(spin-off) Diversity and cooking appliances

[riveralt]

Wow another mountain being created out of a molehill!!
What a load of hot air JohnW2 - over the simple design of a domestic kitchen circuit and that 'thorny?' issue of diversity. Which merits just one sentence in whole of the BGB.

Whichever method is used it is down to the installation designer to determine the best way of safely energising the appliances and more importantly adopt a 'real world' common sense attitude - which for once the BGB seems to support when its keeps its nose out of the issue.

I have read this post and the original a number of times and cannot understand what the big fuss is about.
For some reason the 3kw microwave has appeared and apparently thrown the whole world of circuit design into turmoil - how many domestic microwaves come anywhere near 3kw - very few if any I will wager.
Even when one comes close in total power - like EXEMPLARISK (I have a similar model) from Ikea with its Grill power 1600w, forced air power 1200w and microwave 900w. It still comes with a 13A plug attached and like in my house is plugged into the kitchen ring final circuit - though I would have no problem in incorporating it into the oven and hob circuit as well.
Yes there will be times when those in 3 phase mansions like you have several kitchens about the house (one for the servants I hope ) or even one supersized kitchen and it may be more prudent share the work load with different circuits - but that is down to the designer.

 

[JohnW2]

...that 'thorny?' issue of diversity. Which merits just one sentence in whole of the BGB.

Indeed - and even that nothing to do with diversity in final circuits - yet so many people seem to think that the guidance in the OSG represents some sort of gospel truth that can and should be applied to the design of final circuits without any thought.

Whichever method is used it is down to the installation designer to determine the best way of safely energising the appliances and more importantly adopt a 'real world' common sense attitude - which for once the BGB seems to support when its keeps its nose out of the issue.

Precisely - yet I've never seen/heard anyone suggesting that, by application of such 'real world common sense', they judged that an allowance for diversity as large as suggested by OSG was not appropriate in the particular circumstances concerned. All I ever hear of is boxes being ticked on the basis that 'the design is OK after (OSG) diversity'.

For some reason the 3kw microwave has appeared and apparently thrown the whole world of circuit design into turmoil - how many domestic microwaves come anywhere near 3kw - very few if any I will wager.

We're talking about combination microwave ovens and, AFAIAA, they generally are (like mine - see below) between 2kW and 3kW when operating in 'combination mode'. I think that many only just manage on a 13A plug. Am I mistaken?

Yes there will be times when those in 3 phase mansions like you have several kitchens about the house (one for the servants I hope ) ...

Nope, the servants must have been stuck with candles and camping gas stoves ... one kitchen for me, one still surviving in what was the 'granny flat' and the 'remnant' in what was the 'great-granny flat', but since both of the latter have been dead for 20+ years, the latter two kitchens have not been used for a good while! ... and, ourselves, we don't have an electric cooker at all, other than a combi-microwave (IIRC, 1,700W of elements plus 900W output microwave).

Kind Regards, John

 

[JohnW2]

It still comes with a 13A plug attached and like in my house is plugged into the kitchen ring final circuit - though I would have no problem in incorporating it into the oven and hob circuit as well...

Maybe you (and many otehrs) wouldn't have a problem - but it's interesting that, in other discussions, some people are only two quick to cite Appendix 15 of the regs as saying that "cookers, ovens and hobs with a rated power exceeding 2kW [should be] supplied by their own dedicated radial circuit".

Kind Regards, John

 

[riveralt]

It still comes with a 13A plug attached and like in my house is plugged into the kitchen ring final circuit - though I would have no problem in incorporating it into the oven and hob circuit as well...

Maybe you (and many otehrs) wouldn't have a problem - but it's interesting that, in other discussions, some people are only two quick to cite Appendix 15 of the regs as saying that "cookers, ovens and hobs with a rated power exceeding 2kW [should be] supplied by their own dedicated radial circuit".

Appendix 15 is informative = apply some real world common sense.

 

[ericmark]

In general we design so it will fail safe and even if the diversity calculations are wrong it will not really matter some MCB/RCBO will trip. OK a little different with a fuse but unlikely we are today fitting a kitchen on a fused supply other than the DNO.

The only real problem is built in ovens. The question is do they fail safe? If we look at Christmas with a turkey which has been in many hours and the power fails will the oven over heat the carcass when there are no cooling fans.

Because the carcass is made by a different manufacturer to the oven there is no way to work out what heat will end up in the carcass should the cooling fans fail much will depend on the mass of the oven.

This is about the only danger in wrong estimation of diversity. And to be honest silly County Council instructors for the elderly who tell them to switch off the oven at the wall once finished are far more dangerous than any electrician who gets the diversity wrong.

 

[JohnW2]

In general we design so it will fail safe and even if the diversity calculations are wrong it will not really matter some MCB/RCBO will trip.

