TT or PME

[aliensbrains]

Just been to look at a job and found this. It's an overhead supply so could be pme at the post but no earth back to the cutout and (although admittedly i didn't have much time to have a proper look) I didn't spot a rod.

I'm thinking advise the customer to get the dno to come and have a look and fit an isolator while they're at it.

Any thoughts?

Thanks!

 

[mainline]

Just been to look at a job and found this. It's an overhead supply so could be pme at the post but no earth back to the cutout and (although admittedly i didn't have much time to have a proper look) I didn't spot a rod.

I'm thinking advise the customer to get the dno to come and have a look and fit an isolator while they're at it.

Any thoughts?

Thanks!

View attachment 119116

I think one of the first things would have been a Zs and rcd test, it would have taken a few minutes.

 

[aliensbrains]

I think one of the first things would have been a Zs and rcd test, it would have taken a few minutes.

Agree but customer hasn't exchanged yet and it's occupied. I couldn't do any testing during this visit.
When they exchange I will try to get back and test but that doesn't help me in the meantime.

 

[James]

The label on the incomer states that it is PME.

however,
in the absence of any visible earthing cables, then it must be assumed that the installation is one of the following.

1. TT with an earth cable hidden from view
2. no earth connected
3. a dodgy PME link inside the DB.

as the label states PME then in all honesty, the cost of fixing the earthing is negligible anyway.

 

[mainline]

Agree but customer hasn't exchanged yet and it's occupied. I couldn't do any testing during this visit.
When they exchange I will try to get back and test but that doesn't help me in the meantime.

What was the point in you going there ? If you didn't have 20 seconds of time to even do a quick Zs.

 

[Pretty Mouth]

Agree but customer hasn't exchanged yet and it's occupied. I couldn't do any testing during this visit.
When they exchange I will try to get back and test but that doesn't help me in the meantime.

It's not your customer's installation yet, and as you can't carry out any testing, then I don't see as there's any more you can do. Report your suspicions to your customer, and suggest they have an EICR carried out at the earliest opportunity.

 

[James]

Agree but customer hasn't exchanged yet and it's occupied. I couldn't do any testing during this visit.
When they exchange I will try to get back and test but that doesn't help me in the meantime.

If i was purchasing a property and organised with the seller to allow access for an electrical report to be done,
I would be wary and disappointed that the engineer was prevented from doing the job correctly.

a quick visual of the board internals and 10 minutes of sticking your meter probes in and you would be able to give a proper report. albeit with some restrictions.

 

[Tom256]

as the label states PME then in all honesty, the cost of fixing the earthing is negligible anyway.

Not necessarily. You need the history on this which the DNO can provide.
Engineering Recommendation G12 allows a DNO to refuse connection to the supply Neutral if the DNO considers it inappropriate for reasons of safety. One such reason could be a TN-C-S Earthing non-compliance against BS 7671. Could be no insulating section in a metal gas supply pipe and so on.

It could be if rural, single premises, a PNB at the pole transformer creating the TN-C-S from the transformer to customers intake (?). Regardless, connection can be refused if it is not considered by the DNO safe to do so.

Looking at the top of the consumer unit I can see what would be 3 x Earth conductors which could suggest a possible TT arrangment in place (?)

 

[Tom256]

If i was purchasing a property and organised with the seller to allow access for an electrical report to be done,
I would be wary and disappointed that the engineer was prevented from doing the job correctly.

Yes indeed James.
Side topic - apologies - Be worth checking if those BS 3871's are suitable for Safe Isolation. Not all are. Example: The Mk1 Merlins as shown (picture) and recognised by the toggle width cannot be confirmed safe by Schneider for use in Safe Isolation.

 

[aliensbrains]

Not necessarily. You need the history on this which the DNO can provide.
Engineering Recommendation G12 allows a DNO to refuse connection to the supply Neutral if the DNO considers it inappropriate for reasons of safety. One such reason could be a TN-C-S Earthing non-compliance against BS 7671. Could be no insulating section in a metal gas supply pipe and so on.

It could be if rural, single premises, a PNB at the pole transformer creating the TN-C-S from the transformer to customers intake (?). Regardless, connection can be refused if it is not considered by the DNO safe to do so.

Looking at the top of the consumer unit I can see what would be 3 x Earth conductors which could suggest a possible TT arrangment in place (?)

It's not rural, it's in a town so I'm hopeful this is just an old TT system but I couldn't spot the rod as it was dark. I think the DNO have stuck PME notices on these cutouts as it's PME'd at the poles.

I think I'll stick to my original plan and let the customer know what I've spotted and advise they call the DNO out on completion to confirm PME and install an isolator. I can then get in and have a proper look.

 

[westward10]

That traffolyte label glued to the service head is the same as those screwed to the wooden support poles.

 

[aliensbrains]

I now have a bit of update on this one.

The customer completed this week so found an hour to go back and test. Generally it all tested out well. Maybe too well in some ways...

