0.4s. A rod and connection that could be damaged outdoors or disconnected. And some bright spark coming along and reconnecting the load CPC back to the pme at later date.

In summary - use the supplier earth that is excellent for 99.99% of the time or stick a brass spike in the ground and rely on an RCD for the off chance that the DNO lose the neutral only to your property while you are in the garden still wet and touching ...touching ...can’t think of what you would be that has a potential versus ground ?

 

[Ian1981]

You may roll your eyes and maybe every electrician on you tube does suggest/do this.
But that doesn’t make it a good idea necessarily. The regs suggest as “recommendation” in a PME supply to provide an earth electrode < 20 ohm for a swimming pool supply. But do a risk assessment- are there any exposed conductive parts liable to be touched by the user of hot tub (likely to be class 2) ? Is it an overhead supply /rural installation ? What is the likelihood of a lost neutral in the supply ? In the event of the lost pen conductor is the bonding to water and gas in place and reliable ?

Weigh up pros/cons of using the DNO earth and bonding any extraneous conductive parts (Using 3 core SWA, marshalling terminal if required) and having a Ze of 0.2 Ohm, having protection by ADS fault protection and additional protection via an RCD ...or isolating this excellent earth , having a variable earth rod Ra of 100 ohms plus in most cases and relying solely on an RCD to achieve fault protection disconnection time in 0.4s. A rod and connection that could be damaged outdoors or disconnected. And some bright spark coming along and reconnecting the load CPC back to the pme at later date.

In summary - use the supplier earth that is excellent for 99.99% of the time or stick a brass spike in the ground and rely on an RCD for the off chance that the DNO lose the neutral only to your property while you are in the garden still wet and touching ...touching ...can’t think of what you would be that has a potential versus ground ?

You do realise that was a tongue in cheek remark regarding most YouTube electricians who insist on installing electrodes as the only option because they all copy each other?

 

[Mark.fardon]

You do realise that was a tongue in cheek remark regarding most YouTube electricians who insist on installing electrodes as the only option because they all copy each other?

Ha no I didn’t I’m about 20 posts behind so didn’t get the context ?

 

[Ian1981]

Ha no I didn’t I’m about 20 posts behind so didn’t get the context ?

Ha no problem

 

[Mark.fardon]

So those that would install the rod your thoughts on the above risk assessment and preference for isolating the DNO earth and reliance on a long tent peg for safety from shock under normal conditions ?

 

[suffolkspark]

So those that would install the rod your thoughts on the above risk assessment and preference for isolating the DNO earth and reliance on a long tent peg for safety from shock under normal conditions ?

If the regs point towards using a long tent peg as you call it, then that's what gets used. I live in the countryside where most properties I work in are TT anyway

 

[Mark.fardon]

Well that makes sense of course

 

[davesparks]

The way I read the regs is its fine on pme if you can supplement it with a rod etc under 20ohm. I have recently achieved this on a couple of rods but dont often, so I always opt to seperate it off the pme

If you go back and read the regs again you will see that the >20ohm earth electrode is a note and that they do allow a hottub to be connected to a supply fed by PME without it.

 

[Ian1981]

I still don’t see the insistence of installing an electrode to an all insulated glorified paddling pool, so your In the water and step onto the grass??
Surely they would be included in part 7 if special supplementary requirements need to be considered ( I don’t consider them to fall into 702, if they did they’d be in there)

 

[suffolkspark]

If you go back and read the regs again you will see that the >20ohm earth electrode is a note and that they do allow a hottub to be connected to a supply fed by PME without it.

Ah I read it that it was a requirement but will have another look, just as an aside, the manufacturers of 3 out of the 4 I have installed in the last month have said they must have an electrode, 1 said the opposite. I'll look back in regs in a bit

 

[davesparks]

Ah I read it that it was a requirement but will have another look, just as an aside, the manufacturers of 3 out of the 4 I have installed in the last month have said they must have an electrode, 1 said the opposite. I'll look back in regs in a bit

I might be wrong, I haven't actually looked at it since the change to the 18th edition but it always used to be a note at the end of a regulation saying that you could install an earth electrode with Ra <20ohms connected to the equipotential bonding system of the installation.

A key point on this for me is that it says 'connected to the equipotential bonding system' not to create a seperate TT earthing system or anything like that. As far as I am concerned this means an earth electrode system connected to the MET of the installation.

 

[richy3333]

The regs suggest as “recommendation” in a PME supply to provide an earth electrode < 20 ohm for a swimming pool supply.

Which section of 702 - swimming pools and other basins are you quoting as I cant see this in my BBB?

 

[suffolkspark]

Our favourite word "reccomend" makes an appearance.....

