installation of a ring cicuit
- Thread starter davidbrideaux
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By anaolgy with 433.1.5 that would (to my mind) make total design sense, and I'm sure would be at least as safe as the deemed-to-satisfy 2.5mm²/32A MCB RFC (as I said, I suspect a designer could probably justify a 26A MCB, if (s)he could find one). However, I suspect there may be dissenting voices here.Use 2.5mm for the ring and fit a 20 amp MCB or RCBO, job done
Kind Regards, John.
I think you'll find you've got that completely AAF.
I wasn't debating anything for its own sake - I was trying to get ricicle and JohnW2 to see that they were wrong to suggest to the OP that a solution might be to install a ring final which does not comply with 433.1.5.
I wasn't debating anything for its own sake - I was trying to get ricicle and JohnW2 to see that they were wrong to suggest to the OP that a solution might be to install a ring final which does not comply with 433.1.5.
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That 25A Hager breaker is in fact a 16A...
Yes you have, over and over again.
That is the purpose of this topic.
Every time you suggested, or supported the suggestion, that a non-conventional design could be used you were suggesting it, or supporting the suggestion of it, to the OP.
Unless of course your increasingly tedious insistences were not, in your mind, anything to do with the OPs question and as far as you were concerned this topic was to become another vehicle for your vexatious meanderings.
IN
THIS
CASE
IT
IS
THE
ONE
AND
ONLY
ACCEPTABLE
BASIS.
BAS, I know you can read, so you really ought to be able to understand the difference between (the two quotes from me which you seem to think are contradictory):
Kind Regards, John
and
If you need a hint, try studying the conditional clause at the start of the first one.
Kind Regards, John
The whole point of 433.1.5 is to allow ring final circuits which would otherwise contravene 433.1.1 to exist.
Yes, I agree that it indicates a way in which such circuits can be allowed to exist.
Kind Regards, John.
Oh, go on; I can't resist this...
John,
Trained electricians* are universally** incapable of designing ring circuits 'outside the box'. Indeed most of them are utterly unaware of the existence of Reg 433.1.5 and have never been taught any relevant electrical theory regarding the design of rings.
This is mostly because their lecturers do not understand either and because rings are installed for one reason and one reason alone - it's been done, without thought or question, since 1947.
Because of this general ignorance, rings should not be the design of choice for socket circuits, as B-A-S has spent some considerable time trying to convince you. I'm not saying (and have never said) you should never use a ring, but I am adamant that our electricians install rings precisely because they do not understand clearly what a ring is all about.
Notes:
* That's electricians, as distinct from, say, graduate electrical engineers.
** As close to 100% as is statistically measurable - I've never met one and I meet hundreds.
John,
Trained electricians* are universally** incapable of designing ring circuits 'outside the box'. Indeed most of them are utterly unaware of the existence of Reg 433.1.5 and have never been taught any relevant electrical theory regarding the design of rings.
This is mostly because their lecturers do not understand either and because rings are installed for one reason and one reason alone - it's been done, without thought or question, since 1947.
Because of this general ignorance, rings should not be the design of choice for socket circuits, as B-A-S has spent some considerable time trying to convince you. I'm not saying (and have never said) you should never use a ring, but I am adamant that our electricians install rings precisely because they do not understand clearly what a ring is all about.
Notes:
* That's electricians, as distinct from, say, graduate electrical engineers.
** As close to 100% as is statistically measurable - I've never met one and I meet hundreds.
Who would be capable of designing them completely out of the box?
Imagine that we had never had them.
Imagine that 433.1.5 and its predecessors had never existed.
Imagine that all we had (in this narrow context) was 433.1.1.
Who would be capable of designing a ring final, to which they could with professional hand on heart apply the declaration on an EIC?
And how would they go about it?
In the absence of any genuine information to the contrary we have to assume that the IEE changed 433-02-04 from requiring Iz to be ≥ 0.67xIn to requiring it to be ≥ 20A in order to be deemed to comply with 433-02-01 for a reason.
The 0.67 ratio may once have given some support for a "non-standard" ring (even though the standard then was still an OPD of 30/32A), but that support was explicitly removed in February 2002.
I think you're being a little harsh on electricians, dingbat. Even if they've been taught the theory, or even if they'd intuitively acquired an understanding of why ring finals are allowed (they aren't that hard to get your head round), and even if the electrician can see that a ring final with a 20A OPD and a cable with a CCC ≥ 13.4A is just as safe as a 32-20 one, on what basis could he install one and declare it to be compliant with BS 7671?
I'm not an electrical engineer either, so I don't know what's involved in doing the design and verification.
But I do know that it's not "Feels OK to me, I can see how it would work in the same way, I'll do it and say it complies".
Imagine that we had never had them.
Imagine that 433.1.5 and its predecessors had never existed.
Imagine that all we had (in this narrow context) was 433.1.1.
Who would be capable of designing a ring final, to which they could with professional hand on heart apply the declaration on an EIC?
And how would they go about it?
In the absence of any genuine information to the contrary we have to assume that the IEE changed 433-02-04 from requiring Iz to be ≥ 0.67xIn to requiring it to be ≥ 20A in order to be deemed to comply with 433-02-01 for a reason.
The 0.67 ratio may once have given some support for a "non-standard" ring (even though the standard then was still an OPD of 30/32A), but that support was explicitly removed in February 2002.
