Resistive element OCPD

[JohnW2]

... Most heaters with mineral insulation can run with the core(s) red hot for long periods of time in an over current fault situation

I thought that many such heating elements ran at, or close to, 'red heat' in normal operation, which is why I was surprised that Sunray's hadn't 'failed' ('melted') if four times the normal operating current had been flowing through a part of one for an appreciable period of time.

Kind Regards, John

 

[JohnW2]

I'd hoped I worded the response in a way that I disagreed only in that particular situation....

If that was the case, then I think your 'wording' was perhaps far from an ideal way of expressing it. You wrote that you "vehemently disagreed" with part of my sentence which you selectively quoted ...

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However, if you had also included the initial part of my sentence in your quote, you would have seen that what you were "vehemently disagreeing with" was a sentence which, in full, read:

In any event, in the face of a 'direct short' (which what was being talked about) the difference (if any) in (very short) disconnection times between a 16A and 32A OPD would have no noticeable impact on 'fire risk',

Anyway, reverting to the previous discussion to which you are harping back, as I've said, 'anecdotes are anecdotes'. In regulator,or even just 'common sense', terms, do you really believe that happens such as you have described in this thread are "likely" to happen?

Kind Regards, John

 

[SUNRAY]

If that was the case, then I think your 'wording' was perhaps far from an ideal way of expressing it. You wrote that you "vehemently disagreed" with part of my sentence which you selectively quoted ...

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However, if you had also included the initial part of my sentence in your quote, you would have seen that what you were "vehemently disagreeing with" was a sentence which, in full, read:

Kind Regards, John

So If you had not selectively read this:

several post would have been saved. I'm simply saying that correctly protecting the cable (which is the sole purpose of the OCPD and quoted on this forum with boring regularity) this fire hazard wouldn't have existed.

Anyway, reverting to the previous discussion to which you are harping back, as I've said, 'anecdotes are anecdotes'. In regulator,or even just 'common sense', terms, do you really believe that happens such as you have described in this thread are "likely" to happen?

Kind Regards, John

Forget ****** common sense, whoever installed that circuit and/or ovens installed a fire hazard, there's nothing "likely" about it.

 

[bernardgreen]

In regulator,or even just 'common sense', terms, do you really believe that happens such as you have described in this thread are "likely" to happen?

It isn't a case pf preventing something that other wise would be likely to happen. It is a case of preventing anything that may happen, even something unlikely to happen should be prevented from happening or if it does happen the adverse effects ( damage and or injury ) of it happening should be prevented.

 

[Lectrician]

We’ve concentrated on MCBs here, but the original post is about RCBOs, which will both disconnect within 40ms for a L to E fault, no matter what the actual current rating.

For MCBs, it’s EFLI dependent.

 

[JohnW2]

It isn't a case pf preventing something that other wise would be likely to happen. It is a case of preventing anything that may happen, even something unlikely to happen should be prevented from happening or if it does happen the adverse effects ( damage and or injury ) of it happening should be prevented.

We know that such is your attitude to risk, which is your prerogative.

However, many others (including BS7671) adopt a more pragmatic/practical view, which takes the magnitude (probability) of risk into account. If one wanted to take account of any thing that may/might happen (no matter how incredibly low the probability - i.e. however "incredibly unlikely"), then we could not have electricity at all - LV would certainly be a no-no, and even ELV would come with some risks of firer.

As you presumably understand, I mentioned, and asked about, the word "unlikely" specifically because "unlikely to create an overload" is the very phrase that BS7671 uses for determining when a load is such that omission of overload (as opposed to 'fault') protection of cables is acceptable.

Happy New Year!

Kind Regards, John

 

[JohnW2]

So If you had not selectively read this: ..... several post would have been saved.

My apologies. When you wrote that you "vehemently disagreed with" part of my sentence which you quoted, I made the apparent mistake of believing that you were disagreeing with my words which you had quoted, regardless of what you had written earlier in your post.

Forget ****** common sense, whoever installed that circuit and/or ovens installed a fire hazard, there's nothing "likely" about it.

See my recent response to bernard.

You make it sound as if what you have described is an "unthinkable" practice. However, the existence and wide availability of "45A Dual Cooker Outlet Plates" surely serves as a reminder that it is common (and, presumably 'considered acceptable' by many) practice to supply, say, an oven and hob from the same supply, with common OPD (probably rated 'too high' to adequately protect the flexes of the appliances from overload, should it happen).

