Another Earth Rod or Export from MET?

[walec]

Hi All,
Just installed an new CU in the garage, my problem is the earthing, I used 6mm SWA with a plan to bang an earth rod in. The garage is not attached to the house and has water and gas which are supplied from the house mains. The garage is only 2M away but as it is a seperate building I thought it had to be earthed by TT for equipotential bonding of the pipes. The problem I have is I am getting a reading of 313ohms from the rod (in wet ground). So I am wondering do I bang another rod in or as the gas and water are supplied from the house can I just export the earth to the CU and ignore the equipotential for the garage as it is bonded back to the MET in the house at its origin anyway?
Any help most welcome

 

[Amp David]

As you have services in the garage, pretty sure you're not supposed to export from the MET.

 

[flukey]

you could have brought a 10mm bond from house for the services

 

[walec]

Hello Flukey, I could still bring 10mm bond from the house without too much hassle but I thought that as the services were in the garage (even though they are supplied from the house) I could not export bonding, though I can not find anything in the regs on it...

 

[lamb]

The answer to this is explained in NICEIC snags and solutions part 1, Snag 47. Also, Reg 411.3.1.2 applies "... where an installation serves more than one building the requirements shall be applied to each building".

The first building contains the Earthing Conductor and MET, which is connected by suitably sized main bonding conductors to a BEMT (building earth marshalling terminal) in each additional building.

Common pipes (or other extraneous conductive parts) are then main bonded at each building they enter to the BEMT.

 

[diditrain]

What type of supply do you have in your house?

 

[walec]

diditrain, Supply to the house is PME (TN-C-S)

lamb, am I reading your reply right? you are saying that I can export to an earthing terminal in the garage and use that as the main earthing point for the garage? I have never heard of a BEMT. Sorry if I am a bit slow on the uptake.

 

[diditrain]

I believe that you need to enquire with your utility supplier to see if they are ok with you exporting

 

[lamb]

That would be the suggested solution from NICEIC Snags and Solutions, it even has a diagram of protective conductor paths and terminations.

However, many on the forum believe that a protective conductor connected to the earthing terminal of a TNCS(PME) supply must not be 'exported' outside the equipotential zone (or building).
Must admit I have never been able to find a Reg. pertaining to this from either BS7671, OSG, GN3, Part P or ESQCR.

Obviously certain special installations and locations are not permitted to form part of a TNCS(PME) system, in which case a TT system will be established, with earth electrode and RCD.

 

[Des 56]

Why dont you terminate the swa into a plastic enclosure for the garage
TT the garage
Install main bonding to the services in the garage at that pont of entry

The house is Tnc-s so main bond the services where they enter at the house as per regs
The garage sub main cpc connected to the house earthing system

The sevice pipes being common to the garage and the house doesn't make any difference
A serice neutral disconnection will be shared by umpteen parralell earth connections as well as the garage services of your TT system

The 313 ohms is above the minimum recomended but that reason is a possible unstable earth
If it is in wet ground and unlikely to dry out substantially, that would not be unstable and I would be happy to accept that reading.being way below the operating value of the 30 m/amp RCd

Just a bit more of my opinion to add
If you decide to export the Pme the services in the garage would require a 10mm bond back to the Met at the house as well as local to the garage, because the 6mm armouring wont be adequate

 

[flukey]

sounds like the run back to the MET is quite short so if its easy enough to run a 10mm bond back thats the way i would go mate

 

[walec]

Thanks Des,

I have terminated the SWA in a plastic enclosure and bonded the SWA to the house MET.
It is the 313ohms I was worried about but I take your point about below the operating value of
a 30ma rcd, which is my main switch in the garage, I had not thought of it that way, but I don`t know if the ground will stay wet, its between the garage and the house and there is a concrete path.
I would run a seperate 10mm bond back to the MET as well.
I want to make sure I get the safest most reliable option here and I am not sure what that is so I am grateful of all the help.

Thanks Flukey,
The cable run from MET to CU is about 15 meters.

 

[diditrain]

How many earth rods have you banged in so far? May be easier just to knock another one in

 

[walec]

Only one earth rod at the moment. I was not sure a second one would make that much difference.

 

[diditrain]

I done one recently and just by knocking another one half way in was able to get the value down by over 50 ohms.
In the on site guide it recommends that the earth fault loop impedance for TT installations does not exceed 200 ohms, as above this it is deemed unstable