Is there an error in 312.2.1.1?

If the dotted line was redrawn through the middle of the circles, then it would be right.  As drawn, it implies that the DNO's cutout is part of the consumer's installation.

Simon Barker said:

If the dotted line was redrawn through the middle of the circles, then it would be right.  As drawn, it implies that the DNO's cutout is part of the consumer's installation.

The diagram in Fig 3.9A isn't new. It is unchanged from Fig 3.9 in BS 7671:2018+A2:2022 ... in fact, image below is from 16th Edition BS 7671:2001+A2:2004. I believe it was introduced in BS 7671:2001.

So, the image has been incorrect in respect of UK public PME supplies since it was first introduced over 25 years ago.

Would that warrant a corrigendum for BS7671 18th AMD4 to correct the long standing issue?

Some more thought might be  needed.

Not all countries that use BS 7671 * have an ESCQR type act that prohibits a customer side NE conductor sharing, and in some places, as well as in many other countries wiring regs, the equivalent link is legally under the control of the consumer.  

(Note that in other places  - Australian MEN for example - you are allowed more than one NE link by design, so long as what is in effect a private PEN to an outbuilding or similar  is suitably installed and sized. ) 

In this country, unless the supply is at HV,  it is clearly the DNO's to own, but to move the dotted lines to make the drawing UK only, might not be desirable. 

If the dotted lines were removed,  it could  also could be read as allowing one consumer to have two main supplies, and that is not really what is wanted either.

It is also perhaps worth pondering the intention of the ESCQR, which I suspect was more to discourage earthed concentric wiring and the LV equivalent of single wire earth return, than be too detailed about how DNOs interconnect their TNS and TNC wiring 

regards Mike

* to name a few, Qatar, Malta, UAE Cyprus Mauritius, St Lucia,Saint Vincent and the Grenadines, Sierra Leone, Sri Lanka, Trinidad and Tobago, Uganda,  ..  Not sure how rigorously the rules are enforced in all these places however, having seen some odd things on my travels..

Sergio Fernandez said:

Would that warrant a corrigendum for BS7671 18th AMD4 to correct the long standing issue?

Wouldn't that cause a stir?

I think it is unlikely to meet the criteria for Corrigendum in BS 0.

mapj1 said:

Not all countries that use BS 7671 * have an ESCQR type act that prohibits a customer side NE conductor sharing, and in some places, as well as in many other countries wiring regs, the equivalent link is legally under the control of the consumer.  

We only need to concern ourselves with the UK when considering changes in BS 7671. Although it's true that other countries do use BS 7671, or cite it in specifications for construction design and build, it would be up to that country, or the specifier, to address any national inconsistencies.

mapj1 said:

In this country, unless the supply is at HV,  it is clearly the DNO's to own, but to move the dotted lines to make the drawing UK only, might not be desirable. 

I agree, for a number of reasons. Not least, trying to show this "on or before" the border of the consumer's installation is quite difficult.

mapj1 said:

It is also perhaps worth pondering the intention of the ESCQR, which I suspect was more to discourage earthed concentric wiring and the LV equivalent of single wire earth return, than be too detailed about how DNOs interconnect their TNS and TNC wiring 

Or, perhaps, discourage multiple earthed neutrals in the consumer's installation too. Diverted neutral currents can cause safety issues for those working on the distribution network cabling under certain circumstances ... particularly true now we are moving to more installations capable of islanding.

gkenyon said:

So, the image has been incorrect in respect of UK public PME supplies since it was first introduced over 25 years ago.

My issue really is with Note 4(b) rather than the Figure. The exact position of the N-PE link is unimportant.

I think that first, the note is inaccurate. More importantly, an enthusiastic consumer might be misled into inserting a second N-PE link on the assumption that it is only the network's neutral which cannot be used as a protective conductor in the installation.

gkenyon said:

In addition, because of Regulation 411.3.1.1

Which points to Reg 411.4 and at 411.4.3 is a note: "Regulation 8(4) of the Electricity Safety, Quality and Continuity Regulations prohibits the use of PEN conductors in consumers' installations."

Nothing wrong there, so could the same words not be used in 312.2.1.1?

mapj1 said:

Not all countries that use BS 7671

Fair point, so preface the note accordingly: "In UK, Regulation 8(4) ..."

Sergio Fernandez said:

Would that warrant a corrigendum for BS7671 18th AMD4 to correct the long standing issue?

I'd leave the Figure as it is, but change the note.

Incidentally, Note 2 seems to be at odds with R.8(3)(b) of ESQCR: 

"(3) A generator or distributor shall, in respect of any low voltage network which he owns or operates, ensure that—

...

