Main Protective Bonding to incoming Gas and Water services

The consideration is "where could an extraneous potential come from, as there are no extraneous parts" from your own definition? Extra bonding is not required, but its presence is not a problem. All of the earthing systems in the block are connected anyway, so there is no risk to anyone. This is why the regulation says "external to the building" not any other definition. The situation is in no way dangerous, any more than touching the building walls, etc. In some cases, bonding gives extra risk, as all the bonded metallic parts are raised in potential during an Earth fault, and the disconnection time may be 5 seconds if this is on a distribution circuit. The client should be referred to the big brown book! ( or the blue or yellow ones!).

Hello David,

Thank you for your swift response.

It's just clarifying that an existing (and new)service pipework exiting the ground outside the building in Poly, jointed to metallic pipework, is considered in this case as "entering" the building at that specific point, and not as it "enters" each domestic property (which serves copper pipework throughout each domestic installation). 

I'm if bonding is not installed, I'm sure The Client will receive annual Gas safety Certificates identifying the absence of Main Protective Bonding to the Gas service within the domestic property, as a defect?

Hi HMREES, the Gas Industry Unsafe situations Procedure, to be found here IGEM/G/11 Edition 2 - Gas industry unsafe situations procedure - IGEM says;

"3.3.2 Equipotential Bonding Electrical Safety Issue Regulation 18(2) of GSIUR places a duty on gas engineers to notify the responsible person that equipotential bonding may need to be connected to gas installations in domestic premises. Note: Where non-metallic (non-conductive e.g.PE) service pipes/service pipelines enter the building and are then connected to metallic pipes within the building, the metallic pipes within the building do not require protective bonding. Where required the purpose of equipotential bonding is to ensure the gas installation, other metallic services and parts within the premise remain safe under electrical fault conditions. Where bonding is necessary it should preferably be connected: within 600 mm of the outlet of the gas meter, before any branch, or  where the meter is fitted outside the building, as near to the point of each entry of the pipework, inside the building, before any branch. Where a gas engineer cannot confirm that adequate bonding arrangements where necessary, exist, HSE guidance is that the responsible person be notified in writing of the requirement for equipotential bonding. This can be achieved by leaving a bonding notice as described in BS 6891, which advises that the bonding be checked/carried out by an electrically competent person."

So it is not a fail, and only requires a notice.

Edit; You could provide your own notices at gas meters to state that bonding is not required and giving the reason why

HMREES said:

either sleeved or installed in poly

thinking further on this. The contractor/designer(s) need to satisy themselves that they aren't extraneous, especially if they are sleeved. Enquiries might need to be made.

Also, they need to satisfy themselves that they won't become extraneous in the future, ground level rises, and fitting of mechanical metallic protection (possibly where cars park, or bins placed) spring to mind.

I can see both angles here. You can either consider that the entire building consists of one single electrical installation, or as multiple separate installations that just happen to be physically stacked next to each other - the first akin to a big hotel the second more like a terrace of houses.

In the 'one big' installation approach you'd have one single MET for the entire building and main bonding considered at an whole building level - so if the entering pipework was insulating and had no other paths to the outside the main bonding could reasonably be omitted. Even where there is main bonding at the building entry points, it's not uncommon to see (additional) main bonding at a per-flat level - even if in BS 7671 terms it's more akin to supplementary bonding - to deal with any worries about the large distances and impedances involved between flat and building entry points.

In the 'many small' installations approach each flat would have its own MET - which may or may not be directly connected to the METs of other flats. Large buildings may have multiple risers, possibly fed from different transformers. Where the old TN-C to the flat system is still in place, there really can't be any assumption that all the METs are at about the same potential in any event. In this case with metallic pipework between flats, that pipework would be liable to transfer potentials from one flat (installation) to another, so would require main bonding.

The words of 411.3.1.2 aren't "external to the building" but "In each consumer's installation within a building".

   - Andy.

