Out building earth

[Robo]

Sorry to bring this up again, i just got very confused with the mass input i had on my last post.

So

You can export the tnc-s earth to an outbuilding. Any gas water or metal work needs to be earthed back to the outbuilding db. However its best to get permission from building control.

I could rod the installation but if i did i would have to keep the tncs earth seperate ( use insualted board. And again all metal worked bonded to outbuilding db so protected from rod.

I think this is correct. Please clarify.

If i came across and tns system is it fine to export that earth.

Thanks

 

[ezzzekiel]

http://www.electriciansforums.net/e...159-good-guide-exported-pme-outbuildings.html


THREAD CLOSED

 

[Robo]

Y has my thread been closed, im sorry it causes disruption but i read that many different things im unsure whats right. Iv read the outbuilding guide.

If you have tnc-s supply to outbuilding its ok. If there are extraneous parts a seperate 10mm bond must go back to main dwelling consumer or MET.

Or if there are extraneous parts the tncs must not be exported and a rod installed and bond all metal work back to outbuilding db.


And i will assume its ok to export tns as the conductors and earth are sepearte. Thanks

 

[Spudnik]

Robo,

Your original thread was closed because it was getting way out of hand.

This one may also be closed if it goes a similar way, and i will also start issuing temporary or even permanent bans to anyone who feels they deserve one.

I am getting very bored with having to repeat the fact that everyone has and is entitled to an opinion, however there is no need to make it "personal" if someone doesnt agree.

i/we have made this forum what it is, and i will NOT see it pulled down again.

If anyone doesnt understand, please feel free to pm myself or any mod who will be happy to explain in more detail.

 

[rocker]

Right I don't claim to be an expert on this and normally have to refer to a guide any time I work with outbuildings, as it's not something I do all the time, but I think the gist is this:

NORMALLY, you can export earth on a TN-C-S system to an outbuilding. If you do this you can earth the armour on the cable from either or both ends.

IF there is any pipework or anything requiring bonding in the outbuilding, for simplicity you're probably better off rodding the outbuilding and making it a TT. In this case, the earth from the TN-C-S side and the earth from the TT side MUST NOT BE CONNECTED. So you'd only earth the armour from one side.

Do not EVER export earth and then bond to rod.

I think the confusion has arisen because you should not export earth when the earth and neutral are the same cable...... this is the case on TN-C-S cable that comes into the property, but as soon as it gets to the service head THESE ARE SEPERATED, so after this point, you may export earth.

If you're worried about whether your particular DNO allow export or not, your best bet is to give them a ring. They will be able to give you a straight answer straight away.

As I say it's not something I have a great deal of experience in, and obviously this is very basic and you should read the guide. I am more than willing to be corrected on this.

 

[antimatter]

Y has my thread been closed, im sorry it causes disruption but i read that many different things im unsure whats right. Iv read the outbuilding guide.

If you have tnc-s supply to outbuilding its ok. If there are extraneous parts a seperate 10mm bond must go back to main dwelling consumer or MET.

Or if there are extraneous parts the tncs must not be exported and a rod installed and bond all metal work back to outbuilding db.


And i will assume its ok to export tns as the conductors and earth are sepearte. Thanks

I'm in agreement with you on this Rob

 

[antimatter]

Right I don't claim to be an expert on this and normally have to refer to a guide any time I work with outbuildings, as it's not something I do all the time, but I think the gist is this:

NORMALLY, you can export earth on a TN-C-S system to an outbuilding. If you do this you can earth the armour on the cable from either or both ends.

IF there is any pipework or anything requiring bonding in the outbuilding, for simplicity you're probably better off rodding the outbuilding and making it a TT. In this case, the earth from the TN-C-S side and the earth from the TT side MUST NOT BE CONNECTED. So you'd only earth the armour from one side.

Do not EVER export earth and then bond to rod.

I think the confusion has arisen because you should not export earth when the earth and neutral are the same cable...... this is the case on TN-C-S cable that comes into the property, but as soon as it gets to the service head THESE ARE SEPERATED, so after this point, you may export earth.

If you're worried about whether your particular DNO allow export or not, your best bet is to give them a ring. They will be able to give you a straight answer straight away.

As I say it's not something I have a great deal of experience in, and obviously this is very basic and you should read the guide. I am more than willing to be corrected on this.

Thats what ive been saying all along.

I think the confused came because a lot of the time an outbuilding may not have extraneous metalwork, or there is a tn-s supply present, as in older buildings, but i believe this is, or so im told a 2391 question, so it is important to clear it up i suppose.

 

[rocker]

It doesn't matter who's said what. If that's a pretty good basic guide for installation then brill. If you are PIRing an outbuilding with an exported earth, you will need to have a lot more information and experience as this ain't the be-all-and-end-all.