Those 3 10mm earth cables are bonding and main supply earth... I assumed it would be a rod under the floorboards but measured 0.27 Ze. Now I'm confused as to where it's PME'd and unfortunately didn't have time to trace the cable.

As was the original plan, the customer has raised an enquiry with the DNO on the basis there's a PME sticker but no earth at the cutout.We haven't mentioned the Ze reading.

 

[Tom256]

Just seen in your post of the 19th of December

I think the DNO have stuck PME notices on these cutouts as it's PME'd at the poles

In my post prior to yours on the 19th I mentioned the possibility of a PNB external at a pole transformer is that indeed the case?
The neutral - earth link need not be at the cutout. A point made often when what presents as TN-S is not.

 

[buzzlightyear]

No PME earths to the to the main tails cut out ,so your testing 27 ohms
so you saying earth rod under flooring ,with out confirmation then what if you are doing a EICR.

 

[Tom256]

Confirm the arrangment as there can be several variations even with PNB.
From that sure would be nice to know if the DNO have offered / provided an Earth terminal

 

[aliensbrains]

Confirm the arrangment as there can be several variations even with PNB.
From that sure would be nice to know if the DNO have offered / provided an Earth terminal

And where it is!

 

[Teddy Lop]

Is "main supply earth" an assumption or confirmed? could it not be another bond?

Low reading could be a parallel path of connected metalwork, eg earthing of boiler/gas pipe (by both bonding and/or cpc's) going to a neighbour's TNC-S connection? Did you do a proper Ze ie disconnecting the "main supply earth" to test it?

If unsure about a TNC-S connection, a low resistance check N-E is a first thought...

 

[aliensbrains]

Yes, I disconnected the (assumed) main earth and measured 0.27ohms.

 

[aliensbrains]

Update from customer: National grid have been round, removed the PME sticker and informed the customer they'll need an an earth rod installing.

 

[HappyHippyDad]

Update from customer: National grid have been round, removed the PME sticker and informed the customer they'll need an an earth rod installing.

One small sticker causing so many problems!
I know we can't guess but, if I had to guess, I bet there's an earth rod somewhere, probably beneath a new porch or conservatory!
If you really want to @aliensbrains you could let us know what Ra you get on your new rod, as we like things like that

 

[aliensbrains]

One small sticker causing so many problems!
I know we can't guess but, if I had to guess, I bet there's an earth rod somewhere, probably beneath a new porch or conservatory!
If you really want to @aliensbrains you could let us know what Ra you get on your new rod, as we like things like that

Interestingly, national grid also recorded 0.27ohm L-N, the exact same reading I got when I measured Ze from the 10mm 'main earth' in the consumer unit.

So now I'm thinking, what is the best thing to do with that 'main earth' once I've installed a rod...

 

[HappyHippyDad]

Interestingly, national grid also recorded 0.27ohm L-N, the exact same reading I got when I measured Ze from the 10mm 'main earth' in the consumer unit.

So now I'm thinking, what is the best thing to do with that 'main earth' once I've installed a rod...

I'm guessing the quotation marks for 'main earth' are because you know you can't assume it is the main earth without seeing what it is attached to. I can't see a reason to disconnect it once the rod is in place, but may stand corrected.

 

[aliensbrains]

I'm guessing the quotation marks for 'main earth' are because you know you can't assume it is the main earth without seeing what it is attached to. I can't see a reason to disconnect it once the rod is in place, but may stand corrected.

Correct. Neither I or the DNO can confirm where this is connected but given the readings it seems sensible to assume it's an unofficial PME connection.

Therefore as you say, my current thinking is to leave it in place, and make a note on the certificate referencing the DNO job number, to explain the situation (and the low Zs values) for a TT system.

 

[mainline]

Correct. Neither I or the DNO can confirm where this is connected but given the readings it seems sensible to assume it's an unofficial PME connection.

Therefore as you say, my current thinking is to leave it in place, and make a note on the certificate referencing the DNO job number, to explain the situation (and the low Zs values) for a TT system.

Leave the rod in place as recommended in bs7671

 

[aliensbrains]

Leave the rod in place as recommended in bs7671

Other way round. I need to install a rod. The question is whether or not to leave the previous earth (which the DNO will not confirm PME for) in addition to the new rod.

 

[mainline]

Other way round. I need to install a rod. The question is whether or not to leave the previous earth (which the DNO will not confirm PME for) in addition to the new rod.

Either way install a rod and leave the existing.

 

[Teddy Lop]

Correct. Neither I or the DNO can confirm where this is connected but given the readings it seems sensible to assume it's an unofficial PME connection.

Therefore as you say, my current thinking is to leave it in place, and make a note on the certificate referencing the DNO job number, to explain the situation (and the low Zs values) for a TT system.

Surely an "unofficial" TNCS connection still has to go to the incoming supplies neutral/PENl conductor, not "disappear off somewhere?"

