Moving this away from a thread

[SUNRAY]

I have seen many times a shower on a MCB technically too small, and I would guess a 16 amp oven would run on a 13A fuse, however I would not say that is the way it should be done, diversity is size of largest device then percentage of others, so if one set of heating elements is 3.5 kW then in theory should not be on 13A fuse.

But the manufacturer of my cooker says full load 65A but still says supply from 32A MCB/RCBO.

Yes you are correct.
I have often seen/been party to running 4KW of stage lighting on 13A fused plugs but it's not right.
A 10.5KW shower on 30/32A OCD and 4mm² cable is also very wrong but it works and I came across one a while ago but customer refused to change it as it had been working troublefree for a number of years without signs of melting/scorching. I informed the freeholder and hope it got followed up.
My daughter's range quote 14.7KW max MI and suggests 32A to 45A [7.4 to 10.4KW] supply depending on usage.

 

[SUNRAY]

I have seen many times a shower on a MCB technically too small, and I would guess a 16 amp oven would run on a 13A fuse, however I would not say that is the way it should be done, diversity is size of largest device then percentage of others, so if one set of heating elements is 3.5 kW then in theory should not be on 13A fuse.

But the manufacturer of my cooker says full load 65A but still says supply from 32A MCB/RCBO.

Yes you are correct.
I have often seen/been party to running 4KW of stage lighting on 13A fused plugs but it's not right.
A 10.5KW shower on 30/32A OCD and 4mm² cable is also very wrong but it works and I came across one a while ago but customer refused to change it as it had been working troublefree for a number of years without signs of melting/scorching. I informed the freeholder and hope it got followed up.
My daughter's range quote 14.7KW max MI and suggests 32A to 45A [7.4 to 10.4KW] supply depending on usage.

 

[JohnW2]

Ah that depends entirely on what the appliance is. For a Cooker with a selection of heating elements [10 is not uncommon these days] it is very unlikely that all elements will be in use at one time. However an appliance with a single load of 16A, incuding fan & light which will always represent a single load, then surely, that load must be fully catered for regardless of whether it's a cooking appliance or not. I'd consider it bad practice to place a known 16A load on a 13A fuse.

For a start, even 'ovens' can today have two or three elements.

However (and we've been through this before), even with a single element one can have 'diversity over time' as well as 'diversity across loads', the concept being that the 'after diversity load' is the average load over a period of time (15min, 30 min or whatever).

I have no electric cookers, ovens or hobs, so, as I have reported before, I have undertaken experiments using a 1,700 W deep fat fryer, which I switched on from cold and ran for an hour or so at ‘chip frying temperature’. which is probably not a bad model of the likely behaviour of an oven. What I found is illustrated in the graphs below. I won't bother to describe (again) what those graphs show (which should be fairly self-evident), but any interested can see my original description (here) .

Kind Regards, John

 

[scousespark]

If the oven is rated at 3.7KW @240v, it is over the limit for a 13amp fuse and should be on a dedicated circuit. Even converting the power rating to the value at 230V only drops the load by a small amount.

We don’t apply diversity when determining the breaker on a single circuit. We apply diversity over the total load in the installation to ensure the main fuse can cope with this load. Diversity is applied to take into account that the installation will seldom be running at full load. We don’t apply it to reduce the calculated load.

 

[JohnW2]

We don’t apply diversity when determining the breaker on a single circuit.

Maybe you don't, but a lot of people do, particularly in the case of cooking appliance(s).

If one adopted the policy you mention above, it would mean that a circuit protected by a 32A OPD could only support cooking appliance(s) with a maximum load under about 7.4 kW - hardly enough for some induction hobs, let alone ovens as well.

As EFLI (and myself) are often pointing out, the 'standard' diversity guidelines theoretically allow for cooking appliances with a maximum load up to about 19kW (about 83A at 230V) to be supplied by a circuit (with suitable cable) protected by a 32A OPD. If we didn't apply diversity to single circuits, there wouldn't really be any point in the existence of the 'standard diversity guidelines' for cooking appliances (etc.) I mentioned.

We apply diversity over the total load in the installation to ensure the main fuse can cope with this load. Diversity is applied to take into account that the installation will seldom be running at full load. We don’t apply it to reduce the calculated load.

Diversity can be applied at a whole host of levels - within a particular piece of equipment, across all loads on one final circuit, across all circuits in an installation or across parts of the distribution network. You are mentioning just one of those.

Kind Regards, John

 

[scousespark]

I’m talking specifically about installing a 37kw oven on a RFC and overloading the fuse in the plug. In this case we would not apply diversity to drop the load below 13amps.

 

[JohnW2]

I’m talking specifically about installing a 37kw oven on a RFC and overloading the fuse in the plug. In this case we would not apply diversity to drop the load below 13amps.

[I presume that you mean 3.7 kW, not 37 kW! ]

Why is protecting a cooking appliance with a maximum load of about 16A with a 13A fuse 'worse' than protecting one with a maximum load of 83A with a 32A MCB?

Kind Regards, John

 

[EFLImpudence]

As I have said before, no reason logically or theoretically but (some of) the plugs and sockets are not good enough.

 

[JohnW2]

As I have said before, no reason logically or theoretically but (some of) the plugs and sockets are not good enough.

