Outside tap

[marcuswareham]

Hi all

I am on a TN-C-S / PME supply and have an outside tap, I have fitted it with 5 speedfit plastic elbows and a speedfit all plastic valve. So having so much plastic in place that should be the recommended insulated section of pipe to prevent dangerous voltages being on the tap in the event of a PEN failure

With the pipes full of water the resistance between the MET and the outside tap is 1726 ohms, so I work that out to be a potential current of 133mA which is still enough to be potentially fatal?

Please correct me if I am wrong here?

I know people say the risk of a PEN fault is so low you don't worry about it, but if it never happened why all the regulations for EV charging from PME, also I live in a very windy little village with individual overhead cables from the transformer down the street and UK power networks are often out to repair them. Information from UK power networks would indicate there is an electrode at the transformer and 2 more down the road, it seems the nearest to my house is 8 poles away and some of those poles look as if they could fall down lol, so if its gonna to happen anywhere I guess it would be here.

Thanks Marcus

 

[davesparks]

If the poles come down then you'll far more likely loose all conductors, not just the PEN.

Is it ABC or seperate cables on the poles?

What do you mean by a recommended insulated section of pipe? I'm not aware of any recommendations regarding this.

 

[radiohead]

I think you are worrying unnecessarily. However a simple step you could take is to install an earth electrode to the earthing system which would reduce or eliminate the miniscule risk. This was in fact considered, and rejected as a requirement of the recent 18th edition of the regulations.

 

[davesparks]

As I've mentioned on your other threads, a couple of earth electrodes connected to the MET of the installation will help reduce the risk.

 

[radiohead]

What do you mean by a recommended insulated section of pipe? I'm not aware of any recommendations regarding this.

I suspect this is a confusion regarding main bonding, where it has been clarified in the 18th that an insulating section at the point of entry negates the requirement for main bonding, however that is not the same as recommending an insulating section should be present.

 

[marcuswareham]

If the poles come down then you'll far more likely loose all conductors, not just the PEN.

Is it ABC or seperate cables on the poles?

What do you mean by a recommended insulated section of pipe? I'm not aware of any recommendations regarding this.

Hey no its is not aerial bundled cable it is separate cables and from what I can tell to an untrained eye the PEN is the bottom one, so that was what I meant about the likely hood of just one cable i.e the PEN breaking. Yes exactly see what you mean that they would most likely all go if a pole falls down, but maybe something less dramatic might kill the PEN

Sorry the insulated section of pipe recommendation is an un-official recommendation I have read on a few forum posts etc, so what people say they do etc
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As I've mentioned on your other threads, a couple of earth electrodes connected to the MET of the installation will help reduce the risk.

Exactly doing the math to get under touch voltage the earth electrodes would have to be around 1-3 ohm mark, I am fairly sure I will not achieve that
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[telectrix]

why not fit a plastic tap?

 

[davesparks]

Sorry the insulated section of pipe recommendation is an un-official recommendation I have read on a few forum posts etc, so what people say they do etc

Exactly doing the math to get under touch voltage the earth electrodes would have to be around 1-3 ohm mark, I am fairly sure I will not achieve that
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I've never seen that recommended on this forum.
Obviously it's dependant largely on exactly what is in the water at any one time, so hard to calculate exactly, but I think for 15mm pipe the plastic section needs to be well over half a metre to have any chance of achieving a high enough resistance with the 'average' hard water in the UK.

 

[marcuswareham]

Does anyone know of any other way to mitigate against it, our outside tap is in regular use and it is above soil and a few loose stones and of course the floor gets wet increasing conductivity

I did try a plastic tap, but they are hard to get hold of (or at least good ones) the one I got hold of was a bit pants, it was on for about 4 days then the treads broke and it shot its self across the drive.

Also how would you work out the touch voltage on the tap should you have a PEN fault, I know the impedance of my extraneous conductive parts without them being connected to the MET, however what would you put in for the current as who knows how many houses would be on the same conductor
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why not fit a plastic tap?

Sorry I think I was typing at the same time, I tried plastic tap, didnt end that well

 

[davesparks]

I suspect this is a confusion regarding main bonding, where it has been clarified in the 18th that an insulating section at the point of entry negates the requirement for main bonding, however that is not the same as recommending an insulating section should be present.

Which is still something I don't like, they really should have specified how long the insulating section should be, or at least that testing must still be carried out.
As has been proved in this thread a short section of plastic water pipe isn't enough as the water itself has some conductivity

 

[marcuswareham]

I've never seen that recommended on this forum.
Obviously it's dependant largely on exactly what is in the water at any one time, so hard to calculate exactly, but I think for 15mm pipe the plastic section needs to be well over half a metre to have any chance of achieving a high enough resistance with the 'average' hard water in the UK.