Kind Regards, John

 

[JohnW2]

We’ve concentrated on MCBs here, but the original post is about RCBOs, which will both disconnect within 40ms for a L to E fault, no matter what the actual current rating. For MCBs, it’s EFLI dependent.

Whilst that is all true, I don't see it's relevance to this discussion, which has been entirely about the inability of a device providing 32A over-current protection to give 'adequate protection' to the flex of a ~12A load.

The possibility of residual currents due to L-E faults has not, as far as i am aware, arisen anywhere in this discussion.

However, you do rise an interesting point. I'm sure that the sort of fault in heating element that Sunray has described is extremely rare in the first place (certainly for any significant time, since the current-carrying portion of the element would be expected to 'burn out' pretty quickly), in practice it would often be accompanied by an L-E fault (to casing of element) - in which case the residual-current part of an RCBO would operate, regardless of over-current protection.

Kind Regards, John

 

[EFLImpudence]

Just to say;

The sort of fault Bernard mentioned regarding bifilar elements is not really relevant to domestic ovens, and

Sunray states that the oven in question did not have an earth fault nor a short-circuit so one can assume that it was a usual element with a terminal at each end - yet it is 5.3Ω - that really is unlikely.

Perhaps it is a 45A oven that someone has connected with the small flex.
If not, then there is also the matter of the internal wiring which will be manufactured with the smallest possible conductors for the intended load.

 

[JohnW2]

Just to say; ... The sort of fault Bernard mentioned regarding bifilar elements is not really relevant to domestic ovens, and ....

Agreed...

Sunray states that the oven in question did not have an earth fault nor a short-circuit so one can assume that it was a usual element with a terminal at each end - yet it is 5.3Ω - that really is unlikely.

Quite so. As I've been saying, I would regard it as "incredibly unlikely' - which is why neither I nor BS7671 feel that one needs to anticipate such extremely rare anecdotal occurrences.

... there is also the matter of the internal wiring which will be manufactured with the smallest possible conductors for the intended load.

Indeed. Those who get concerned about 'inadequate protection against overload' in external flex supplying loads which are extremely unlikely to create such an overload never seem particularly concerned about the internal wiring of the connected equipment.

Happy New Year!

Kind Regards, John

 

[EFLImpudence]

Thank you. Happy New Year to you, yours and everyone else.

 

[Lectrician]

Whilst that is all true, I don't see it's relevance to this discussion, which has been entirely about the inability of a device providing 32A over-current protection to give 'adequate protection' to the flex of a ~12A load.

The possibility of residual currents due to L-E faults has not, as far as i am aware, arisen anywhere in this discussion.

However, you do rise an interesting point. I'm sure that the sort of fault in heating element that Sunray has described is extremely rare in the first place (certainly for any significant time, since the current-carrying portion of the element would be expected to 'burn out' pretty quickly), in practice it would often be accompanied by an L-E fault (to casing of element) - in which case the residual-current part of an RCBO would operate, regardless of over-current protection.

Kind Regards, John

The RCBO tripping in 0.4 seconds prevents earthed metalwork from rising in potential for too long while the fault clears. The flex trapped under an oven for example.

 

[JohnW2]

The RCBO tripping in 0.4 seconds prevents earthed metalwork from rising in potential for too long while the fault clears. The flex trapped under an oven for example.

Well, it would if that happened, but I'm becoming rather confused -I suppose partially because of the problem with RCBOs is that one often doesn't know for sure what has caused it to operate.

However, I thought that Sunray's whole point was about over-current (and teh adequacy of over-current protection), rather thsn anything to do with residual current - particularly since he wrote (with his emboldening) ...

... In the case of the '3 ovens' there is no earth fault (other than the 3MΩ measured with a multimeter) and no 'direct short' circuit, the fault is ~5Ω L/N on one appliance which on it's own was not low enough to blow the 30A fuse or trip a 32A RCBO ....

Kind Regards, John

 

[ericmark]

Thank you @SUNRAY this is some thing I have talked about before, but not actually had a case of it happening.
I have looked at many installation instructions for ovens, and non seem to give a maximum overload size, they have the minimum, but not maximum, typical cooker supply is 30/32 amp, and as shown the duel cooker outlet has been the standard method to have a hob and oven from a supply which was installed for a stand alone cooker, which all seems to make sense as the load for two independent units is in general going to be same as one unit doing all. Well actually my stand alone cooker the oven can draw well over 3 kW.

