UK TN-S Looped supply - earth upgrade. Need to isolate neighbour/refer to DNO?

Msitekkie · Feb 2, 2022

I have come across what I think is a looped supply - like a normal incomer but two of them coming up side by side. It's a TN-S arrangement with an earth cable looped between the two incomers and on the the consumer unit. Trouble is it's only about 1mm, although I got a decent ZE of 0.35.

It would seem to me that as I replace the earth I may also be disconnecting the earth for any neighbours upstream... Yikes!
Has anyone else come across a similar scenario, who can advise? Should I just refer it to the DNO?

timhoward · Feb 2, 2022

A photo would help.
First question - If it's TNS, who does that main earth belong to? It sounds as though you shouldn't be touching it at all.
In any case, crude maths suggest that if a 60 amp supply, PFC of 657A it needs to be at least 3mm to meet adiabatic equation.
I'd speak to DNO.

Msitekkie · Feb 2, 2022

timhoward said:
A photo would help.
First question - If it's TNS, who does that main earth belong to? It sounds as though you shouldn't be touching it at all.
In any case, crude maths suggest that if a 60 amp supply, PFC of 657A it needs to be at least 3mm to meet adiabatic equation.
I'd speak to DNO.

Thanks Tim.
Yes, upgrading the earth is the issue I'm trying to address - I was going to make it 10mm (16mm tails). I was coming round to the view myself perhaps I should be asking the DNO to do it, but as this is the first time I have come across a looped supply & I have never heard of such a scenario, I also wanted to sanity check that's what it is.
Sorry I don't have a pic at the moment - it only dawned on me after I got home, that it might be a looped supply.

Msitekkie · Feb 3, 2022

timhoward said:
A photo would help.
First question - If it's TNS, who does that main earth belong to? It sounds as though you shouldn't be touching it at all.
In any case, crude maths suggest that if a 60 amp supply, PFC of 657A it needs to be at least 3mm to meet adiabatic equation.
I'd speak to DNO.

Thanks Tim,
I was going to upgrade to 10mm (has 16mm tails) but was coming round to the view it maybe should be the DNO. I Wanted to sanity check first as I haven't come across a looped supply before

timhoward said:
A photo would help.
First question - If it's TNS, who does that main earth belong to? It sounds as though you shouldn't be touching it at all.
In any case, crude maths suggest that if a 60 amp supply, PFC of 657A it needs to be at least 3mm to meet adiabatic equation.
I'd speak to DNO.

[ElectriciansForums.net] TN-S Looped supply - earth upgrade. Need to isolate neighbour/refer to DNO?

westward10 · Feb 3, 2022

Under no circumstances touch those adjustable earth clamps. They are not a suitable means if connection and are putting undue stress on the cables.

timhoward · Feb 3, 2022

In the meantime you can at least upgrade the earthing from that MET to the CU.
My guess is that the barely visible uninsulated wire is the original earth and someone has later added those clamps and the ?4mm earth. They look dodgy to me, and as above, don't touch them.
Without seeing next door we can't say for certain (might be TNCS next door!) but the ?4mm between the two suppliers cables could indeed be the basis for TNS next door.
I would call the DNO as I'm 99% certain they are not DNO clamps, and you are right that next door could be reliant on them.

In terms of conductor size, it's actually likely the 4mm is adequate to meet adiabatic requirements, but the way it's connected (to my eyes) is not adequate.

snowhead · Feb 3, 2022

As Tim above, the Yellow / Green and clamps have been added most likely by a non DNO person thinking the original bare conductor is not adequate.
I suspect the DNO will say it is.

littlespark · Feb 3, 2022

That’s not what the holes in the tags are for…..

UNG · Feb 3, 2022

The last time I did some work on an installation with those clamps on the main cable I found when I did an earth loop at the board I heard a noise which with the light off you could clearly see an arc between the clamp and the cable and the earth loop reading was ok, I called the DNO and left it to them to sort out

timhoward · Feb 3, 2022

I'm also pretty certain the DNO will be happy with the conductor size.
A long-standing quest for me has been finding a reliable set of time/current curves for BS1361 fuses - there was a graph in the 16th edition, but it was much more conservative than every other one I have found online. (If anyone has recommendations I'd love to know!)

Anyway, with 60A fuse, PFC of 657amps, the old 16th edition graph says it trips in 0.6 seconds. Other graphs have as low as 0.2 seconds.
For an 80A fuse most graphs online say about 1 second.

Bung all that in here : CPC Sizing Tool - https://myelectrical.com/tools/cpc-sizing-calculator
If 0.2 seconds, You get 3mm for copper, 3mm for aluminium and 4mm for steel.
If 1 seconds, you get 3mm for copper, 5mm for aluminium, and 8mm for steel.
Good chance it's ok.

For other reasons above I'd still suggest the DNO should have a look at that, in only to confirm that an approved earth is still connected to next door, and that those clamps have not done any damage.

pc1966 · Feb 3, 2022

Probably the original was 4mm (or imperial equivalent) but bare, so upper temperature limit for adiabatic quite high!

Still, in the increasingly PME world there could well be concerns that segments of TN-S earthing carry neutral currents as well for long periods under open PEN conditions so they might want it sized up a bit?

Msitekkie · Feb 3, 2022

timhoward said:
I'm also pretty certain the DNO will be happy with the conductor size.
A long-standing quest for me has been finding a reliable set of time/current curves for BS1361 fuses - there was a graph in the 16th edition, but it was much more conservative than every other one I have found online. (If anyone has recommendations I'd love to know!)

Anyway, with 60A fuse, PFC of 657amps, the old 16th edition graph says it trips in 0.6 seconds. Other graphs have as low as 0.2 seconds.
For an 80A fuse most graphs online say about 1 second.

Bung all that in here : CPC Sizing Tool - https://myelectrical.com/tools/cpc-sizing-calculator
If 0.2 seconds, You get 3mm for copper, 3mm for aluminium and 4mm for steel.
If 1 seconds, you get 3mm for copper, 5mm for aluminium, and 8mm for steel.
Good chance it's ok.

For other reasons above I'd still suggest the DNO should have a look at that, in only to confirm that an approved earth is still connected to next door, and that those clamps have not done any damage.

Thanks again Tim and everyone else. UK Power Networks are booked in to go have a look on 18th Feb. Will be interesting to see what they do.
The unsheathed cable disappears under the floor. If it was the DNO earth wouldn't it be a "sweated" connection to the sheath at the top of the cable where the other clamps are? I'm guessing (as no one else has picked up on the fact it doesn't do that) I'm probably wrong, but interested to get that 1. confirmed if so
& 2. get benefit of other's experience here as to how else it would be connected.

timhoward · Feb 3, 2022

Msitekkie said:
Thanks again Tim and everyone else. UK Power Networks are booked in to go have a look on 18th Feb. Will be interesting to see what they do.
The unsheathed cable disappears under the floor. If it was the DNO earth wouldn't it be a "sweated" connection to the sheath at the top of the cable where the other clamps are? I'm guessing (as no one else has picked up on the fact it doesn't do that) I'm probably wrong, but interested to get that 1. confirmed if so
& 2. get benefit of other's experience here as to how else it would be connected.

It wasn't really possible to see from the photos where the unsheathed cable went.
Various possibilities - it could be sweated on lower down, it could have snapped off and been shoved down the hole by the same person that threw the clamps on, or it could be an or current old water/gas bond.
I think you've done the right thing getting the DNO out. Please do report back!

UK TN-S Looped supply - earth upgrade. Need to isolate neighbour/refer to DNO?

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