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.

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).

I am sure that Mike will tell us, but it does seem sensible to label water bonding, gas bonding, etc.

AJJewsbury said:

I thought the N/PE cover on most cutouts was normally sealed

This is after the N/PE split, so it is just the PE.

I shall post pictures tomorrow.

Chris Pearson said:

This is after the N/PE split, so it is just the PE.

Usually, at that point, it's often the same block for N and PE ... at least at the service head itself.

So, perhaps labelling it as "PE" alone is not quite right?

However, at my sister's house, there is a DNO earthing terminal, that's brought off the Neutral, and has a DNO earth electrode connected ... because it's PNB.

Again, all belongs to the distributor, but ...

I see what you mean. I had not quite remembered the service head properly. However, the PE terminals are accessible to an ordinary person.

Perhaps the terminal constitutes the MET? "The terminal ...provided for the connection of protective conductors ... to the means of earthing."

I see what you mean. I had not quite remembered the service head properly. However, the PE terminals are accessible to an ordinary person.

Perhaps the terminal constitutes the MET? "The terminal ...provided for the connection of protective conductors ... to the means of earthing."

Chris Pearson said:

However, the PE terminals are accessible to an ordinary person.

In some designs of service heads. In others, even from the same manufacturer, you need to remove a screw that is accessible only when the service fuse carrier is removed from the head (in fact, there is one of those Series 7 with a fixed block and no access to the consumer on the demo board I pictured earlier in the thread).

Chris Pearson said:

Perhaps the terminal constitutes the MET?

Not in my opinion, it's not part of the electrical installation.

In my opinion, in BS 7671 speak, the earth block on the service head is the means of earthing, see Fig 2.1 in BS 76771 ... but if I remember correctly, in ESQCR it's the Distributor's Earthing Terminal.

I cannot argue with that interpretation, but is not exactly crystal clear.

gkenyon said:

Series 7 with a fixed block and no access to the consumer

Like this one!

Clearly, in this instance (TN-S/PILC), there has to be a separate MET.

Chris Pearson said:

Clearly, in this instance (TN-S/PILC), there has to be a separate MET.

Really? Henley do an SNE service head (that has a CNE link too) where the link from the PILC or armour of TN-S can be brought into the service head earthing terminal.

The absolute best bit about that kit, is you have no clue from the outside whether you have TN-S or TN-C-S !

The absolute best bit about that kit, is you have no clue from the outside whether you have TN-S or TN-C-S !

Or for that matter the occasionally seen weedgrowers special edition config , where the provision for looped supply can be misused to export an unfused live via the CPC terminal....  the giveaway is the G/Y disappears behind the wooden board as fast as possible with no bare metal MET, and perhaps a neutral derived earth from somewhere else.
In some err, perhaps 'less trusting' countries than the UK, meter boxes, supply terminal covers and similar kit are made transparent so that the internal wiring, and any ingress of water or insects, can be seen and visually inspected by any interested party to be correct and in good condition. Such an approach has much to recommend it.
Mike