Neutral earth links in feeder pillars

Are both the neutral and earth cables sized for full potential current? You maybe need just a few to be low in capacity to provide additional reasons for 'fixing' any problems. Just a thought.

mapj1 said:

"building networks" of some older blocks of flats that used to be maintained by the area boards, neither of which fell/fall under the wiring regs / '7671

Although, BS 7671 is often specified for new/upgrade works on these systems ... BS 7671 does cover TN-C systems, so can be used, but TN-C-S-c-...-s isn't permitted.

TN-C-S-c-...-s

Ignoring any colour coding, I guess you could deem it to be simply TN-C-S with the PEN consisting of two (or more) conductors in parallel for part(s) of the route. As long as they're not designated as separate N and PE conductors, but rather as PENs, BS 7671 wouldn't object I think. (As Mike suggests, common practice in DNO land where old 4-core + armour cables are converted to PME with "N"-armour links scattered here there any everywhere).

   - Andy.

cannot do that, if there is a load part way along, between links, as that makes use of the '-s' nature of things.
Its all splitting hairs, it may or may not be illegal, its not a good approach technically, and as a country we don't like it.

M.

AJJewsbury said:

gnoring any colour coding, I guess you could deem it to be simply TN-C-S with the PEN consisting of two (or more) conductors in parallel for part(s) of the route.

Not really ... especially if exposed-conductive-parts, or extraneous-conductive-parts, connected to different bits of the TN-C-S-c-...-s, are simultaneously-accessible (in any combination). It's back effectively to 411.3.1.1 2nd para.

if there is a load part way along, between links, as that makes use of the '-s' nature of things.

But not a problem if all the tap off points have their own N-PE links? (as the OP seems to suggest might be the case?) Even if a few don't and a "presented as TN-S" supply is tapped off mid way between links... I'm not entirely seeing what the problem is (at least any problems that wouldn't already be present in a conventional TN-C-S system with a single conductor PEN). I'll have a think... 

especially if exposed-conductive-parts, or extraneous-conductive-parts, connected to different bits of the TN-C-S-c-...-s,

Is that any more of a problem that adjacent installation tapped off from different positions along a conventional PME main? (I think I might have a different picture in my mind's eye, so might well be overlooking something significant...).

  - Andy.

AJJewsbury said:

But not a problem if all the tap off points have their own N-PE links? (as the OP seems to suggest might be the case?) Even if a few don't and a "presented as TN-S" supply is tapped off mid way between links... I'm not entirely seeing what the problem is

If conditions for PME as presented in ENA Engineering Recommendation G12 are not met, then there can be excessive voltages between parts of the PEN conductor. If these are transferred by cpc's or bonding so they are simultaneously-accessible, this can be a problem. Bonding parts of the system together resolves this ... but provides its own problems through diverted neutral currents.

Problems can be exacerbated by damp ground of the type we have in lots of the UK (but not all) ... which is why we don't often use TN-C systems here (except the TN-C portion of the system in the supply network) and why ESQCR is written as it is.

It's worth, at this point, talking about PNB ... when used in private systems (where supplies have multiple sources with supplies from DNO at HV), this is usually really only TN-C-S on a "technicality" that the N conductor from the source can carry earth fault current back to the source, rather than actually having a PEN conductor that's intentionally used for neutral and protective provisions ... it is actually this latter "combining neutral and protective functions in the same conductor' that's actually prohibited under ESQCR.

If you take things too literally, you could never have a generator or transformer in a private network, because there's always a neutral conductor popping out of the source for termination of the neutral conductor (and system referencing conductor where appropriate).

is usually really only TN-C-S on a "technicality" that the N conductor from the source can carry earth fault current back to the source

Down that train of thought lie dragons I fear. This is the trouble of thinking in terms of earth fault loops - follow that logic and even TT should be considered TN-C in part - which cannot have been the original intention. If rather you think in terms of the path the installation has to Earth (which I think is the key issue when worrying about what can result in exposed-conductive-parts attaining a hazardous voltage relative to Earth).

Consider...