 

[antimatter]

I do agree its not an everyday occurrence, but i suppose we all have the potential to be called to a job like this any day, so clarity on this subject is of the utmost importance.

 

[Robo]

Robo,

Your original thread was closed because it was getting way out of hand.

This one may also be closed if it goes a similar way, and i will also start issuing temporary or even permanent bans to anyone who feels they deserve one.

I am getting very bored with having to repeat the fact that everyone has and is entitled to an opinion, however there is no need to make it "personal" if someone doesnt agree.

i/we have made this forum what it is, and i will NOT see it pulled down again.

If anyone doesnt understand, please feel free to pm myself or any mod who will be happy to explain in more detail.

I appreciate what ur saying Jason, ppl oobviously had very different views. Rocker has Sorted it for me. Much appreciated. Cheers

 

[ezzzekiel]

whats an outbuilding?

 

[rocker]

I think it's a type of avocado?

 

[Deleted member 26818]

Sorry to bring this up again, i just got very confused with the mass input i had on my last post.

So

You can export the tnc-s earth to an outbuilding. Any gas water or metal work needs to be earthed back to the outbuilding db. However its best to get permission from building control.

I could rod the installation but if i did i would have to keep the tncs earth seperate ( use insualted board. And again all metal worked bonded to outbuilding db so protected from rod.

I think this is correct. Please clarify.

If i came across and tns system is it fine to export that earth.

Thanks

The gas and the water has to be bonded back to the house. This can be achieved by bonding back to the outbuilding DB, and then taking an appropriately sized bonding conductor back to the house DB or MET. Remember as the supply is TN-C-S and PME conditions apply, and Table 54.8 should be used.
The bonding conductor can be either separate from the SWA, or one of the conductors of the SWA, obviously as Table 54.8 indicates a minimum CSA of 10mm², then the SWA will have to be 10mm² if one of the conductors is used for bonding. The armour will not have a copper equivalance large enough to meet the requirements of Table 54.8.
The people you have to get permission from are the DNO.
As for installing an earth electrode at the outbuilding along with exporting the TN-C-S earth, there is nothing in either BS7671 or ESQCR prohibiting such.
However, individual DNOs have their own requirements, and often the requirement is that the earth electrode must have a resistance below 20Ω.

 

[malcolmsanford]

Spinlondon are you refering to the DNO 21ohm value at the transformer or the earth electrode at the installation.

 

[rocker]

Why would anyone export earth to an outbuilding, and then bang a rod in for bonding? It just seems to defy logic. It's going to be a minefield of different earth potentials for a start, but also if you are putting in a stake why waste all the cable connecting to the TN-C-S's MET? Just run a little bit of 16mm to the spike. It couldn't be simpler!

Right I'm not wasting my Friday night on this, I'm off out for a beer or 8

 

[Deleted member 26818]

The earth electrode at the installation.
This was the guidance we were given many years ago for installing street furiture. At the time we worked in two DNO areas, the old LEB and Eastern.
The LEB had the 20Ω requirement whereas Eastern just required an earth electrode.
Probably more applicable where the outbuilding is metal clad, such as a shipping container.

rocker I have no Idea why someone would want to do such, I am only responding to a previous post, where it was advised that such should never be done.

 

[Dave_]

As for installing an earth electrode at the outbuilding along with exporting the TN-C-S earth, there is nothing in either BS7671 or ESQCR prohibiting such.

Yes there is.

An exposed conductive part connected to one means of earthing must not be simultaneously accessible with an exposed conductive part connected to another means of earthing

Regulation 413-02-03 refers

 

[Deleted member 26818]

Sorry Dave, but making up Regulations is childish.

 

[malcolmsanford]

I see that your going into the DNO territory and there is talk that they may reduce the LV value of below 20ohms in line with the HV one of 10ohms.

As the not exceeding the 21 ohm value includes the phase conductor, the transformer winding and the DNOs earth rod. it will not include the earth electrode within the installation. It really means that even if you were able to achieve a perfect 0 ohm reading on the installation earth electrode you should still expect to have a Ze of not exceeding 21 ohms.

These values though are not in the remit of the installation electrician as they are external of the installtion and the values we should be concerend with is the 200 ohm maximum that the BS 7671-2008 require.

I'll leave everything before the meter down to the DNO and everything after is down to me, and the DNO have the same philosphy thankfully.

 

[Deleted member 26818]

I take it then Malcolm that you do not accept that you should first contact the DNO before exporting a TN-C-S earth, and that you would be quite suprised if they were to disconnect your supply, or just remove the PME connection after discovering that youhad exported such an earth without their permission?