Omission of overload protection for resistive heating elements.

[EFLImpudence]

In view of what has been written in the other threads, do we now conclude that omission of overload protection for ovens should not be done?
If so, for what sort of appliances was 433 intended?

I obviously cannot argue with SUNRAY's experiences but that a fault as described caused a 13A fuse to blow does not mean that the faulty appliance would have sustained, for example, four times its design current 'for ever' or until the (undersized for OPD) flex melted.

I still do not see how an oven element filament can fault to earth somewhere along its length and bolt itself satisfactorily to the casing as if it were the manufacturers joint at the end. Surely the first incidental very small CSA contact would arc and blow itself away.

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 does seem to be a very odd way of doing things. Why would you not want a 12" 1kW one? Why would you want a six feet long one?
Is there a specific reason?

Such is definitely not the case in things like ovens, immersion heaters and showers, even if a 1kW oven element might be twice as long (coiled) as a 2kW one - if it is.

 

[SUNRAY]

In view of what has been written in the other threads, do we now conclude that omission of overload protection for ovens should not be done?
If so, for what sort of appliances was 433 intended?

I obviously cannot argue with SUNRAY's experiences but that a fault as described caused a 13A fuse to blow does not mean that the faulty appliance would have sustained, for example, four times its design current 'for ever' or until the (undersized for OPD) flex melted.

I still do not see how an oven element filament can fault to earth somewhere along its length and bolt itself satisfactorily to the casing as if it were the manufacturers joint at the end. Surely the first incidental very small CSA contact would arc and blow itself away.


That does seem to be a very odd way of doing things. Why would you not want a 12" 1kW one? Why would you want a six feet long one?
Is there a specific reason?

Such is definitely not the case in things like ovens, immersion heaters and showers, even if a 1kW oven element might be twice as long (coiled) as a 2kW one - if it is.

Those elements I indicated are just one of the ranges provided by the manufacturer produced for the AHU environment, generally speaking a multitude of elements running across a large duct is the way it's done. So for example if the designer has come up with duct of say 1mx1m and requires 12KW 3ph of heating, the manufacturer will come up with a design using a combination of elements to present a fairly uniform heat across the aperture. In this instance it may be 12x 72" or 6x 48" + 3x 72" or also but less likely 6x 36".
Yes they have other ranges but that is their most popular in my experience, most AHU manufacturers seem to have standardised on the 1 & 1.5KW and it's also quite common to round the heater bank up, for example from 8KW to 9KW or build a 6KW 3ph battery and an additional pair of 1KW elements which can be switched in if/when required to avoid custom elements.

I've worked on small ducts, say 300mm which include a short heating element like 1KW, so yes they certainly exist.

As to the faults, yes I'd say the majority short out and burn a hole which is usually quite visible, often very spectacular! especially on those protected by a large OCD. Bear in mind AHU electrics are not generally protected against residual current. However I've worked on plenty that have gone short L-E and continued working at a higher current, sometimes only found due to the unbalanced 3ph power metering.

 

[JohnW2]

In view of what has been written in the other threads, do we now conclude that omission of overload protection for ovens should not be done? ... If so, for what sort of appliances was 433 intended?

I've been wondering about that, too, but I think I have an answer which satisfies at least myself ....

As you imply, if 433.3.1(ii) does not allow omission of overload protection when the load is a resistive heating element, then what on earth types of load could it ever apply to?

We have to accept that, as bernard and Sunray have pointed out, it is "not impossible" that a heating element would result in a 'moderate impedance' L-N fault, which did not involve 'earth', and therefore could result in an overload current, whilst not causing an RCD (or OPD-based 'fault protection') to operate. However, as I have said, I regard that as an extremely unlikely (i.e. 'rare) event, and that is the basis of my 'answer'; to the question above ...

... I would say that the crucial thing about 433.3.1(ii) is that it says that overload protected may be omitted if "... because of the characteristics of the load or the supply, is not likely to carry overload current". As above, my personal belief is that the nature of a heating element is such that it is not (indeed, 'not remotely') "likely" to result in overload current, hence satisfying the condition for omission of overload protection.

"Not impossible" and "Likely" are clearly very different things.

Kind Regards, John

 

[SUNRAY]

Somehow I managed to post before completed writing so above has been editted

 

[EFLImpudence]

We have to accept that, as bernard and Sunray have pointed out, it is "not impossible" that a heating element would result in a 'moderate impedance' L-N fault, which did not involve 'earth',

I don't think Sunray said that and Bernard's example was for a bifilar element - not ovens.

The only way an oven element can cause an L-N (not fault to earthed casing) overload is for the filament resistance to reduce; this is surely not possible.