I can't remember if it was on these forums or another, but iv read it somewhere in a few posts, they mentioned just using a plastic coupler

I see what you mean about the length of plastic, I might have some 15mm barrier pipe kicking about somewhere and its somthing to try while I am stuck at home

 

[davesparks]

I can't remember if it was on these forums or another, but iv read it somewhere in a few posts, they mentioned just using a plastic coupler

Then it's clearly someone who hasn't got a clue what they are talking about.
A plastic coupler would only work with pure water as it's non conductive. Mains water has all sorts of things dissolved in it making it relatively conductive.

 

[Wilko]

With the pipes full of water the resistance between the MET and the outside tap is 1726 ohms, so I work that out to be a potential current of 133mA which is still enough to be potentially fatal?

Hi - when I made a similar measurement some time back I recall I got about 200K Ohms rather than the 2K (ish) you’ve measured. I will revisit, but could there be a measurement problem? Or perhaps another path separate from the pipework?

 

[marcuswareham]

As I've mentioned on your other threads, a couple of earth electrodes connected to the MET of the installation will help reduce the risk.

How would you work out how much you have reduced the risk by?

if say i have earth rod of 30ohms connected to the MET , in aim to bring the touch voltage down from PEN fault, how much would that bring the touch voltage down to?

 

[davesparks]

Hi - when I made a similar measurement some time back I recall I got about 200K Ohms rather than the 2K (ish) you’ve measured. I will revisit, but could there be a measurement problem? Or perhaps another path separate from the pipework?

How long was the section of insulated pipe you measured this with? And also what is your water supply like for hardness?
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How would you work out how much you have reduced the risk by?

if say i have earth rod of 30ohms connected to the MET , in aim to bring the touch voltage down from PEN fault, how much would that bring the touch voltage down to?

That all depends on the specific details of the PEN fault, how many installations remain connected to your earth electrode, what the resistance to earth of any bonded metalwork is, what loads are connected during the fault etc etc

 

[marcuswareham]

Hi - when I made a similar measurement some time back I recall I got about 200K Ohms rather than the 2K (ish) you’ve measured. I will revisit, but could there be a measurement problem? Or perhaps another path separate from the pipework?

humm, of course there could be a problem with my measurements. lucky enough I can reach MET in meter box and the outside tap just about with the length of the leads from my MFT, not sure how i would be getting the measurement wrong although

I live in quite a hard water area, that might make the difference

 

[Pretty Mouth]

I can't remember if it was on these forums or another, but iv read it somewhere in a few posts, they mentioned just using a plastic coupler

I see what you mean about the length of plastic, I might have some 15mm barrier pipe kicking about somewhere and its somthing to try while I am stuck at home

Marcus, it appears you are correct on this one. Engineering Recommendation G12 is guidance for DNOs, to help them decide when PME earthing can be provided. This is quoted from it:

6.2.11 External exposed metalwork connected to the internal earth system (including outside water taps)

Under an open-circuit supply neutral condition the potential of external metalwork could rise above earth potential if it is connected to the internal earth. A person coming into contact with it could receive an electric shock and the shock could be severe if that person were barefooted. The probability of these two conditions occurring together is considered to be so small that the use of PME where external exposed metalwork exists is not precluded.

An insulating insert may be incorporated in the pipe to an outside water tap. However, care should be taken to ensure that simultaneous contact with metal pipework on each side of the insert is not possible if there is likely to be a potential across the insert under this condition.

 

[marcuswareham]

That all depends on the specific details of the PEN fault, how many installations remain connected to your earth electrode, what the resistance to earth of any bonded metalwork is, what loads are connected during the fault etc etc

Doh, Yes I was trying to use the example in the regs for working out what resistance would be needed for an earth electrode and a car charger on PME, but didn't know what figure to use as the load
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Marcus, it appears you are correct on this one. Engineering Recommendation G12 is guidance for DNOs, to help them decide when PME earthing can be provided. This is quoted from it:

6.2.11 External exposed metalwork connected to the internal earth system (including outside water taps)

Under an open-circuit supply neutral condition the potential of external metalwork could rise above earth potential if it is connected to the internal earth. A person coming into contact with it could receive an electric shock and the shock could be severe if that person were barefooted. The probability of these two conditions occurring together is considered to be so small that the use of PME where external exposed metalwork exists is not precluded.

