Extending the equipotential Zone of PME installation.

[HappyHippyDad]

I was going to just add on to Voltz's thread but it seemed to come to a natural end but I had some questions.

1. A few experienced members say that you should not extend the (equipotential zone) EPZ of a PME system. Am I right in thinking that this is because of the possible potential between the earth of the PME and true earth? If so, then would this potential not be dealt with by supplying the required 10mm bonding from the MET of the originating building? Both 'Wiring Matters' View attachment 2005_16_autumn_wiring_matters_electrical_installations_outdoors.pdf and GN8 (page 66, figure 5.15)give an example of extending the EPZ of PME.

2. Whilst I'm asking I'd also be interested in why this would not be an issue with a TNCS (PNB) but it would with a TNCS (PME), I cant quite get me head around why there is a greater chance of a potential difference between system earth and true eart with a PME? How do the additional earthing points of a PME cause this?

3. Lastly, Some have mentioned putting in a rod 'along' with extending the EPZ of a PME. GN8 (Fig 5.15) shows this scenario but states the CPC of the extension should not be connected to the earthing terminal of the additional building if TT is going to installed. This makes sense as you will not have a large PD between system earth and true earth (given a good Ra).

 

[Marvo]

3. Lastly, Some have mentioned putting in a rod 'along' with extending the EPZ of a PME. GN8 (Fig 5.15) shows this scenario but states the CPC of the extension should not be connected to the earthing terminal of the additional building if TT is going to installed. This makes sense as you will not have a large PD between system earth and true earth (given a good Ra).

Okay, I'll make a start with your last question. If you extend the equipotential zone of a TNCS (PME) installation to an outbuilding and you also install a supplementary rod, this doesn't make it a TT installation, it makes it a PME installation that happens to have an extra rod. Therefore your question about connecting the CPC to the earthing terminal of a building with a TT earth arrangement is moot because the building would be PME.

 

[HappyHippyDad]

Okay, I'll make a start with your last question. If you extend the equipotential zone of a TNCS (PME) installation to an outbuilding and you also install a supplementary rod, this doesn't make it a TT installation, it makes it a PME installation that happens to have an extra rod. Therefore your question about connecting the CPC to the earthing terminal of a building with a TT earth arrangement is moot because the building would be PME.

Thanks Marvo,
So what is the benefit here of 'adding' the rod when you have a good PME system? I'm assuming it must decrease the PD between true earth and system earth?

 

[Leesparkykent]

Thanks Marvo,
So what is the benefit here of 'adding' the rod when you have a good PME system? I'm assuming it must decrease the PD between true earth and system earth?

Should a discontinuity occur in the DNO supply PEN conductor (under very exceptional circumstances I know) all metalwork will rise to approximately 230 V with respect to true Earth, which will in itself mean that there could be a lethal touch voltage present between the exposed-conductive parts, extraneous-conductive-parts and true Earth. In an outdoor environment there is much more likelihood that a person will come into contact with the earth of the electrical system and true Earth. A separate earth electrode connected the MET will help to minimise the potential.


Secondly, under normal operating conditions, it is possible, due to small differences in potential between the earth of the electrical system and true Earth, for a small voltage to be present. This perceived electric shock that could potentially happen can be minimised by installing the additional earth electrode/s. This difference would normally only be detectable, for instance, by a wet person creating a reduction of body resistance due to being wet and no shoes, etc.

 

[Darkwood]

I tackle the question 3 - if you read again the TT section

An exposed conductive-partconnected to one means of earthingmust not be simultaneously accessible with an exposedconductive-partconnected to another means of earthing(Regulation 413-02-03 refers)

So if you have a supply armour connected to the origin then leave it isolated so a TT can be formed at the load end then you must ensure that you cannot touch the earthed armour/gland and the earthing of the TT system.... there maybe a potential between the 2 set of earths as they are independent and sourced differently.

I'm open for comment on that but thats how I read this and can understand the reasons why.