I think it becomes much clearer that only in a true TN-C-S system can the N current have any effect on the potential on exposed-conductive-parts, or a broken N result in earthed parts becoming live.

   - Andy.

AJJewsbury said:

I think it becomes much clearer that only in a true TN-C-S system can the N current have any effect on the potential on exposed-conductive-parts, or a broken N result in earthed parts becoming live.

N currents having effect on touch-voltages is a good reason why TN-C-S-c-...-s is a problem.

AJJewsbury said:

follow that logic and even TT should be considered TN-C in part

Agreed

AJJewsbury said:

which cannot have been the original intention

Really? Have a look at Regulations 312.2.1.1, 312.2.2.1, which require earthing 'at the source'.

How far from 'the source' does earthing for TN-S have to be before it is classified as "TN-C-S PNB"?

And we know that we have have 'TN-C-S' supplies that are (to use a commonly-used, but perhaps no-longer valid term) "exported" using TT earthing arrangements (with the PME earthing terminal as "source earth" but in fact it's not necessarily earthed at source, and hence perhaps not strictly fully conformant to the "ideal" systems presented in BS 7671?

Is "PNB" actually a thing? Capacitances and fault-current flows in broken neutral (between source and eart) situations in three-phase systems do lead to huge safety problems

AJJewsbury said:

think in terms of the path the installation has to Earth

And we should be properly aware that "earth" (aka the planting medium) isn't a very good conductor on the cm^3 scale, even if it is good at the km^3 scale.

We can all fall into the easy-mind trap of thinking all wires are good (zero impedance) and all Earths (chassis, bonds, etc) are even more perfect, when it's the opposite.

How far from 'the source' does earthing for TN-S have to be before it is classified as "TN-C-S PNB"?

I suspect the DNO think about things in a different way - not so much in terms of lengths, but number of electrodes - if the system doesn't have multiple electrodes, it can't be PME, but N is still earthed, so it must therefore be PNB. Even on "ordinary" PME system, if you look a rural overhead installations, the 1st electrode is sometimes a whole span (a few tens of metres) away from the transformer (to keep HV and LV earthing separate).

Is "PNB" actually a thing?

In DNO land, certainly. It doesn't always map nicely onto BS 7671 nomenclature (TN etc) nicely though. I've seen some PNB arrangements that look like textbook TN-S to me (especially where there's only a single customer on a transformer). But I've seen other diagrams that are definitely TN-C-S (but not PME as there's only one deliberate electrode). The water seems to get muddied further by many DNOs seemingly insisting on the right to change supply arrangements in the future (perhaps not unreasonably) and so to cover themselves say that TN-S PNB customers treat the supply as if it were TN-C-S anyway (even saying "PME conditions" apply, when clearly the physics and maths of the initial setup screams otherwise). Customers with their own private transformers can make up their own minds of course.

   - Andy.

How far from 'the source' does earthing for TN-S have to be before it is classified as "TN-C-S PNB"?

I suspect the DNO think about things in a different way - not so much in terms of lengths, but number of electrodes - if the system doesn't have multiple electrodes, it can't be PME, but N is still earthed, so it must therefore be PNB. Even on "ordinary" PME system, if you look a rural overhead installations, the 1st electrode is sometimes a whole span (a few tens of metres) away from the transformer (to keep HV and LV earthing separate).

Is "PNB" actually a thing?

In DNO land, certainly. It doesn't always map nicely onto BS 7671 nomenclature (TN etc) nicely though. I've seen some PNB arrangements that look like textbook TN-S to me (especially where there's only a single customer on a transformer). But I've seen other diagrams that are definitely TN-C-S (but not PME as there's only one deliberate electrode). The water seems to get muddied further by many DNOs seemingly insisting on the right to change supply arrangements in the future (perhaps not unreasonably) and so to cover themselves say that TN-S PNB customers treat the supply as if it were TN-C-S anyway (even saying "PME conditions" apply, when clearly the physics and maths of the initial setup screams otherwise). Customers with their own private transformers can make up their own minds of course.

   - Andy.