 

[Ian1981]

Yes if you took recommend to mean shall, then we all would need to install AFDD’s in most installations, but that’s only a recommendation as well as I’ve not installed one yet!

 

[Mark.fardon]

If you go back and read the regs again you will see that the >20ohm earth electrode is a note and that they do allow a hottub to be connected to a supply fed by PME without it.

And the key here is if you can’t get 20ohms - which you won’t with an normal earth rod in majority of cases, then don’t install one .

Remember the regs are a guide not statutory .

 

[Ian1981]

And the key here is if you can’t get 20ohms - which you won’t with an normal earth rod in majority of cases, then don’t install one .

Remember the regs are a guide not statutory .

But part p a statutory document requires that the installation complies to bs7671 which gives a reason to follow it.

 

[Mark.fardon]

But part p a statutory document requires that the installation complies to bs7671 which gives a reason to follow it.

Really ? Where does it state that ?

 

[Ian1981]

Really ? Where does it state that ?

Here it’s an older copy mind
Should be designated and installed in accordance with bs7671.

 

[davesparks]

And the key here is if you can’t get 20ohms - which you won’t with an normal earth rod in majority of cases, then don’t install one .

Remember the regs are a guide not statutory .

What would you class as a 'normal' earth rod?

I have installed many earth electrodes, made up of multiple standard rods, with Ra of less than 20 ohms.

Plus of course there are the many lightning protection installations which use standard rods to achieve even lower values.
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Our favourite word "reccomend" makes an appearance.....

I think that has changed, it now states supplementary equipotential bonding whereas I am sure it used to just say equipotential bonding.
To my mind this possibly moves the intended point of connection of the earth electrode to be local to the swimming pool or other basin.

 

[Mark.fardon]

...I think that has changed, it now states supplementary equipotential bonding whereas I am sure it used to just say equipotential bonding.
To my mind this possibly moves the intended point of connection of the earth electrode to be local to the swimming pool or other basin.

Yes was just looking at 702 again and actually states where you are USING PME earth as the supply to the "basin" then it recommends the electrode is added to the supplementary bonding :
"NOTE: Where a PME earthing facility is used as the means of earthing for the electrical installation of a swimming
pool or other basin, it is recommended that an earth mat or earth electrode of suitably low resistance,"

So it states the opposite of the lengths most prescribe of isolating the PME earth from the hot tub supply using stuffing glands, cutting back the armour etc, you just could/should add an electrode in the ground and bond it loacally at the sub board or close by...


ps I'm still unsure the logic/reason for seperating the PME earth from a "TT"'d installation for eaxample to an outbuilding when you could equipotentially bond it back or via a marshalling terminal to the PME earth ? (providing you have the correct size bonding conductor)

 

[pc1966]

ps I'm still unsure the logic/reason for separating the PME earth from a "TT"'d installation for example to an outbuilding when you could equipotentially bond it back or via a marshalling terminal to the PME earth ? (providing you have the correct size bonding conductor)

The usual reason for TT-ing an out building is the 10mm size of the CPC if it needs bonding to extraneous conductive parts in the out building from a PME-derived supply.

If you are feeding via 10mm or more SWA (with a CPC, or armour of equivalent rating) no issue, but if its a 4mm run then you either need a supplementary CPC or simply isolate it via a TT-ing RCD device and use a local earth rod.

As for the earth mat/rod for an outdoor pool it makes sense. The issue for safety is the voltage difference between the water/metal in the event of a PME fault, and the Earth around the pool. Even if the earth rod/mat is unable to pull the PME fault down close to true 0V, it will still raise the surrounding soil to (hopefully) a small enough difference in potential for anyone stepping out of the pool.

 

[Mark.fardon]

Here it’s an older copy mind View attachment 59055
Should be designated and installed in accordance with bs7671.

Fair point !
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The usual reason for TT-ing an out building is the 10mm size of the CPC if it needs bonding to extraneous conductive parts in the out building from a PME-derived supply.

If you are feeding via 10mm or more SWA (with a CPC, or armour of equivalent rating) no issue, but if its a 4mm run then you either need a supplementary CPC or simply isolate it via a TT-ing RCD device and use a local earth rod.

As for the earth mat/rod for an outdoor pool it makes sense. The issue for safety is the voltage difference between the water/metal in the event of a PME fault, and the Earth around the pool. Even if the earth rod/mat is unable to pull the PME fault down close to true 0V, it will still raise the surrounding soil to (hopefully) a small enough difference in potential for anyone stepping out of the pool.