 

[Darkwood]

lol - posted all same time think we all tackled the same point unk:

 

[HappyHippyDad]

Should a discontinuity occur in the DNO supply PEN conductor (under very exceptional circumstances I know) all metalwork will rise to approximately 230 V with respect to true Earth, which will in itself mean that there could be a lethal touch voltage present between the exposed-conductive parts, extraneous-conductive-parts and true Earth. In an outdoor environment there is much more likelihood that a person will come into contact with the earth of the electrical system and true Earth. A separate earth electrode connected the MET will help to minimise the potential.


Secondly, under normal operating conditions, it is possible, due to small differences in potential between the earth of the electrical system and true Earth, for a small voltage to be present. This perceived electric shock can be minimised by installing the additional earth electrode/s.

So this applies to both PME and PNB then as they both have the earth connection made to the neutral at the consumers end? It seems that PME gets mentioned as the 'dangerous' one of the 2 TNCS? This must solely be due to the possible PD of true earth and syatem earth rather than the example in your first paragraph? Although why PME is more dangerous than PNB when extending the EPZ I cant see?

 

[HappyHippyDad]

lol - posted all same time think we all tackled the same point unk:

I think my head is going to be spinning in a minute Darkwood, especially if the other questions get tackled and I try and respond to all! :crazy:

 

[Leesparkykent]

PMB stands for Protective Neutral Bonding. This is a variation of the TN-C-S system in that the customer is provided with an earth terminal connected to the supply neutral, but the neutral is connected to earth at one point only, normally at or near to the customer's supply point. This arrangement is normally reserved for use when a customer has a dedicated TX.

Do you mean between TNCS and PME?

Edit: my head is spinning already lol. just for clarity

1) PME supplies are TNCS
2)not all TNCS are PME
3)PNB is TNCS

 

[HappyHippyDad]

PMB stands for Protective Neutral Bonding. This is a variation of the TN-C-S system in that the customer is provided with an earth terminal connected to the supply neutral, but the neutral is connected to earth at one point only, normally at or near to the customer's supply point. This arrangement is normally reserved for use when a customer has a dedicated TX.

Do you mean between TNCS and PME?

Edit: my head is spinning already lol. just for clarity

1) PME supplies are TNCS
2)not all TNCS are PME
3)PNB is TNCS

No, I meant between PNB and PME (the 2 different TN-C-S systems). Perhaps I'm getting a bit muddled up, but I'm sure some of the chaps mentioned that it was ok to extend the EPZ if it was a TNCS, but then added the proviso but not if it was PME (TNCS), so I assumed this must mean its OK if its PNB? Perhaps I'm misreading! Because the same problems will exist with both TN-C-S's (in conjunction with extending the EPZ) wont they?

 

[Leesparkykent]

No, I meant between PNB and PME (the 2 different TN-C-S systems). Perhaps I'm getting a bit muddled up, but I'm sure some of the chaps mentioned that it was ok to extend the EPZ if it was a TNCS, but then added the proviso but not if it was PME (TNCS), so I assumed this must mean its OK if its PNB? Perhaps I'm misreading! Because the same problems will exist with both TN-C-S's (in conjunction with extending the EPZ) wont they?

Maybe Davesparks or NBP can clear it up? If I remember correctly the posts were between Davesparks and Netblindpaul.

 

[HappyHippyDad]

I'm going to move on to the earth rod!

As in Voltz's example if we have a metal container fed from a seperate building (PME) :

1. Would it be a good idea to connect a rod to the earthing terminal in the outbuilding as well as using the CPC from the PME supply (and add a seperate 10mm bonding conductor from originating MET to bond the container and other extraneous), so in the case of a broken neutral in the DNO's supply there is still an earth?
2. Would it be a good idea to make the metal container a TT

 

[Leesparkykent]

I'm going to move on to the earth rod!

As in Voltz's example if we have a metal container fed from a seperate building (PME) :

1. Would it be a good idea to connect a rod to the earthing terminal in the outbuilding as well as using the CPC from the PME supply (and add a seperate 10mm bonding conductor from originating MET to bond the container and other extraneous), so in the case of a broken neutral in the DNO's supply there is still an earth?
2. Would it be a good idea to make the metal container a TT

As volts situation was a metal container. IMO this should be treated similar to a caravan and be made in to a TT system.

