[note - I wrote this after the 12:00 posting, before seeing later ones: it's about the general case of PNB with 4 conductors from a transformer, not about what things might be happening with the unknown connections in the OP's system]


Surprising and rather interesting.  I see John's point about BS7430 etc. From a following-standards perspective it indeed seems that "PNB implies TNCS" is the dominant view.  Below I note that standards seem to see a PEN in PNB too.


That said, I agreed with most of the reasons that have been given here for the above being a strange choice of definition.  In particular, people associate TNCS with a shock danger in the event of a single conductor fault without need of any other fault: that wouldn't happen for any broken conductor in a PNB system (excepting leakage currents). The argument that topology rather than length should be decisive (the cable from transformer to earthing point just "extends" the transformer terminals) is also strong.


Regarding whether there's a 'PEN' in PNB...  The BS7671 definition of PEN is simply about sharing protective and neutral conductor functions; and the definition of protective conductor functions includes connecting an earth electrode to a source neutral point. If the neutral point is the point where the three phases' neutral-ends meet (in or on the transformer), then the conductor in question in PNB is a PEN by this definition. Here, again, this would not have been my choice when thinking about the effects of a break in this conductor with no other fault in the system. Even a TT system could by this definition have a 'PEN conductor' if the neutral from the source runs as a wire to the first earth electrode and then on to the customers, as is often found in overhead supplies with separate HV/LV earthing where the LV earth is at the "next pole". 


Diagrams about multiple sources (rather than specifically PNB) in BS7671 2008 Figs.9A/9B have a note (b) that "the function of this conductor is similar to a PEN". I don't have a more up to date BS7671, but I see the Swedish 2018 equivalent keeps that diagram and much the same note, so it's not just a locally added choice of definition. If the standards believe their own definitions that I mentioned above, they should probably drop "similar to" and say "is a PEN". This diagram of multiple sources appears the most similar diagram to PNB, since the simpler diagram showing a single-source TNCS includes optional earth electrodes at multiple points without distinguishing the case where there are no electrodes on the source side of the PNB/N/PE/electrode point. The interest of the standards appears to be about whether the N and PE emerge separately at a place called a source, or a distribution supply cable, or whatever, rather than by where this point is relative to the earth electrodes.


Although it still sounds strange to me, I do see some mitigating reasons for the views of the standards now that I've been pushed to think about it:


Notation.

    If, as typically explained, we treat the 'T' of TNCS as meaning that "the source has a part [neutral] connected to earth" and the 'N' as "protective earthing of installations connects to the source neutral", then it's true that in PNB there's a conductor 'C'ombining both of these functions, besides other places where a 'S'eparate conductor has just the PE function. That's taking the fussy view that the source neutral is the point where the neutral ends of the separate source-phases meet. I like the "extending the terminals" argument in its way, but can't deny that a cable with its associated connections, extending tens or hundreds of metres from the actual neutral "point", introduces further opportunities for a break compared to a simple chunk of copper with bolts.  I don't think anyone has stated what (exact) basic definition of the TN* naming system we're using, or what we'd regard as the authority for this.


Safety.

   In the event of a fault from a higher-voltage system (interwinding fault in the transformer, or faults between different systems' conductors) the PNB neutral connection might be essential to reliable fault disconnection of the higher voltage supply, besides to limiting overvoltage on the loads during the fault. This is a bit of a cheat example as it's not the protective function we'd normally think of, and wouldn't arise if the source were a generator away from other electrical systems. However, it does give this piece of conductor a special significance beyond just normal operation.

   Without significant load, an earth fault (L-PE) in a PNB system with broken transfomer-neutral connection would not cause dangerous voltage on exposed parts in the installation, although it would cause harmful voltage on some loads. However, with plenty of load, there might be enough current through a L-PE fault to cause potentially dangerous touch voltage at the end of a circuit (rated much less than the installation load), while not giving good disconnection time. A little straw-clutching, but it's not out of the question, and indicates the conductor in question (PNB N-conductor) to have a role in earth-fault protection. Whether it's preferable to have a classic TNS system (earth at source) with broken main PE path is another matter.