That's probably true but, if it is, those plugs/sockets are almost certainly not compliant with BS1363, which requires them to tolerate 14A continuously for 'hours'.

Pragmatically, you might be right, but to what extent should we design on the basis of a component maybe not complying with the Standard with which it claims to comply?

What do we know about how a 32A MCB will 'fare' with more than double its "In-worth" of current flowing through it for brief periods?

By the way, I'm not particularly condoning a plug/socket for a '16A' cooking appliance - and I wouldn't do it myself.

Kind Regards, John

 

[SUNRAY]

That's probably true but, if it is, those plugs/sockets are almost certainly not compliant with BS1363, which requires them to tolerate 14A continuously for 'hours'.

That may be the reg but I have frequently had to replace plugs and/or sockets which have failed and it's not only cheap/knockoffs. Personally I will not install the likes of MK because I haven't found them to be any more reliable than homebase/screwfix etc and see no point in paying 2 or 3 times the price of a cheaper product for no apparent gain. Several years back I kept all the failed items with a view to completing some sort of survey but in the end i got fedup with tripping over the boxes and dumped them. I still have a stock of very elderly 'Bill' DSO's for use in full current or Tuff situations.

Pragmatically, you might be right, but to what extent should we design on the basis of a component maybe not complying with the Standard with which it claims to comply?

We should design to the standards/regs and use components which do comply.It's difficult sometimes but we should make every effort.

What do we know about how a 32A MCB will 'fare' with more than double its "In-worth" of current flowing through it for brief periods?

Indeed, working in controls/industrial makes one very aware of inrush and 'B' type MCB's are rare.

By the way, I'm not particularly condoning a plug/socket for a '16A' cooking appliance - and I wouldn't do it myself.

Kind Regards, John

Again working in commercial/industrial environments it's very normal for appliances to be connected using a plug/socket. Commercial kitchens frequently have rows of CeeForms on the wall.

 

[JohnW2]

That may be the reg but I have frequently had to replace plugs and/or sockets which have failed and it's not only cheap/knockoffs.

Agreed - but, as I've said, one would hope to be able to rely on a product that was said to comply with a Standard to actually be compliant with that Standard.

Maybe one problem is that the compliance-confirming tests are (presumably) undertaken on 'brand new' components, and that things might change after they're been in service for some years?

We should design to the standards/regs and use components which do comply.It's difficult sometimes but we should make every effort.

It's 'difficult' (or impossible!) if one cannot rely upon a product complying with the requirements of a Standard with which it claims to comply - particularly when it has been in service for some time.

Again working in commercial/industrial environments it's very normal for appliances to be connected using a plug/socket. Commercial kitchens frequently have rows of CeeForms on the wall.

It's not the 'connecting with a plug/socket' were talking about (which, as you say is very common practice in many situations) but, rather, connecting with a plug/socket whose 'rating' is less than the peak current that can be drawn by the load - do you see that (legitimately) being done in commercial/industrial environments?

Kind Regards, John

 

[SUNRAY]

John.
Your comments make sense to me but a brand new MK SSSO provided for a 2.9KW water boiler [for making hot drinks] shouldn't fail within 2 months.
Electricians blamed the moulded plug and replaced it with an MK to go with the new socket, that lasted for a month longer.

I didn't realise your comment about the plug still related to an overloaded 13A. No I wouldn't expect to find that in a non domestic situation, in fact I'd expect to see something better than a 13A socket to power a 3KW device.

 

[JohnW2]

Your comments make sense to me but a brand new MK SSSO provided for a 2.9KW water boiler [for making hot drinks] shouldn't fail within 2 months.

Indeed not - and if it actually complied with the requirements of the relevant standard, if the plug and socket had been wired competently and 'matted' correctly, and neither had been subjected to any physical or environmental abuse, then it presumably wouldn't fail anything like that rapidly, unless one was extremely unlucky (no manufactured product can be guaranteed to be faultless).
Electricians blamed the moulded plug and replaced it with an MK to go with the new socket, that lasted for a month longer.

I didn't realise your comment about the plug still related to an overloaded 13A.

Isn't that what the last few pages of this discussion have been about? I admit that there is some confusion, in as much as there are two or three somewhat 'overlapping' threads here at the moment, and it's easy to get confused between them!

Kind Regards, John

 

[SUNRAY]

One of mine from 2009

No I am not happy with BS1363 etc running at 13A, even the good ones like MK.
View media item 14453 View media item 14452This pair was installed at easter in a schools new tuck shop to run a 3KW Burco urn. It originally had a moulded plug which was replaced with MK as that is the spec. When they did the PAT in summer hols it failed because the 2 are welded together.?

 

[SUNRAY]

Another comment redirected from a thread.

With the low current taken by modern lighting plus the fact that they continue to operate over a very wide voltage range that would not be a problem.

If it was why stop at 1.5, what about 2.5, 4, or 6?

Because it's a PITFA getting 2x4mm² into the average lighting fitting... and yes I have been there and done it.

I don't understand why Winston has such an aversion to lighting circuits on what I believe the most popular size cable to use on lighting circuits. Especially as, generally speaking, the cost of 1.5mm² T&E is frequently negotiated down below the cost of 1.0mm² T&E.