 

[SUNRAY]

I've been wondering about that, too, but I think I have an answer which satisfies at least myself ....

As you imply, if 433.3.1(ii) does not allow omission of overload protection when the load is a resistive heating element, then what on earth types of load could it ever apply to?

We have to accept that, as bernard and Sunray have pointed out, it is "not impossible" that a heating element would result in a 'moderate impedance' L-N fault, which did not involve 'earth', and therefore could result in an overload current, whilst not causing an RCD (or OPD-based 'fault protection') to operate. However, as I have said, I regard that as an extremely unlikely (i.e. 'rare) event, and that is the basis of my 'answer'; to the question above ...

... I would say that the crucial thing about 433.3.1(ii) is that it says that overload protected may be omitted if "... because of the characteristics of the load or the supply, is not likely to carry overload current". As above, my personal belief is that the nature of a heating element is such that it is not (indeed, 'not remotely') "likely" to result in overload current, hence satisfying the condition for omission of overload protection.

"Not impossible" and "Likely" are clearly very different things.

Kind Regards, John

All I can answer to that is my personal experiences of crawling around inside air handling unit and ducts (And I promise they look nothing like the highly polished silver as depicted in films) fault finding and repairing such elements. Ok it's not a daily occurrence but enough to be able to quote my experiences and the reason I'd use the smallest sensible OCD. However that OCD also by design considers the cable size nich wahr?.

 

[SUNRAY]

I don't think Sunray said that and Bernard's example was for a bifilar element - not ovens.

The only way an oven element can cause an L-N (not fault to earthed casing) overload is for the filament resistance to reduce; this is surely not possible.

It certainly sounds impossible, providing it is an enclosed element. Is there any rule to say that open elements are no longer used in ovens?

 

[SUNRAY]

I don't think Sunray said that and Bernard's example was for a bifilar element - not ovens.

The only way an oven element can cause an L-N (not fault to earthed casing) overload is for the filament resistance to reduce; this is surely not possible.

Again my experience include other heating products and an example of a number of resistive elements, each with their own thermostats, being operated on a larger OCD which conformed to the rule was trace heating cables where a leak had gone unnoticed due to the lagging, the fluid (Not water) had rotted the insulation over time and the wires did short together. As it happened it did trip the MCB which was reset a number of times before the regularity became noticed by the various maintenance staff.
Let's not get into keeping records, this sort of thing happens.

 

[bernardgreen]

Bernard's example was for a bifilar element - not ovens.

I do not recall the makes and models but I have seen large domestic ovens which had bifilar elements.

 

[JohnW2]

I don't think Sunray said that and Bernard's example was for a bifilar element - not ovens.

I was talking in general about 'resistive heating elements'. A 'bifilar' one such as bernard described is the only situation in which I can think of that ....

The only way an oven element can cause an L-N (not fault to earthed casing) overload is for the filament resistance to reduce; this is surely not possible.

... could arise and, although I have certainly never knowingly seen such an animal, I can't swear that no oven elements are of such a design (it would seem daft if there were, for the very reason we are discussing).

However, I don't think that any of this alters what I said and believe - namely that, whatever else, oven elements are NOT "likley" to result in overloads, so, in my opinion, the reg in question would allow overload protection to be omitted in such a case.

Kind Regards, John

 

[EFLImpudence]

Again my experience include other heating products and an example of a number of resistive elements, each with their own thermostats, being operated on a larger OCD which conformed to the rule was trace heating cables where a leak had gone unnoticed due to the lagging, the fluid (Not water) had rotted the insulation over time and the wires did short together. As it happened it did trip the MCB which was reset a number of times before the regularity became noticed by the various maintenance staff.
Let's not get into keeping records, this sort of thing happens.

Maybe, but the subject is a small domestic oven and other similar domestic appliances.

 

[JohnW2]

I do not recall the makes and models but I have seen large domestic ovens which had bifilar elements.

As implied in what I have just written, I struggle to understand why they would adopt such a design (for the very reason we are discussing). 'Unnecessarily' bringing 'L' and 'N' things close together would not seem a desirable thing to do. What would be a reasonable justification?

Kind Regards, John

 

[JohnW2]

Maybe, but the subject is a small domestic oven and other similar domestic appliances.

... AND, whether such a load is "likely" to result in an overload current.

Kind Regards, John

 

[bernardgreen]

What would be a reasonable justification?

Single ended element

Reduced electromagnetic field

 

[JohnW2]

It certainly sounds impossible, providing it is an enclosed element. Is there any rule to say that open elements are no longer used in ovens?

Rule or no rule, I haven't seen such an animal for decades. Toasters and some small standalone 'grills' etc. (and fan heaters etc.) are a different matter, but we're not discussing them

Kind Regards, John