Repost - Earth Rod not taken account in TN systems

Question

Hi All, 
 I realise the function of the earth rod in a TN system is to provide a close reference to true earth for the neutral 
 The thing that has confused me slightly is the TNCS PNB, which has an earth rod located at the consumer end. When i looked at the old forums there was a debate between whether this was TNCS or TNS, as the neutral carries no current due to the earth rod, and therefore by definition cannot be a combined conductor. If the current is not dissipated into the ground via the rod, why would no current flow in the neutral of this system prior to the rod 
 Thanks in advance 
 EDIT: My question wasn&rsquo;t phrased very well and I&rsquo;ve tried to clean it up for future readers, but i think this is the correct summary. 
 Fault current CAN flow between the neutral/earth link and the neutral point of the transformer in a PNB earthing arrangement. The previous forum posters were essentially saying is that even though though the link is remote, fault current will still flow in the CNE cabling, but we can note that it also would in a pure TN-S system but more likely an internal section of busbar within the TX and the neutral bar, instead of external cabling and by that logic TN-S would be a form of TN-C-S if semantics were involved. 
 Link to thread 
 What earthing arrangement is this?

mapj1 · Accepted Answer

I do consider PNB to be TN-S like, or at least more like TNS than TNC-s I may be a minority ! 
 The part between the transformer star/ neutral and the first electrode is carrying current that is in perfect balance with the phase current (s) so In that sense it is carrying the fault current as well as the load current. However, the same is true somewhere in the neutral path of a 'pure' TN-S as well. 
 But the severity of breaking that extended transformer neutral connection is not the same as breaking a PEN in between two earthing points in a TNCs or PME system, as there is no-where that the chassis of equipment become live relative to terra-firma, In fact in a single phase PNB system, if you break the transformer neutral it is all quite safe, in a 3 phase PNB less so, delta connected loads will be fine but star connected ones will fight to pull the neutral off- balance. Again, the same risk is true of breaking the neutral in TN-S. 
 For me at least the thing that makes PME what it is is the 'M' - the multiple points where N and E meet. This is both the strength and the weakness - for if the CNE breaks between 2 such points then a lot of earthed metalwork is now very metallically connected to an off- earth voltage. This does not happen in TNS or PNB, regardless of which side of the one and only NE bond you sever the neutral. 
 For me one could take a TNS and slide the point of separation along away from the TX, and it becomes PNB at some point, 
 Also TNC-s is not always the same as PNB either, as we do not take the CNE all the way to the consuming loads or even multiple loads and split it there - the split is very much done at the origin of the distribution side of things. 
 Mike.

gkenyon · Answer

AMK said: PME and PNB are both variants of the TN-C-S earthing system, where the PEN conductor is split into separate neutral and earth conductors at the consumer’s premises. 
 AMK , this isn't quite true. 
 Even in some public PNB systems, the distributor provides an SNE cable. 
 WPD, at least, used to publish this openly. 
 The key differentiator, is that the transformer star point (or mid-point) is NOT earthed directly in PNB systems (whatever the variant), but is in true TN-S and TN-C-S, but the N conductor is connected elsewhere from the transformer. 
 You can then argue whether the "N" conductor (I will not call it a Neutral at this juncture, because strictly, according to the International Electrotechnical Vocabulary, it's perhaps not a Neutral in single-phase or split-phase secondary transformers) in a PNB system acts as a PE conductor for the transformer itself ... but the wiring (which is what BS 7671 is concerned with) can be SNE, and therefore it's possible for PNB to be TN-S (but PME conditions would still apply). 
 
 The following diagrams might help illustrate what I'm talking about:

Repost - Earth Rod not taken account in TN systems

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