A BIT OF A PUZZLER !!

Now we all know that we cannot transport the earth to a detached garage with TNCS but what do you do in this case.

Main head in house with what looks like TNS head with armoured sheath but with a earth linked as well to neutral, disconnected and checked the Ze on armour of incomer with the neautral disconnected from it, Reading 0.15 ohms, so we established it was probably a TNS so will phone DNO to find out if it is either TNS or TNCS.

Next problem is if it is TNS then no problem with the exporting of the earth but if it is TNCS what do you do as the detached garage houses the gas meter and has 2 gas pipes to the house, both are bonded on point of entry to the house back to the MET the small CCU in the garage has a 10mm bond to the gas as well, so what happens in this situation with TNCS, normally I would break the earth on the supply to the garage and rod the garage on it's own but what happens in this situation.

Many thanks all, Mike.

In this situation for TNCS you will need to have a bonding conductor between the two buildings as they can not be segregated. This can be the CPC betweeen the two IF it is big enough.

You would need at least a 2 x 6mm PVC/SWA/PVC cable to fulfil the 10mm PME extraneous (equipotential) bond requirement. But i would use a 3 core SWA, and use both the SWA and the 3rd core as my CPC and earthing...

Mike, your not exporting the earthing arrangement, your extending it. So No, everyone doesn't know that you can't extend a PME earthing arrangement, ...because you most certainly Can!!!

Many thanks for your input when you have not come across a situation I always think it best for a second opinion with someone that has, I am only renewing the consumer unit so thought I would check, the SWA is only 2.5mm 3 core and I thought thatI would have to bond the exterior gas meter by running a 10mm from the main header due to the situation.

Cheers all, Mike.

Hi Larnacaman

I am a bit slow today, can you explain please how you come to specifically 2 x 6mm PVC/SWA/PVC cable to fulfil the 10mm PME extraneous (equipotential) bond requirement.

Cheers

A 10mm SWA cable has a wire requirement of 22.6mm CSA to meet CPC requirements, it actually has 41mm. The next size down, 6mm 2 core has a CSA of 24mm, The 6mm 3 core SWA has a CSA of 44mm.

So if a 10mm SWA cable can use it's armour as both a CPC and as the bonding conductor, then the 6mm 2 core , SWA CSA will satisfy the 10mm bonding requirement. The 3 core would be even better if using the 3rd core and the SWA.... That is Unless i'm missing something here?? ...lol!!!

Im afraid neither the 10mm or 6mm swa armor will meet the requirements for PME main bonding

remember that SteelWA has a higher resistance than copper,

plus, Im not sure so dont take it as fact, but are you even allowed steel as a PME earth?

I was under the impression it either had to be copper or aluminium,

could be wrong though, I often am. :C

its a rather obscure phrase for Protective Multiple Earthing which the DNO in the UK INSIST on their TNCS supply being, even though its NOT.

According to my book to use the armouring for 10mm pme bond the cable would need to be 95mm for two core and 70mm for three core if you have four core it would only need to be 50mm but it is a 16th edition book but sure things have not changed that much.

According to my book to use the armouring for 10mm pme bond the cable would need to be 95mm for two core and 70mm for three core if you have four core it would only need to be 50mm but it is a 16th edition book but sure things have not changed that much.

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but are you actually allowed to use steel for a main bond in PME?

you know, on thinking about it a bit more, I can see no reason not to, apart from the obvious one you have noted, and I pointed out, higher resistance,

I'd never worked it out before, but it is a lot,LOT, larger than I think most people assume.

but are you actually allowed to use steel for a main bond in PME?you know, on thinking about it a bit more, I can see no reason not to, apart from the obvious one you have noted, and I pointed out, higher resistance,

I'd never worked it out before, but it is a lot,LOT, larger than I think most people assume.

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Not very cost effective if you need to use 95mm two core to get the 10mm bond. Other option would be 10mm three core and use third core for earth and bond or run a seperate 16mm earth. What ever you do its not going to be a cheap job.

Just read the OPs original Thread post again, and it seems he already has a 10mm bond to the gas at the garage. So there's no need to bring another one in.... He can just use the SWA of his cable as he's CPC!!

As i said before if i'm missing something here and i'm wrong, about the CSA areas of SWA then please explain.... Personally, depending on the distance involved, i'd always provide a separate bonding conductor in these situations

right,

if you realise the inherent risks in this scenario, are know how to deal with them,

then IMHO you need to bond your garage CU back to MET with a suitably sized cable, ie, same CSA as your earthing conductor, I would assume 16mm?

this is only a preconception on my point and as I have not seen or inspected your install then this advice can only be taken with appropriate knowledge of the issues involved.

Just read the OPs original Thread post again, and it seems he already has a 10mm bond to the gas at the garage. So there's no need to bring another one in.... He can just use the SWA of his cable as he's CPC!!As i said before if i'm missing something here and i'm wrong, about the CSA areas of SWA then please explain.... Personally, depending on the distance involved, i'd always provide a separate bonding conductor in these situations

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For a bonding conductor you use the copper equivalence, now steel is around 8 to 1, so PME 10mm copper would require a copper equivalent of 80mm steel, neither 6mm or 10mm csa armor meets this requirement.

There is an issue with using the armor in PME for bonding, this is due to the possible current which may flow, and the effect it will have on the temperature of the live conductors contained within.

Steptoe, its perfectly acceptable to use steel as a bonding conductor.

Thanks plumber, just not economical though due to sizing!

And the heating issue you have pointed out, well noted.

Thanks plumber, just not economical though due to sizing! And the heating issue you have pointed out, well noted.

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Sometimes the steel may already be in place(structural), you may utilize this for bonding to save costs.

The heating issue can be a problem, i suppose it far easier just to use an extra core.

Right the setup now is Rcbo then 2.5mm 3 core armoured to garage 2 way consumer unit with 16amp & 6amp Bs 60898 Mcb's the garage houses the gas meter with 1 x 1"iron gas pipe to house and a 28mm copper pipe to the house which is bonded to the garage consumer unit, a 10mm from the house consumer met bonded to the gas pipe just before it leaves the house then goes out into the garage to the main met of the small garage consumer unit, the copper pipe is all soldered connection.

Regards all, Mike.

So just to get things clear does the 10mm Bond from the House CU go all the way to the Garage MET connecting to Gas pipe at both ends?

Yes but bonds to the gas as it goes out the house to the garage then to the garage CU Met.