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Topic Title: What is the earthing arrangement, if any?
Topic Summary: Would you stay in this B&B?
Created On: 14 May 2018 09:05 PM
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 14 May 2018 09:05 PM
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chrispearson

Posts: 116
Joined: 15 February 2018

Bear with me, 'cos there is a bit of explaining to do, and there may be more to follow, but here is the origin. (It has been cropped and reduced in size to fit on the page in here.)



The property, which is semi-detached, is in a hamlet of 10 - 12 dwellings. There is a transformer up a pole in a field about 120 m down the road. Two cables pass from the transformer via one or two intermediate poles to the corner of next door. They then pass along beneath the soffits to this property. As you can see in the photo, two separate cables reach the service head.

I am not familiar with this ancient equipment, but what is that wire that goes from the service head to the MET? Surely it cannot be TN-C-S? Please forget for the time being that the earthing conductor and bonding are far too small if it is. I really cannot see how there could be a PME arrangement given the layout of the hamlet: almost all overhead and no street lamps.

One of the green and yellows goes off to the fuse box and the other is bonded to the gas supply to the boiler. The gas supply is only about 20 years old, but I am told that it is metal rather than plastic.

ETA - no bonding to the water pipes!

So on the face of it, we have an unearthed installation.

Edited: 14 May 2018 at 09:15 PM by chrispearson
 14 May 2018 09:10 PM
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dustydazzler

Posts: 1970
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Is this one of those new fangled diy pme (men) jobbies
 14 May 2018 09:37 PM
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alancapon

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It could be PME. You need to ask the DNO. The seal looks like a proper seal, although the image doesn't show whether there is a code impressed in it. Unless it has been modified it could well be a fused neutral. Trying to take the fuse out isn't wise. I have known all four terminals come away from the back board before with cutouts like that. When you have a live double pole fuse carrier in one hand with four cables still attached it starts to get exciting. . .

Regards,

Alan.
 14 May 2018 10:13 PM
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mapj1

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Joined: 22 July 2004

If it dates back to the era of cotton covered incomer, then black can be an earth colour. (before green, before green and yellow)
Are those two pink furry things to the right hand side low the incomers from the overhead ?
Yes I'd stay there, but then I've stayed in South America too. Are there any RCDs?

Maybe walk the poles outside with a pair of binoculars, and look for the lines earthing the neutral.

-------------------------
regards Mike
 14 May 2018 11:33 PM
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gkenyon

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Would a description of T?N??C?S be appropriate?

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
 15 May 2018 09:16 AM
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chrispearson

Posts: 116
Joined: 15 February 2018

Originally posted by: alancapon

It could be PME. You need to ask the DNO. The seal looks like a proper seal, although the image doesn't show whether there is a code impressed in it.


DNO doesn't know, but they will send a man round to find out if required. The seal was probably put on when the meter was changed a few years ago.

Originally posted by: mapj1

If it dates back to the era of cotton covered incomer, then black can be an earth colour. (before green, before green and yellow)

Are those two pink furry things to the right hand side low the incomers from the overhead?


Yes they are.

Are there any RCDs?


Please see below. There is a main switch, then two lighting circuits, then the RCD.

Maybe walk the poles outside with a pair of binoculars, and look for the lines earthing the neutral.


It's not possible to see all the way round the poles, but there is no obvious earthing - it is just two cables to a pair of semi-detached cottages.

I was hoping that you more experienced folk would recognise this ancient equipment, but I cannot quite see how it could be TN-C-S.

Incidentally, the owners are building a house just down the road. Although the house is next to the field with the transformer up a pole, the supply is underground. PME might be available there, but a TT earth has been installed, presumably because of 704.411.3.1

 15 May 2018 10:27 AM
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Legh

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I've seen rural overhead PME systems like this before.
My first port of call might be to loop test L-PE and L-N and see what results.
Then get the DNO out to verify or confirm the earthing system.

Legh

-------------------------

http://www.leghrichardson.co.uk

de-avatared
 15 May 2018 10:35 AM
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AJJewsbury

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I really cannot see how there could be a PME arrangement given the layout of the hamlet: almost all overhead and no street lamps.

All it would need to comply with today's PME regulations would be one electrode where the last service line branches off from the main distribution main (in addition to the electrode at the transformer) - and that service line could itself supply several consumers (up to 4 from memory). Earlier versions of the regulations might not have been even that demanding.

