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?

 

[JBW175]

You can’t use the adiabatic equation for sizing bonding conductors.

Table 54.8 on page 205 of the Big Brown Book is your friend here.

You need to run in 10mm bonds.

 

[westward10]

The neutral is the conductor in the 'concentric' service cable, your 'earthing conductor' should not be less than half the size of this. Main protective bonds not less than half the size of your earthing conductor subject to a minimum csa of 10.0. Yes you can use the adiabatic equation for the earthing conductor but not the main protective bonds, the earthing conductor is subjected to a minimum csa of 10.0 if it supports the bonds.

 

[Aaron b]

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?

I think you are remembering that you can calculate the protective conductor using the adiabatic equation and if it wasn't PME you could use a protective bonding conductor not less than half the CSA of that. Unfortunately that doesn't help you here.

Can you fit an insulating section (PEX pipe) to the water?

 

[MMecca]

I think you are remembering that you can calculate the protective conductor using the adiabatic equation and if it wasn't PME you could use a protective bonding conductor not less than half the CSA of that. Unfortunately that doesn't help you here.

Can you fit an insulating section (PEX pipe) to the water?

I suppose so. And the gas!

 

[Aaron b]

I suppose so. And the gas!

I'm not sure of a solution for the gas... That's usually outside though so I thought it might have been easier.

I'm not sure if extraneous-conductive-parts need to be simultaneously accessable to be an issue but I don't like that idea with gas or water anyway.

 

[MMecca]

Thanks for all your replies so far. I did use the adiabatic equation to calculate the earthing conductor size, and I agree that this calculation is somewhat irrelevant when you consider the BS7671 regulations on PME main and bonding cable sizes. (132.16 etc.) Which give a value of 10mm csa for both.
Now this particular installation is a new consumer unit only. The property was wired according to the 14th edition and the requirements then were for the bonding cables to be at 6mm csa according to table D2M. According to the NICEIC, there is an allowance, when working on existing installations wired to a previous edition of the regs, for the bonding cables to be left as they are and not increased to 10mm. This is provided that a) continuity is verified and at an acceptably low value (in this case both are <0.05 Ohms) and b) there is no sign of thermal damage to the cable. There is no sign of overheating of the bonding cables in this instance.
So basically the existing 6mm bonds can be kept in place, with a note on the EIC to confirm that they are only 6mm but have not been subject to thermal damage. This avoids a large amount of disruption running new 10mm cables which aren't in fact needed.

 

[JBW175]

Thanks for all your replies so far. I did use the adiabatic equation to calculate the earthing conductor size, and I agree that this calculation is somewhat irrelevant when you consider the BS7671 regulations on PME main and bonding cable sizes. (132.16 etc.) Which give a value of 10mm csa for both.
Now this particular installation is a new consumer unit only. The property was wired according to the 14th edition and the requirements then were for the bonding cables to be at 6mm csa according to table D2M. According to the NICEIC, there is an allowance, when working on existing installations wired to a previous edition of the regs, for the bonding cables to be left as they are and not increased to 10mm. This is provided that a) continuity is verified and at an acceptably low value (in this case both are <0.05 Ohms) and b) there is no sign of thermal damage to the cable. There is no sign of overheating of the bonding cables in this instance.
So basically the existing 6mm bonds can be kept in place, with a note on the EIC to confirm that they are only 6mm but have not been subject to thermal damage. This avoids a large amount of disruption running new 10mm cables which aren't in fact needed.

Have the NIC technical desk actually told you that? Or have you heard it from a 3rd party?

Because as far as I’m aware, you need to upgrade the bonds to the current regs before carrying out any additions or alterations, CU upgrade included.

Now don’t get me wrong, and I’m happy to be corrected, but I can’t for the life of me imagine taking my assessor to look at a job where the earthing & bonding has not been upgraded, and not getting a major non compliance.

As a disclaimer, I’m with NAPIT not the NIC.

 

[MMecca]

Have the NIC technical desk actually told you that? Or have you heard it from a 3rd party?

Because as far as I’m aware, you need to upgrade the bonds to the current regs before carrying out any additions or alterations, CU upgrade included.

