Extending the equipotential Zone of PME installation.

[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