I don't think street lighting makes much difference - lights on timber or concrete poles wouldn't provide much of an earth electrode and even modern steel columns are often inserted into plastic sleeves in the ground (to make replacement easier) so can't be relied upon to provide a decent earth electrode.

What sort of Ze reading are you getting? (and how does it compare with L-N loop readings?)

- Andy.
 15 May 2018 11:10 AM
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mapj1

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Pink furry company incomers will be from the era well before PME was dreamed up, and may well be black earth as well. Round here we had PVC for about 10- 20 years before we had PME. (PVC coming about 1960, no cotton covered rubber at all after about '65. PME first available 1970 something, the normal thing by about 1980. )
From that era if there is an NE link , it will be a conversion.
Far more likely it will be TT from the era of a low current supply and supposed to have an earth via the lead water pipes, which may have been replaced at some distant past.. In the days when an earth was good if the test lamp between live and earth lit up. Blowing the fuse on a 15A or even a 30A circuit with a horizontal maximum contact electrode hundreds of metres long, often self watering due to leaks at the stopcock stuffing glands, is more or less practical (maybe not in modern ADS timescales however, but a few seconds ). The modern 4 foot plated steel rod that does not reach the water table in summer with a resistance of 100 ohms needs an RCD.

Of course in an era of 2 pin sockets and unearthed lights, no one really bothered as much.
This sort of thing .. flameport's archive


-------------------------
regards Mike


Edited: 15 May 2018 at 11:33 AM by mapj1
 15 May 2018 11:37 AM
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chrispearson

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Gentlemen, thank you for your responses.

Originally posted by: AJJewsbury

All it would need to comply with today's PME regulations would be one electrode where the last service line branches off from the main distribution main (in addition to the electrode at the transformer) - and that service line could itself supply several consumers (up to 4 from memory). Earlier versions of the regulations might not have been even that demanding.

What sort of Ze reading are you getting? (and how does it compare with L-N loop readings?)


Unfortunately I didn't have my Megger with me when I was visiting and the location is 320 miles away.

I found a very useful document from ENWL, which explains PME, but there isn't a distribution main as such - just two cables to two cottages.

Originally posted by: mapj1

Far more likely it will be TT from the era of a low current supply and supposed to have an earth via the lead water pipes, which may have been replaced at some distant past..


Perfectly possible. I shall ask.

Of course in an era of 2 pin sockets and unearthed lights, no one really bothered as much.


We all survived doing the ironing with it plugged into a lighting fixture, but of course that's not really the point.
 15 May 2018 11:41 AM
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daveparry1

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I'd assume TNCS, that black wire from the MET is going to the neutral in the cut-out. It probably was TT in the past but has been upgraded by the supplier, a Ze test would confirm it.
 15 May 2018 01:16 PM
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mapj1

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We all survived doing the ironing with it plugged into a lighting fixture, but of course that's not really the point.


Oh, memories. My own shadow rocking back and forth across the page of the book I was reading (probably a Biggles or Enid Blyton) at my grand parents place before it was rewired in the very early 1970s. A really lethal looking iron as well. Given their age it would have been a mid 1930s installation.
But in a way it partly is the point- the risk of single fault to danger, class 0 equipment with unearthed metal cases etc is not infinite, it is just a rather higher risk that the modern double fault to danger way of thinking allows.

If you accept odds of a fatal accident at 10,000 to one rather than demanding 10,000,000 to one, then life gets much simpler and less stressful. So anyone who is happy to go on a climbing holiday should be OK for a holiday in a house that is off-earth, at least for a short period.

PME on overhead singles will only be permitted if the neutral joints are suitably robust, so probably they will need to be double crimped.

-------------------------
regards Mike


Edited: 15 May 2018 at 01:25 PM by mapj1
 15 May 2018 08:36 PM
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Zoomup

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Joined: 20 February 2014

Originally posted by: chrispearson

Originally posted by: alancapon



It could be PME. You need to ask the DNO. The seal looks like a proper seal, although the image doesn't show whether there is a code impressed in it.




DNO doesn't know, but they will send a man round to find out if required. The seal was probably put on when the meter was changed a few years ago.



Originally posted by: mapj1



If it dates back to the era of cotton covered incomer, then black can be an earth colour. (before green, before green and yellow)



Are those two pink furry things to the right hand side low the incomers from the overhead?




Yes they are.



Are there any RCDs?




