HMO earthing arrangements.

[Darkwood]

I am having a debate about the requirements of earthing in an HMO -

The incoming supply is 3ph 200amp and Gas and Water are bonded 50mm as needed in this case.
The building is one block with its internals divided into 20 flats, each flat has its own distribution board rcd protection split-board, the supply to the flats comes from a local sub mains via a 16mm T&E, this does not have rcd cover as it does not route anywhere along its install where it would require the need for rcd protection.
The flats have a common plumbed in central heating system in metal pipework and no gas fixtures.

The discussion or debate is that the T&E requires an additional 10mm earth cable to run with it but I disagree, as there is no individual bonding requirement for the flats from the flat DB, I came to this conclusion because it is 1 common supply to the building and that has been bonded as needed at source.
Earth Loop Impedance for the 16mm T&E and as long as the ELI is met then there is no additional 10mm needed for the flat supply.
In my view the cable earth is good enough as it satisfies the adiabatic equation but he has contacted his own scheme providers and they say it needs the 10mm running with it.

Now either I am missing something here or Nappit are pushing out the wrong advice to this guy... as my field is machinery control and design I am a little lacking in this area now and just need my conclusion verifying or correcting... feedback appreciated.
I think I have included all the info needed.

 

[Strima]

Had this before and it was a bit confusing to say the least. NIC told me that each flat is to be treated as it's own installation with internal bonding, then in turn each one is bonded back to the supply MET which was read into the supply circuit must comply for bonding requirements.

But this was before the requirements had changed for earth potential so I would say test back to the MET to see if they require bonding at various points throughout the building. You'll probably find they don't need bonding.

 

[Darkwood]

On paper and test values it satisfies requirements, 5 flats already live and tested, this is a new group brought on site to speed job up with very different thinking..
I simply find these provider schemes repeatedly contradict each other on guidance in various areas and don't seem to use hard logic and references to justify themselves so take it with a pinch of salt what they say.

Bonding is not required to ensure the pipework is effectively earthed, it is needed to ensure incoming ground potential does not differ from supply earth potential so their original excuse that each flat needs to 10mm bond to the rads locally was not justified, as the install already has been equipotentially bonded at source and it is a common supply that feeds the flats then bonding of the rads is not required imo.

I can check if later if they need earthing been exposed conductive parts and simply run a 4mm from the board if needed but as a complete metal piped install system this is highly unlikely, these are industrial sized pipes on a loop with radiators tapped of in standard size... I just cannot see their case warrants 10mm additional earth running.

 

[loz2754]

I agree with Darkwood on this.
The NICEIC position (requiring main bonding at the intake position and at each individual flat, where each flat has its own metered supply) relates to self contained flats. However an HMO is a generally a single property divided into individual dwelling units with shared facilities.
I find the general lack of readily available up to date guidance for HMOs quite frustrating.

 

[Darkwood]

The second defense for them was the 16mm itself needed additional earthing as it was a submain, again if the earth in the 16mm satisfies the needs of the fault current tripping the protective device IE it meets ELI requirements then it does not need to be upgraded... there is no fault current within the distribution board on this 16mm supply that needs a larger earth...

I realise this is a confusing area of debate in the past and thus I do not knock them for questioning it but find the lack of real justification a little concerning, it is fine believing a requirement needs to be met if one thinks that but one should also be able to demonstrate it either through reference to regulations or calculations off the back of regulation.

 

[Rockingit]

If the maths complies, so does BS7671.

However, GN8 5.7 states "It is clear that BS7671 requires protective eq.bonding in each and every installation connecting together extraneous-conductive-parts to the MET, and this would apply equally to separate installations of a multi-occupancy building."

Sounds like you might have to bend on this one.

 

[loz2754]

If the maths complies, so does BS7671.

However, GN8 5.7 states "It is clear that BS7671 requires protective eq.bonding in each and every installation connecting together extraneous-conductive-parts to the MET, and this would apply equally to separate installations of a multi-occupancy building."

Sounds like you might have to bend on this one.

The argument is that an HMO is one installation with sub circuits.

Edit: unless this HMO has separate metering for each individual "flat".

 

[Wilko]

My thought too - in the HMO is there a single Supplier’s meter or individual MPAN? Not saying that’s definitive but it might push one way or the other.

 

[loz2754]

I have worked on several HMOs of different sizes and configurations, and I have come to the conclusion that it's best to to speak directly to the local council's HMO inspector, as he will be the one signing it off (or not).

 

[Darkwood]

All flats are common to one meter or MPAN so effectively it is classed as one installation, it is like having a large house with a upstairs and a downstairs DB, you wouldn't have to run a extra earth to the upstairs and equipotentially bond the radiator/water pipe as it came onto that floor, this is exactly the same principle here, even if the flats were landlord metered individually then that does not change the requirements imho as they are private meters, I would only agree if the flats had their own metered supply direct to the DNO IE their own MPAN... this would then make it a self contained flat in the eyes of equipotential bonding requirements.

 

[Darkwood]

I have worked on several HMOs of different sizes and configurations, and I have come to the conclusion that it's best to to speak directly to the local council's HMO inspector, as he will be the one signing it off (or not).

I assume you refer to building control, unfortunately it is pot luck with them, many just want that certificate of compliance signed of by the electrician and their backs are covered, others however will seek advice from other sources and it comes down to how accurate the source is, many people simply do a belts and braces attitude to cover complicated requirements regardless of the actual requirements been demonstrably satisfied.
I have had many a run in with building control and tbh I now find if they tell you that is what is required it is better to simply ask for written confirmation and get on with it as long as you have a paper trail, many a time I have proven them wrong but this loses them face and causes issues further downstream... they are a rule to themselves, although I have met a few who are genuinely there to help and will take critique and listen to your reasoning.

 

[westward10]

As you say one meter, one source of supply therefore one installation and one set of main protective bonding. Your analogy is correct in #10.
The only mistake you have made is contacting a scheme with an optimistical view that the person holding the phone is actually more knowledgeable than yourself.

 

[Darkwood]

@westward10 - not my mistake, this was done by the other team on site who disagree with me, I am not at all surprised tbh if they are not getting the right info back but I suspect they have not been very clear of the circumstances for the scheme to give that response in the first place but it wouldn't surprise me in the slightest if they were indeed just dishing out belt and braces advice.

 

[Rockingit]

No different to any other building with one source of energy and a bunch of submains. Go for it.