AJJewsbury said:

But the point I was trying to make was that it can be confusing to label earth bars in each control cabinet within the same building as METs (as in the OP's diagrams), especially when trying to interpret the wording of BS 7671.

The control cabinets cannot be made to BS 7671 as they are assemblies, and are outside its scope. Whilst the earth bars in them ought to be identified as either PE, PB, FE, or FB terminals using appropriate symbols, nothing in the usual standards for control panels (BS EN 60204-1 and BS EN IEC 61439 series) prevents someone additionally labelling it as a Main Earth Terminal (and in fact I've seen 'MET' and 'MFET' in control cabinets in the past).

AJJewsbury said:

I prefer the term Building Earth Marshalling Terminal (BEMT) for subsidiary buildings

That''s fully not in alignment with the current BS 7671 requires main protective bonding to be connected to the MET for each building ... in BS 7671 itself there's no requirement to 'bond' one building to another, so the only conclusion is each building has its own MET.

(This is completely separate to minimum csa of conductor to 'export' PME - that is not a 'main protective bonding conductor' although ought to be sized as such and based on the main service neutral conductor size because of diverted neutral currents in PME systems - I would give the same sizing dvice regarding the minimum csa of a conductor connecting a supplementary earth conductor to the incoming service MET in a single building installation.)

I can see where you're coming from, though, given the different approach in 16th Ed and earlier versions of the standard, but we've been on the current approach to main protective bonding (applicable only to buildings) since 2008.

so the only conclusion is each building has its own MET.

I can see that from the point of view of 411.3.1.2, but then wording of 542.4.1 and the very definition of an Earthing Conductor seem still to be in the era of one MET per installation. I guess it all fitted together rather more neatly going back 4 or 5 editions when each building was considered to have its own installation (with its own DP isolation). Sometimes it is useful to be able to distinguish a building's earth terminal from an installation's (despite the recent changes to 544.1.1).

   - Andy.

AJJewsbury said:

but then wording of 542.4.1 and the very definition of an Earthing Conductor seem still to be in the era of one MET per installation.

I'm not 100 % with that. I can see that 542.4.1 requires at least one MET to connect the earthing conductor an an installation that has earthing of any description (most installations). That doesn't preclude other METs being provided. The definition of earthing conductor doesn't preclude other earthing conductors (to other earth electrodes, per the definition of earthing conductor ... but they could be for FE as well as PE and they are required to also be connected to an MET).

542.4.1 would certainly require an MET in each building having any of the following not connected to an MET elsewhere in the installation:

• cpc's in the case of TT or IT systems with independent earth electrode for that building.
• extraneous-conductive-parts
• FE conductors
• LPS

You could also take the view that the cpc of an incoming distribution circuit to a building provides the means of earthing for that building (where connected), and therefore that wouldn't preclude an MET in a separate building supplied from another building having an MET if only for connection of cpc's per 542.4.1 (i)

Can you have a branching earthing conductor?

If the CPCs are all inserted into the earthing bar in a DB, and all the bonding is inserted into a terminal strip which is outside the DB, which is the MET? (542.4.1)

Chris Pearson said:

Can you have a branching earthing conductor?

Not sure it's an earthing conductor as defined in BS 7671

Chris Pearson said:

If the CPCs are all inserted into the earthing bar in a DB, and all the bonding is inserted into a terminal strip which is outside the DB, which is the MET? (542.4.1)

It could be considered (say in a domestic installation) that the conductor you're talking about is the cpc associated with the tails for that distribution board?

It then connects the other cpc's back to the MET.

All arbitrary really ...

"Arbitrary" sums it up nicely.

In my old supply, the PILC is connected to a terminal bar (the MET??) by some 4 rather weedy looking bare copper wires (I have checked and they are 4 mm² each) in the meter cabinet. From there, a bit of green and yellow goes to a switch-fuse in an adjacent cabinet. The downstream side continues under the breakfast room floor where 3 cores of a 5 core SWA are used. The CPC  terminates in a strip adjacent to the first DB where it is joined by umpteen bonding conductors (and a short CPC to the board). So is that the MET or BET or BMET? It would never have made sense to take all the bonding under the floor to one of the external cabinets.

The Lucy service head of my new supply has 2 earth terminals. 2 terminals = 2 earthing conductors!

So I think that we can forgive the OP for having 3 METs.

Ok so we cannot agree what to call the earth terminals, I suspect the original questioner is not so bothered about that but it is interesting.
More important is where to bond things to. This will depend rather on how things are laid out. If cabinet 1 and cabinet 2 are within reach of each  other, and their supplies are taken from adjacent places on the main board,  then really they are as if  they had been in one larger box, and one and could/ should be at similar voltages. The the extraneous parts bond back to either or both of them and no-one blinks. (assuming the cabinets are adequately earthed for the supply that feeds them)
But, if they are apart, and the extreme would be separate buildings, then the 2 cabinets both need to each be bonded to the services and any building steel in each zone - you cannot rely on the bonding in a far away place, nor that no-one will ever work on the plumbing and end up with a hand on either side of a break where a tap is replaced or something.

And ideally if the water or gas  pipes passes near the main incomer, bond there as well as then that becomes the full size main bond and reduces the requirement to bond locally.

M.

mapj1 said:

More important is where to bond things to. This will depend rather on how things are laid out. If cabinet 1 and cabinet 2 are within reach of each  other, and their supplies are taken from adjacent places on the main board,  then really they are as if  they had been in one larger box, and one and could/ should be at similar voltages.

It also depends on what is in scope of BS 7671, and what is in the scope of other standards. If the control cabinets are for anything classed as Machinery, which includes parts of HVAC systems, then the protective bonding circuit for the Machinery should comply with BS EN 60204-1.