Indeed that's exactly the point I made to Detlef. Diversity itself is NOT (or should not be!) a safety issue – which is perhaps why the regs are so silent about diversity. As I wrote, provided that cable in question is protected against overload by an appropriate OPD, the actual size of the load is irrelevant as far as safety of the cable (or anything else) is concerned. In a domestic environment, appropriate application of diversity is therefore essentially only a convenience issue.

If, rarely, (as per your Christmas turkey example) dangers arise because an appliance loses power, I don’t think that one can/should blame the cause of the power loss for that danger (whether the cause be over-generous application of diversity, resulting in OPD operation, a ‘power cut’ or anything else). In such a situation the culprit is a dangerous (poorly designed) appliance.

How this is fits in with the regs (which don’t mention diversity in final circuits at all) is a more complicated question. 433.1.1(i) requires that a circuit’s OPD should have an In no less than the ‘design current’ of the circuit (which therefore defines minimum cable CSA) – and "design current" is, in turn, defined as the “current to be carried by the circuit in normal service”. There is no mention of diversity. If someone is excessive/stupid in applying diversity, then I presume that would be non-compliant with 433.1.1(i), but what the regulations really ‘think’/intend about the application of ‘standard’ (OSG) diversity is anyone’s guess. Unless the appliance has built-in current limiting, we all know that it may well sometimes draw more current than the calculated ‘after-diversity’ figures for at least some period of time – but whether the regs would regard that as “in normal service” or not, I haven’t got a clue.

The ideal would presumably be to take the statistical (hence uncertain) elements out of the equation and to require that all cooking appliances which drew more than 10A had built-in current limiting – in which case we could forget about diversity and use that ‘current limit’ as the design current (essentially as we already effectively do with appliances designed to work with a 13A pug).

Kind Regards, John

 

[JohnW2]

... it's interesting that, in other discussions, some people are only two quick to cite Appendix 15 of the regs as saying that "cookers, ovens and hobs with a rated power exceeding 2kW [should be] supplied by their own dedicated radial circuit".

Appendix 15 is informative = apply some real world common sense.

Indeed so (and the OSG could be said to have even lower 'status', not even being part of 'the same book' as the regs). However, as I said before, it seems common to see ‘blind’ adherence to guidances, rather than the application of common sense.

I doubt that I am by any means unique in having a combi-microwave (rather than a simple microwave) primarily so that it can serve as a back-up cooking facility in the event that my ‘primary cooker’ becomes unusable. If that primary cooker were electric (which, as it happens, mine isn’t), I would therefore regard it as common sense to have the combi-microwave on a different circuit.

This is particularly relevant in relation to the reason that this whole discussion arose. EFLI suggested that an OP’s hard-wired combi-microwave should be on the same circuit as his oven and hob, specifically so that all three appliances could be served by a single ‘emergency switch’/isolator. If that advice were taken, not only would failure of the oven/hob supply result in the microwave also being unusable, but a fault in oven or hob which required that appliance to be ‘isolated’ would also take out the entire cooking facilities, including the microwave. You and others may disagree, but this does not correspond with my view of ‘common sense’.

Indeed, one can even argue (for similar reasons) that if one just has an oven and separate (electric) hob, common sense would favour having those two things on separate circuits (or, at the very least, with separate isolators), to maximise the chances of being left with at least some cooking facility in the event of a circuit or appliance problem.

... but maybe what I call 'common sense' you would regard as stupidity?

Kind Regards, John

 

[riveralt]

I would therefore regard it as common sense to have the combi-microwave on a different circuit.

This is particularly relevant in relation to the reason that this whole discussion arose. EFLI suggested that an OP’s hard-wired combi-microwave should be on the same circuit as his oven and hob, specifically so that all three appliances could be served by a single ‘emergency switch’/isolator. If that advice were taken, not only would failure of the oven/hob supply result in the microwave also being unusable, but a fault in oven or hob which required that appliance to be ‘isolated’ would also take out the entire cooking facilities, including the microwave. You and others may disagree, but this does not correspond with my view of ‘common sense’.

I'm not sure that EFLI actually said 'should' I think he said what his preference was and was there a reason why the OP had not thought of that.

I have no reason to disagree with your view of common sense. The point I am making is that EACH installation is different and every customer has different requirements. Diversity is just another tool in the bag for calculating the most appropriate circuit design to meet the customers needs. As long as the pros and cons are discussed, the installation is safe and the customer is happy - carry on.

I think paragraph 2 of the Red OSG say it all (about the application of diversity):

"The information and values given in this appendix are intended only for guidance because it is impossible to specify the appropriate allowances for diversity for every type of installation and such allowances call for special knowledge and experience.
The values given in Table 1b, therefore, may be increased or decreased as decided by the installation designer concerned.
No guidance is given for blocks of residential dwellings, large hotels, industrial and large commercial premises; such installations should assessed on a case by case basis."

 

[JohnW2]

... EFLI suggested that an OP’s hard-wired combi-microwave should be on the same circuit as his oven and hob, specifically so that all three appliances could be served by a single ‘emergency switch’/isolator.

I'm not sure that EFLI actually said 'should' I think he said what his preference was and was there a reason why the OP had not thought of that.

