TN-s or TN-C-S

13 Feb 2010
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United Kingdom
Hi all, new to the forum and have a question for you

While adding some sockets to a conservatory and locking off the CU I observed the main earth comming off the supply sheath and assumed a TN-S, however the earth cable from the sheath then goes into the suppliers cut out box with another earth comming out and attaching to the MET block on the wall TN-C-S. Any one seen similar?

I have a feeling that the whole setup has been interfered with by a DIY'er as the tails going into the meter have about 4mm of copper conductor showing and the CU wiring is a mess.
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It sometimes happens with a TN-C-S supply, that the sheath is connected up to the MET in order to bond it, rather than it being used to provide the earth in a TN-S arrangement. My parents house for example is like this.

Alternatively of course, it could be a DIY TN-C-S conversion, which may be dangerous if the supply is not done to PME standards. I'm not sure what the best approach would be here - you could try calling the DNO and asking them what earthing type you should have, but they probably won't be much help...

As for the copper conductor showing at the meter - is it on the tails going in to the meter from the cutout, or out of it to the CU - if the former you'll need to get your supplier to sort it as those are their responsibility, if the latter it's your problem (although technically if you need to break the seal on the meter you should get your supplier to do it).
you could try calling the DNO and asking them what earthing type you should have, but they probably won't be much help...
Write them a letter, a real one on paper.

It'll get passed to the right department, and they won't be able to ignore it. Tell them that a qualified electrician has told you that you might have an unofficial TN-C-S provision, as that will get their attention.
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thanks for the useful comments

After inspecting some of the internal wiring and finding this worse than what I had already found the customer has required a full rewire. I have advised them to contact the DNO and get them to sort out the earthing and tails and to fit a double pole switch so the installation can be isolated without getting them involved again.

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