Relying on loads not being able to overload

[EFLImpudence]

.... and is less worrisome since the reduction in nominal voltage. ... Is the nominal voltage now 220V + 14.5% - 1.8%

Wot's that all about?! Have I missed something? Whatever, it makes sense to me that the 'maximum Zs' figures quoted should be such that acceptable disconnection times are achieved even at the 'lowest permitted' supply voltage, regardless of how the permitted range of supply voltages is expressed!

Well, why was not this perceived worry about the voltage being less than nominal a concern when the nominal was 240V and the lower limit was more of a difference?

If the nominal voltage is 230 but values (Zs) to which we must work are calculated using a lower voltage (218 according to your chart) then 230 isn't really the nominal is it?

 

[JohnW2]

Well, why was not this perceived worry about the voltage being less than nominal a concern when the nominal was 240V and the lower limit was more of a difference?

You're asking the wrong person! As you imply, it should have been even more of a worry when there was a bigger difference between 'nominal' and 'minimum permitted'.

If the nominal voltage is 230 but values (Zs) to which we must work are calculated using a lower voltage (218 according to your chart) then 230 isn't really the nominal is it?

'Nominal' is surely just a bureaucratic/regulatory word which means absolutely nothing in electrical terms - given that any supply voltage between 216.2V and 253V is currently 'permitted', hence possible.

I'm not sure what you mean by "218 according to your chart".

Kind Regards, John

 

[EFLImpudence]

'Nominal' is surely just a bureaucratic/regulatory word which means absolutely nothing in electrical terms - given that any supply voltage between 216.2V and 253V is currently 'permitted', hence possible.

Nominal (230) means that which is used for calculation but in this case it is not 230.

I'm not sure what you mean by "218 according to your chart".

As you said, the maximum Zs's in 411 are Uo/Ia.
The values in your chart are 218/Ia.

 

[JohnW2]

'Nominal' is surely just a bureaucratic/regulatory word which means absolutely nothing in electrical terms - given that any supply voltage between 216.2V and 253V is currently 'permitted', hence possible.

Nominal (230) means that which is used for calculation but in this case it is not 230.

I reckon one can debate what 'nominal' means in this context. If one really wanted to be safety-conservative, one presumably would use 'worst-case' voltages i.e. the 'minimum permitted' or 'maximum permitted', depending on what one was calculating), regardless of the (essentially arbitrarily) stated 'nominal'.

I'm not sure what you mean by "218 according to your chart".

As you said, the maximum Zs's in 411 are Uo/Ia. The values in your chart are 218/Ia.

Ah, I see - when you say 'my chart', you mean the Amd3 proposals! I should have explained in more detail. In their infinite wisdom, in order to take into account the possibility that supply voltage is less than nominal, the IET are proposing to introduce a 'correction factor' of 0.95 (not 0.94), thereby allowing for voltage to be as low as 230V-5% (rather than 230V-6%) - as you say, 218.5V (not the 'permitted minimum' of 216.2V). I presume that this is not meant to make sense See....

Untitled

• JohnW2
• 6 Apr 2014
• 1

Kind Regards, John

 

[stillp]

John, you're quoting from a draft for public comment. If you think it doesn't make sense, please send your comment to BSI together with a proposal.
Please remember that BS7671 is not written by the IET, but by JPEL/64 of BSI.

 

[JohnW2]

John, you're quoting from a draft for public comment. If you think it doesn't make sense, please send your comment to BSI together with a proposal.

I certainly would have done so, but it's nearly a month since the stated closing date (8th March) for comments on the DPC. Do you think there is still a point in submitting a comment?

Unfortunately, when I read the DPC, I subconsciously thought that all the proposed changes we are discussing resulted from application of a correction factor ("Cmin") of 0.94, which would have made total sense (to me). It was only when EFLI pointed out yesterday that the proposed new Zs figures were based on a voltage of 218.5V (not 216.2V) that I revisited the document, and saw that they were actually proposing Cmin=0.95. Had I realised that a few weeks ago, I would certainly have submitted a comment (and proposal that they should consider Cmin=0.94).

Please remember that BS7671 is not written by the IET, but by JPEL/64 of BSI.

As you are aware, I know that, and I apologise for falling into the very common sloppiness of attributing everything to do with BS7671 to IET and/or BSI!

Kind Regards, John

 

[stillp]

John, as I've said to you previously, there's more point in submitting a comment after the cut-off date than posting on here. The former might have some result, the latter won't!

The point about BS7671 being drafted by the JPEL is not just a point of pedantry, since it is not often appreciated that the process for the OSG, GNs and so on is different, and is somewhat less 'democratic'.

 

[JohnW2]

John, as I've said to you previously, there's more point in submitting a comment after the cut-off date than posting on here. The former might have some result, the latter won't!

That's obviously true, but I think you're being a bit unfair in your implications. In no way did I set out to comment on, or criticise, the proposals in what I "posted on here" yesterday. I was merely explaining to EFLI (once I realised myself!), when he raised the point, the 'reason' why the proposed new "maximum Zs figures" were based on a supply voltage of 218.5V, rather than the permitted minimum of 216.2V - merely adding a comment which implied (as is true) that this didn't seem to make much sense to me.

However, in terms of my question to you, I was hoping that, with your experience, you might have some idea as to how strictly the BSI applies the 'closing dates for comments', but it seems that you don't. Do you by any chance know the date of the committee meeting which will consider (or has already conidered!) comments on the DPC?

The point about BS7671 being drafted by the JPEL is not just a point of pedantry, since it is not often appreciated that the process for the OSG, GNs and so on is different, and is somewhat less 'democratic'.

