Outside tap

[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.

 

[pirate]

Those look like JG Speedfit elbows.
You can open them up, and re-use without replacing "O" rings, but i would only do that once, otherwise just replace...there's no room for being stingy, just replace as necessary.
Also, B&Q sell many different types of pipe and fittings...I used some of their grey ones 20 years ago and they are still fine, but I do believe it is crucial to keep the same manufacturer for pipe and fittings as there are tolerances which are important.
Also, just a minor point, I always buy the longest pipe in straight lengths available. The coils are a fkn nightmare, unless you are doing very long runs, in which case they are invaluable.
Just been out to a job where the guy was screwing down floorboards and punctured the hot supply twice and the cold supply once...it's secure now, for a day or two, but I will have to return to fix it properly. Under a bathroom floor which is tiled...say no more!
Thank Rothenberger for Kibosh!

View: https://www.youtube.com/watch?v=JkNsL6CDdXk

 

[pc1966]

Thank Rothenberger for Kibosh!

Wow, not seen them before but that looks a great idea!

 

[marcuswareham]

Yeah you can re-use the JG speedfit without opening up and replacing o-ring, I have used load and loads of there stuff as I re-plummed my house. In the places where you can see the pipe work I did nice solder joints, But in the crawl space where you can hardly move, its has to be speed fit.

If it replace all of the copper down there for plastic I am not gaining that much really, unless I take the pipe on a little tour of the back of some kitchen units to make it a bit longer, at the min I think it has 3 or 4 elbows and a speedfit isolation valve (plastic also)
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The strange thing is, why is the outside tap resistance less, when there is more plastic then used on my test piece

Dont think it is water under pressure as iv measued it with the tap on and the tap off

Very strange, it is copper though the wall, but i cant think what it would be touching though the wall, i drilled the hole and it was easy, didnt hit anything suspicious

 

[davesparks]

How long have the plastic pipe fittings been in place?
It's possibly deposits from the water on the inside of the fittings decreasing the resistance

 

[marcuswareham]

How long have the plastic pipe fittings been in place?
It's possibly deposits from the water on the inside of the fittings decreasing the resistance

Ahh good thinking it might be something like that. probably between 6 months and a year

I might swap out one of the elbows tomorrow and do the same test on the bench with the older (deposit built up) elbow and see if there is a difference

 

[pc1966]

If you are replacing any section, I would go for the one through the wall to the outside. You get the biggest electrical isolation advantage (as inside unlikely to have a risk to ground even if PME raises the earth potential) and most likely frost damage is outside (plastic less likely to rupture than copper).

I see little advantage versus cost/effort for the rest.

 

[marcuswareham]

If you are replacing any section, I would go for the one through the wall to the outside. You get the biggest electrical isolation advantage (as inside unlikely to have a risk to ground even if PME raises the earth potential) and most likely frost damage is outside.

I see little advantage versus cost/effort for the rest.

Cant replace that bit unfortunately as it is a hose union backplate like so:

https://www.NoLinkingToThis/p/hose-union-back-plate-15mm/56415

I would love to have a plastic version, but have had a quick search and cannot find

 

[pc1966]

Tricky. You could take the pipe out, 90 deg bind then something like this:

https://www.NoLinkingToThis/p/jg-speedfit-plastic-push-fit-adapting-90-wall-plate-elbow-15mm-x/17692

Not as neat & elegant though.

Or just replace the 1st copper section after it comes through the wall.

If you are measuring impedance from tap to Earth you might well be seeing some parallel resistance in the wall. You would need to measure tap to 1st copper section after the plastic bend (floating from Earth) and even then as already said there might be deposits inside making resistance lower than the water alone.

 

[marcuswareham]

Tricky. You could take the pipe out, 90 deg bind then something like this:

https://www.NoLinkingToThis/p/jg-speedfit-plastic-push-fit-adapting-90-wall-plate-elbow-15mm-x/17692

Not as neat & elegant though.

Or just replace the 1st copper section after it comes through the wall.

If you are measuring impedance from tap to Earth you might well be seeing some parallel resistance in the wall. You would need to measure tap to 1st copper section after the plastic bend (floating from Earth) and even then as already said there might be deposits inside making resistance lower than the water alone.

Yeah not as neat, its nice being flush to the wall but I see your point

Yes that is a good test, I will try that between tap and next copper section inside. If it is the case there is parallel resistance in the wall, would that be beneficial in a PEN fault (seems crazy brick wall could conduct) although the bricks were put there in a long time ago and they are the breathable kind so iv been told (house was built in 1840)

Outside Tap Kit 15mm x ½ - https://www.NoLinkingToThis/p/outside-tap-kit-15mm-x/33139

Could try that if not, however, I don't much like the idea of mains pressure water connected with a hose and jubilee clip ! (or the self cutting idea)

 

[pc1966]

Test first.
I would also be reluctant to use the Jubilee clip approach!