Using pre installed cooker circuit for 13amp or 16amp oven.

[SUNRAY]

Yep.

This is, of course, a situation in which RCD protection would solve the problem, whereupon the rating of the OPD would become essentially irrelevant. It is all-but-inconceivable that an L-N fault 'of appreciable impedance' (i.e too high impedance to magnetically trip a MCB) could arise in/around a heating element, so any such fault would inevitably be L-E, and an RCD would immediately respond appropriately.

Kind Regards, John

That becomes a totally moot point in installations like these.

I haven't tried to count how many enquiries there have been in the last year about them and this represents a teeny weeny insy wincy proportion of those still in use. FWIW I have 2 in service in my home, only one has a leading RCD.

So a fault current of no more than 30mA then the RCD trips. Not 45 amps.

If there is one that is indeed the situation.

 

[JohnW2]

That becomes a totally moot point in installations like these. ... I haven't tried to count how many enquiries there have been in the last year about them and this represents a teeny weeny insy wincy proportion of those still in use. FWIW I have 2 in service in my home, only one has a leading RCD.

Sure. I merely wrote "This is, of course, a situation in which RCD protection would solve the problem ...". If there is not any RCD protection, then that solution obviously is not present

However, if manufacturers are concerned about the sort of faults you have described, when they sell their product in the UK they should (in my opinion) at least offer the option of RCD protection (rather than a "16A fuse"!), since few people in the UK are likely to install a 16A circuit (let alone one with a 16a fuse ) just for an oven/whatever.

Kind Regards, John

 

[EFLImpudence]

I take it you've not worked on many heating devices then.

Why do you say that?

Enclosed elements develop short circuits between the current carrying part and the earthed outer. Just before Christmas I changed a 1.5KW element in a heater battery which was tripping its 10A MCB.

Did it take three hours?

And I've had several immersion heaters which have developed faults and operated the OCD

Of course they have.

I truthfully don't know. I did'nt take note of the resistance but if the short circuit occurred at ¼ of the length of the element it would be 6KW which would not trip a 32A in a month of sundays, whether the element would fail in 3 hours I wouldn't put money on.

What do you think happens to an element filament designed to get very hot at 1.5kW when it carries 6kW.
Not to mention how it bolts itself to the casing in a satisfactory joint.

EDIT: But that doesn't change the fact that a 1.5KW could justabout be wired with 0.75mm² wire which would not like to be running at 32A

Has your kettle ever run at 32A for three hours?
What do you think would happen to a kettle and its flex if it were wired to the ring without a plug?

 

[SUNRAY]

As I see it you have either completely missed the points mentioned or you have little or no working knowledge or you're trying hard to pick holes. I'm assuming it's the middle option.

Why do you say that?

From your comments it seems you are unaware they can go faulty

Did it take three hours?

No but my initial tests found it to be faulty and I tested no further than that. IF it had been on a 32A OCD I know not how long it would have taken to trip, or even if it would have tripped at all.

Of course they have.

Yes, that's what the OCD is there for

What do you think happens to an element filament designed to get very hot at 1.5kW when it carries 6kW.
Not to mention how it bolts itself to the casing in a satisfactory joint.

That particular heating element is from a manufacturers standard range of: 12" 6KW, 14.5" 5KW, 18" 4KW, 24" 3KW, 29" 2.5KW, 36" 2KW, 48" 1.5KW, 72" 1KW. (Rated at 240V) Notice any pattern emerging? Yes indeed they are all made using different lengths of the same product.

That particular range has various mounting options which is not consistant across the range. The highest power I've knowingly encountered is the 2.5KW but generally the company that installs them into AHU's tend to go for the 1KW or 1.5KW and add more elements as required. AIUI the elements of 3KW and below are brazed into brass threaded fittings which are typically bolted through the side of the heater battery frame or an angle bracket where the electrical connexions are made using silicon or ceramic bead insulated wiring.

Has your kettle ever run at 32A for three hours?

NO and I wouldn't expect it to either as it's fitted with a 13A fuse.

What do you think would happen to a kettle and its flex if it were wired to the ring without a plug?

it would work properly. However under fault conditions is a different story.

So let's get back to what I actually wrote:

That is until it developes a fault and the cable rated for 16A is being asked to carry45A. The 32A over current device will happily pass such current for 3 hours.

All I've stated is:
1) Under fault conditions with an element shorting to earth the fault current could be anywhere from the original rating of the element to the full potential earth fault current (could be thousands of amps)

I happen to have chosen a random value of 45A.

2) The circuit consists of a 32A MCB, 6mm² T&E and the 1.5mm² flex fitted by the oven manufacturer, (as per the Hygena oven I Installed a while back, it may have even been 1.25mm²)

3) Type B MCB tripping curve is approx 1.4 at 3 hours. 32x1.4=44.8A

That's all I wrote.
My conclusion is: Under fault conditions such a fault current will not trip the MCB for 3 hours and the poor undersized flex (for the OCD) is sitting there running at 3 times it's CCC.

As winston1 keeps ramming down out throats "the OCD is only there to protect the cable". However he is now advocating ignoring his own pedancy and saying a thin cable is safe to use on a large OCD. I have a different opinion.

 

[SUNRAY]

Sure. I merely wrote "This is, of course, a situation in which RCD protection would solve the problem ...". If there is not any RCD protection, then that solution obviously is not present

However, if manufacturers are concerned about the sort of faults you have described, when they sell their product in the UK they should (in my opinion) at least offer the option of RCD protection (rather than a "16A fuse"!), since few people in the UK are likely to install a 16A circuit (let alone one with a 16a fuse ) just for an oven/whatever.