(b) every supply neutral conductor is connected with earth at, or as near as is reasonably practicable to, the source of voltage except that where there is only one point in a network at which consumer’s installations are connected to a single source of voltage, that connection may be made at that point, or at another point nearer to the source of voltage" [my emphasis].

If you worry about a drawing from 2001, it is quite instructive to compare the current rules  with its  ancestor on that topic (The Electricity Supply Regulations 1988 - only superseded in 2002, when the ESCQR  came into law - so about a year after the offending diagram was created ).

The earlier (now repealed) legislation was far more nuanced about NE bonds, and had a lot of technical details, maximum cable distances, copper areas and so on, but did not actually prohibit a PEN on  the consumer side, subject to certain limitations. - see regulation 7. The same legislation also enshrined in law, the 20 ohm maximum electrode impedance and 1 ohm hot site/cold site distinction that are now both reduced to more of a rule of thumb.

Edit,

I agree the new text and the current legislation are not saying the same thing either.

perhaps of course the ESCQR is going to be edited at some point and there is advance information.

regards Mike

The 1988-2002

'The supplier shall not connect or permit the connection of, or continue, a supply to any part of a consumer’s installation where the supply neutral conductor is, or is intended to be, used to connect any circuit protective conductor in that part of the consumer’s installation with earth unless he is satisfied so far as is reasonably practicable, that  .. '

and a list of minimum cross-sections, and maximum resistances Is not at all the same as the post 2002 blanket ban.

'A consumer shall not combine the neutral and protective functions in a single conductor
in his consumer’s installation.'

so maybe when the figure was drawn, it was OK.

well this DTI guidance document to the ESCQR is interesting.  I realize the DTI has since been subsumed, but the guidance has not as far as I know been replaced or updated.

For each major section of the then fairly new ESCQR, it sets out the intention.
Page 19 has the relevant note

Consumers may not combine the functions of neutral and protective
conductors in their installations,
i.e. consumers must not operate TN-C systems or use CNE cables within their installations. Distributors operating such systems must comply with additional requirements (see regulation 9 Protective multiple earthing).

So at the time of drafting, the concern was TNC, not really diverted neutral current.
It is therefore entirely possible that the guidance, or the legislation, should now be revised for the new world order. 

It does not however explicitly say 'cannot ever have a customer side NE link' or indeed mention the abbreviation  PEN. Which is good, as private LV generators still require an NE link somewhere, even if it is a switched one for when the mains fails. The problems arise of course when normal neutral currents and fault-only earth currents get confused about where they should be flowing, and some part of that split path is inadequate. 

Mike.

mapj1 said:

It does not however explicitly say 'cannot ever have a customer side NE link'

Indeed not. However, all the systems in 312.2.1 have a N-PE link somewhere (even TN-S at the star point). If you add another one in the consumer's installation, do you not then have two conductors in parallel between the two links? So now, which is which?

two conductors in parallel between the two links? So now, which is which?

Well indeed, labelling is a problem, though of course we all know that the electrons neither know nor care about the colour of the insulation being blue or green & yellow, so the current splits between the paths in inverse proportion to their resistance, and so, to be safe, both conductors must be sized accordingly much like any other conductors in parallel. The problem is one of naming - rather like the PNB - TNS  thing. 

Actually I understand it is not an uncommon situation on campus type installations with multiple parallel transformers with both HV and LV mains connected between buildings  to allow any one transformer to be bypassed and taken out of service if required during maintenance or building works. Such systems may not quite meet the strictest interpretation of the ESCQR, although if the two rings are called "distribution", rather than "consumer" wiring, it is probably OK, and maybe the bulk of in-service designs that are like that pre-date 2002 anyway. Given the sites tend to be managed and all the maximum current levels are known the risk is better controlled than the random house on a street situation.

Mike

two conductors in parallel between the two links? So now, which is which?

Well indeed, labelling is a problem, though of course we all know that the electrons neither know nor care about the colour of the insulation being blue or green & yellow, so the current splits between the paths in inverse proportion to their resistance, and so, to be safe, both conductors must be sized accordingly much like any other conductors in parallel. The problem is one of naming - rather like the PNB - TNS  thing. 

Actually I understand it is not an uncommon situation on campus type installations with multiple parallel transformers with both HV and LV mains connected between buildings  to allow any one transformer to be bypassed and taken out of service if required during maintenance or building works. Such systems may not quite meet the strictest interpretation of the ESCQR, although if the two rings are called "distribution", rather than "consumer" wiring, it is probably OK, and maybe the bulk of in-service designs that are like that pre-date 2002 anyway. Given the sites tend to be managed and all the maximum current levels are known the risk is better controlled than the random house on a street situation.

Mike