TN-S earthing query

Mate, you’ll need to use what you have.
If you want the DNO to upgrade their earth you’ll be waiting until the deserts freeze and the camels take up skating.

Stephen-d said:

Hi everyone,
This might be a silly question but I have been asked to look at job where they would like a new garage supply, its a old property with the re-wireable fuse board but previously someone has tapped off the meter to an additional RCD unit which was used for a shower (no longer required). They do plan on doing a board upgrade, but at the moment they could do with power in the garage. So my thought are i could use the RCD unit to feed the garage and use a main switch consumer unit in the garage for the time being. My question is in regards to the TN-S, it looks like a old supply with stranded cable with no insulation? Would this earth still be ok to use providing the earthing to the RCD unit is 16mm? Or would the DNO have to upgrade their earth first?
Thanks

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As long as the Ze reading is fine then nothing wrong with using it. Those stranded uninsulated cables are still pretty common, as are the braided tape type. (They are often soldered (not exactly, but can't think of the right word) to the incoming cable and rather more reliable than the ones that have had a bonding clamp added at some point)

Is there a separate earth bar to act as the MET? If not, that's the best way to set it up, with 16mm to each CU (or what ever is there currently if it's appropriate) - that way if they ever do upgrade the earth there will be no additional wiring to do.

You can request an upgrade and if the head allows an easy PME upgrade then they will come out and do it without too many problems. Otherwise if the head is fine and not ancient they'll probably leave it as it is unless you are requesting a fuse upgrade.

Nothing wrong in principle with using the shower unit. If the garage is separate and running SWA to it then better to have the RCD at the far end, but since it's already there makes sense to reuse it....

Incoming earth no problem, as long as it tests OK,
If they've been naughty and 'tapped off the meter' direct, it sounds like Henleys would be practical.

Taylortwocities said:

Mate, you’ll need to use what you have.
If you want the DNO to upgrade their earth you’ll be waiting until the deserts freeze and the camels take up skating.

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Haha ok thats great, thank you

Dartlec said:

As long as the Ze reading is fine then nothing wrong with using it. Those stranded uninsulated cables are still pretty common, as are the braided tape type. (They are often soldered (not exactly, but can't think of the right word) to the incoming cable and rather more reliable than the ones that have had a bonding clamp added at some point)

Is there a separate earth bar to act as the MET? If not, that's the best way to set it up, with 16mm to each CU (or what ever is there currently if it's appropriate) - that way if they ever do upgrade the earth there will be no additional wiring to do.

You can request an upgrade and if the head allows an easy PME upgrade then they will come out and do it without too many problems. Otherwise if the head is fine and not ancient they'll probably leave it as it is unless you are requesting a fuse upgrade.

Nothing wrong in principle with using the shower unit. If the garage is separate and running SWA to it then better to have the RCD at the far end, but since it's already there makes sense to reuse it...

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Thanks for your reply.
Yes theres is a MET there, I was planning on doing with the earths it made sence to me but just wanted to confirm it ?

The RCD unit would only be a until they upgrade the board then the garage can have a RCD main switch, just thinking of the safest and best way to do it at at the moment.

Stephen-d said:

Thanks for your reply.
Yes theres is a MET there, I was planning on doing with the earths it made sence to me but just wanted to confirm it ?

The RCD unit would only be a until they upgrade the board then the garage can have a RCD main switch, just thinking of the safest and best way to do it at at the moment.

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your plan looks good to me.henley block the tails if not already done. just make sure the armour of the SWA is earthed. myusualmethod would be to use 3 core SWA and use both the 3rd conductor and the armour as cpc. if it;s pme, though, you'd need equivalent of 10mm for bonding any extraneous parts in the outbuilding.

ipf said:

Incoming earth no problem, as long as it tests OK,
If they've been naughty and 'tapped off the meter' direct, it sounds like Henleys would be practical.

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Thank you, completly agreed. Ideally that would be a better situation, but there isn't a isolator there so would mean pulling the main fuse out, which I assume the previous person had done

telectrix said:

your plan looks good to me.henley block the tails if not already done. just make sure the armour of the SWA is earthed. myusualmethod would be to use 3 core SWA and use both the 3rd conductor and the armour as cpc. if it;s pme, though, you'd need equivalent of 10mm for bonding any extraneous parts in the outbuilding.

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Hi tel,
I am pretty sure this is a TN-S, is there anyway to see if it has been converted? I have a picture but it isn't the best.

Thats great thank you ?

Looks like TNS. Not converted to anything else.
Cannot see from the photo how the main earth is connected to the supply cable sheath. Has it been sweated on?

PS horrible big holes in the top of that Wylex mini CU.
Not IP compliant - needs sorting

Taylortwocities said:

Looks like TNS. Not converted to anything else.
Cannot see from the photo how the main earth is connected to the supply cable sheath. Has it been sweated on?

PS horrible big holes in the top of that Wylex mini CU.
Not IP compliant - needs sorting

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I knew someone would spot that haha, the wylex cu is not being used the 6mm twin and earth is disconnected and no power going to the cu so I will remove that.
It is at the back of the cable, is 'sweated on' like soldered?

Stephen-d said:

It is at the back of the cable, is 'sweated on' like soldered?

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Exactly

Is it attached in that way?

Taylortwocities said:

Exactly

Is it attached in that way?

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I think so, but I will double check when I go back there. I will test it before doing any work as well

Stephen-d said:

Thank you, completly agreed. Ideally that would be a better situation, but there isn't a isolator there so would mean pulling the main fuse out, which I assume the previous person had done

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Whatever's needed, it wants sorting. Can't see it on the photo.

Seems a bit unusual......someone breaking into a meter to pick up a supply and not by passing.?? Thicko or DIY, maybe?

I'd say that's a definite TN-S - the stranded cable makes it appear original, although the amount of bare cable sheath on show makes me wonder if it was a repair at some point, either by DNO or someone else.

When I first saw one like this I assumed the earth block was internally connected to make a nice TN-C-S, but no - just provided for convenience of connecting earths...

Might be worth a call to 105 and picture - they might come out upgrade the earth to 16mm - they usually use some form of clip these days to reconnect the earth - then they'd probably tape everything up so no metal on show. Or there might even be a way to convert it to PME, though can't see an immediate option in that photo.

They are fairly good for upgrading earths around here (UKPN) - about 8 weeks wait though unless it's an actual issue with no earth - but you don't need to be there if you've left everything ready....

No chance that they'll upgrade the actual head though I'd say -

Looks like the seal fairy has paid a visit previously as you say, so might be a good time to try and get a henley block in somewhere, if only to make it easier later.

Are there two feeds coming out of the meter? Is the meter seal broken, or does it have separate ones for incoming/outgoing? If nothing else then I'd get that notified to the electricity supplier so they can choose if they want to correct that.

Can't see from the photo, but if there's enough slack from the meter to the boards, it might be possible to insert a henley block now, and then the extra feed can be removed by the meter supplier?