Conclusion: "PNB -> TNCS", and "TN*-system source neutral connection = PEN", seem 'pandemic' in standards, and rather confusing regarding our usual first-thoughts about the implication of 'TNCS' and 'PEN', but not as strange as I thought yesterday. And I'll stop this legalistic study now, having been appreciative of the discussions here and of this little surprise. It almost certainly will never matter to me, but it's good to be warned yet again that much that seemed clear is not.

 "The transformer tank and associated HV metalwork shall be connected to the HV earth electrode." If there is a combined HV/LV earth electrode, that might explain things, but is it allowed?

Yes it is, but the Rise of Earth Potential must be calculated first to make sure the site is “cold” less than 430V or less than 650V with fast protection. 


Regards,


Alan.

Nathaniel:



 If the neutral point is the point where the three phases' neutral-ends meet (in or on the transformer), then the conductor in question in PNB is a PEN by this definition. Here, again, this would not have been my choice when thinking about the effects of a break in this conductor with no other fault in the system.

 

No it's not - by any definition it would need to be carrying load or imbalance current and earth fault current


It could only be PEN conductor if the neutral was offered for earthing purposes - akin to a larger version of a PME cut out


What were are discussing is the case where the neutral is earthed at a point outside the transformer casing - which is pretty well every distribution transformer ever manufactured  - the debate is that the conductor that creates that star point connection is a PEN, if it is earthed at one location after it leaves the transformer - regardless of that means of earthing being a sperate or combined earth


Regards


OMS


 

Would some kind soul do a quick sketch of how they think the OP arrangement is configured?

Hi, Alan and OMS.


In my text that you quoted, please note that "by this definition" refers to a BS7671 definition of PEN (and of protective conductor PE) that I gave in an earlier sentence. The conductor in question in PNB, between the transformer neutral point and the switchboard + earth electrode, connects an earth electrode and a neutral point: so the definition says it's a protective conductor. And it connects to the source neutral and 'contributes to the transmission of electrical energy', so it's a neutral conductor. By being both of these, it's a PEN.  The definition of PE seems a little permissive.


Then I went on to note that by this same definition it seems that even in a TT system with its first system earth electrode some distance along the neutral, the neutral conductor between that electrode and source neutral would deemed a PEN (regardless of whether the network owner offers installation earthing to the neutral). The BS7671 definition of PEN isn't linked explicitly to TNCS, which was initially a surprise.

 

OMS  No it's not - by any definition it would need to be carrying load or imbalance current and earth fault current

... which is what it does in the cases I gave (of certain conductors in PNB and TT systems): it carries load/imbalance currents and is in the earth fault loop. 


The reason I laboured the matter of "if the neutral point is where the three phases' neutral ends meet" is because I thought someone would claim that the load-end of the PNB "conductor in question"  is also just a part of the neutral point. That's a neat way to avoid the strange definition of this conductor, but it doesn't strike me as the spirit of the definitions.


I don't claim anything about the BS7671 definition being sensible, or the best, or in keeping with people's 'instinct'. I was just interested to see the divide between what many of us consider the common-sense definitions of TNCS/TNS/PEN and what appears to come from an influential standard.  I know you don't agree with PNB being TNCS: I also suggested TNS to be the sensible option in my first posting in this topic.  But if you can show based on BS7671 definitions of PEN rather than your own or some other undeclared ones that the two controversial PENs mentioned above are not PENs, I'd be interested to know!  It's seldom that I get into a discussion of definitions rather than actual technical details of good and bad points of a system, but this case surprised me into a little interest.

I was back on site briefly today and had chance to have a quick look at the transformer. It is an ESI 35-1 transformer so it seems that there is only one n-e bond. It would therefore seem identical to the TN-S diagram on page 48 of the regs, albeit that the line between N and PE would be a bit longer than I have thought of it before. Is there any reason to still bond as if PME?

No it is TN-S. So any bonding can be 1/2 the CSA of the earthing conductor subject to a minimum of 6mm and need not exceed 25mm.