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]

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.