But it is common for the oven to be wired in cable unable to take 32 amp.

What would be good is a fused connection unit which could take a 15 amp fuse this would allow us to use ovens designed for 16 amp supplies in the UK, but over 13 amp the fuse box jumps in size, with my mothers house the people refitting the kitchen took a 10 mm² SWA around outside of house and put a small consumer unit in the kitchen for just the kitchen. This was because the house needed a re-wire, with rubber covered cables, and no earth to lights, but it did allow 32 amp RCBO to hob and 16 amp RCBO to oven, but that was a lot of extra work and the unit at door was not very ascetically pleasing. However it was a good safety feature as the isolators for oven and hob were placed behind the hob so you needed to reach over hob to isolate it.

But until reading this thread I would have said 32 amp was OK, it was when 45 amp supplies were used that worried me.

But so often the oven could not take 10 mm² cable in the terminal box, and even if it did, the internal cable would not take 32 amp never mind 45 amp.

We would not wire a oven to a 100 amp supply, but there seems to be no limit published, 20/25 amp is normally OK for 2.5 mm² depending on cable type and route, I looked up and found.

So it seems 2.5 mm will take 32 amp, with LSHZ cables, you can get twin and earth as LSHZ, but normally it is thermal plastic so not suitable. But it would seem 32 amp in the main is the upper limit.

However for most installations when moving from a stand alone cooker to eye level hob and oven (can't really see the point if oven put under the hob, may as wll be stand alone.) there is little option but use the twin outlet, I still think we need a fused outlet at 15/16 amp, but can't see a practical way to do it.

 

[EFLImpudence]

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

Ok. Nor I.

I have looked at many installation instructions for ovens, and non seem to give a maximum overload size, they have the minimum, but not maximum,

Why would they?
The trouble is people think a requirement for a 16A supply means it must have a 16A OPD.

typical cooker supply is 30/32 amp, and as shown the duel cooker outlet has been the standard method to have a hob and oven from a supply which was installed for a stand alone cooker, which all seems to make sense as the load for two independent units is in general going to be same as one unit doing all. Well actually my stand alone cooker the oven can draw well over 3 kW.

That is the case but this thread is about what size separate flexes might be used for the oven and the hob.

But it is common for the oven to be wired in cable unable to take 32 amp.

Yes, for a 12.5A load it would.

What would be good is a fused connection unit which could take a 15 amp fuse this would allow us to use ovens designed for 16 amp supplies in the UK

Well, yes, so 16A would be better.

, but over 13 amp the fuse box jumps in size, with my mothers house the people refitting the kitchen took a 10 mm² SWA around outside of house and put a small consumer unit in the kitchen for just the kitchen. This was because the house needed a re-wire, with rubber covered cables, and no earth to lights, but it did allow 32 amp RCBO to hob and 16 amp RCBO to oven,

Again, the flex sizes are what the thread is about.

But until reading this thread I would have said 32 amp was OK, it was when 45 amp supplies were used that worried me.

Ok.

But so often the oven could not take 10 mm² cable in the terminal box,

Why would it be necessary?

and even if it did, the internal cable would not take 32 amp never mind 45 amp.

You seem to be thinking that Amps can be forced into the appliances. They cannot.

We would not wire a oven to a 100 amp supply, but there seems to be no limit published,

Well, we wouldn't for practicality and likely fault current problem.
Are people forgetting that even with omission of overload, the fault current conditions still have to be met?

20/25 amp is normally OK for 2.5 mm² depending on cable type and route, I looked up and found.
So it seems 2.5 mm will take 32 amp, with LSHZ cables, you can get twin and earth as LSHZ, but normally it is thermal plastic so not suitable. But it would seem 32 amp in the main is the upper limit.

Ok.

However for most installations when moving from a stand alone cooker to eye level hob and oven (can't really see the point if oven put under the hob, may as wll be stand alone.) there is little option but use the twin outlet, I still think we need a fused outlet at 15/16 amp, but can't see a practical way to do it.

So, back to square one: why do we need 15/16A?
If you are concerned, you can use the cable you have found.

Plus if such a thing as a 16A fuse were needed, would they not have been introduced by now?