haeyun kim said:

Q) I am wondering where does (iii) refer to,

In my house, which is TN-C-S, the green-and-yellow cable from the service head goes to an earthing block, rather than the earth terminal in the consumer unit. The main bonding is connected to this block, an a connection taken to the consumer unit. This is not uncommon. This illustration is from Figure 4.9 of IET Guidance Note 8 Earthing and Bonding, and I've circled in red the separate MET:

haeyun kim said:

and why it is necessary to install the warning notices there?

So no-one disconnects the earthing arrangement, or the connection to the consumer unit, as this may well cause exposed-conductive-parts to have a voltage of 110 V AC from leakage currents in the installation (which can sometimes be fatal). The warning notice can be a durable label applied just above or below the MET, for example as this picture (which is a part of a setup I use to demonstrate a method for testing a consumer's earth electrode):

'Durable' of course depends on the external influences pertaining to the particular installation, so it's not uncommon in commercial/industrial installations to have a more durable plastic, or engraved laminated, notice, perhaps secured with screws.

we seem to have lost a reply from Graham, but I'll have a go.  the MET is any sort of marshalling block where the green and yellow wires come together that is not inside another box. although the regs don't make it very clear there is often more than one - perhaps in the basement of a block of flats, and then again inside each flat. this sort of thing. The warning is because taking the site off-earth without isolating the supply first may well allow all the earthed metalwork to drift to a  painful voltage.

Mike.

mapj1 said:

we seem to have lost a reply from Graham

Yes, not sure why?

EDITED ... but it has reappeared, perhaps popped onto the moderation queue?

And now the original post has re-appeared, I can link the two responses, which just happen to be complementary, by pointing out that the picture in the post from mapj1  clearly shows a more durable plastic notice, secured with screws ...

I love the wee flags!

Most SP CUs nowadays seem to have a couple of terminals in the earth bar for bonding, but I am not sure that it was always so.

The other thing is that my (ex-) TN-S supply has 4 weedy looking strands for insertion into the MET. They are not long, but do have a diameter of about 2¼ mm. It follows that a junction block (even with no bonding) was required.

With modern TN-C-S service heads, there is a terminal for the earthing conductor.

Should that have a warning notice?

Chris Pearson said:

With modern TN-C-S service heads, there is a terminal for the earthing conductor.

Should that have a warning notice?

The service head is not part of the consumer's electrical installation, and isl also outside the scope of BS 7671 ... so perhaps a question for the relevant DNO ?

Equally well, the earthing conductor is part of the consumer's installation, so where do you draw the line?

The earth terminal is not sealed, in fact there is nothing more than a sort of dust cover over the terminals.

I had never thought about this before, but the OP prompted the question.

I thought the N/PE cover on most cutouts was normally sealed - and even if it isn't it is rather more obviously part of electrical equipment that even ordinary persons shouldn't be tempted to mess with (similar to the earth bar in a CU not needing a label). Some larger (industrial) METs do look rather more mechanical than electrical to the untrained eye.

Don't over estimate non-electrical types. I've seen an example of a plumber doing exactly what the label said, when replacing a gas pipe - they left 6" of the original pipe hanging in mid-air just so they didn't have to remove the original bonding clamp. Maybe the wording could have been better... (e.g. ensure it remains connected, rather than do not remove perhaps).

   - Andy.

AJJewsbury said:

ve seen an example of a plumber doing exactly what the label said, when replacing a gas pipe - they left 6" of the original pipe hanging in mid-air just so they didn't have to remove the original bonding clamp.

Chris Pearson said:

I love the wee flags!

Permitted by 514.5.2 even if they contain only numbers, provided those numbers pertain to the relevant circuit for circuit identification ... The main purpose of 514.5.2  being to stop someone numbering the PE conductor and using it as a live conductor, and 514.4.2 of BS 7671:2018+A2:2022 aligns with this to prevent "oversleeving" of the PE in a multicore cable to use it as a live conductor, as well as using GNYE single-core cables for any purpose other than PE or CPFE (the latter being combined protective and functional earthing).