Safety issue with domestic appliance

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I sort-of addressed that in my recent post. Eric is, of course, presenting a fairly unusual argument - that existance of an internal fuse could, in certain circumstances, theoretically create a hazard.
How?

To extend what I wrote before, I have to say that I have seen appliances ikn which the internal fuses is quite physically distant from where the supply enters the appliance.
Surely that would be a very low rated fuse to protect something delicate.

In that situation, should a fault to earth occur in that wiring (upstream of the internal fuse), then the hazard which eric has mentioned could theoretically arise.
I don't see how.
That would presumably be similar to a fault where no internal fuse was fitted.

Whether or not any internal fuse would be likely to blow before the circuit's OPD operated is a different question - but I suppose it's not impossible if, as eric was suggesting, there were a ~13A internal fuse in the oven and a B45 protecting the circuit.
Why would there be an internal 13A fuse if that were what was expected in a plug (or 16A supply)?
If a 13A fuse is necessary then the manufacturer should fit it covering all the vulnerable parts.

It's those 'not quite negligible impedance faults' that could be the issue - a fault current of, say, around 100A would presumably blow a 13A fuse pretty smartish, maybe before it caused a B45 to operate (thermally).
How would there be a 100A fault current in an oven (oven being discussed but not necessarily what the OP has)?
 
Thanks, I missed that. I do not then see how his fears can be justified.
I'm not so sure about that. IF he is correct in saying there is a design or manufacturing fault whereby a live conductor comes in contact with the casing when the oven heats up, then that is obviously unacceptable, regardless of the fact that protective device(s) ought to rapidly deal with the problem.

However, he says that no protective device is operating, hence all the discussion about types of earths, OPDs and RCDs etc..

We just don't know what we are discussing. For all we know, it might just be a case of a 'neon screwdriver' lighting up!

Kind Regards, John
 
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As I have subsequently clarified, it would probably need 'two faults' (one on either side of the internal fuse) - and that is an incredibly unlikely scenario.
Why would there be an internal 13A fuse if that were what was expected in a plug (or 16A supply)? If a 13A fuse is necessary then the manufacturer should fit it covering all the vulnerable parts.
We were not talking about 'protecting vulnerable parts' (I presume you mean 'of the appliance') but, rather, the facilitation of ADS.
How would there be a 100A fault current in an oven (oven being discussed but not necessarily what the OP has)?
We're talking about faults, not overloads. There is this assumption that when two conductors come into contact, there will be an electrical connection of 'negligible impedance' between them, but that is not necessarily the case. Particularly if the Zs of the circuit is fairly 'marginal', we're talking about difference between a fault impedance of 'zero' ('negligible') and probably not much over 1Ω to make the difference between a fault current of 225A (to guarantee magnetic tripping of a B45) and 100A - and a slightly 'poor contact' between conductors (or a bit of tarnishing on one of them) could easily result in such a fault impedance. Let's face it, 'loose connections' in terminals can, and do, result in a sufficiently high fault impedance to result in serious overheating, even fires.

Kind Regards, John
 
But call me a cynic if you will, but I don't think they do so selflessly - they do it to protect the financial interests of the people who fund them.
Yes, of course - that's why manufacturers join them.
Can a cost-benefit analysis be made which justifies changing the requirements to increase that percentage?
Possibly, but we don't hear a lot of statistics to show that the existing percentage is inadequate do we?
No, I seem not to be.
Seemed like that to me, although I was surprised, given your views expressed in other threads.
 
We were not talking about 'protecting vulnerable parts' (I presume you mean 'of the appliance') but, rather, the facilitation of ADS.
Yes, but you mentioned a fuse 'not at the start of the internal wiring'.
Why would there be a 13A fuse at such a position if the oven could all be on a plug.

We're talking about faults, not overloads.
Exactly.

There is this assumption that when two conductors come into contact, there will be an electrical connection of 'negligible impedance' between them, but that is not necessarily the case. Particularly if the Zs of the circuit is fairly 'marginal', we're talking about difference between a fault impedance of 'zero' ('negligible') and probably not much over 1Ω to make the difference between a fault current of 225A (to guarantee magnetic tripping of a B45) and 100A
Well, it is quite hard to get a conductor to 'fall', touch another or the carcass and maintain 1Ω or 2.3Ω.

- and a slightly 'poor contact' between conductors (or a bit of tarnishing on one of them) could easily result in such a fault impedance. Let's face it, 'loose connections' in terminals can, and do, result in a sufficiently high fault impedance to result in serious overheating, even fires.
A loose spade terminal will just heat until it or the conductor burns away.
It cannot withstand 100A or 225A.
Plus, of course, that would not be fault impedance until it fell off and touched something else.
 
As I have subsequently clarified, it would probably need 'two faults' (one on either side of the internal fuse) - and that is an incredibly unlikely scenario.
On reflection, I don't even think that is possible. I need to think a bit more, but I suspect that I allowed myself to be seduced by eric's statement into thinking there was a potential, hazard such as he suggested, but I no longer think that is the case ... if there is an L-E fault downstream of an internal fuse, that internal fuse should clear it (if the circuit'd devices didn't do so first, and if there is an L-E fault upstream of an internal fuse then the circuit's protective device(s) should clear this. If, very unlikely, both faults existed, then the respective devices should clear them.

So, if I've now got my thinking straight, I think I need to apologise for partially agreeing with eric and thereby confusing things a bit!

