Exporting earths?

[ezekiel]

just a hypothetical question regarding previous post about electrics to garage and exporting earths -

if a rod needed to be installed for earthing arrangements would the cpc on the incoming supply still be connected into the db in garage?

 

[RF Lighting]

No.

If it was decided to TT the garage supply then the CPC would be to protect the supply cable only.

This earthing system must remain completely separated from the supply earthing system.

 

[ezekiel]

so how would they be seperated surely all the earthing would be linked from earth bar?

 

[RF Lighting]

You do not connect the earth of the supply cable to the MET in the garage.

 

[ezekiel]

ha now i feel stupid - thanks

 

[ceraunic_level]

Sorry to dig up an old topic, but this issue still causes me some confusion

So just to clarify.

TNCS is OK to supply an outbuilding as long as no earthy metal that need main protective bonding is in the out building

If earthy metal is there and is main bonded back to the house CU then TNCS still OK.

If main bonding back to House CU is not practical, then you

TT the outbuilding...

Isolate the supply earth

Supply an earthing conductor from the outbuilding CU to the earth rod

Main bond metalwork back to outbuilding CU.


If that is all correct, I am still a little lost on isolating the supply earth

If the supply cable is SWA, and this armour is earthed at the house CU,
is that not exporting the earth to the outbuilding CU via the armour?

I can understand not connecting the supply CPC to the second CU, but, if the second CU were metal this would have two earths via the connecting gland of the supply and the TT.

I guess you must get isolating glands?

How is it done?

Thanks

 

[JohnD]

and what if the garage had a water pipe going ito it?

very likely the pipe would be metal, and bonded at the house, as well as making good contact with the ground.

 

[plugwash]

TNCS is OK to supply an outbuilding as long as no earthy metal that need main protective bonding is in the out building

right

If earthy metal is there and is main bonded back to the house CU then TNCS still OK.

Right, you can also bond to the outbuilding CU but if you do then the earth from the main CU to the outbuilding CU has to be sized to PME bonding conductor requirements.

I can understand not connecting the supply CPC to the second CU, but, if the second CU were metal this would have two earths via the connecting gland of the supply and the TT.

If doing this you should use a plastic CU both to avoid the gland conducting to the enclosure and to avoid having non rcd protected conductors in a metal enclosure only protected by the TT earth. I would also suggest using some sealant to stick the gland cover in place ensuring that the gland does not accidently become an exposed conductive part connecting to the wrong earthing system.

 

[ceraunic_level]

Thank you plugwash most appreciated

'avoid having non rcd protected conductors in a metal enclosure only protected by the TT earth'

Yes, that makes sense

OK
JohnD I don't know?, but I guess you could test to see if the water pipe is continuous with the house earth, and if yes, The main bond back to main CU and forget about TT

or alter the pipe coming into the outbuilding, cut at floor level and replace a short length with plastic...

 

[Taylortwocities]

This article may help your understanding.
Note this was written during 16th edition times..

http://www.theiet.org/publishing/wiring-regulations/mag/2005/16-elect-inst-outdoors.cfm?type=pdf

 

[ceraunic_level]

Thanks Taylortwocities