I think most of the points have been already covered - just to add:

1. Any electrode has a very considerable resistance to the general mass of the Earth - generally several tens of Ohms (with a lot of effort you might get down to an Ohm or slightly lower). It's all down to the soil that surrounds the electrode rather than the resistance metallic electrode itself. So two (or more) electrodes on the same PEN conductor normally make a very small difference indeed  (tens of Ohms in parallel with probably less than 1/10th of an Ohm) - for all practical purposes the effect on normal loop impedance etc. is ignored. Forget any notion of there being no potential difference along a N or PEN conductor - there will be if there's any N or PE current flowing - Ohm's Law still applies even if the numbers are small.

2. To be TN-C-S there must be a conductor in the system that carries both N current and provides a connection to Earth. To my mind that's down to the order between the electrode and the N-PE link, so TN-S looks like this:

and TN-C-S:

The PEN (or CNE) conductor only exists between points A and B to my mind (not between 0 and A).

Note that there's nothing in those diagrams (or BS 7671's definitions) that say how far apart the transformer, N-PE link and electrode can be.

Also we're talking about the path of the connection of the installation to Earth rather than the route taken by earth fault currents (whose complete path will of course be back to the transformer star point). It's the presence of N currents between the installation and Earth that causes some of the biggest problems with TN-C-S, (especially during broken PEN events, but some smaller problems under normal conditions too).

Take the TN-C-S diagram and add a few more electrodes to the PEN conductor and you have PME.

PNB can be either (in the limiting case, a single bolt provides the N-PE link and the connection point for the electrode's conductor) so the cases converge - it can be argued either way).

"PME conditions apply" typically means the "DNO want you to treat it as if it were PME" - whether it actually is at the present time or not - i.e. they're keeping their options open if they want to convert to fully fledged PME at a later date (typically to allow additional consumers to be connected to the same transformer in the future). Where there isn't a DNO involved on the LV side (e.g. industrial site with a private transformer) it's then feasible to adopt a TN-S version of PNB and not worry about PME conditions.

   - Andy.

Hi Andy,

Thanks for the post; some further useful diagram and helpful text. In the industrial park example, I’d presume wavecon or similar DNO cabling would be used for the CNE? I must admit I haven’t come across it before for Non-DNOs.
Myself and I bet most people not involved in designing utilities would lean towards the use of SWA or AWA singles in the standard TNS (Non PNB) arrangement, what would be the deciding factors in you using this PNB TNS arrangement with the CNE? Probably length of run would be a factor 

fiftyhertz said:

what would be the deciding factors in you using this PNB TNS arrangement with the CNE? Probably length of run would be a factor 

Or just DNO policy

what would be the deciding factors in you using this PNB TNS arrangement with the CNE?

In TN-S there is no CNE.

Have a search of old posts on the forum for examples of PNB style TN-S (it gets talked about quite a bit) - the classic is an industrial setup (with their own transformer) where the HV and LV earths need to be kept separate (usually because the HV earth isn't metallic all the way back to the HV source - i.e. there are overhead sections on wooden poles in the HV supply). In this case, the N-PE link and connection to the LV earth electrode is often done at the first tier distribution board - which is often a distance away from the transformer (e.g. in a LV switch room inside the building, whereas the transformer is outside across the card park somewhere, or at least in a different compartment). Many consider the arrangement to be pure TN-S - just with rather long connections to the transformer's secondary windings.

As others have already mentioned, PNB is common for rural domestic supplies too (think a remote house or farm with a pole mounted transformer) - although typically the DNOs say that everyone should treat those as "may be PME in the future" so slap on PME conditions anyway.

   - Andy.

Hello Andy,

Many thanks for this.  Earthing is one area where I find I need to "re learn" it every time I come back to it and so the community guidance I find very useful!

GN-8 explanatory diagrams do seem to indicate that it is the path of the "Earth Fault Loop" current that is a deciding factor when considering what type of earthing system is in use (i.e. the red earth fault paths etc)...

• TN-C - Earth fault current flows in the PEN all the way back to the source TX neutral to complete the circuit (TX Neutral earthed at star point)
• TN-S - Earth fault current flows in the separate CPC all the way back to the source TX neutral to complete the circuit (TX Neutral earthed at star point)
• TN-C-S - Earth fault current flows in the separate CPC but also relies on a PEN conductor to make it's way back to the source TX neutral to complete the circuit (neutral can be earthed at multiple points (PME) or a single point (PNB))
• TT system - Earth fault is via separate CPC in the installation but via Terra back to the TX Neutral

When you say:

"To be TN-C-S there must be a conductor in the system that carries both N current and provides a connection to Earth. To my mind that's down to the order between the electrode and the N-PE link, so TN-S looks like this..."

1. When I look at your sketch for TN-S it seems to be very close to showing the GN-8 figure 4.7 for TN-C-S albeit you are earthing the earth bar not the neutral directly?
2. Also from the transformers point of view isn't the neutral wire in your sketch carrying both neutral current and providing a connection to Earth?(i.e. references TX neutral to "earth" potential for the installation)

I work in industrial installations whereby we own and operate our own HV switches and transformers and regularly include N/E links in the LV panel.  In BS7671 it suggests that where you own and operate the transformers then the transformer and the LV bushings are considered part of the "installation".  

Again I find this topic a challenge and so thought I would ask for clarification!

Cheers,

Rich

From the transformers point of view - and it may be easier to visualise with single phase, but the same is true of 3 phase, all return current comes back to the other end of the secondary winding - it cannot tell if it is 'fault' current or a legitimate load at  that point - just that the total winding current  had better add up to zero - in the sense that for any one winding, what goes in one end of it , had jolly well better match whatever comes out at the the other - it is just a wire wrapped around a magnetic stick after all...

The is always  true of TT, TNS TNCs and indeed any transformer ..(on a non faulty transformer and ignoring displacement current/ inter winding capacitance effects)

Once you get very close to the NE bond there is a fuzzy sort of bridge between TNC-s and TNS that can be almost at the level of what order the lugs are placed on the terminals, and depends on how many things need come undone before the system loses its terra-firma earth reference, and if it loses the neutral connection to the load first..

You may at one extreme have a shared PEN that is really only the length of one bolt or one link bar between 2 studs. Or with the same hardware assembled in a different order, you may not have a PEN at all. Or as per the more textbook case, you may have a PEN that is many metres or even hundreds of metres in length.

Mike

From the transformers point of view - and it may be easier to visualise with single phase, but the same is true of 3 phase, all return current comes back to the other end of the secondary winding - it cannot tell if it is 'fault' current or a legitimate load at  that point - just that the total winding current  had better add up to zero - in the sense that for any one winding, what goes in one end of it , had jolly well better match whatever comes out at the the other - it is just a wire wrapped around a magnetic stick after all...

The is always  true of TT, TNS TNCs and indeed any transformer ..(on a non faulty transformer and ignoring displacement current/ inter winding capacitance effects)

Once you get very close to the NE bond there is a fuzzy sort of bridge between TNC-s and TNS that can be almost at the level of what order the lugs are placed on the terminals, and depends on how many things need come undone before the system loses its terra-firma earth reference, and if it loses the neutral connection to the load first..

You may at one extreme have a shared PEN that is really only the length of one bolt or one link bar between 2 studs. Or with the same hardware assembled in a different order, you may not have a PEN at all. Or as per the more textbook case, you may have a PEN that is many metres or even hundreds of metres in length.

Mike