Education please... or why Neutral?

[Donkmeister]

They could try it but they have entered into a contract by taking the money to provide the service.

Hmmm, wonder if they'll have a get-out clause... "We'll supply you with PME provided it isn't too difficult or we don't fancy it."

 

[jabuzzard]

I was thinking around that concept - if you have a shortish TN-CS system with 2 electrodes, one at the sub and one at the remote end each reading 20Ω resistance in contact with earth then the path via earth will be 40Ω. The path via metal is less than 0.35Ω. Parallel the two together and 40Ω doesn't make much difference to 0.35Ω.
Even if you halve the 40Ω it still doesn't make much difference.
Even with few over a network I am not convinced that they make much of a difference.

Wrong, it does not work like that.

Parallel resistances are not added together, they done as the addition of the reciprocals such that 1/r=1/r1+1/r2. So if you have two 20Ω connections to earth that are linked then your total resistance to earth is ~10Ω Hence putting in more earth connections dramatically lowers your impedance to earth. This is why heavy duty surge protection will have multiple earth connections all linked by copper bus bars. It does make a difference.

The correct answer to the above would be 1/((1/20)+1/(20+0.35)) which works out at 10.08674102Ω, but there are only two significant figures in the source material so the final and correct answer is 10Ω

This is pretty basic circuit theory, and would have been covered in any O level physics class and one would hope GCSE, but they have cut so much of the syllabus these days who knows.

 

[Spark123]

In order for current to flow there needs to be a circuit. In the example I used the PD is created in the secondary of the DNO transformer and all circuits on the same system are common to this point.

The parallel path along the CNE for current to flow will be from one electrode via mother earth to the other electrode and back to the transformer secondary hence the two electrodes I used in the example are in series.

If we measure the resistance of the electrode path and the impedance of the CNE, the resultant parallel resistance will not be that much different to the resistance of the CNE conductor which is why I was not suprised by what NotHimAgain wrote here.
When we measure Ze in a TN we are mainly concerned with measuring the impedance of the path including the phase and earth conductors to the DNO transformer - not the effectiveness of the connection to mother earth.

 

[DCC]

As yet I had not seen any comments from DCC have we helped or hindered him!

Mmm sorry chaps, post a question then lose internet access for a few days - good timing.

Some interesting stuff here, thanks. No time this morning for trying to understand it although I will sit down tonight and see how much of it I can follow.

I think my confusion stemmed from thinking that at some point between me and Drax the return path was mother earth therefore, I pondered, why not derive earth locally but it seems I was mistaken merely that neutral is (may be) tied to earth at various places, although BAS comment somewhere on page 1 about earth sometimes being used as the return path got me wondering again.

 

[ban-all-sheds]

http://en.wikipedia.org/wiki/SWER

 

[DCC]

Thanks BAS thats exactly how I had thought it was always done - not sure where I picked up the notion though but you can see why I wondered why neutral wasn't derived locally.

Interesting stuff.