As we seem to be in a bit of a quiet period between revisions to BS 7671, I thought it might be a good opportunity to review what we already have and see if it could be improved.
There are a few of the existing regulations where the wording doesn't entirely make sense to me - I'm sure others have had similar problems. So what's your "favourite" cryptic regulation? What's wrong with it and (if you can) how could it be improved?
- Andy.
As we seem to be in a bit of a quiet period between revisions to BS 7671, I thought it might be a good opportunity to review what we already have and see if it could be improved.
To be honest, it's probably more efficient to comment on up-coming HD 60364 series standards, as these will be incorporated into BS 7671 in the future. At this stage, we are looking at accepted HD 60364 series standards, for which the UK is obliged,. by agreement with CENELEC, to adopt in BS 7671 at the next available update.
The only other thing we can do, is correct errors, or try and address ambiguities ... but if there is an issue with understanding the technical intent of the requirement in the relevant HD 60364 series document, that has to go back to CENELEC (and possibly IEC) committees to get a steer, or, sadly, wait until after that is ratified in the international standard!
For openers I'd like to nominate 462.1 (isolating/switching N):
This is the subject of a Special National Condition (SNC) in the Harmonized standard ... so, what does that mean?
Well, we can resolve the issue nationally, but if it results in an adjustment to the BS 7671 requirement, that JPEL/64 deems is a modification to the technical intent of the SNC, then we will have to apply to CENELEC for a change to the SNC, and potentially this has to happen prior to amending it in BS 7671.
it's probably more efficient to comment on up-coming HD 60364 series standards
Very interesting point. What's the process for HDs? - do they go through a DPC process like BS 7671 or are changes usually sponsored by national committees? (From purely an administrative view, with such a wide potential audience, I can see an advantage in having things filtered a bit...)
...that JPEL/64 deems is a modification to the technical intent of the SNC
So, in the case of 462.1, perhaps the first stage is to work out what the technical intent was. Yes, I know, ultimately only a court of law can decide on the meaning, but presumably there should be someone in the UK who knows what was originally intended (the current wording isn't that old IIRC) - the wording of the 2nd part especially simply doesn't make sense to me (mea culpa no doubt, but I'm unlikely to be the only one, and if a significant number of those that are meant to implement the regs can't understand what is required, we have a problem, regardless of the semantic correctness).
- Andy.
do they go through a DPC process like BS 7671 or are changes usually sponsored by national committees? (From purely an administrative view, with such a wide potential audience, I can see an advantage in having things filtered a bit...)
Yes, they are put out for public comment through the same BSI Standards Development Portal as BS 7671.
So, in the case of 462.1, perhaps the first stage is to work out what the technical intent was.
It is as simple as it's not necessary to switch the neutral in cases where bonding is in place, and either (a) or (b) applies.
(a) and (b) are in effect the same thing ... it's a reliably earthed TN system. BS 7671 can't second-guess a distributor or legislation, but with respect to (b) see Regulation 114.1 (2nd para).
So, if there is no bonding, you must switch the neutral, or it's not a TN system, you must switch the neutral (unless there's another exception ... there are odd exceptions for TT systems dotted around).
Second interpretation
In a TN-C-S or TN-S installation in which protective equipotential bonding has been installed, the neutral conductor may be left unswitched provided that either condition (a) or condition (b) applies.
It follows that if there is no protective equipotential bonding (e.g. because the other mains services arrive in plastic pipes) the neutral conductor may be left unswitched even if neither condition (a) nor condition (b) applies.
I think I'm allied with this one.
Some of this is the language of standardization, though ... which is dictated to us by ISO, IEC and BSI. I agree that the language used is not ideal for users of the standard, although we do provide training in the industry that is supposed to address that.
Some of this is the language of standardization, though ... which is dictated to us by ISO, IEC and BSI. I agree that the language used is not ideal for users of the standard, although we do provide training in the industry that is supposed to address that.
I did wonder at the beginning of this thread, whether there might be an element of linguistics. Some languages have moods which do not exist in others, which may apply particularly to conditional clauses. So, something may be clearer in one language than another.
It is as simple as it's not necessary to switch the neutral in cases where bonding is in place, and either (a) or (b) applies.
That bit I think I understand the words (even if I have queries about the logic), it's the "the neutral conductor is reliably connected to Earth by a low resistance to meet the disconnection times of the protective devices according to the requirements of Chapter 41" that's got me confused.
Say I have a rural PME (or PNB) supply where the supply's electrode has a resistance of close to 20Ω - how does that mesh with Chapter 41?
- Andy.
Say I have a rural PME (or PNB) supply where the supply's electrode has a resistance of close to 20Ω - how does that mesh with Chapter 41?
see Regulation 114.1 (2nd para).
In a TN system, the earth fault path does not include Earth, so I'm not sure how a particular earth electrode resistance value helps ...
I think that there is another facet to this discussion, which was not obvious to me initially.
It is easy to think of protective equipotential bonding in terms of 411.3.1.2. However, that is only concerned with extraneous-conductive-parts.
The definition of equipotential bonding (page 39) includes exposed-conductive-parts as well. So, the bolts which attach the earth bar inside a DB to the casing provide not only support, but also equipotential bonding.
It follows that virtually all modern domestic and light commercial installations will have some bonding.
461.2 does little more than re-state Reg 8 of ESQCR 2002.
Given the exception in 461.2, 462.2 would seem to apply almost entirely to TT and IT systems.
It follows that virtually all modern domestic and light commercial installations will have some bonding.
Agreed, where ADS is used.
see Regulation 114.1 (2nd para).
Yes, but that only mentions the reliability of that connection, not its resistance.
In a TN system, the earth fault path does not include Earth, so I'm not sure how a particular earth electrode resistance value helps ...
Exactly, so what are they trying to get at with the phrase "the neutral conductor is reliably connected to Earth by a low resistance to meet the disconnection times of the protective devices according to the requirements of Chapter 41" when the that resistance has no relevance to disconnection times?
- Andy.
The definition of equipotential bonding (page 39) includes exposed-conductive-parts as well.
But is that just covering the case of supplementary bonding (to/between exposed-conductive parts) rather than c.p.c.s. in general - given that a very significant proportion of the supply voltage can be dropped along a c.p.c. during a fault (and in the case of distribution and larger final circuits, for a significant length of time).
- Andy.
But is that just covering the case of supplementary bonding (to/between exposed-conductive parts) rather than c.p.c.s. in general - given that a very significant proportion of the supply voltage can be dropped along a c.p.c. during a fault (and in the case of distribution and larger final circuits, for a significant length of time).
Since BS 76781:2008, bonding to e-c-p's is only applied 'to each building' ... but all we are doing here, is highlighting potential ambiguities in the web of related requirements, not a single requirement.
But is that just covering the case of supplementary bonding (to/between exposed-conductive parts) rather than c.p.c.s. in general - given that a very significant proportion of the supply voltage can be dropped along a c.p.c. during a fault (and in the case of distribution and larger final circuits, for a significant length of time).
Since BS 76781:2008, bonding to e-c-p's is only applied 'to each building' ... but all we are doing here, is highlighting potential ambiguities in the web of related requirements, not a single requirement.
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