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).

 

[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

 

[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).

Secondly, PNB you move the star point connection to earth of the tx up to the customers panel. This negates the issue with voltage dropped accross the neutral supply cable.


Cheers

 

[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).

Thirdly, the rod is used to reduce the touch voltage upon a broken pen. It wont affect the voltage drop accross the neutral. You need to assess the load impedance and then calculate the Ra to acheive a required touch voltage.

Cheers