Now don’t get me wrong, and I’m happy to be corrected, but I can’t for the life of me imagine taking my assessor to look at a job where the earthing & bonding has not been upgraded, and not getting a major non compliance.

As a disclaimer, I’m with NAPIT not the NIC.

Yes, I have confirmed this with the NICEIC technical help desk today.

 

[westward10]

Thanks for all your replies so far. I did use the adiabatic equation to calculate the earthing conductor size, and I agree that this calculation is somewhat irrelevant when you consider the BS7671 regulations on PME main and bonding cable sizes. (132.16 etc.) Which give a value of 10mm csa for both.
Now this particular installation is a new consumer unit only. The property was wired according to the 14th edition and the requirements then were for the bonding cables to be at 6mm csa according to table D2M. According to the NICEIC, there is an allowance, when working on existing installations wired to a previous edition of the regs, for the bonding cables to be left as they are and not increased to 10mm. This is provided that a) continuity is verified and at an acceptably low value (in this case both are <0.05 Ohms) and b) there is no sign of thermal damage to the cable. There is no sign of overheating of the bonding cables in this instance.
So basically the existing 6mm bonds can be kept in place, with a note on the EIC to confirm that they are only 6mm but have not been subject to thermal damage. This avoids a large amount of disruption running new 10mm cables which aren't in fact needed.

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

 

[Pretty Mouth]

Thanks for all your replies so far. I did use the adiabatic equation to calculate the earthing conductor size, and I agree that this calculation is somewhat irrelevant when you consider the BS7671 regulations on PME main and bonding cable sizes. (132.16 etc.) Which give a value of 10mm csa for both.
Now this particular installation is a new consumer unit only. The property was wired according to the 14th edition and the requirements then were for the bonding cables to be at 6mm csa according to table D2M. According to the NICEIC, there is an allowance, when working on existing installations wired to a previous edition of the regs, for the bonding cables to be left as they are and not increased to 10mm. This is provided that a) continuity is verified and at an acceptably low value (in this case both are <0.05 Ohms) and b) there is no sign of thermal damage to the cable. There is no sign of overheating of the bonding cables in this instance.
So basically the existing 6mm bonds can be kept in place, with a note on the EIC to confirm that they are only 6mm but have not been subject to thermal damage. This avoids a large amount of disruption running new 10mm cables which aren't in fact needed.

This tallies with guidance in best practice guides 1 and 4. If the main bonding conductors are at least 6mm, and no thermal damage present, then it's not considered potentially dangerous.

However, if they are less than 6mm, then it's considered a C2, potentially dangerous. This is a problem, as in your OP you said the bonding conductors are only 4mm.

 

[westward10]

Have the NIC technical desk actually told you that? Or have you heard it from a 3rd party?

Because as far as I’m aware, you need to upgrade the bonds to the current regs before carrying out any additions or alterations, CU upgrade included.

Now don’t get me wrong, and I’m happy to be corrected, but I can’t for the life of me imagine taking my assessor to look at a job where the earthing & bonding has not been upgraded, and not getting a major non compliance.

As a disclaimer, I’m with NAPIT not the NIC.

I agree for TN-S or TT it may be acceptable but not TN-C-S. BS7671 does not give specific guidance on this except a general note in 132.16 and as such my opinion is 4.0 would not be adequate when consulting this Regulation and has never been so.
And whilst schemes give opinions I am not one to have any faith in them.

 

[MMecca]

This tallies with guidance in best practice guides 1 and 4. If the main bonding conductors are at least 6mm, and no thermal damage present, then it's not considered potentially dangerous.

However, if they are less than 6mm, then it's considered a C2, potentially dangerous. This is a problem, as in your OP you said the bonding conductors are only 4mm.

You're right, I did. On closer inspection they are both 6mm.

 

[MMecca]

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

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

 

[MMecca]

I agree for TN-S or TT it may be acceptable but not TN-C-S. BS7671 does not give specific guidance on this except a general note in 132.16 and as such my opinion is 4.0 would not be adequate when consulting this Regulation and has never been so.
And whilst schemes give opinions I am not one to have any faith in them.

Apologies, both bonding cables are 6mm, not 4mm as I first stated.

 

[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.