So the reason for isolation the PME earth in the undersized bonding probable with the above example of a 4mm cpc to the remote tub/building supply is that it may not have capacity for an earth fault PFC current ?
Nothing to do with the dreaded lost Neutral/ PEN conductor in a PME supply ?

if this is the case then shouldn’t the best practice to ensure 10mm or equivalent CPC size and in the case of a tub put in a supplementary electrode as low ohm as possible in the supply to the remote application and ensure all extraneous parts are bonded - rather than go with the TT approach the latter seems to be the go to choice for most?

a

 

[pc1966]

So the reason for isolation the PME earth in the undersized bonding probable with the above example of a 4mm cpc to the remote tub/building supply is that it may not have capacity for an earth fault PFC current ?
Nothing to do with the dreaded lost Neutral/ PEN conductor in a PME supply ?

It is not the earth fault PFC as that will be cleared by your local MCB no problem, it is the PME fault resulting is a sustained current to true Earth via your externally conductive stuff.

Sure if its only tens of ohms on a small bit of metal then you won't roast a 4mm cable in this example, but the CPC under PME faults potentially carries the residual neutral current of everyone in the faulted section, so it could be tens or hundreds of amps if your Ra is low enough. Having Ra below a couple of ohms is unlikely though, so high tens of amps for long periods is the typical scenario needing 10mm CPC size.

There might well be issues of external metalwork on an out building going high under PME faults as well with a risk for high touch potential like the hot tub case, so TT-ing is an option also.

if this is the case then shouldn’t the best practice to ensure 10mm or equivalent CPC size and in the case of a tub put in a supplementary electrode as low ohm as possible in the supply to the remote application and ensure all extraneous parts are bonded - rather than go with the TT approach the latter seems to be the go to choice for most?

TT-ing is much easier than adding Earth rods/mats for sure, so is the easier option in most case.

If every building had an earth rod on their PME earth then we would have less of a debate/worry as full-on PME disconnection and voltages above 50V to Earth would be much less likely!

 

[Mark.fardon]

Great thanks for that. So purely for the potential danger under lost PEN. In the case of an older house/neighbourhood then it’s likely that gas and water services will be metal and many will be bonded to their respective ETs so I suspect under the DNO fault scenario these would pull the extraneous voltages down very low especially as would be acting in parallel ? I guess all the above applies to EV charging issue also

 

[pc1966]

Great thanks for that. So purely for the potential danger under lost PEN. In the case of an older house/neighbourhood then it’s likely that gas and water services will be metal and many will be bonded to their respective ETs so I suspect under the DNO fault scenario these would pull the extraneous voltages down very low especially as would be acting in parallel ?

Certainly many houses will be bonded with low impedance together in older setups with metal pipes for gas & water, there you might see sub-1 ohm Ra but equally you would have many parallel bonding paths to the shared service pipes (assuming properly wired homes).

But it can't be depended upon and most gas and water mains these days are plastic, so over time that route for voltage equalisation will go as leaky/corroded mains pipes are replaced.

 

[davesparks]

ps I'm still unsure the logic/reason for seperating the PME earth from a "TT"'d installation for eaxample to an outbuilding when you could equipotentially bond it back or via a marshalling terminal to the PME earth ? (providing you have the correct size bonding conductor)

Don't forget that regulations cover all installations, not just little domestic ones.

The point is that sometimes running a main bond back to the MET is not possible.
For example you are installing a supply to a portakabin type classroom at a school which requires main bonding to an extraneous part.
There's a suitable DB to supply it from around 20 mtrs away and you calculate that you need a 10mm 4 core SWA, however the size of the incoming supply dictates that the main bonding needs to be 50mm and the DB you are coming from doesn't have a 50mm connection back to the MET.

So you are faced with a choice of running a 50mm bond 120m long back to the MET, or setting up a seperate TT system for the portakabin.

Hopefully it is obvious that it's simply not economically viable to run that main bond, it woukd probably quadruple the price of the job.

These are the kind of situations where setting up a seperate TT system is the only realistic option.

Scaling it back to domestic installations where main bonds are usually 10mm it is often possible to run that main bond, but people choose not to for various reasons. Sadly the reason is often a lack of knowledge or understanding.

 

[Mark.fardon]

Great post really good point

 

[Midwest]

I’ve been reading threads like this with some personal interest. I’m thinking of buying one of those inflatable hot tubs, primarily as a glorified paddling pool for my grandson.

I was quite happy to plug into my outdoor socket on my decking (pme supply), but I keep reading these sorts of threads Another Hot Tub Thread which makes me re think.

A 10mm supply cable might be difficult to get to the back of my place, but I might be able to get an earth rod in at the front.

The manufacture recommends ‘connecting the spa pump to an equipotential bonding terminal with a solid 2.5mm copper conductor’.

Your thoughts, or am I over thinking this?