Ovens, plugs, kitchen rings and diversity

[ban-all-sheds]

If one tried to apply this to the sort of multiple cyclic load (e.g. hob) cooker situations that EFLI has been talking about, this could get very complicated, because one wouldn't simply have a dichotomy of Imin and Imax. Rather, there would be several different total currents, each with different (and probably unknown!) durations, two or more of which may well be above In.

Only insofar as any load can be considered cyclic if it has a regular cycle.

 

[winston1]

There is no need for such rudeness.

First of all it is not rudeness - it is an accurate assessment of you. I accept that you might be upset by it - people like you often find that the truth hurts.

Secondly there is every need for such an assessment.

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.

And there you go again - still thinking that there are people here who don't realise what an uncomprehending lump of uselessness you are, and that therefore you need to show us once more.

I give up. You don't seem to be capable of comprehending that the nominal voltage does not exist and is something dreamt up by eurocrats. The UK uses 240volts nominal and has for many decades. There are no plans to change it.

 

[ban-all-sheds]

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.

 

[JohnW2]

If one tried to apply this to the sort of multiple cyclic load (e.g. hob) cooker situations that EFLI has been talking about, this could get very complicated, because one wouldn't simply have a dichotomy of Imin and Imax. Rather, there would be several different total currents, each with different (and probably unknown!) durations, two or more of which may well be above In.

Only insofar as any load can be considered cyclic if it has a regular cycle.

Yes, I know you wrote that, and I should have commented at the time. because I'm not at all sure that such is what 533.2.1 intends. In practice, very few, if any, loads which 'cycle' (continuously oscillate between on and off) have regular cycles - so if 533.2.1 considered loads to be cyclic if they had regular cycles, it would hardly have been worth writing, since it would very rarely be applicable.

Kind Regards, John

 

[ban-all-sheds]

I dare say that there are enough machines in industrial environments where there is a regular cycle, and where the potential savings in dropping from, say, 400mm² cable to 300mm², make it a worthwhile concept.

 

[JohnW2]

I dare say that there are enough machines in industrial environments where there is a regular cycle, and where the potential savings in dropping from, say, 400mm² cable to 300mm², make it a worthwhile concept.

Maybe (I nearly restricted my comment to domestic loads) but, even in industrial environments, I suspect that there are relatively few loads whose 'cycling' is totally time-determined, hence regular.

However, where did your statement come from - is it just your view? In general, cycles (of any sort) can be either regular or irregular (time-wise), so I don't see any fundamental reason why a "cyclic load" necessarily has to have a regular cycle. Of course, if the cycle is not regular, calculations would have to involve some assumptions - but that is an inevitable fact-of-life in relation to all issues related to diversity.

Kind Regards, John

 

[ericmark]

I missed this, so to start with original question what ever size the appliance is when fitted with a 13A plug the fuse in the plug limits the power that can be used. I am sure many have used a welding set with small rods which could take well over the 13A but bit at a time allowing the fuse to cool in spite of welder being well over 13A rating the job has been done without any damage or blowing fuse.

So in theory it does not matter what size the appliance is if the fuse does not blow it can be used on a 13A plug.

There is in practice however a problem in that the 13A plug with finger protectors added can no longer get rid of the heat generated by the fuse without damage to the plug.

This is where the use of an oven becomes a problem. Clearly the socket should be installed where the plug will be in free air. Having a socket inside any carcass which reduces cooling of the plug will cause a problem both in the fact it's not getting the cooling and also it's not visible.

When we fit flex the rating of the flex assumes the flex is visible throughout it's length and is in free air to be cooled so with a Baby Belling cooker placed on a counter top the cable is visible throughout it's length and the plug is in free air.

Once we build a cooker into the kitchen then the supply cable is no longer visible throughout it's length. However if an oven is fitted into a carcass and the supply lead is fed through a protected hole as often found in office furniture for computer leads and the like and then into a 13A socket in full view it is unlikely to cause a problem.

As to total loading on a ring final this will depend on what else it feeds. How a washing machine and tumble drier as considered as kitchen appliances I don't know? The Dish washer yes but it holds so little water that in real terms the heater is on for a very short time.

When I first bought this house the tumble drier was in the bedroom until the kitchen extension was built. The extension would be considered as a utility room not a kitchen and the sockets in the utility room come from a different MCB to the kitchen sockets.

For those with a utility room washing machine, tumble drier will likely not be on the kitchen ring final and so we can't assume in the discussion that they will be on the kitchen ring final.

I am lucky when my house was built the immersion heater had both a local and kitchen isolator as as well as the ring I also have two other supplies into the kitchen. Plus a separate supply to utility room. But the kitchen ring also supplies sockets in the rest of the house as does the utility ring.

So since there are so many variations on how houses are wired considering the load from other appliances in a general discussion does not work.

what would people think about such an appliance having a 13A plug and being plugged in to a ring final circuit?

I think if the plug is visible then although not ideal then assessing the risk I think it is acceptable. However I don't feel that it is acceptable where the plug is hidden from view.

 

[JohnW2]

I missed this, so to start with original question what ever size the appliance is when fitted with a 13A plug the fuse in the plug limits the power that can be used. .... So in theory it does not matter what size the appliance is if the fuse does not blow it can be used on a 13A plug.

That may be pragmatically true, but, unless there is a legitimate application of diversity considerations, I don't think that it is either acceptable or compliant with regulations to deliberately overload a circuit or accessory for appreciable periods of time. As recent discussed, a 13A BS1362 fuse may allow up to 25A or so to flow for an hour.