Kind Regards, John

It wouldn't occur to me to use an OCD other than one suited to the job. I must be one of the few.

 

[JohnW2]

It wouldn't occur to me to use an OCD other than one suited to the job. I must be one of the few.

So you would install a 16A circuit (maybe a number of 16A circuits) in a UK installation if the householder had a one or more appliances (of which there are many around, given 'international marketing') whose MIs said that they required "16 fuse(s)"?

... and does it mean that you are one of those (I suspect few!) people who, like me, have substantial stocks of 1A and 2A BS1362 fuses?

Kind Regards, John

 

[JohnW2]

... Type B MCB tripping curve is approx 1.4 at 3 hours. 32x1.4=44.8A

I'm not sure where that comes from. Such a device is required to trip within 1 hour at a current of 1.45 x In.

Kind Regards, John

 

[SUNRAY]

I'm not sure where that comes from. Such a device is required to trip within 1 hour at a current of 1.45 x In.

Kind Regards, John

Yep , looking back at notes from doing 16th that tallies. The curves are all but vertical at 10000 seconds (2¾ hours)

If you like I'll change my numbers to the hour rates but it's still a ruddy long time for a cable to be cooking at 3 times its CCC.

 

[SUNRAY]

So you would install a 16A circuit (maybe a number of 16A circuits) in a UK installation if the householder had a one or more appliances (of which there are many around, given 'international marketing') whose MIs said that they required "16 fuse(s)"?

Yes I've worked on a few machines running at 14-15A complete with manufacturers fitted/moulded 13A plugs or installed on 13A FCUs.
If appropriate yes I do run seperate circuits, however if ratings allow I'd also go down to 13A OCD.

... and does it mean that you are one of those (I suspect few!) people who, like me, have substantial stocks of 1A and 2A BS1362 fuses?

Kind Regards, John

Sad maybe but yes albeit not substantial stocks.

 

[JohnW2]

Yes I've worked on a few machines running at 14-15A complete with manufacturers fitted/moulded 13A plugs or installed on 13A FCUs. If appropriate yes I do run seperate circuits, however if ratings allow I'd also go down to 13A OCD.

That's not quite what I meant - I was talking about a situation in which the appliance clearly did potentially need more than 13A (hence plug/socket or FCU inappropriate), with the MI calling for a "16A fuse". Would you refuse to connect that to a 32A circuit and, instead, install a new "16A" one - or what?

Sad maybe but yes albeit not substantial stocks.

In that case, you can join my 'sad club'

Kind Regards, John

 

[SUNRAY]

That's not quite what I meant - I was talking about a situation in which the appliance clearly did potentially need more than 13A (hence plug/socket or FCU inappropriate), with the MI calling for a "16A fuse". Would you refuse to connect that to a 32A circuit and, instead, install a new "16A" one - or what?
Kind Regards, John

It totally depends on the situation. But remember most of my work is commercial where additional circuits don't tend to be frowned upon. My choice is always to run the additional circuit but where it's not possible there are plenty of ways of adding a 16A MCB to a circuit.

 

[JohnW2]

It totally depends on the situation. But remember most of my work is commercial where additional circuits don't tend to be frowned upon. My choice is always to run the additional circuit but where it's not possible there are plenty of ways of adding a 16A MCB to a circuit.

Fair enough, but this is a DIY forum and a 'domestic thread' and the discussion is all about the installation of appliances in domestic installations.

I may be wrong, but I would doubt that many electricians would install a new dedicated 16A circuit for a new cooking appliance which a householder happened to have acquired and, although, as you say, there are other ways in which 16A protection could be provided, I doubt that would often be done, either. Speaking personally, I can't recall having seen either of those things done in a domestic situation.

Kind Regards, John

 

[SUNRAY]

Fair enough, but this is a DIY forum and a 'domestic thread' and the discussion is all about the installation of appliances in domestic installations.

I may be wrong, but I would doubt that many electricians would install a new dedicated 16A circuit for a new cooking appliance which a householder happened to have acquired and, although, as you say, there are other ways in which 16A protection could be provided, I doubt that would often be done, either. Speaking personally, I can't recall having seen either of those things done in a domestic situation.

Kind Regards, John

Yes I assume this thread is a domestic kitchen but that has not been stated as such, I was only indicating my easy acceptance of running additional circuits in situations where the customer is likely to agree/accept such at the drop of a pin.

In domestic situations I've quite often seen a small CU dedicated to the kitchen or a small ABS MCB enclosure hidden away in a cupboard as an alternative to the ubiquitous 8w switch panel.

 

[Mottie]

Blimey, you sparkies do like to argue amongst yourself, don't you? I bet your Christmas parties always end with a good old punch up after you’ve all downed a few.

 

[Mark armitage]

Yes I assume this thread is a domestic kitchen but that has not been stated as such, I was only indicating my easy acceptance of running additional circuits in situations where the customer is likely to agree/accept such at the drop of a pin.

In domestic situations I've quite often seen a small CU dedicated to the kitchen or a small ABS MCB enclosure hidden away in a cupboard as an alternative to the ubiquitous 8w switch panel.

Yes it's a domestic kitchen there is already a dedicated circuit with 6mm cable going from 32mcb to 45cooker switch then 45a cooker connection plate then 6mm cable to cooker. But we are getting a smaller single oven which is only 16amp but need be hardwired. So should I change 32mcb to 16mcb I'm consumer unit or just get electrician to wire it straight into connection plate with cable supplied.