Resistive element OCPD

[JohnW2]

Thank you @SUNRAY this is some thing I have talked about before, but not actually had a case of it happening.

Nor, I strongly suspect, have many people - underlining the fact that it is probably an exceedingly rare occurrence.

For a fault to develop which resulted in a substantial portion of a heating element being 'bypassed' ('shorted out') without there being any contact between live conductors and their (almost inevitably present) earthed metal containment must be extraordinarily improbable.

In any event, even if that "extraordinarily improbable" happening were to occur, as I have said I would think it very likely that the remaining current-carrying part of the element would rapidly 'burn out' (melt) - before you or anyone else had a chance to see it drawing a very high current. I presume that, like most of us, you have come across 'burnt out elements'?

Furthermore, it was purely by chance that, in the anecdote Sunray cited, the extent of the fault ('shorting out' much of the element) was not quite enough to result in very rapid operation of a 32A MCB/RCBO. He mentioned 5.1Ω, hence about 45A at 230V. Had that (equally by chance) have been about 4Ω (about 57.5A), then a B32 should have operated within about 5 minutes, and if it had been about 3Ω (about 77A), within 1 minute or so.

Hence, not only is a fault of the type described extremely improbable but, even if it does happen, it would only be by chance that it did not result in rapid burning out of the element and/or operation of a B32 - which all adds up to the fact that it is "extraordinarily improbable" that a situation such as Sunray has described will both arise and persist for an appreciable period of time.

As we have both said, the use of 'dual cooker outlets' connected to 32A (sometimes 40A or 45A) supplies has been very common practice for a very long time (and remains so today) - so, if the scenario being discussed were more than "extraordinarily improbable", we would surely be encountering it and/or hearing about it?

As I've said (and as is also the case with bernard), I think we have to ask Sunray whether he ever crosses roads, travels in cars, climbs ladders or uses power {or even hand} tools etc. etc.

Kind Regards, John

 

[ericmark]

Well, yes, so 16A would be better.

OK I found a picture of a 15 amp.

In any event, even if that "extraordinarily improbable" happening were to occur

Would it be reported? With loss of PEN it is reported, and that is a rare event, think 250 per year if I remember, so with 67 million in the UK, so some thing like 0.0003% chance, very low anyway even if my maths it out, yet we have been getting worried about it with EV car chargers.

As to 16 amp fused connection units, we know why not made, people would used them to take a supply from a ring final, and overload the ring final.

 

[bernardgreen]

As I've said (and as is also the case with bernard), I think we have to ask Sunray whether he ever crosses roads, travels in cars, climbs ladders or uses power {or even hand} tools etc. etc.

I cross roads, use power tools and climb ladders. So far no accident.... probably because I know the risks involved and take care to negate them.

 

[JohnW2]

I cross roads, use power tools and climb ladders. So far no accident.... probably because I know the risks involved and take care to negate them.

Same here, but no matter how much one attempts to mitigate risks, some residual risk will always remain, so it comes down to judgements about probabilities.

In the case of Sunray's anecdote, not only is the scenario he described incredibly unlikely to occur in the first place but, even if it does occur, the probability of anyone coming to harm as a result would be very small.

Kind Regards, John

 

[JohnW2]

Would it be reported?

Why would the 'burning out' of a heating element be 'reported'? Reported to whom?

With loss of PEN it is reported, and that is a rare event, think 250 per year if I remember, so with 67 million in the UK, so some thing like 0.0003% chance, very low anyway even if my maths it out ....

There's nothing wrong with your arithmetic, but I'm not sure about the perceived relevance of expressing the annual number of 'lost PENs' as a percentage of the number of people in the UK population!

I would suspect that the number of deaths (if any) attributable to 'lost PENs' is extremely low, quite probably no more than one per year, so the probability of such a death in the UK population could well be more like 0.0000015% per year. That's lower than the probability of dying as a result of being struck by lightning in the UK.

... yet we have been getting worried about it with EV car chargers.

Particularly given the figures above, I can't speak for the "we" to which you refer

Admittedly, EV changing presumably does significantly increase the risk of electric shock (which may be fatal) in the event of a 'lost PEN', but it does still seem to be an extremely small risk for 'us' (not necessarily me!) to be getting excited about. In fact, I'm not necessarily convinced that, in this respect, EV charging represents a risk (in the presence of a 'lost PEN') much greater than that posed by the millions of 'outside taps' in the UK!

Kind Regards, John