Little confused about Earthing - Some advice Please

daver · Sep 24, 2011

Hi Guys

Go easy, I'm learning.

Went to look at a little job today. In brief TNS, split 25mm tails. First CU is next to service head with 16mm Earth. 2nd CU is about 5m away in new extension with 16mm Earth.
First CU is bonded to Gas with 6mm, and the 2nd CU is bonded to water with 10mm.

I was a little confused, as I understood that both CU's had to be bonded, or if i upgrade the 6mm to gas, does this cover regs.

Although the customer didnt have the certs, he had this signed off late last year.

All the guy wants is a small radial from the extension consumer unit, so I just wanted t clarify.

And is it correct, that I would only need to do Installation Cert for that additional circuit.

Thanks in advance

Dave

MarkieSparkie · Sep 24, 2011

Hi Dave, Apply the adiabatic equation to your 6mm[SUP]2[/SUP] Gas main protective bonding conductor, it may well comply. If it complies BS7671, all well and good there would be no need to upgrade it. If you use the adiabatic equation for compliance, make sure your calculation and statement of compliance are included in your EIC. If not replace your Gas main protective bonding conductor with 10mm[SUP]2[/SUP].
Yes, you only have to do an EIC with a full set of test results for the final circuit you install. However, it is considered good practice to identify the other final circuits of the installation on the Schedule of Test Results, but you are not required to include any test results for the other circuits. This approach would aid clarity in your case where the bonding is split between two DB earth-bars rather than from a single MET.

Deleted member 9648 · Sep 24, 2011

I may stand corrected but bonding conductors are sized by selection...not the adiabatic equation....(543.1.1).
As PME conditions do not apply main bonding conductors must be not less than half the cross sectional area required for the earthing conductor,and not less than 6.0mm. (544.1.1).
Main bonding should really go to the MET at the intake,however as bonding is present...(and within the minimum 6.0mm)...and as long as there are no signs of any problems,you have no worries if you are only carrying out an alteration or addition,just note on the cert any deviations from 7671,but that those deviations do not compromise the safety of the installation.

Chr!s · Sep 24, 2011

In TT and TNS there will be very little current flow within the bonding conductors, 6mm is is there pretty much for mechanical strength. As for sizing, Wirepuller's excellent post is spot on.

If the second CU is fed from the first via a 16mm cpc then that may well be suitable as a combined cpc/bonding conductor, this will allow extraneous conductive parts to be connected at that CU.

Its not a case as such that both CU's need bonding just that all extraneous conductive parts need to be connected to your met.

Keeping your bonding localised, keeps the touch voltages down. Though it would be no real issue if all bonding was carried out at the first CU.

Regards Chris

Engineer54 · Sep 24, 2011

As Chris has stated you only need one bonding cable from each of the main services (Gas/Water) to the installations MET or EMT, but NO need for both as they are already linked via the 16mm tail from the MET!! So if you have a 6mm and a 10mm bonding cable from the main water service point, you can safely remove the 6mm leaving the 10mm in place serving the installation. The second CU should be classed as a sub-main and it's earthing bar classed as the installations EMT!!

Unknown Parellel earth paths are difficult enough, without adding to them.