Thanks Andy, 

The second view you offered has most in most common with what is in place. The block of flats is supplied via a single TN-S 3ph supply to a dedicated intake, with bus bar riser supplying a Ryefield every third floor, which in turn feeds each flat. The containment for the lateral supplies from the Ryefields to each flat is original steel conduit (Bonded), which has been rewired within the past 10-15 years, with a separate CPC connected to an MET in each flat, and also back to a single MET within the Ryefield. There are obsolete steel Gas and Water services entering each flat (passing through adjoined properties), which most local authority owned properties have historically also been bonded back to that particular flats MET. The new water supply will be fed from a new stand alone building housing a "pumped supply" installation fed from the intake. There is also the fact that there are still properties that have an original 1960's installation within the block, in unknown states of condition. To not bond those properties at least, arguably goes against the requirements of BS7671? or doesn't it?

So every flat is connected to the same Earthing system, so the situation is exactly as I described. If a building is large enough to require separate supplies, the Earthing systems of each will be very solidly bonded together, for example in a hospital. The reason is simple, this would otherwise be an example of introducing an external potential, which is banned by 411.3.1.2.

There is a slight definition problem here Andy, which is leading to your comments, and that is that you are defining multiple electrical installations (each flat), whereas the building distribution is not in principle separate, just who is paying the bill. I do not think that BS7671 says that these are separate in any way, the metering position is not acting as some kind of break in the installation. The change from DNO (which has different rules) to BS7671 is an obvious break, but the building distribution is also covered by BS7671, so cannot logically be a different installation. The introduction of another fuseboard does not change the status of a section of an installation.

If we had a case where each flat was supplied by a separate DNO supply your argument would be possible, but I struggle to even think of such a case, in any reasonably sized block such would be very unlikely, but perhaps possible in small converted properties. In such a case compliance with 411 would be more tricky without verifying that the supplies all came from the same supply main.

All these obsolete pipes, whether bonded or not, are largely irrelevant, a fault to these is realistically extremely unlikely, and unless deliberately isolated from bonds from the past are probably Earthed, but this is very easily verified. In either case, it is a very tiny risk, and if passing through a bathroom should be bonded anyway from past connections and in any case, the area should also be RCD protected.

The description does not say if the supply is PME, but if so the E-N bond will be at the DNO head, and the installation MET is close to here also.

All this is why I said building in reference to 411.3.1.2, and I think that this wording in the new Brown Book has subtly changed to that which I said, but is probably still not fully clear, having also added consumer into the sentence. A consumer is someone supplied by the DNO, even if that is a BNO who also has consumers connected.

but the building distribution is also covered by BS7671, so cannot logically be a different installation.

I'm not sure sure. It seems logical to be to be able to have one BS 7671 installation feeding another BS 7671 installation - it's not just abut physics but ownership and responsibility too. You wouldn't want to be reliant on someone else's (parts of the) installation for say isolation or RCD protection where you didn't necessarily have ready access.  We have enough problems with people complaining about not being able to access the incomer to check bonding, earthing arrangements and so on where it's actually part of the same organisation, never mind where it's owned by some completely different 'management company' which sees no benefit in co-operating with piddling little access requests from strangers.

A consumer is someone supplied by the DNO, even if that is a BNO who also has consumers connected.

A consumer is someone 'which uses electricity for its own needs' (BS 7671) or 'any person supplied or entitled to be supplied by a supplier' (ESQCR) so in neither case is the BNO a consumer (which is quite convenient where BNO have been obliged to adopt TN-C distribution systems from DNOs, otherwise they'd be in breach of the ban on CNE conductors in consumer's installations in the ESQCR).

I still think that where the distribution system is TN-C with the N-PE link in each flat (which was common up to the 1980s I think) you'd certainly want main bonding in each flat - if only to avoid 'tingles' between exposed- and extraneous-conductive-parts (never mind the results of any nasty broken PEN situations), so I still think consideration of the individual circumstances is needed.

    - Andy.