And just for clarity - in BS 7671 terminology an installation only has one MET (Main earth terminal) - earth terminals or bars elsewhere don't get the "Main" prefix.

   - Andy.

That nomenclature is fine for a "classic" domestic setup with the earthing conductor connected to a 4-way terminal strip beside the meter and continuing to the CU; with gas and water main bonding. However, with utilities coming in plastic pipes, all we have is the earth bar in the CU, which I suppose could be the "main" earth terminal even though it is the only one.

By contrast, once you have separate buildings with separate distribution circuits, and earth terminals in one or more of them, which is the MET? And what do you call the terminals in each building? In a sense, at least in the sense which is commonly understood, each is a MET. "Building earth terminal" and "earth marshalling terminal" or even "building earth marshalling terminal" are alternatives, but they do not particularly appeal to me.

AJJewsbury said:

And just for clarity - in BS 7671 terminology an installation only has one MET (Main earth terminal) - earth terminals or bars elsewhere don't get the "Main" prefix.

I think I agree with Chris Pearson , because main protective bonding connects extraneous-conductive-parts to the MET, and Regulation 411.3.1.2 requires main protective bonding to be applied to each building.

So, there's possibly only one LV MET per building (in some small buildings like control buildings, substations, etc., this may well be a bonding ring conductor) ... in fact, in very large buildings with multiple private transformers, it may well be that there are a number of MET's, which are connected together with a common bonding network of a design relevant to the design of the building and installation(s) contained in it.

For distinct buildings, I prefer the term Building Earth Marshalling Terminal (BEMT) for subsidiary buildings, extraneous-conductive-parts in the remote building being connected to the MET via the building's submain protective conductor, although I can see the argument for calling it a MET as well. Certainly where the outbuilding has a distinct earthing system (e.g. TT'd from a TN main building) calling it a MET makes sense.  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. (Maybe I was being too subtle for once)

  - Andy.

For distinct buildings, I prefer the term Building Earth Marshalling Terminal (BEMT) for subsidiary buildings, extraneous-conductive-parts in the remote building being connected to the MET via the building's submain protective conductor, although I can see the argument for calling it a MET as well. Certainly where the outbuilding has a distinct earthing system (e.g. TT'd from a TN main building) calling it a MET makes sense.  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. (Maybe I was being too subtle for once)

  - Andy.

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.