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!

 

[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.