EFLI did, indeed, write exactly what you say, but is that really materially different from what I wrote? - I did say that he 'suggested that they should be on a single switch', not that he said that they 'should be on a single switch'. Whatever, regardless of words, my point remains that my personal version of common sense would not have a common isolator for an appliance and its potential 'back-up'!

I have no reason to disagree with your view of common sense. The point I am making is that EACH installation is different and every customer has different requirements. Diversity is just another tool in the bag for calculating the most appropriate circuit design to meet the customers needs. As long as the pros and cons are discussed, the installation is safe and the customer is happy - carry on.

I couldn't agree more but, as I keep saying, it seems only too common for people to 'blindly' follow guidances and regulations, without necessarily having applied enough thought, let alone 'common sense'. For example ....

I think paragraph 2 of the Red OSG say it all (about the application of diversity): "The information and values given in this appendix are intended only for guidance because it is impossible to specify the appropriate allowances for diversity for every type of installation and such allowances call for special knowledge and experience. ... The values given in Table 1b, therefore, may be increased or decreased as decided by the installation designer concerned.

As you say, that paragraph should 'say it all' - but, again, that is, IMO, precisely what so many people don't do - even when the guidance tells them to! Rather, they take those 'guidances' as 'commandments', not the least becasue they can then use them as 'justifications' of what they have done (i.e. to protect their backsides). Despite what that paragraph of the OSG says, I would suggest that you would probably have to look pretty hard to find (m)any cases in which an electrician has used his/her 'special knowledge and experience' to decide that a greater allowance for diversity than suggested in the OSG was applicable in a particular situation. Am I wrong?

As ought to be apparent from some of the things I write, I am almost a 'champion' of professional electricians. I would love to see them making more use of their knowledge, experience, personal opinions and 'common sense' to make decisions of their own (what I would call 'professionalism') rather than relying on 'rules, regulations and guidances' - whether that be in relation to decisions about diversity, deciding what non-compliances with current regs they 'personally recommend' should be addressed, or whatever.

Kind Regards, John

 

[EFLImpudence]

I know you and we all like a good discussion but I do think it somewhat unfair to pick on a perfectly reasonable thread where nothing dangerous or outlandish is being proposed and question the proposals by introducing all possible situations where the said proposals would not be applicable.

Suggesting that higher tolerances for diversity should be allowed when the situation in question was nowhere near the limit in the first place is not really appropriate.

This is much the same as threads about bonding and earthing where someone will quote a very unlikely occurrence where it would have been better if your bath had been earthed.
I say 'earthed' because you cannot 'bond' what does not require bonding.


Also, while it is true that the topic title referred to isolators which you can have as many as you want anywhere you want, my posts were regarding emergency switches.
I do not know if the OP knows the difference or meant what he wrote.

My first reply -
'Cooker switches' are not actually mandatory so you can put them anywhere you want (within reason) although it defeats the object of them if you hide them.
Personally, I would prefer only ONE switch as they are (it is) for emergency switching.
Do you have a reason for not putting all three appliances on the same circuit.

 

[JohnW2]

I know you and we all like a good discussion but I do think it somewhat unfair to pick on a perfectly reasonable thread where nothing dangerous or outlandish is being proposed and question the proposals by introducing all possible situations where the said proposals would not be applicable.

That's precisely why I moved this out of that thread. As I'm sure (hope) you understand, there is nothing 'personal' in any of what I've said. Anyone could have written what you did - it just happened to be you.

Suggesting that higher tolerances for diversity should be allowed when the situation in question was nowhere near the limit in the first place is not really appropriate.

Yes, I probably did get a bit carried away with the diversity argument (to the extent of omitting to make an important point - see below) - although I do have some uneasinesses about it. As someone (was it viewer) suggested yesterday, I have a feeling that the guidance in the OSG may not be anything like as 'appropriate' in 2013 as it was when it was conceived. As I wrote to eric today, in an ideal world we would probably have only current-limited cooking appliances, and hence no need (or much less need) to play statistical games with diversity in final circuits (diversity always will be a crucial consdieration at the level of whole installations or networks). Whatever, as you will have gleaned from this thread, one of my main reservations about having a common isolator for all of the cooking appliances has nothing to do with diversity - although I got so carried away with diversity that I forgot to mention it in the original thread!

This is much the same as threads about bonding and earthing where someone will quote a very unlikely occurrence where it would have been better if your bath had been earthed.I say 'earthed' because you cannot 'bond' what does not require bonding.

As you've probably seen, I've just contributed to such a thread, and I think/hope that you'll be happy with what I wrote. I am not Bernard.

Also, while it is true that the topic title referred to isolators which you can have as many as you want anywhere you want, my posts were regarding emergency switches. I do not know if the OP knows the difference or meant what he wrote.

Again, please don't feel the need to justify yourself or be 'defensive' in any way. As I said, I was not 'getting at you' in any way, and if it appeared as such, I apologise.

Kind Regards, John