An insulating insert may be incorporated in the pipe to an outside water tap. However, care should be taken to ensure that simultaneous contact with metal pipework on each side of the insert is not possible if there is likely to be a potential across the insert under this condition.

ah very good thanks for that, shame they dont say what plastic insert and how long
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https://www.NoLinkingToThis/p/pegler-pb50-hose-union-bib-tap-15mm-x/6263v

maybe an outside tap like that, with rubber/plastic over the handel so dont come in-contact with the metal

or a big rubber mat to stand on :- D

 

[Pretty Mouth]

Doh, Yes I was trying to use the example in the regs for working out what resistance would be needed for an earth electrode and a car charger on PME, but didn't know what figure to use as the load
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ah very good thanks for that, shame they dont say what plastic insert and how long

This is from one of Chris Kitcher's books:


It is a common belief that water in pipe work will conduct: in fact the current which will flow through water across a plastic fitting filled with water is very small.
To find out how much current would flow I carried out a controlled experiment using two short lengths of copper pipe. These were joined using a plastic push fit coupler. Once fully pushed home the pipes were no more than 2mm apart (Figure 4.24).

The pipe was then filled with tap water and the ends were connected to a 230v supply. The current flowing was so low that my clamp meter would not measure it, as it only measures down to 0.1mA. The current flow would increase if the water had central heating additives in it, but not considerably.

 

[marcuswareham]

This is from one of Chris Kitcher's books:


It is a common belief that water in pipe work will conduct: in fact the current which will flow through water across a plastic fitting filled with water is very small.
To find out how much current would flow I carried out a controlled experiment using two short lengths of copper pipe. These were joined using a plastic push fit coupler. Once fully pushed home the pipes were no more than 2mm apart (Figure 4.24).

The pipe was then filled with tap water and the ends were connected to a 230v supply. The current flowing was so low that my clamp meter would not measure it, as it only measures down to 0.1mA. The current flow would increase if the water had central heating additives in it, but not considerably.

Interesting, I might do my own version of that exsperimemt tomorrow (i know dont try at home, i will be careful) maybe i have supper conductive water, or somehow an earthed metal part in my wall touching the copper pipe where it goes though. I suspect not to both of those, i do like an exsperimemt though

0.1mA is extremely low

 

[davesparks]

Doh, Yes I was trying to use the example in the regs for working out what resistance would be needed for an earth electrode and a car charger on PME, but didn't know what figure to use as the load

The average house will normally have an average demand less than 20A, maybe peaking around 30A occasionally. So I've run that equation for 20A 30A and 40A
For 20A resistance would need to be less than 5ohms
For 30A, 3.35ohms
For 40A, 2.51ohms

Depending on where you are in the country these may or may not be achievable figures.
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This is from one of Chris Kitcher's books:


It is a common belief that water in pipe work will conduct: in fact the current which will flow through water across a plastic fitting filled with water is very small.
To find out how much current would flow I carried out a controlled experiment using two short lengths of copper pipe. These were joined using a plastic push fit coupler. Once fully pushed home the pipes were no more than 2mm apart (Figure 4.24).

The pipe was then filled with tap water and the ends were connected to a 230v supply. The current flowing was so low that my clamp meter would not measure it, as it only measures down to 0.1mA. The current flow would increase if the water had central heating additives in it, but not considerably.

Yes, he demonstrated that exact experiment to us during the 17th edition course, it was all very impressive but I'm not sure how relevant it is to the point.

There was no mention of how long the water had been in the apparatus or how pure/impure that water was. So there was absolutely no scientific basis to the experiment leaving his conclusion completely useless.

 

[marcuswareham]

The average house will normally have an average demand less than 20A, maybe peaking around 30A occasionally. So I've run that equation for 20A 30A and 40A
For 20A resistance would need to be less than 5ohms
For 30A, 3.35ohms
For 40A, 2.51ohms

Depending on where you are in the country these may or may not be achievable figures.

Think would struggle to achieve those i think, also is that for just one house? there is no mains gas in my village so alot of electric showers etc , so demand might be higher i dont know (although the bigger houses have oil, alot of smaller are electric only) maybe

so 2 houses at 20A would need to be 2.51ohms, but a pen fault could be like 10 houses i guess the hope is they all have low resistance extraneous conductive parts bonded

or go round and install an electrode for everyone hahahah :- P

Yes, he demonstrated that exact experiment to us during the 17th edition course, it was all very impressive but I'm not sure how relevant it is to the point.

There was no mention of how long the water had been in the apparatus or how pure/impure that water was. So there was absolutely no scientific basis to the experiment leaving his conclusion completely useless.

well i will give it ago tomorrow haha

 

[davesparks]

so 2 houses at 20A would need to be 2.51ohms, but a pen fault could be like 10 houses i guess the hope is they all have low resistance extraneous conductive parts bonded

or go round and install an electrode for everyone hahahah :- P

Out of 10 houses there will likely be some extraneous parts in good contact with earth and also at least one of the DNO's earth electrodes connected.
The DNO distributing main will have a low resistance earth electrode at the subtstation and an earth electrode at its furthest end. So any break in the PEN affecting multiple houses must still have at least one DNO earth electrode on either side of the break.