Please see below. There is a main switch, then two lighting circuits, then the RCD.



Maybe walk the poles outside with a pair of binoculars, and look for the lines earthing the neutral.




It's not possible to see all the way round the poles, but there is no obvious earthing - it is just two cables to a pair of semi-detached cottages.



I was hoping that you more experienced folk would recognise this ancient equipment, but I cannot quite see how it could be TN-C-S.



Incidentally, the owners are building a house just down the road. Although the house is next to the field with the transformer up a pole, the supply is underground. PME might be available there, but a TT earth has been installed, presumably because of 704.411.3.1





The 100mA R.C.D. suggests an original TT supply. What it is now is anyone's guess. The black earth wire ages the installation a bit, probably back to the 1950s/1960s.

Z.
 15 May 2018 09:07 PM
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chrispearson

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Originally posted by: Zoomup

The 100mA R.C.D. suggests an original TT supply. What it is now is anyone's guess. The black earth wire ages the installation a bit, probably back to the 1950s/1960s.


Ah yes, but it's split load innit?
 15 May 2018 10:46 PM
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kellyselectric

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Two pink furry things? Thankyou that made me smile first time in ages!!
 15 May 2018 11:08 PM
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mapj1

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woven cotton used to be a popular serving on rubber over tinned copper. However, over time the rubber perishes and the cotton covering degrades to fluff. And adding insult to injury, if the tinning of the cores was not up to scratch, the copper gets attacked by the sulphur in the rubber, forming a black non-conducting sulphide, and the stands become pitted and hard to get a good contact to.
Apart from that, great stuff. Last in common use before I was born, and as my kids keep reminding me, that must mean it is old.
If it went in the late 50s it may be Poly Butyl Jute insulated cable, which also pink and furry, but stands the test of time a bit better than simple rubber. None then less still out of date by now.

Whatever you do, don't move it until it is isolated. It may be fine, but after this many years I rather doubt it.

-------------------------
regards Mike


Edited: 15 May 2018 at 11:19 PM by mapj1
 15 May 2018 11:57 PM
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alancapon

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Originally posted by: chrispearson
DNO doesn't know, but they will send a man round to find out if required. . .

There is your answer then. They need to check to see what the earthing type is, and they are legally bound to do so, if their records are not clear. Any of these other suggestions are pure guesswork.

Regards,

Alan.
 16 May 2018 08:33 AM
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Zoomup

Posts: 3964
Joined: 20 February 2014

Originally posted by: chrispearson

Originally posted by: Zoomup



The 100mA R.C.D. suggests an original TT supply. What it is now is anyone's guess. The black earth wire ages the installation a bit, probably back to the 1950s/1960s.




Ah yes, but it's split load innit?



Wozthatthen?

Z.
 19 May 2018 10:42 AM
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chrispearson

Posts: 116
Joined: 15 February 2018

I thought that it would be useful to find out what's next door.



As you can see, it does look like TN-C-S. (That earthing conductor looks a little thin to me.)

Originally posted by: AJJewsbury

All it would need to comply with today's PME regulations would be one electrode where the last service line branches off from the main distribution main (in addition to the electrode at the transformer) - and that service line could itself supply several consumers (up to 4 from memory). Earlier versions of the regulations might not have been even that demanding.


I think that the answer to this connundrum is that the pair of cables that come direct from the transformer are a distribution main. ENWL documents specify that a supply from a transformer to a single customer is a distribution main and that it may be extended by mural wiring without requiring an earth at the far end.

So there we have it : TN-C-S, but not PME (or even PNB).
 19 May 2018 11:04 AM
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Zoomup

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Originally posted by: chrispearson

I thought that it would be useful to find out what's next door.







As you can see, it does look like TN-C-S. (That earthing conductor looks a little thin to me.)



Originally posted by: AJJewsbury



All it would need to comply with today's PME regulations would be one electrode where the last service line branches off from the main distribution main (in addition to the electrode at the transformer) - and that service line could itself supply several consumers (up to 4 from memory). Earlier versions of the regulations might not have been even that demanding.




I think that the answer to this connundrum is that the pair of cables that come direct from the transformer are a distribution main. ENWL documents specify that a supply from a transformer to a single customer is a distribution main and that it may be extended by mural wiring without requiring an earth at the far end.



So there we have it : TN-C-S, but not PME (or even PNB).


The earthing conductors look untidy, one is too short and less than professionally installed. D.I.Y?

Z.
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