gas bond from local distribution board

Guest112 · Nov 21, 2011

My engineer wants me to fit a gas bond from a local distribution board in an office but as we know it should be direct from the MET. What if the distribution board is moved or removed? Needless to say i wont be putting my name to it. This isnt right is it?? How can i make the best of a bad situation?

the reason is cost i believe, my view if thay arent willing to pay we shouldnt be doing it and shouldnt be doing it wrong either!

malcolmsanford · Nov 21, 2011

Providing the CPC to the board is big enough to also take any likely fault currents on the bonding scenario then you could create a Building Earth Marshaling Terminal (BEMT) similar to what you would do in individual flats.

If it is a metal clad building you could also try and use the steel frame work to provide a bonding conductor

Guest112 · Nov 21, 2011

ok so say the cpc to the board is big enough how would i calculate this, the system is a tt the line conductor is a 50mm I checked and the minimum is a 16mm so i need to check the cpc for the existing board I think the swa feeding the board is using the armourings as its earth i think the swa is a four core 35mm tp+n

so using a sub main cpc is acceptable considering its size for a supplementary bond then? ive always been under the impression that it had to be on its own but im guessing thats the best practice and the other mentioned method is acceptable though?

Rockingit · Nov 21, 2011

malcolmsanford said:
you could create a Building Earth Marshaling Terminal (BEMT) similar to what you would do in individual flats. /QUOTE]

Do these exist under 17th? I've just *scanned* 411 and 54 and didn't immediately see anything.

seb77 · Nov 21, 2011

as far as i am aware there is no problem running the gas bond from the earth bar of the consumer unit????.

seb77 · Nov 21, 2011

jusr read your thread again..you mean sub board!!!. ignore my last post mate.

malcolmsanford · Nov 22, 2011

Subcategory - 1. BS7671 - Voltimum UK - Electrical Installation Products and Contracting

This quite a good if not a lengthy article on the subject and explains much better than I could.

BEMTs are normally used in large multi building complexes where you have the MET at the origin of the installation and then sub mains going out to various buildings. The services in those buildings by the regulations should all go back to the MET, and there is only one MET as the regs tell us, and so to achieve that they would do via a combined CPC/Bonding cable.

It's obviously a design concept, I believe that it is in GN 8, but I don't have the book with me, also there is an example of it in the NICEIC snags and solutions for Earthing and bonding.

You would have to carry out some calculations using the adiabatic equation on the CPC to determine it's CSA and then from there the CSA of the bonding conductor. As it is a TT then you have a good chance that you could do this. If the SWA has a dedicated core for the CPC, then you could combine both this and the armour to achieve your goal. Obviously if your relying on the SWA Armour alone for a CPC then you may not be able to do it.

Engineer54 · Nov 22, 2011

You can use either the earthing bar in the CU/DB, or the standard external earthing block as your installations MET!! Same thing with EMTs on on multi dwelling buildings or other installations, with several sub mains feeding other small building installations....

So long as the MET or EMT bar/block is of a sufficient size and CSA to take the largest Earthing cable and bonding cables etc....

IQ Electrical · Nov 22, 2011

peterdaniels said:
ok so say the cpc to the board is big enough how would i calculate this, the system is a tt the line conductor is a 50mm I checked and the minimum is a 16mm so i need to check the cpc for the existing board I think the swa feeding the board is using the armourings as its earth i think the swa is a four core 35mm tp+n

so using a sub main cpc is acceptable considering its size for a supplementary bond then? ive always been under the impression that it had to be on its own but im guessing thats the best practice and the other mentioned method is acceptable though?

This is for a main protective bond, not supplementary bonding.