Spur to outbuilding TN-S

[D Skelton]

you seem to have the belief that you cannot use a tnc-s earth connection beyond the building where the supply is located , yes ?

Erm, sort of... My belief has always been that you cannot extend a circuit supplied by a tnc-s system beyond the equipotential zone. Where you are extending outside the equipotential zone I was taught that you should then be creating a TT system.

If the place where you are extending to has services that form part of the equipotential zone then by definition you are not extending outside of the equipoential zone thus making it ok to use the TNC-S.

In the scenarios shown in both Fig 1 and Fig 2 the detached garage has no services therefore making it outside of the equipotential zone and yet the PME has been extended to it??

Am I being thick?

 

[Guest55]

if there are no extraneous pipes at the outbuilding , then there can be no difference in poten......
oh i give up lol.

 

[D Skelton]

if there are no extraneous pipes at the outbuilding , then there can be no difference in poten......
oh i give up lol.

View attachment 14506

Yeah I get that lol, ok, I guess where I'm going wrong is that my black and white views on the subject are flawed.

Seems I may have bashed in a couple of unnecessary earth rods, oh well

 

[D Skelton]

To add to this, after a bit of scrolling through various other forums and online debates about exporting the earth on a TN-C-S system, it seems I'm not the only one who is a little confused on exactly what is and what isn't allowed.

It appears that most of the time, like someone else on here mentioned, it comes down to the DNO and whether they will allow it or not.

For simplicities sake however, I don't think I have done anything wrong (according to the regs) by adopting a blanket view that when extending a PME system outside of an equipotential zone it should be separated and rodded.

if there are no extraneous pipes at the outbuilding , then there can be no difference in poten......
oh i give up lol.

After some reflection an answer to this is that there could be a difference in potential between the earth on which the main dwelling is sited and the earth on which a detached outbuilding is sited. I could be wrong but surely a distance between two seperate buildings would mean in some situations there could theoretically exist a difference in potential?

 

[Deleted member 9648]

Is this a wind up?....I thought this had been put to bed bleeding years ago.....:censored:

 

[D Skelton]

Is this a wind up?....I thought this had been put to bed bleeding years ago.....:censored:

No, not a wind up mate, I'm genuinly interested in the topic and after scrolling through many other threads I still haven't come across a definitive answer. If you can point me to the right thread, or offer an answer yourself, that would be much appreciated.

Like I said earlier, my views until now have been very much black and white as I hadn't researched the subject at any great length, I only relied on what I was taught at college and as an apprentice.

 

[Guest55]

you still here lol ?
ok , ok , i'll give you my ultra simple view on this ;
if there is exposed / extraneous metalwork at the outbuilding , then tt the supply and fit earth rods at the shed.
if no metalwork , then extend pme / use suppliers earth.
are we there yet ?
;-)

 

[D Skelton]

are we there yet ?
;-)

Sorta' haha

 

[Deleted member 9648]

you still here lol ?
ok , ok , i'll give you my ultra simple view on this ;
if there is exposed / extraneous metalwork at the outbuilding , then tt the supply and fit earth rods at the shed.
if no metalwork , then extend pme / use suppliers earth.
are we there yet ?
;-)

Partly true....if there are ECP's in the outbuilding you can still use the TNCS earth, but as you are extending the zone main bonding will need to go back to the MET and meet the requirements for PME supplies (ie...min 10.0mm).....so if your supply cable to the outbuilding is <10mm you will need to run a separate main bond back to the MET.

No, not a wind up mate, I'm genuinly interested in the topic and after scrolling through many other threads I still haven't come across a definitive answer. If you can point me to the right thread, or offer an answer yourself, that would be much appreciated.

Like I said earlier, my views until now have been very much black and white as I hadn't researched the subject at any great length, I only relied on what I was taught at college and as an apprentice.

The definative answer is in bs7671 which we work to....some special locations as part of section 7 prohibit the use of a TNCS earth.....there is no mention of general outbuildings anywhere in 7671,therefore there are no special requirements and they are not special locations......definitive enough for me.

 

[D Skelton]

Thank you, that's the definitive answer I was after lol. I was beginning to question my own knowledge of equipotential zones!?! Like I said in one of my earlier posts, I haven't got my BGB to hand as I'm sort of on holiday, if I had, I probably wouldn't have started the thread.

I trust you are correct wirepuller, but it does go to show that you can never rely fuilly on something that is taught whereas you can always rely on the BGB!