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Ian1981

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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)
 
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suffolkspark

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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

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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

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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?
 

Ian1981

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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!
 
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

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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.
 
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 ?
 

davesparks

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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.
 
...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

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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.
 
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
 
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pc1966

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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!
 
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

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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

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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.
 
Great post really good point
 
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