Ovens, plugs, kitchen rings and diversity

[JohnW2]

So what we are looking at is how far can we stretch the limits. So a built in oven in a carcass with nothing above the oven having a 16A oven on a 13A supply is unlikely to cause a problem even if the fuse blows. But same oven with cupboard above is another question. In ones own home where you can see if anything daft is put in the cupboard OK but in some one else's home is it worth the risk?

I have to say that I find all this a bit far-fetched. Once the fuse has blown, there is no longer any electrically generated heat, and do we really need to worry about what, say, a hot turkey in the (no longer heated) oven might do to the contents of a cupboard two or three (or more) feet above?? !!

So what we as saying is should we follow BS7671 blindly or should we use some common sense when we see for a set situation breaking the rules will cause no danger.

I know the answer to that for myself, but, as I'm always saying, some (maybe many) electricians feel constrained by employers and/or insurers and/or Scheme operators to work to the strict word of the regs, even when that prevents them from exercising common sense.

Kind Regards, John

 

[EFLImpudence]

This is perhaps what you were implying, but it's just occurred to me that maybe the person(s) who came up with the "10A + 30%" formula, a long time ago, may have thought like you (i.e. didn't 'like' time-diversity alone) ...

oooooo kkkkkkkk

IF they shared your feeling that it was not appropriate to apply diversity to a single oven, and IF it was an era when most cookers had single 2-3 kW oven plus hob (separate ovens and hobs, and double ovens, then being unusual), then "10A + 30%" would more-or-less mean that they weren't allowing any diversity for the oven, but were allowing everything else to be considered as only 30% of its rated power.

No, I wouldn't think that; there is no reason to do it like that as the whole appliance would be on a 30A fuse and all the elements (oven and hob) would be cycling together - as you say below.

Of course, even for you, that probably doesn't work properly - since, if I understand correctly, the moment one has multiple independent loads (oven + hob) you then would feel it appropriate to apply diversity to everything (including the oven).

I admit I have not really thought about the time factor before because I have always though/presumed that diversity (10A+30%) was a method of judging a limited current throughout the time rather than limit the time the full current is drawn.

After all, after the possible, although unlikely, occurrence of switching on all the loads at once the maximum current will never (well, highly unlikely) be drawn

 

[JohnW2]

IF they shared your feeling that it was not appropriate to apply diversity to a single oven, and IF it was an era when most cookers had single 2-3 kW oven plus hob (separate ovens and hobs, and double ovens, then being unusual), then "10A + 30%" would more-or-less mean that they weren't allowing any diversity for the oven, but were allowing everything else to be considered as only 30% of its rated power.

No, I wouldn't think that; there is no reason to do it like that as the whole appliance would be on a 30A fuse and all the elements (oven and hob) would be cycling together - as you say below.

Fair enough - it was just a thought!

I admit I have not really thought about the time factor before because I have always though/presumed that diversity (10A+30%) was a method of judging a limited current throughout the time rather than limit the time the full current is drawn.

Well, what I suspect is going on mathematically is that 'they' are looking at 'current x time' (or, more precisely, current integrated over time), which is effectively the 'average current' over a period of time - and that is influenced by both the number of loads 'on' simultaneously and the 'duty cycle' (proportion of 'on' time) for each individual load. However, as far as the cable (and maybe also OPD) is concerned, it's probably I², rather than just I, that matters, since that will be a measure of the amount of heat generated and dissipated.

After all, after the possible, although unlikely, occurrence of switching on all the loads at once the maximum current will never (well, highly unlikely) be drawn

This may be another problem in your thinking. Whether "highly unlikely" is even remotely true depends crucially on the 'duty cycle' (on/off time ratio) of each of the loads. If that ratio is fairly high, you might be surprised by how often, and for how long, all were on simultaneously.

If you, or anyone else, can give me some clue as to what sort of duty cycles to expect of, say, a hob element in service (I have no electric hobs, so I can't experiment myself!), I'll run some simulations to illustrate to you (and discover for myself) what the situation actually is!

Kind Regards, John

 

[ericmark]

I am still getting my head around the time theroy. OK with a mark/space of 1/3 and a time between each on time of say 60 seconds then an element with a resistance of say 6 Ω would average out at 13A. But as we increase the time between the on time then we get to the point where the peaks can rupture the fuse.

I have noted the induction hob has a much shorter time between the on period then a halogen hob and the halogen hob has a shorter time than with a hot plate.

To say lights are only used at night so 2300W is OK as average is only 1150W is clearly flawed but my computer power supply rated at 5A can be considered as 2A because the mark/space ratio is on average only drawing that current.

So with the oven we are looking at the time between switch on and the thermostat taking over at the highest temperature it can be set to. Is the current drawn on initial heat up enough to cause degrading of the fuse? Answer that and we can progress.

 

[JohnW2]

I am still getting my head around the time theroy. OK with a mark/space of 1/3 and a time between each on time of say 60 seconds then an element with a resistance of say 6 Ω would average out at 13A. But as we increase the time between the on time then we get to the point where the peaks can rupture the fuse.

Yes, with the figures you suggest, it's very possible that the fuse would blow. The 'on' current would be about 38A and, per the curves I linked to recently, a 13A BS1362 carrying that current would blow in something between 0.5 and 90 seconds - hence quite likely to blow with the 30 second 'on' time you suggest.

