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Separate main bonding to outbuilding

Former Community Member over 2 years ago

Hi, I just wanted people’s thoughts on this please.

I have been to a job with a TN-S supply. A sub main feeds a separate building with an SWA, the armour being used as the CPC. I have calculated the armour has not the additional cross sectional area to used as the required main bonding. I understand from GN8 that the main bonding needs to be extended to the building as specified in size by BS7671. I realise that this main bond will run from the MET to the Earth marshalling terminal in the second building. My question is this:

The SWA is buried and cannot be dug up (well easily). There is a second duct that runs between the buildings which is empty, but it follows a different route to the SWA. I seem to remember reading that it is best to run a separate bonding cable in close proximity to the existing SWA, am I correct or did I dream this? Is it advisable not to run the additional cable via a different route? If so, my only option will be to TT the second building.

Thanks in advance.

KS

0 Legh Richardson over 2 years ago

If the green and yellow wire or equivalent is needed to supply a protective function( earth faults) then reg 543.6.1 applies, running the conductors as close to the main circuit conductors.

See 544 for the sizing of earth bonding conductors in TNS systems

Legh
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0 alanblaby over 2 years ago

What do you mean by 'main bonding'?

Only the largest of SWA cables need an additional core for cpc purposes.

Are there extraneous pipes/structure that needs bonding in the 2nd building?
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0 Former Community Member over 2 years ago

There is a second duct that runs between the buildings which is empty

Welcome KS

Another option would be to use this duct for installing another length of SWA and associated separate conductor, but only you can work out the economics of a new supply (it might facilitate future load creep) versus the cost of providing an adequate number of RCDs.

Regards

BOD
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0 Alcomax over 2 years ago

I have calculated the armour has not the additional cross sectional area to used as the required main bonding.

As Alanblaby said, we need to be sure what you mean by "main bonding".

Your statement about "additional cross section" may suggest there is concern about main protective bonding being additional to the present protective earthing? I may have misunderstood, but the armour as a CPC is sized for the largest requirement, be it as a CPC or main protective bonding, but it would accommodate both, at the same time, so to speak.
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0 Former Community Member over 2 years ago

Thanks for this, it seems like it should be in close proximity then.

KS
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0 Former Community Member over 2 years ago

Thanks for the reply. And use there are a number of extraneous parts in the second building.

KS
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0 Former Community Member over 2 years ago

Thanks, I think they may end up putting a new supply in. I was just considering options.
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0 Former Community Member over 2 years ago

Alcomax and Alanblaby, thanks for the reply. Just to clarify.
Alcomax you said “but the armour as a CPC is sized for the largest requirement, be it as a CPC or main protective bonding, but it would accommodate both, at the same time, so to speak”.
So in the case I am talking about the steel wire of the armour has sufficient cross sectional area to provide a low impedance for the protective device to disconnect in the required time. This has been confirmed by manual calculation, software and now measured. The armour also has an equivalent cross copper cross sectional area for the required bonding. The csa of the armour is not sufficient to fulfil both requirements if they are additive. My concern was that in GN8 figure 5.14 buildings B1 and B3 (which best represents my scenario) the narrative states:
“Where a circuit protective conductor also acts as a bonding conductor the requirements for both functions will have to be met. In other words, as well as meeting the requirements of Regulation 543.1 for a circuit protective conductor (CPC), the requirements of Regulation 544.1.1 have to be met”
Perhaps I am misinterpreting this but what I concluded this to mean is that enough steel armour is required to provide the CPC and in addition an amount of steel armour must be available to provide the equivalent cross sectional area of copper required for the protective bonding.
Using a hypothetical bonding and cpc example
10mm2 of armour for CPC purposes, protective bonding requires 20mm2 of armour, total armour required would be 30mm2.
So really the nub of the questions is the armour is doing two jobs, does it have to be a size to fulfil both as above (30mm^2) or does it require to be large enough to achieve the most onerous, 20mm2 in the example above.
Hope that makes sense and thanks again.

KS
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0 Chris Pearson over 2 years ago

Kawaspark:
So really the nub of the questions is the armour is doing two jobs, does it have to be a size to fulfil both as above (30mm^2) or does it require to be large enough to achieve the most onerous, 20mm2 in the example above.

The latter. It has to be big enough for the most onerous requirement, not the two added together.
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0 Former Community Member over 2 years ago

Thanks Chris, now re looking at GN8 and the sizes of conductors in their example that makes sense.

KS
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Separate main bonding to outbuilding

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