Yes, I accept (and know/'appreciate') all that. As I said, I was just being 'sloppy'.

Kind Regards, John

 

[stillp]

John, I was merely trying to encourage you to make a formal comment about the apparent inconsistency in the value chosen for Cmin. Explaining where you found it to EFLI is interesting perhaps to the two of you, but will not result in a change to the draft of BS7671 unless someone else makes the comment as a result of your posting.
I have asked one of the members of the JPEL for their comments on the choice of 0,95 for Cmin.

I thought you'd have interpreted my earlier remark as expressing the possibility that a late comment would be accepted. Some BSI Secretaries will accept late comments, others won't. I'm not a member of JPEL/64 so I don't know their Secretary, and I don't know the dates of their meetings.

 

[JohnW2]

John, I was merely trying to encourage you to make a formal comment about the apparent inconsistency in the value chosen for Cmin. Explaining where you found it to EFLI is interesting perhaps to the two of you, but will not result in a change to the draft of BS7671 unless someone else makes the comment as a result of your posting. I have asked one of the members of the JPEL for their comments on the choice of 0,95 for Cmin.

Fair enough. I'm pleased to hear that you feel the question is worth asking. It occurs to me that the reason why the proposal 'does not make sense' to me could be the result of a misunderstanding on my part, so maybe you or someone else (westie?) could clear this up for me?

As you will realise, my 'surprise' is based on my understanding that a UK electricity supply is ‘permitted’ (by whom?) to be as low as 230V - 6%, namely 216.2V. The proposed new Note 1 to 411.4.5 indicates that, for LV supplies “in accordance with ESQCR (2002, as amended)”, minimum Zs values should be calculated from U0 x 0.95 - i.e. 230V -5%, namely 218.5V. This almost implies that ESQCR imposes a minimum supply voltage of 230V - 5% (i.e. 218.5V), rather than 230V - 6% (i.e. 216.2V).

Can you (or someone) possibly clarify exactly what regulations say what about the minimum supply voltage in the UK? I really need to be clear about that before I can comment on the DPC without potentially making a fool of myself!

I thought you'd have interpreted my earlier remark as expressing the possibility that a late comment would be accepted. Some BSI Secretaries will accept late comments, others won't. I'm not a member of JPEL/64 so I don't know their Secretary, and I don't know the dates of their meetings.

Fair enough - I'll have a go (if/when my mind is clear about 'permitted minimum supply voltages' etc.). As you've said, there is nothing to lose. The frustrating thing, of course, is that they almost certainly won't act on my 'proposal', yet will provide me with no feedback regarding the rationale behind their proposal of Cmin=0.95. Maybe you will get a better understanding of their thinking from your JPEL colleague - in which case I would obviously be very interested if you were able to share that with us.

Kind Regards, John

 

[stillp]

The ESQCR and amendments are at http://www.legislation.gov.uk/all?title=electricity safety quality

I can't help you with the precise reference though.

Yes, if I get an answer from my JPEL friend I'll post it here.

 

[stillp]

From reg 27 of the ESQCR 2002:
(2) Unless otherwise agreed in writing between the distributor, the supplier and the consumer (and if necessary between the distributor and any other distributor likely to be affected) the frequency declared pursuant to paragraph (1) shall be 50 hertz and the voltage declared in respect of a low voltage supply shall be 230 volts between the phase and neutral conductors at the supply terminals.

(3) For the purposes of this regulation, unless otherwise agreed in writing by those persons specified in paragraph (2), the permitted variations are—

(a)a variation not exceeding 1 per cent above or below the declared frequency; .
(b)in the case of a low voltage supply, a variation not exceeding 10 per cent above or 6 per cent below the declared voltage at the declared frequency;

 

[JohnW2]

From reg 27 of the ESQCR 2002: ... (b)in the case of a low voltage supply, a variation not exceeding 10 per cent above or 6 per cent below the declared voltage at the declared frequency;

That's what I need. Very many thanks.

Kind Regards, John

 

[JohnW2]

I thought you'd have interpreted my earlier remark as expressing the possibility that a late comment would be accepted.

I clearly should have interpreted it in that way, and it seems that my efforts were not in vain ....

The committees are working on the comments, but I think yours is new, so I'll add it to the list. Thanks for sending it in.

I have to say that I'm very surprised - not so much because they are going to look at my late comment, but by the suggestion that my comment is 'new'. My comment that (in view of the ESQCR-permitted minimum supply voltage) I would have expected 'Cmin' to be be 0.94, rather than 0.95 seems so 'obvious' that I had imagined that many people would already have raised the same issue.

I suppose the upside of this is that, should the regs end up with Cmin=0.94, I will be able to satisfy myself with the knowledge that it was probably 'all because of me' (with some encouragement from stillp). Now we wait! Of course, if that happened, some electricians would be less pleased (by the requirement for 'even lower' Zs), and might send out a 'hit squad' to find me

Kind Regards, John

 

[EFLImpudence]

I'm not really bothered what figure we have to use as maximum Zs.


However, I am slightly irked by the methods being employed.

Why are new tables not just issued which apply this new nominal voltage?
Will there be ANY occasions when this new method will not be necessary?

So, we look in the table at the maximum Zs for a device and reduce it to 76% of this so-called maximum which now can never be used.

Some rewording is necessary in my opinion.
Perhaps three tables which will be headed:

'Nominal Zs' for OPDs. just for curiosity value
'Actual Zs' for low load circuits and
'OSG Zs' to be certain.