Exporting earth to another dwelling

[Tomm]

Hi,

i Have been reading about exporting earths whilst surfing this forum on a quiet night shift. It seems to me that you can only export earth to an outbuilding if it doesn't have any extraneous conductors. So in theory I could supply a garage 20 m away in the garden and use the earth from the main dwelling for a two way board for sockets and lights only, but if I then decided to put in a water supply or run gas for a boiler I would then have to install an earth rod because I would then need to bond the incoming water and gas?
If I was to install an earth rod would you then get rid of the supply earth from the main dwelling and use the earth rod as your main earth? looking back on previous threads seems some people say you must use an earth rod, some say just use the armour of the swa and some say to run a 3 core swa as well as earthing the armour?

thanks

 

[telectrix]

it's only when it's a pme earthing system that you need to bond any extraneous metal services with a 10mm bondibg conductor from the house MET. the SWA armour may be OK as cpc, but not adequate for bonding.

 

[Leesparkykent]

Extending the Equipotential zone to an out building with extraneous conductive parts is perfectly acceptable.
when selecting a cable to install to the outbuilding you will just have to also consider if the CSA of the cable meets at least the minimum requirements by bs7671 for bonding.


When using the steel armour of the swa for bonding you have to remember the resistivity of copper at 20C is 17.2 x10-9. The resistivity of steel varies with carbon content from 100 to 1000 x 10-9. GN8 suggests using a factor of 8 for the ratio of the resistivity of copper to steel.


For example if I calculated that the minimum sized cable needed was say 4mm due to volt drop, load, installation method, etc but I knew the outbuilding had extraneous conductive parts and I've looked in the book and know the minimum sized main bonding required was 10mm then I would select a 10mm 3core swa instead and use one of the cores as a combined CPC and main bonding conductor.


If you used 2core SWA and wanted to use the armour as combined CPC and main bonding conductor then due to using it for main bonding you have to remember the x8 rule for conductivity so if the minimum sized bonding had to be 10mm then due to the x8 rule the CSA of the steel armour would have to be 80mm
without looking I think the CSA of 70mm 2core xlpe swa is 80mm (I would have to check).


Another solution would be to use 2core SWA and run in a separate main bonding conductor to the extraneous conductive parts from the MET.

 

[uksparks]

I was under the impression that you should really treat TNS as a PME as you cannot guarantee it will not get converted after you have been.

 

[Taylortwocities]

If your new incoming services are run in PVC then you don't have a problem!

Should you decide not to export the earth but to use an earth rod, then you'll need to install the garage wiring as a "TT".

 

[Tomm]

The reason I ask is because I have just moved into a new house with a decent sized garage to the side of the house. It is supplied via a 6mm 3 core swa cable from the main house. It is on a 40a mcb in the house with a 40a rcd and a smal 3 way cu in the garage. We want to put hot water and a couple of radiators into the garage so I have Ben recommended a oil boiler to be placed outside the building. Reading through the regs and osg it seems to me that I will have to bond all copper pipe work entering the garage from outside. Because the earth supply to the house is only 6mm2 it seems I will have to install a bigger earth conductor from the house or install a earth rod. Please can you advise the way to go. The house is tncs

thanks

 

[telectrix]

you could get round it by having any pipework entering the garage done in plastic.

 

[Leesparkykent]

The reason I ask is because I have just moved into a new house with a decent sized garage to the side of the house. It is supplied via a 6mm 3 core swa cable from the main house. It is on a 40a mcb in the house with a 40a rcd and a smal 3 way cu in the garage. We want to put hot water and a couple of radiators into the garage so I have Ben recommended a oil boiler to be placed outside the building. Reading through the regs and osg it seems to me that I will have to bond all copper pipe work entering the garage from outside. Because the earth supply to the house is only 6mm2 it seems I will have to install a bigger earth conductor from the house or install a earth rod. Please can you advise the way to go. The house is tncs

thanks

You could TT the garage end if you wanted to.

In comparison though would the cost of installing a bigger supply to the garage, a hot water cylinder with an immersion heater and a couple of panel heaters be cheaper than installing an oil boiler?

 

[Tomm]

You could TT the garage end if you wanted to.

In comparison though would the cost of installing a bigger supply to the garage, a hot water cylinder with an immersion heater and a couple of panel heaters be cheaper than installing an oil boiler?

if I was to run a 16mm cable then I would also be able to use the earth from the house rather than install a ttsystem would I not? Need to price up the different options, I want to install something that I can expand in the future as the longer term plan is to convert it into a small bungalow. Won't be for a few years yet tho.

 

[shanky887614]

if I was to run a 16mm cable then I would also be able to use the earth from the house rather than install a ttsystem would I not? Need to price up the different options, I want to install something that I can expand in the future as the longer term plan is to convert it into a small bungalow. Won't be for a few years yet tho.

16mm it is then.

a tt is not as good as supliers earth


rule of thumb 16mm earth is around £1.70-2 a metre

 

[davesparks]

if I was to run a 16mm cable then I would also be able to use the earth from the house rather than install a ttsystem would I not? Need to price up the different options, I want to install something that I can expand in the future as the longer term plan is to convert it into a small bungalow. Won't be for a few years yet tho.

Why 16mm?

 

[UKMeterman]

If you want 16mm, it isn't too expensive 3 Core 16.0mm SWA Cable (Blue, Brown, Green/Yellow)| MeteorElectrical.com

 

[shanky887614]

Why 16mm?

he is going to convert to a bungalow later

like meterman says use 3core swa but still earth the swa at least on one end

 

[davesparks]

he is going to convert to a bungalow later

like meterman says use 3core swa but still earth the swa at least on one end

Better off with a new supply from the DNO then!

But I still wonder why he thinks 16mm earth is required?

 

[Tomm]

Better off with a new supply from the DNO then!

But I still wonder why he thinks 16mm earth is required?

The bungalow will be for family it won't ever be sold separately from the house, So just as well be supplied from the house no additional meter or anything. Because I have 40m of 16mm 3 core in the store lol. Would you go 10mm or TT?

 

[davesparks]

The bungalow will be for family it won't ever be sold separately from the house, So just as well be supplied from the house no additional meter or anything. Because I have 40m of 16mm 3 core in the store lol. Would you go 10mm or TT?

Good luck convincing the planning committee!

I don't know what I would use, there is not enough information! What is the Ib of the circuit, length, Ze etc etc?