This is where the use of an oven becomes a problem. Clearly the socket should be installed where the plug will be in free air.

what would people think about such an appliance having a 13A plug and being plugged in to a ring final circuit?

I think if the plug is visible then although not ideal then assessing the risk I think it is acceptable. However I don't feel that it is acceptable where the plug is hidden from view.

As has been said, this discussion was not meant to be about whether or not one uses a plug. Rather, it's about whether it is acceptable (using the example cited) to apply diversity to enable a 4.68 kW (about 20A) single oven (not a cooker with multiple elements) to be fed (even if hard-wired) from a "13A circuit" (or, come to that, a 16A circuit). It was EFLI's view that, since a single oven has only one thermostat (hence all elements are either on or off), diversity cannot/should not be applied - although I have been trying to argue that 'time diversity' is an acceptable concept. I certainly haven't seen anyone suggesting, in general, that 'standard' diversity can be applied to hobs and cookers, but not standalone ovens.

Kind Regards, John

 

[ericmark]

I see you point about time diversity, but calculations start at the largest single item so it would no be as shown in OSG.

One thing I missed is the danger involved with loss of supply. If we look at the Christmas Turkey there is a huge mass inside the oven. Once the supply fails all cooling fans stop. So let us consider cooking over night which is common for Turkey in our house and everyone in bed when the fuse ruptures.

Question is will the carcass stand the heat of the oven without cooling fans going with so much heat in the oven?

Clearly where there is storage above the oven anything in that cupboard would also get very hot.

So down to a risk assessment.
1) Damage to supply cables very low risk.
2) Damage to fuse carrier much higher risk.
3) Damage to items kept in storage cupboard above oven even higher risk.
4) Damage to other items because the fuse carrier fails quite high even possible exposure to live parts and burns to personnel if they try to unplug the hot plug.

Swap to a FCU and the risks are reduced but still have the problem of overheating when cooling fans fail.

So although time diversity may be valid because of the risk should the fuse fail I would say using a under size fuse or MCB is not worth the risk as in both cases as we know from showers which are often larger than the supply MCB over time the MCB or fuse becomes weaker and disconnects the device. With a shower all that happens is some one gets cool water with an oven some thing will likely over heat due to loss of cooling fans so on second thoughts I will change my mind and say no we should not fit an oven to a prospective device smaller than the oven rating.

 

[bernardgreen]

Question is will the carcass stand the heat of the oven without cooling fans going with so much heat in the oven?

If the oven is designed to be compliant with the safety guidelines then the outside of the oven should not become hot enough to be an ignition source ( assuming the elements are also without power ) but may become hot enough to be a burn hazard if touched.

 

[JohnW2]

I see you point about time diversity, but calculations start at the largest single item so it would no be as shown in OSG.

I'm not sure I understand your point - we're talking about just one "single item" (the oven).

One thing I missed is the danger involved with loss of supply. If we look at the Christmas Turkey there is a huge mass inside the oven. Once the supply fails all cooling fans stop. ... ... so on second thoughts I will change my mind and say no we should not fit an oven to a prospective device smaller than the oven rating.

Whilst you make some theoretically valid points, they appear far more general than the specific point about diversity we're discussing. Are you now suggesting that diversity should not be used in relation to any cooking appliance (or, perhaps, any cooker with a fan oven), in case the consequence is for an OPD to operate, leaving a hot turkey without a fan (with possible 'thermal consequences')?

I'm not sure that I actually understand this fear about failing oven fans. For a start, they are presumably designed to be safe in the event of fan failure (even if the elements are still powered up and working) - and, furthermore, isn't use of the fan in some (maybe many) of these ovens 'optional' (switchable), anyway? Also, unless you like really burnt and dried out food, no turkey or anything else in the oven is going to get up to a particularly dangerous temperature.

[in passing, has anyone actually experienced this Christmas cooker phenomenon which is so often wheeled out?! - I did once run out of LPG (my main fuel) on Christmas Day, but that's a rather different matter!]

Kind Regards, John °

 

[ban-all-sheds]

Also, don't forget that in the case of the turkey being cooked overnight to a nice degree of dryness, at least one person will be up at the crack to put the sprouts on, so they would notice that the oven wasn't working.

 

[JohnW2]

10A + 30% has nothing to do with time, does it? ... Why is diversity not allowed below 10A?

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) ...

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.

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).

Kind Regards, John

 

[EFLImpudence]

Also, don't forget that in the case of the turkey being cooked overnight to a nice degree of dryness, at least one person will be up at the crack to put the sprouts on, so they would notice that the oven wasn't working.

You say that but the other day I boiled an egg for an hour and it was still hard.

 

[ericmark]

If the carcass is matched to the oven and if there is no cupboard in the carcass then yes losing the oven supply will cause no danger.

As correctly pointed out gas ovens likely have no fan cooling but also this is allowed for in the main placing them under something which can stand the heat.

Where it is normally to have no fan cooling one will not place items likely to be damaged by the heat in a cupboard above the oven. But where normally it stays cool one does not really consider the fact one day it may become warm. I see my mother has spare paper hankies in cupboard above her oven OK no candles or matches but still one would not expect it to get hot.

It does not matter what we do we have to consider others doing daft things. But still there has to be a limit.

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?

BS 7671 is a guide to safe installation and the OSG is a further guide at one time they were combined. I personally think some of the items in the appendix would be better in the OSG and the rules quoted for caravans on the SELV system have gone OTT. Why should there be a minimum battery size and why have a maximum voltage which precludes the use of stage battery chargers?

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.

Well yes I think there are times when we need to turn a blind eye but this has to be a personal decision and there can't be any blanket yes it's OK. We still need to follow regulations where we don't know special circumstances.