UK Selecting bonding conductor size

[MMecca]

This is a question (or discussion probably) regarding selecting the main earthing conductor, and subsequently the main equipotential bonding conductor sizes in a TNCS installation. We are fitting a replacement consumer unit only.
The property currently has 4mm bonding cable installed to gas and water and a 6mm earthing conductor to the MET. The supply fuse is a 60A BS1361. The tails from the cut-out to the meter (and the CU) are 16mm; the csa of the coaxial incomer is unknown, assumed 16mm.
My understanding of BS7671 is that the earth conductor should be no less than half that of the neutral conductor. So 10mm. And subsequently the bonding should be 6mm.
But this is based on nothing in particular other than recommended values as in BS7671.
In order to upgrade the bonding cable would be an expensive and disruptive affair which we'd like to avoid.
So is there any reason why I cannot use the adiabatic equation to calculate the earthing conductor size?
This would be based on a PFC of 1.2kA and a time of <0.1s and a k of 143. The result is 4mm. So the existing installation is OK on paper, but not according to BS7671.

Any thoughts?

 

[Pretty Mouth]

Does Table D2M of the 14th Ed actually quote a minimum size of bonding for TN-C-S.

14th edition doesn't give a minimum size for TN-C-S specifically, it's 6mm minimum for all earthing arrangements

 

[westward10]

Yes, it's 6mm, subject to consulting with the supplier

I suspected something like that the 15th Ed cites consulting with the supplier and with this info you have no idea whether 6.0 was actually correct in the first.

 

[MMecca]

I suspected something like that the 15th Ed cites consulting with the supplier and with this info you have no idea whether 6.0 was actually correct in the first.

I suppose there's no reason to suggest that it wasn't done properly in the first place.

 

[westward10]

I suppose there's no reason to suggest that it wasn't done properly in the first place.

You have no way of knowing. What you must do is follow BS7671 and not some random person from the NICEIC.

 

[Aaron b]

Has anyone here seen a bonding conductor thermally damaged? It sounds odd to me.

 

[westward10]

Has anyone here seen a bonding conductor thermally damaged? It sounds odd to me.

Yes this is an often quoted statement promoted by the Best Practice Guides. Thermal damage may not necessarily be visible to the human eye so must be taken with a pinch of salt, as I say BS7671 is what you must follow.

 

[Aaron b]

To say it's not melted yet whilst there's been no faults seems crazy.

 

[MMecca]

You have no way of knowing. What you must do is follow BS7671 and not some random person from the NICEIC.

So we should assume all installations have been wired up wrongly? I don't think so. Everything points to this installation having been correctly done for the regs at the time. And I disagree that you should blindly follow BS7671 without referring to the Onsite Guides, Best Practice Guides, GN3 etc. Plus the NICEIC technical helpline is there for a reason and is the most qualified advice I have at hand so I'm going to follow it. Especially (I don't mind admitting it) if it saves ripping a house apart in the process.

 

[timhoward]

So we should assume all installations have been wired up wrongly? I don't think so. Everything points to this installation having been correctly done for the regs at the time.

I actually think it's not really relevant that it may have met the regs in 1966. There weren't as many PEN faults then!

Isn't the wider principle that if you are significantly altering an installation, the right thing to do is to ensure the alteration complies with the regs today. BPG1 says this on page 5 before proceeding to give questionable advice.
At the end of the day you are putting your name to an EIC saying it complies with BS7671:2022 (unless you specifically list this as a departure of course, and then it's supposed to offer an equivalent level of protection, so I don't see how that stands up to scrutiny)

Personally, I think the "it will probably be ok" get-out-clause in BPG1 is daft and is exceptionally daft if it's PME, as the point is that with no warning a PEN fault could see the bonding carrying the entire return neutral current. There wouldn't be any signs of thermal damage if it hasn't happened before (as @Aaron b said)

Regarding "ripping a house apart", the regs do allow bonding to be "as close as practicable to the point of entry", and on a couple of occasions I've left original undersized bonding in place and added some new 10 sq mm to the nearest convenient point.

 

[MMecca]

I actually think it's not really relevant that it may have met the regs in 1966. There weren't as many PEN faults then!

Isn't the wider principle that if you are significantly altering an installation, the right thing to do is to ensure the alteration complies with the regs today. BPG1 says this on page 5 before proceeding to give questionable advice.
At the end of the day you are putting your name to an EIC saying it complies with BS7671:2022 (unless you specifically list this as a departure of course, and then it's supposed to offer an equivalent level of protection, so I don't see how that stands up to scrutiny)

Personally, I think the "it will probably be ok" get-out-clause in BPG1 is daft and is exceptionally daft if it's PME, as the point is that with no warning a PEN fault could see the bonding carrying the entire return neutral current. There wouldn't be any signs of thermal damage if it hasn't happened before (as @Aaron b said)

Regarding "ripping a house apart", the regs do allow bonding to be "as close as practicable to the point of entry", and on a couple of occasions I've left original undersized bonding in place and added some new 10 sq mm to the nearest convenient point.

I agree somewhat that the advice that 'if a conductor hasn't suffered thermal damage then it's ok', doesn't make complete sense.
So I'm in a position where I don't quite understand the reasoning behind the guidance, but it is the quidance that I have been given by my governing body. It's not as if I just rang up some random guy and he gave me his personal opinion; he was clearly quoting the NICEIC's stance on the subject of undersized bonding cables and unequivocally advised me that the 6mm bonds need not be changed. I was given the impression that there was no reason to change the bonding cables at all if there was no sign of thermal damage. And it's not recorded as a departure as no alterations have been made yet. I have raised my eyebrows before during DIS assessments at comments made by assessors, but the technical helpline seems to be more from the horse's mouth and I'm not really in a position to doubt their advice. Certainly something worthy of further discussion though.

 

[westward10]

Well you had decided the NICEIC is the advise to follow before you came on here as it is the easy option and as long as you feel you are satisfying 132.16 then all well and good.

 

[MMecca]

Well you had decided the NICEIC is the advise to follow before you came on here as it is the easy option and as long as you feel you are satisfying 132.16 then all well and good.

Actually I hadn't. I've taken on board everyone's comments, thank you.