Yes in an ideal world an electrode would be connected at every installation, that is the way it is done in many other countries. They were trying to bring that in with the 18th edition but sadly it didn't make the final cut.
Personally I think it was a mistake to not make such a requirement when PME was first introduced in this country.

I personally think it should be a requirement that an earth electrode be connected to the first installation earth terminal after the connection to the DNO earth terminal for every TNCS supply.

 

[marcuswareham]

Out of 10 houses there will likely be some extraneous parts in good contact with earth and also at least one of the DNO's earth electrodes connected.
The DNO distributing main will have a low resistance earth electrode at the subtstation and an earth electrode at its furthest end. So any break in the PEN affecting multiple houses must still have at least one DNO earth electrode on either side of the break.

Yes in an ideal world an electrode would be connected at every installation, that is the way it is done in many other countries. They were trying to bring that in with the 18th edition but sadly it didn't make the final cut.
Personally I think it was a mistake to not make such a requirement when PME was first introduced in this country.

I personally think it should be a requirement that an earth electrode be connected to the first installation earth terminal after the connection to the DNO earth terminal for every TNCS supply.

True,

And I also agree with you about it should be required to have an electrode at every first installation. Or better yet why don't DNOs use TN-S instead (guess TN-C-S is cheaper)

If that did become a requirement in the 18th edition they are not retrospective, leaving all existing installations without electrode no?

Apparently in South Africa on TN-C-S supplies they use an RCD before the earth is separated from the neutral (before service head) this RCD will not work to protect against L - E faults but it is solely there to protect in the event of a broken PEN conductor, shame we cannot do that in the UK

 

[davesparks]

True,

And I also agree with you about it should be required to have an electrode at every first installation. Or better yet why don't DNOs use TN-S instead (guess TN-C-S is cheaper)

If that did become a requirement in the 18th edition they are not retrospective, leaving all existing installations without electrode no?

Apparently in South Africa on TN-C-S supplies they use an RCD before the earth is separated from the neutral (before service head) this RCD will not work to protect against L - E faults but it is solely there to protect in the event of a broken PEN conductor, shame we cannot do that in the UK

The UK distribution network is far more reliable than that in South Africa so such things would not be justifiable.

There are advantages to TNCS over TNS, the main one is of course cost.

No the regulations are not retrospective, and a change in the regulations won't magically make a lot of earth electrodes appear overnight. But like anything it has to start somewhere, and if the regulation is well written and clear as to what the intention is then it will lead to the desired result.
As installations are upgraded, new supplies installed on existing networks and EICRs making recommendations for remedial work then earth electrodes will be installed and the overall effect will be positive.

 

[radiohead]

I have never heard of a single instance of anyone being electrocuted from a metal external tap due to breakdown on a PME system. Has anyone? Or is this the equivalent of building a nuclear shelter in the garden just in case?

 

[pc1966]

There was a case of school children getting shocks from a school fence (due to CCTV or something attached to it, etc) that was probably PME fault related, but none died or has serious effects as far as I remember.
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OK, was worse in that teacher got burns:

Burnley primary school pupils suffer electric shocks

Ten children and a teacher suffer electric shocks at a Lancashire primary school.

www.bbc.com

UPDATE: Pupils and teacher suffer electric shocks at Burnley school

THIRTEEN children and a teacher needed treatment for electric shocks after an incident at a Burnley primary school.

www.lancashiretelegraph.co.uk

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Nothing obvious about the cause from Google search just now, but IET forum generally speculated it was a PME fault.
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Back on topic, given the tap is already on plastic pipe and it is unlikely that you would see the full 230V or whatever on it then the shock risk here is pretty small.

 

[pirate]

JW did some videos about this
here's one:

View: https://www.youtube.com/watch?v=MF9DCNkaE8I

here's another:

View: https://www.youtube.com/watch?v=3-3M3r69pOQ

and finally:

View: https://www.youtube.com/watch?v=W1pXsMjF3Dg

 

[marcuswareham]

The JW video is good although thats a fair length of plastic compared to a few plastic elbows

I did the test today seem to be 73mA so still a fair bit and over 30mA. that with 1 plastic elbow on 15mm copper pipe filled from the outside tap

 

[pc1966]

How many of those elbows have you got in the tap already?

Would it be easy enough just to fit some plastic speedfit pipe for, say, the section through the wall, etc. Basically replacing any copper where a good wet touch connection could be made on adjacent sections?

As far as I remember those joints allow opening up, not sure if you need to replace any O-rings, etc. I think the likes of B&Q sold the plastic pipe in 3m lengths or similar.