I think you're being a little harsh on electricians, dingbat. Even if they've been taught the theory, or even if they'd intuitively acquired an understanding of why ring finals are allowed (they aren't that hard to get your head round), and even if the electrician can see that a ring final with a 20A OPD and a cable with a CCC ≥ 13.4A is just as safe as a 32-20 one, on what basis could he install one and declare it to be compliant with BS 7671?
I'm not an electrical engineer either, so I don't know what's involved in doing the design and verification.
But I do know that it's not "Feels OK to me, I can see how it would work in the same way, I'll do it and say it complies".
I don't doubt that is largely true, although I'm sure there are some exceptions. Nor (and this is going to be controversial) do I really think that electricians should be expected to do 'from first principles' design. In many walks of life, we've seen blurring of the distinction between designers and 'do-ers' and I'm far from convinced it's a good thing - for a start because (a) it usually ends up as an imperfect compromise and (b) it often represents inefficient use of dfferent skill sets.
Again, I more-or-less totally agree with all that. I don't think that 433.1.5 is actually a problem. Indeed, it is the one part of the regs which allows an electrician to install an RFC on a 'cookbook' basis without having any knowledge of how the design came about, or how they would set out to design an RFC 'from scratch' themselves.
I suppose the regs themselves are much of the problem. The Building Regs are much better in this respect. Whilst they generally do present a lot of detail about how structures should be designed (by engineers or architects) 'from first principles' they also have countless deemed-to-satisfy provisions, with tables of such all over the place. It clearly is crazy to expect professionals to re-invent the same wheel hundreds of times per day, and all the tabulated deemed-to-satisfy stuff in the building regs avoid the need for that in most ordinary situations. If the Wiring Regs were similar, there would be far less need for many of the debates we see here and, much more to the point, no need for electricians to attempt to undertake design exercises for which they may not be adequately trained.
Even though they have 'done us proud' (despite dramatically increasing numbers of electrical appliances to be supplied) in the UK for around 60 years, I'm not really a lover of rings; I'm sure they are an example of one of those things (like tobacco and alcohol) which would never have been allowed if they had first shown their faces in the later stages of the 20th century! This particular thread has not been about the pros and cons of rings, and BAS has not attempted to convince me, in this thread, that they are not the ideal choice for socket circuits. In any event, as I said, I don't need convincing of that.
Given that adequately trained people are generally not involved in the design process of individual installations, that's a very reasonable attitude. I guess some of this comes back to the inadequacy of the regs - in only offering one deemed-to-supply design for an RFC. If it offered a range of options, electricians would thereby be able to select the one that bst suited the circumstances without needing the engineering knowledge to be able to design one from scratch. As is apparent from the question which started this thread, this is now a particular issue given that installation methods can result in the CCC of 2.5mm² T&E falling below 20A - which currently means that the regs offer no explicit solution which would enable such cable to be used in a ring.
Kind Regards, John
I wasn't debating anything for its own sake - I was trying to get ricicle and JohnW2 to see that they were wrong to suggest to the OP that a solution might be to install a ring final which does not comply with 433.1.5.
Yes - and I did mention later that I shouldn't be advocating that a DIYer should design outside the scope of the regs.
But just clear one thing up for me - are you saying that a competant electrician should not wire one outside the guidance of BS7671 ?
Where did he do that ?
Many thanks for that interesting historical information. It has caused me to think again, and modify my position somewhat (see below).
As you imply, ≥20A is not different from 0.6-0.67*In for an In of 30/32A - but, as you say, there was obviously some reason why they changed it. So that's got me thinking....
....a design problem I had not previously considered is that, whilst it is perfectly reasonable (in engineering terms) to undertake pro-rata 'scaling' of the Iz and In, one cannot similarly scale the size of the individual loads likely to be applied to the circuit (any of which, including those right at one end of the ring, could theoretically be up to 13A). Hence, despite what you say (which is also what I had been thinking), it is not necessarily true that a (Iz/In) 11.4A/20A RFC is "just as safe" as a 20A/32A one.
The problem is this. Design of an RFC necessarily has to make some assumptions/guesses about likely/possible distributions of loads around the ring (which is why, in engineering terms, RFCs are not too good an idea). Since the loads are not 'scaled down', it is just as likely that a 10A or 13A load will be plugged in right at one end of a 20A ring. That means that most/all of the CCC of one leg of the ring would be 'used' up by just that single load, leaving it pretty likely that adding additional loads to the ring (within the 20A design maximum) would result in the current in that cable exceeding its Iz. In contrast, if the same single load were plugged in at one end of a 32A ring, it would not 'use up' much more than about half of the short leg's CCC. Put another way, given that single loads of up to 13A are theoretically possible, it is far easier to exceed the Iz of one leg of the ring with a 20A ring than a 32A one if one fairly large part of the load is applied near one end of the ring - so that needs to be factored into the design process, and this is perhaps why (with 13A single loads possible) the IET decided on a minimum Iz of 20A for RFCs.
I'm not saying that, in engineering terms, an 'unconventional' RFC definitely cannot be designed, but it's now apparent that it's not a simple 'pro-rata' exercise, and it's probably not something that one could reasonably expect the average electrician to undertake.
I guess this further underlines my recent comment that it's a great pity that the regs don't offer a range of deemed-to-satisfy RFS designs, rather than just one.
Kind Regards, John.
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