E95 said:

however the MET is not within the same building

Is that because there are no extraneous-conductive-parts in that building at the moment? I only ask because Regulation 411.3.1.2 requires main protective bonding to be applied to each building.

So, it's correct to consider an MET for each building - this should be taken from the main incoming switchboard for the building ... if there are multiple supplies to the building from another building, this can potentially be problematic for the functional earthing system according to BS EN 50310.

Things might get tricky if you are exporting PME as the main protective bonding in any building in the installation may need to be sized as per the main bonding in the main building into which the DNO supply is provided.

E95 said:

BS 50310 Telecommunications bonding networks for buildings and other structures doesn't (to me) seem to define what a "clean" or a "functional" earth system is and I can't make heads or tails of the standard

In reality, there is no such thing as "clean earth".

'Functional earthing' is quite simply 'earthing for the purposes of other than electrical safety' (IEV Reference 195-01-13).

[Edits in red, after observation made by Jam ]

The purpose for information technology, communications and control systems (ICT as they are collectively named in BS 7671) is to help with electromagnetic compatibility, help with high protective conductor currents (e.g. where a combined protective and functional earthing is used for IT racks etc) and also deal with shield/screen currents picked up on long cable runs.

This is why BS EN 50310 is referenced in Section 444 of BS 7671.

BS EN 50310 summarises the purpose of the functional earthing it facilitates in the Scope (section 1 of the standard), i.e. to:

• minimize the risk to the correct function of ICT equipment and interconnecting cabling from electrical hazards
• provide ICT installations with a reliable signal reference to improve immunity from EMI.

Hi Graham - 

I'm saying no MET within the building as the building is supplied on a private network where the transformer is located in another building, so not METs only "Earthing Marshalling terminals" if my definitions are correct? I believe there would be protective bonding conductor back to the Earthing mashalling terminal at the sub DB within the building if that makes sense?

So realistically are you saying there is no requirement to take the connection back to the MET at the primary switchboard and it can be taken directly to the closest earthing marshalling terminal within the most upstream DB within this building and share the path to the MET as part of the CPC?

E95 said:

So realistically are you saying there is no requirement to take the connection back to the MET

What I'm saying is that Regulation 411.3.1.2 tells us that each building has an MET for that building, regardless of whether you chose to have one  for the whole site. It is this one (in the building) that you connect the bonding system in BS EN 50310 to. The reason for this, is ICT cabling outside (including between) buildings is covered by BS EN 50174-3 not BS EN 50174-2, and under certain conditions may need separate surge protection, or other measures.

Whether that MET is a separate bar or block, or is a terminal bar or busbar in a distribution board or switchboard, is purely academic. Usually, it's a separate bar, to permit additional connections to be made without accessing the main switchboard ... but BS 7671 does not mandate that.

The incoming DB in the building must have an earthing terminal associated with it, and as well (in a TN-S) any main bonding (if any) and functional bonding, and of course cpc's, are connected to that.

Depending on what's going on at the site, application of lightning protection to one or more buildings, presence of HV distribution around the site, etc., it may not be advisable to provide additional copper earthing between each building's MET and the site's MET.

BS 7671 now recommends (Reg 411.4.2) that the MET in buildings with TN systems has an additional earth electrode connected to the cpc (except for outbuildings of dwellings, although the main dwelling should still have one). This may already be the case if the building has lightning protection.

gkenyon said:

Regulation 411.3.1.2 tells us that each building has an MET for that building

I think that the problem here is one of nomenclature.

The definition of MET starts with the definite article, "The" and not the indefinite article, which implies that there is only one MET in any one installation. It does not say, "The terminal or bar ... to the means of earthing in a particular building."

Even "(electrical) installation" is unclear: "An assembly of associated electrical equipment ...". There is nothing which says that a site can have only one installation. Indeed, one might argue that each building has an installation, in which case if each installation has a MET, each building also has one.

What does "connected directly" mean? I don't think that the electrons as they shuffle back and forth are particularly bothered whether the conductor is continuous or has a join in it. Indeed, if you do have a CPC/bonding/earth conductor, do you really have to replace the whole lot in the event of damage, or can it be spliced?

gkenyon said:

'Functional earthing' is quite simply 'earthing for the purposes of electrical safety' (IEV Reference 195-01-13).

Sorry to pick on a typo, but I think you meant almost the opposite, i.e.

Functional earthing = "earthing for purposes other than electrical safety"

gkenyon said:

In reality, there is no such thing as "clean earth".

I agree, and would add that BS IEC 61000-5 would tell you that this practice is largely deprecated for general use.

Jam said:

Functional earthing = "earthing for purposes other than electrical safety"

Yes, I will edit that. THank you

Chris Pearson said:

I think that the problem here is one of nomenclature.

Whilst I have read your response, and through about it carefully (and agreeing with some points, but not others), I'm not 100 % with you on that, given the wording of  the first para of 411.3.1.2 in BS 7671:2018+A2:2022:

In each consumer’s installation within a building, extraneous-conductive-parts liable to introduce a dangerous potential difference shall be connected to the main earthing terminal by protective bonding conductors complying with Chapter 54. Examples of extraneous-conductive-parts may include:

this talks about the "installation within a building" - it would not make sense for the MET in question to be in another building (specifically related to this Regulation ... not the remainder of BS 7671).

I don't see anything there that implies that the MET has to be in the same building, only that everything is connected back to it.

Simon Barker said:

I don't see anything there that implies that the MET has to be in the same building, only that everything is connected back to it.

We''ll have to agree to disagree ... I can offer no official interpretation I'm afraid.