Also, it it were an "unofficial" TNCS connection where PME isn't present (tip; it can't by definition be an unofficial PME, PME and TNCS do not mean the same thing) then connecting it to the houses earth might not be the best idea

 

[aliensbrains]

Surely an "unofficial" TNCS connection still has to go to the incoming supplies neutral/PENl conductor, not "disappear off somewhere?"

Also, it it were an "unofficial" TNCS connection where PME isn't present (tip; it can't by definition be an unofficial PME, PME and TNCS do not mean the same thing) then connecting it to the houses earth might not be the best idea

Agreed. If both me and the DNO recorded 0.27ohms then it appears this cable must be connected to the neutral somewhere. But the DNO have still asked for a rod.

Can you explain why connecting this back to the earth in the CU is not the best idea? What would you suggest is a better approach?

 

[Teddy Lop]

Agreed. If both me and the DNO recorded 0.27ohms then this cable must be connected to the neutral somewhere. But the DNO have still asked for a rod.

Can you explain why connecting this back to the earth in the CU is not the best idea? What would you suggest is a better approach?

It's all to do with the danger of losing the PEN conductor which would make the entire earthed parts of installation live; PME is the mechanism by which TNC-S is considered safe, ostensibly connecting to multiple extra earth rods along the conductors route (hence multiple earthing) but theres also other requirements such as crimped rather than screwed line tap connections. If the DNO have removed the PME label from the service head it suggests the supply side doesn't meet these requirements.

 

[aliensbrains]

It's all to do with the danger of losing the PEN conductor which would make the entire earthed parts of installation live; PME is the mechanism by which TNC-S is considered safe, ostensibly connecting to multiple extra earth rods along the conductors route (hence multiple earthing) but theres also other requirements such as crimped rather than screwed line tap connections. If the DNO have removed the PME label from the service head it suggests the supply side doesn't meet these requirements.

I get what you are saying...

I'm guessing the earth rod would be too high resistance to prevent the issues in the event of a broken PEN?

Therefore it would seem sensible to disconnect this cable from the CU and safely terminate? But this presents new issues with having a green and yellow cable that I suspect is tied to the incoming Neutral hanging in the wall.

 

[Teddy Lop]

I get what you are saying...

I'm guessing the earth rod would be too high resistance to prevent the issues in the event of a broken PEN?

Therefore it would seem sensible to disconnect this cable from the CU and safely terminate? But this presents new issues with having a green and yellow cable that I suspect is tied to the incoming Neutral hanging in the wall.

If it is what you think it is, and a loss of PEN should occur putting 230 onto your earthing, the earth stake is unlikely to worry the service fuse

I'd be inclined to try to see exactly where this mystery conductor goes, I think a parallel path through neighbouring property etc is far more likely than some weird DIY extraneous CNE connection

 

[aliensbrains]

If it is what you think it is, and a loss of PEN should occur putting 230 onto your earthing, the earth stake is unlikely to worry the service fuse

I'd be inclined to try to see exactly where this mystery conductor goes, I think a parallel path through neighbouring property etc is far more likely than some weird DIY extraneous CNE connection

OK so I have been round for a while today to get some more clarity and attempt to trace this cable.

Managed to trace the cable into the bathroom, through a bedroom then it seems to go down towards the kitchen and is buried in a wall so can't trace it further than that.

I found I made an error previously when I said that the main earth read a 'Ze' of 0.27ohm. After retesting, I believe that was actually one of the bonding conductors. Both have a similar low reading <0.30ohm.

The 3rd 10mm earth in the CU, I traced tonight had it had a Ze (Ra) of 122ohms so I'm now assuming it's a rod, possibly under the kitchen floor.

I also knocked on next door both sides - this is a row of fairly small town houses with an overhead supply. Both of the neighbours have TNCS earthing arrangements (despite my customer being refused it last Friday).

Therefore, I believe I have parallel paths via the bonds to neighbours TNCS systems, an old TT rod which I can't verify easily without ripping holes in walls or kitchen flooring and the DNO refusing to install TNCS despite both neighbouring properties having it installed.

 

[Teddy Lop]

This is where TNC-S gets really ugly... The integrity of your earth is now dependant on whether neighbours fiddle with their own installations. I don't know why we don't just mandate non-conductive couplings on incoming services

 

[aliensbrains]

This is where TNC-S gets really ugly... The integrity of your earth is now dependant on whether neighbours fiddle with their own installations. I don't know why we don't just mandate non-conductive couplings on incoming services

Yes. I think it will be one more call to the DNO to see if they will install TNCS after all, given the fact neighbours both sides have it.

If they still refuse, it's back to plan A, install a new rod and an RCBO board. At least I now have a better understanding of the installation.

 

[aliensbrains]

One small sticker causing so many problems!
I know we can't guess but, if I had to guess, I bet there's an earth rod somewhere, probably beneath a new porch or conservatory!
If you really want to @aliensbrains you could let us know what Ra you get on your new rod, as we like things like that

Finally got round to this job today. The DNO doubled down on their refusal to install PME so I fitted a new rod and RCBO board. I got an Ra of 44.5 ohms which I was pretty happy with.