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.