 

[Dave OCD]

My head is spinning too lol, in reply to that I'd say possibly [seems to be the gist of it] and maybe. [As you'd avoid an additional 10mm earth from origin to outbuilding]

Edit - this is in reply to HHDs last post, Lee got in between lol.

 

[HappyHippyDad]

As volts situation was a metal container. IMO this should be treated similar to a caravan and be made in to a TT system.

Would you say there is anything wrong with number 1? It's offering a far better Ze than making into a TT (most probabaly).

 

[Leesparkykent]

Would you say there is anything wrong with number 1? It's offering a far better Ze than making into a TT (most probabaly).

I'm sure many would argue the point that the DNO earth will provide for a better value...which is a valid argument. IMO number 1 isn't technically wrong but I would feel more comfortable making the metal container a TT earthing arrangement.

 

[davesparks]

It might help if you think of adding an additional earth electrode to a PME fed installation as serving the purpose of main bonding the actual ground itself.

 

[HappyHippyDad]

It might help if you think of adding an additional earth electrode to a PME fed installation as serving the purpose of main bonding the actual ground itself.

I did think along those lines but then I thought how could a rod (even a deep one) bond the ground? At least how could it bond a large enough area to be effective at lowering the PD between true and system Earth when it will only be utilising a certain amount of true earth?

Would you extend the EPZ to a metal container and add an earth electrode or would you isolate the system CPC and make the metal container into a TT? And more importantly why would you choose the option that you do?

 

[davesparks]

I did think along those lines but then I thought how could a rod (even a deep one) bond the ground? At least how could it bond a large enough area to be effective at lowering the PD between true and system Earth when it will only be utilising a certain amount of true earth?

Would you extend the EPZ to a metal container and add an earth electrode or would you isolate the system CPC and make the metal container into a TT? And more importantly why would you choose the option that you do?

It won't be only bonding an area or only utilising a certain amount of true earth, you might as well say that a main bond only bonds a certain length of pipe!

As I've said on the other thread my only involvement in installations in containers is our storage site. For our storage licence we have to have at least 2 electrodes connected at opposite corners of each container with a resistance to earth from the top of the container of <10ohms.

Personally I would lean towards installing earth electrodes connected to the MET of the installation and keep the PME connection because 99.9% of the time the supply neutral won't be faulty. If the dreaded supply neutral fault does occur then the installation will remain earthed via the electrodes.

 

[GEOFF S]

I was going to just add on to Voltz's thread but it seemed to come to a natural end but I had some questions.

1. A few experienced members say that you should not extend the (equipotential zone) EPZ of a PME system. Am I right in thinking that this is because of the possible potential between the earth of the PME and true earth? If so, then would this potential not be dealt with by supplying the required 10mm bonding from the MET of the originating building? Both 'Wiring Matters' View attachment 29676 and GN8 (page 66, figure 5.15)give an example of extending the EPZ of PME.

2. Whilst I'm asking I'd also be interested in why this would not be an issue with a TNCS (PNB) but it would with a TNCS (PME), I cant quite get me head around why there is a greater chance of a potential difference between system earth and true eart with a PME? How do the additional earthing points of a PME cause this?

3. Lastly, Some have mentioned putting in a rod 'along' with extending the EPZ of a PME. GN8 (Fig 5.15) shows this scenario but states the CPC of the extension should not be connected to the earthing terminal of the additional building if TT is going to installed. This makes sense as you will not have a large PD between system earth and true earth (given a good Ra).

Hi

Firstly, an equipotential zone is a zone of equal potential, difficult to acheive in most installations. We normally create a protective zone via Protective bonding, that said the potentials are not equal. A VERY important point is to realise we make an assumption that the floor we stand on is prettty much insulated. Not much point bonding items simultaneous to touch when the risk of shock is hand to foot. So you need to create a protevtive zone. The issue you may have with PME is where earthed metal work is at a different potential to true earth. Long supply runs mean more volts dropped along the neutral supply cable, this raises the voltage on earthed metal work(earth neutral link)within the installation. Contact between metal work and true earth may allow enough current to flow to feel a shock. Agricultural installs sometimes require metal grids where pme is used to negate this issue. So ensure the floor is well isulated and any earthed metal work is bonded and you wont have any issues.

Cheers