However, your figures are much more extreme than those in the case we were discussing, and I would not expect one to be able to 'get away with' what you are suggesting - you are talking about trying to run a load of about 8.8 kW (about 38A) off a 13A supply. If one applied 'standard diversity' to your figures, the after-diversity current would be about 18.4A - clearly not suitable for a 13A circuit. ... 'diversity over time'obviously does not give you a carte blanche to put as large a load as you want on as low-rated a circuit as takes your fancy!

What we were actually discussing was a 4.68kW oven which drew around 20A (as compared with your 39A) - i.e. a resistance of about 11.5 Ω. That scenario is totally different - the curves suggest that a 13A BS1362 fuse will always carry about 21A indefinitely - so the fuse wouldn't be expected to blow, no matter what the mark-space ratio and no matter how long the on-times. ... in this case a 'standard diversity' calculation gives an after-diversity current of about 13A, seemingly confirming that a 13A circuit is appropriate.

So with the oven we are looking at the time between switch on and the thermostat taking over at the highest temperature it can be set to. Is the current drawn on initial heat up enough to cause degrading of the fuse? Answer that and we can progress.

I haven't a clue about degrading of the fuse. All I can tell you from the curves is that a 4.68 kW/20A element should never blow a 13A BS1362 fuse, even if that current flowed continuously and indefinitely.

Kind Regards, John

 

[ericmark]

I can see your point. But any fuse or MCB overloaded for an extended time will oxidise the heating element or fuse at an accelerated rate so yes may last 6 months but it will in the end rupture or start tripping more and more.

The shower is the one we have likely all seen where a 32A MCB is feeding a 40A shower and for 6 months to a year all seems OK and then it starts to trip or the fuse blows.

Keeping a 13 amp fuse hot for an extended time will also anneal the contacts and on fuse replacement likely there will be a bad contact on the new fuse and the rot sets in.

Even the socket starts to suffer as the heat is transmitted down the pins.

Yes it will work for 6 to 12 months then it will become a constant source of problems.

 

[JohnW2]

I can see your point. But any fuse or MCB overloaded for an extended time will oxidise the heating element or fuse at an accelerated rate so yes may last 6 months but it will in the end rupture or start tripping more and more.

What you say makes sense, but I have no idea as to how large or significant these effects are in practice. It's obviously a situation likely to arise whenever any sort of diversity is invoked (since the reason is usually to allow one to use a cable and OPD 'not rated' to carry the peak current) - so maybe a price to be paid for applying diversity is always a (I suspect pretty small) decrease in the life of the OPD.

Are you suggesting that you don't like relying on diversity at all?

Kind Regards, John

 

[winston1]

I give up.

Thank God for that. Do you mean it?

I give up too. You don't seem to be capable of comprehending that the nominal voltage does not exist in the (non)sense you "think" it does, and never has. The UK used to use 240V as a nominal value, even though that was only one point in the range of values the electricity supply was allowed to vary between. It now uses 230V, which is also one point in the range of values the electricity supply is allowed to vary between.

I realise that your inability to grasp this is causing you distress, and that you won't find peace until it is changed back to the value it used to be, so I have a suggestion.

Lobby the British Electrotechnical Committee and the IET (specifically the joint BSI/IET Technical Committee JPEL/64) to change U0 to 240V in as early an Amendment as possible. You are hereby granted permission to leave the forum in order that you may devote all of your spare time and energy to this vital task, and permission not to return until you have been successful.

But please note that until you are successful, not using U0=230V in calculations is a contravention of the Wiring Regulations, and you must stop telling people to do that.

Hey, I just found this:


Because of the change in 1995 in nominal supply voltage (U0) from 240 V ±6% to 230 V +10% to - 6%, calculating load current from kW or kVA rating is made more difficult. Whilst the nominal voltage was officially changed, the actual range stayed much the same and the actual distributed voltage to homes and other premises remained unchanged. Manufacturers may state equipment ratings at 240 V. In determining current demand from kW or kVA demand, the voltage at which the demand is calculated may be used and this may be 240 V and not 230 V."



exert is from the IET's installation design and calc book.

So, from the last sentence it seems we should still be basing our calculations on 240 v.

 

[ban-all-sheds]

You should arrange to meet up with that westfield6 nutjob.

Anybody who uses the phrase "At some stage the eurocrats in Brussels decided we should all be the same" has no understanding of the processes involved, is clearly motivated (at least in part) by pathetic, ignorant xenophobia, and is, basically, a complete t**t.

You'd get on well with each other.

Perhaps you should also join the IET forum, then you could ask about the disclaimer at the front of the Electrical Installation Design Guide, and about whether a guide takes precedent over a British Standard, you could ask Parsley where the actual standard allows the use of 240 as the value for U0, and where you can now find tables for maximum EFLI values to use with a U0 of 240v.

 

[securespark]

The people that are stupid are those that dreamed up the idea of a nominal voltage that does not exist and those sparks who follow like sheep and use a non existent voltage in calculations.

Go away and read the current version of 7671 from cover to cover then come back here once you understand it.

 

[JohnW2]

You should arrange to meet up with that westfield6 nutjob. ... Perhaps you should also join the IET forum, then you could ask about the disclaimer at the front of the Electrical Installation Design Guide, and about whether a guide takes precedent over a British Standard, you could ask Parsley where the actual standard allows the use of 240 as the value for U0, and where you can now find tables for maximum EFLI values to use with a U0 of 240v.

I have started a new thread about (hopefully some more sensible thoughts about) the issue of use of "nominal voltage" for calculations ( click here ) and would be grateful if anyone interested in discussing that issue should do so in that new thread, rather than littering this one with such matters. Thanks.

Kind Regards, John