UK High Ze TNS - what to expect from DNO?

samtheman · Aug 28, 2024

Clueless householder here!

My wife told me John Lewis refused to plug in our new oven this morning due to "high readings on sockets". It looks on their paperwork like the "Pre Earth Loop Impedance Test" failed with 2.02 Ohms.

I thought that was strange as we had a new consumer unit fitted and tested after buying the house, so I dug out the installation certificate which apparently says under part 5: "Supply company should be contacted to correct the very High Ze Reading of 1.57 Ohms without delay". Who knew? It says it's a TNS supply.

So not exactly without delay I decided to call 105 (SSEN) and apparently they'll send someone out. What should I expect from them?

Pretty sure I can figure out plugging in an oven myself but if this is dangerous I'd like to get it sorted.

[ElectriciansForums.net] High Ze TNS - what to expect from DNO?

mainline · Aug 28, 2024

samtheman said:
Clueless householder here!

My wife told me John Lewis refused to plug in our new oven this morning due to "high readings on sockets". It looks on their paperwork like the "Pre Earth Loop Impedance Test" failed with 2.02 Ohms.

I thought that was strange as we had a new consumer unit fitted and tested after buying the house, so I dug out the installation certificate which apparently says under part 5: "Supply company should be contacted to correct the very High Ze Reading of 1.57 Ohms without delay". Who knew? It says it's a TNS supply.

So not exactly without delay I decided to call 105 (SSEN) and apparently they'll send someone out. What should I expect from them?

Pretty sure I can figure out plugging in an oven myself but if this is dangerous I'd like to get it sorted.

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Yes it needs sorting out.
The guideline for a TNS Ze is around 0.8 ohms

Can you post a pic of your consumer unit ?

davesparks · Aug 28, 2024

What you should expect is for SSE to replace the whole cutout arrangement (the red cast iron lump and the fuseholder above it) and provide a new earth connection as part of that. They may maintain this as a TNS earth or they may convert it to a TNCS earth.

You may also need your own electrician there to reconnect your earths or SSE might do it themselves

samtheman · Aug 28, 2024

mainline said:
Yes it needs sorting out.
The guideline for a TNS Ze is around 0.8 ohms

Can you post a pic of your consumer unit ?

Here's the whole arrangement - supply helpfully hidden behind that gas meter!

westward10 · Aug 28, 2024

The person who replaced that consumer unit should have addressed the issue.

samtheman · Aug 28, 2024

westward10 said:
The person who replaced that consumer unit should have addressed the issue.

Thanks - do you mean they should have notified the supplier themselves rather than leave it as a footnote in an installation certificate for me to find two years later? To be fair I see now it's also on the EICR that induced me to get the new CU so I probably could have been paying more attention.

Anyway, it is what it is - I just hope SSEN will arrive at some point this week and sort this out for me as if they say it's not their problem I guess it might be an expensive fix!

mainline · Aug 28, 2024

Wouldn't the absence of a reliable and effective means of earthing be a C2 Observation, or at least an F1 so, would result in an overall unsatisfactory report. ??

mainline · Aug 28, 2024

samtheman said:
Thanks - do you mean they should have notified the supplier themselves rather than leave it as a footnote in an installation certificate for me to find two years later? To be fair I see now it's also on the EICR that induced me to get the new CU so I probably could have been paying more attention.

Anyway, it is what it is - I just hope SSEN will arrive at some point this week and sort this out for me as if they say it's not their problem I guess it might be an expensive fix!

The earthing is their problem.

samtheman · Aug 28, 2024

mainline said:
Wouldn't the absence of a reliable and effective means of earthing be a C2 Observation, so, would result in an overall unsatisfactory report. ??

Ah - the original report had all sorts of observations which led me to get the new consumer unit. This observation however was a "Further Investigation" and on the installation certificate it's now just a comment on the existing installation.

Mostly just concerned with getting it put right - preferably not at considerable expense to myself!

Chivers · Aug 28, 2024

I'm surprised the person installed the new consumer unit before the high earth was investigated tbh and I'm also surprised they didn't got to greater lengths to explain the issue. Though nearly all the circuits are at least on RCBOs by the looks of it. what's the circuit closest to the Main isolator?

I hope you get this sorted out quickly

Chivers · Aug 28, 2024

The sockets being able to be used outside imho are C2. There's no immediate danger

samtheman · Aug 28, 2024

Chivers said:
I'm surprised the person installed the new consumer unit before the high earth was investigated tbh and I'm also surprised they didn't got to greater lengths to explain the issue. Though nearly all the circuits are at least on RCBOs by the looks of it. what's the circuit closest to the Main isolator?

I hope you get this sorted out quickly

Thank you! Yes I asked for the "better option" as we were replacing it, just wish this had been made clearer.

davesparks · Aug 28, 2024

samtheman said:
Here's the whole arrangement - supply helpfully hidden behind that gas meter!

That gas meter may well cause some problems, SSE are unlikely to be able to work on their equipment with it hidden behind it like that.

davesparks · Aug 28, 2024

samtheman said:
Anyway, it is what it is - I just hope SSEN will arrive at some point this week and sort this out for me as if they say it's not their problem I guess it might be an expensive fix!

It is SSE's equipment and only they are allowed to work on it so it is very much their problem to sort it out.

However there is a possibility that they will say they are no longer able to supply an earth to your property, if this happens you will need an electrician to install an earth electrode to convert it to a TT earthing system. There shouldn't be too much additional work required for this as the CU is new and will be fine for a TT system (although there will probably be a few posts here soon arguing this point)

UNG · Aug 28, 2024

I suspect that SSE will want the gas meter moving before they do anything with that service head as they will need to get at the PILC termination box with a gas torch to melt the pitch in the termination box before they can terminate it into a new service head

Looking at the 3 earth connections on the PILC termination box they are not the best even the lugged connection has been badly crimped with side cutters and the cable looks to be missing a strand or two, the screws on other two don't look that tight and are probably one of the reasons for the poor earth because the PILC incomer is not properly clamped,

I'm wondering if this has carried some fault current at some time and arced a bit causing the sooty appearance