Extending TT to remote outbuilding ??

Bennewman · Aug 27, 2013

Hi all.
As I understand it, exported earth is only an issue for PME situations? Therefore, when a domestic installation is a TT and power is to be brought out to a remote outbuilding, is there a requirement to fit a second earth electrode and if so, should this be independent of the house TT setup? I've added a crude picture of the installation scenario wiring. As you can see, if a second earth electrode is installed, I've questioned by use of the red zigzag whether there should be continuity in the earth from the house to the cabin CU, or it should be terminated without connection?

Many thanks.

[ElectriciansForums.net] Extending TT to remote outbuilding ??

Engineer54 · Aug 29, 2013

Bennewman said:
So.......a second earth stake has to be fitted. BUT, should this be independent of the original??

A second earth rod position at the new building is the correct way to go, linking the two rod positions either with a 3rd core or the armouring of the supply SWA cable!! It is totally pointless to have 2 independant TT systems on a single installation!! Can't make it any clearer than that for you.

davethsparky · Aug 29, 2013

Engineer54 said:
Why on earth (excuse the pun) would you not link the two earth rod positions via the 3rd core between the MET and EMT??

That was in response to the earlier post which I had quoted which was concerned with the creation of a new tt earth instead of extending what I assume was a pme earth. Not in response to the main topic here.

I thought that was clear from the post I had quoted?

Engineer54 · Aug 29, 2013

davethsparky said:
That was in response to the earlier post which I had quoted which was concerned with the creation of a new tt earth instead of extending what I assume was a pme earth. Not in response to the main topic here.

I thought that was clear from the post I had quoted?

Ooops my post was meant as a reply to this post!! My apologies!! lol
Even then i'm not sure now, if he is talking about PME to TT?? Have to have another read through when i get home tonight!! lol!!

!!

Originally Posted by Squid_uk I recently have done a similar installation to an out building. I installed a new submain to the building with a new DB and earthing stake. I used a 16mm 3 core SWA but did not connect the 3 core (used as earth) to the house earth, therefore keeping it seperate but still protecting the submain via a 3rd core and the steel armour. Also my Ra reading was 60ohms after installing 2 stakes coupled, so as stated in earlier posts, depth is good!

davethsparky · Aug 29, 2013

No worries.

I assumed pme as trying to keep the new earth seperate would not be necessary otherwise. And to the best of my knowledge TNS wouldn't normally require a seperate earth system.

If I have read it right he has run a 3c swa to the outbuilding. Connected the 3rd core of the swa to the new TT earth at the outbuilding and not the earth at the supply end. Which in my opinion doesn't achieve anything.
Hopefully the armour is connected to the supply earth and isolated at the outbuilding end, otherwise the whole game is pointless. And if the armour is solely connected to the new TT earth then the Zs situation for the supply could be a bit squiffy!

Can't see the point of it all myself, a 16mm core is more than enough for a main bond for a normal house supply anyway, not to mention the armour connected in parallel!

Deleted member 9648 · Aug 29, 2013

Engineer54 said:
Tut Tut!! Still trying to convince begineers to be lazy so and so's then!!

No...asking why you are saying in post 15 of this thread that relying on RCD's is not acceptable and yet on another thread falling over yourself to praise an Ra value of 15 ohms. You want it both ways,well you are not having it as long as I'm a member of this forum........seems to me your stance on RCD reliance is somewhat 'relaxed' where the Ra value relying on an RCD is achieved by one of the 'holier than thou clique' you are founder member of.

Squid_uk · Aug 29, 2013

I was given this by one of my tutors at college, he pulled it from the NICEIC magazine.

Engineer54 · Aug 29, 2013

wirepuller said:
No...asking why you are saying in post 15 of this thread that relying on RCD's is not acceptable and yet on another thread falling over yourself to praise an Ra value of 15 ohms. You want it both ways,well you are not having it as long as I'm a member of this forum........seems to me your stance on RCD reliance is somewhat 'relaxed' where the Ra value relying on an RCD is achieved by one of the 'holier than thou clique' you are founder member of.

Nah, as usual you're twisting things to suit your own stance on not bothering to achieve any form of a stable working TT system!! My stance is and always has been that RCD devices (of ALL types) are not as reliable as the manufacturers would like you to believe. Unless you are able to achieve near or TN values, then an added upfront 100/300mA S type is the prudent way to go, to back up that single RCD's device that's looking after several circuits each side of the dual CU's. Nothing ''holier than thou'' about anything to do with providing a working TT system, just those that are too bloody lazy to try, ...and those that are willing to invest in making the effort!! Ridiculing those that are willing to make the effort seems to be your main objection for some god only known reason??...

Paultrim123321 · Aug 29, 2013

Sorry, not having dealt much with the extending of PME, but my take is its permitted as long as the outbuilding your feeding has no extraneous conductive parts, for example building erected with steel framework sunk in ground,incoming metallic pipework e.g. Gas, Water. Etc
If the install does contain any of the above, then you need to isolate secondary earthing system from supply earthing system(pme) and Rod? Again pls don't shoot me down as I don't deal with extending PME systems.

MDJ · Aug 29, 2013

E54 knows his stuff and there's no more to say, however there would be problems on a caravan site haha, just read section 7 about PME regarding that type of site LOL

telectrix · Aug 29, 2013

Paultrim123321 said:
Sorry, not having dealt much with the extending of PME, but my take is its permitted as long as the outbuilding your feeding has no extraneous conductive parts, for example building erected with steel framework sunk in ground,incoming metallic pipework e.g. Gas, Water. Etc
If the install does contain any of the above, then you need to isolate secondary earthing system from supply earthing system(pme) and Rod? Again pls don't shoot me down as I don't deal with extending PME systems.

NO. if there's extraneous you need to ensure that the cpc of the feed cable is adequate to act as a cpc and a bonding conductor for a PME system, or run in a separate bonding conductor. ( with domestics, this is generally 10mm ).