Kind Regards, John
 
Yes, but you mentioned a fuse 'not at the start of the internal wiring'. Why would there be a 13A fuse at such a position if the oven could all be on a plug.
I thought eric was talking generally, not just about ovens that could be on a 13A plug.
Well, it is quite hard to get a conductor to 'fall', touch another or the carcass and maintain 1Ω or 2.3Ω.
We're talking about fault impedances (impedances of the faults) appreciably lower than those - the figures you mention are total Zs values, much of which will already be accounted for by Ze and R1+R2. However, I think you're a bit optimistic about how low necessarily is the resistance/impedance of a 'casual touching' of two conductors. I've just tried 'dropping' the bare innards of a 16mm² conductor onto a copper plate a number of times, and got measured resistances varying from zero (i.e. <0.01Ω) to about 15Ω; most were 0.3Ω or greater. When it was fairly high, the slightest touch (even blowing at it) could result in a dramatic change in the contact resistance.
A loose spade terminal will just heat until it or the conductor burns away. It cannot withstand 100A or 225A.
What happens depends entirely upon 'how loose' and hence 'how high a contact resistance'. The amount of heat (and damage) produced will be related to I²R, R being the resistance/impedance of the connection. Depending on the value of that, anything can happen from very slight warming to rapid 'burning up'. Think of the above figures from my experiment. had I connected, say, 200V (with a very high current providing capacity!) between the two (instead of a meter), the resultant 'fault current' would have varied from about 13.3A up to >20,000A.

Kind Regards, John
 
We're talking about fault impedances (impedances of the faults) appreciably lower than those - the figures you mention are total Zs values, much of which will already be accounted for by Ze and R1+R2.
Yes, I was forgetting that.

However, I think you're a bit optimistic about how low necessarily is the resistance/impedance of a 'casual touching' of two conductors.
Not really.
I thought you were thinking of "touches" that would result in insufficient current to cause the OPD to trip; hence the unsuitability of higher rated MCBs.

I've just tried 'dropping' the bare innards of a 16mm² conductor onto a copper plate a number of times, and got measured resistances varying from zero (i.e. <0.01Ω) to about 15Ω; most were 0.3Ω or greater.
Yes - I think - but to maintain a live carcass without tripping the MCB needs a sustained contact of quite high resistance.
Is this possible with it bouncing about with the sparking? It's more likely that it will be welded to the surface.

It's a pity you can't drop a live conductor with a spade terminal on it and see how many times a 32A (or 45A?) MCB would not trip but leave the plate live.
Perhaps it raises the question - Has an oven ever done this?

When it was fairly high, the slightest touch (even blowing at it) could result in a dramatic change in the contact resistance.
I presume you mean a change to higher?
 
Not really. I thought you were thinking of "touches" that would result in insufficient current to cause the OPD to trip; hence the unsuitability of higher rated MCBs.
We all know that one of the failings of ADS is that it doesn't necessarily work (as quickly as required) if the impedance of the fault is not 'negligible' (which essentially means zero). If the Zs of the circuit is already 'at the limit' for compliant ADS, then anything above zero ohms will not necessarily cause the OPD to operate in the required time.

However, there's no practical solution to that. We could seriously over-engineer circuits in an attempt to get ADS to work with faults of non-negligible impedance but, even if we reduced R1+R2 to 'near zero', we would still be stuck with Ze as a lower bound to Zs.
Yes - I think - but to maintain a live carcass without tripping the MCB needs a sustained contact of quite high resistance.
I suppose that depends upon what you mean by "a quite high resistance" and what you mean by "maintain". A Zs of around 4.6Ω (maybe a contact resistance of around 3.6Ω) would result in a current of about 50A, which could take about 3 hours to thermally trip a B32)
Is this possible with it bouncing about with the sparking? It's more likely that it will be welded to the surface.
I would think that whilst it is 'bouncing about' the ('average') contact resistance is likely to be fairly high. If it welds, then the 'contact resistance' will obviously reduce to very close to zero, so, for a complaint circuit, the OPD would operate 'immediately'
It's a pity you can't drop a live conductor with a spade terminal on it and see how many times a 32A (or 45A?) MCB would not trip but leave the plate live.
I'll leave that to you, or maybe RF! However, as above, a pretty modest contact resistance could result in it taking a very long time for a Type B MCB to trip - I'm not sure whether that counts as "leaving the plate live".
I presume you mean a change to higher?
No - usually lower. As one might expect, if the contact resistance is high ('not very good contact'), the slightest pressure or movement tends to result in a dramatic reduction in contact resistance.

Kind Regards, John
 
A Zs of around 4.6Ω (maybe a contact resistance of around 3.6Ω) would result in a current of about 50A, which could take about 3 hours to thermally trip a B32)
Well, yes, but that's playing with numbers - although obviously correct.
How do MCBs ever trip, then?

Why not a Zs of 0.9Ω (with a contact resistance of 0.3Ω) and a current of 256A so all's well?
I (only) once blew a 60A main fuse with a 1mm² CPC which sprang out of my hand and touched the incoming line screw at the main switch.

No - usually lower. As one might expect, if the contact resistance is high ('not very good contact'), the slightest pressure or movement tends to result in a dramatic reduction in contact resistance.
Oh, ok - depends which way you were moving/blowing it, I suppose.
 
It seems we have scared him off, he has not returned to say exactly what happened. It has however made me change my thoughts, if a manufacturer does not give the maximum size of a protective device, then it is up to him to internally protect it. Never looked at it like that before.

I looked at the instructions for my mothers Zanussi oven, it clearly states max overload size 20 amp. It uses less than 13A but clearly states 20A max.

It is fed from a 13A FCU so fitted correctly. Since I don't have a stand alone oven over 13A I don't know what they state.
 
Well, yes, but that's playing with numbers - although obviously correct.
How do MCBs ever trip, then?
They trip if the faul impedance is 'negligible'. The more 'non-negligible' it becomes, the more likely is that it won't trip (quickly). Of course, any MCB should trip eventually with a current over 1.13In, but that could take hours for small currents (high fault impedance).
I (only) once blew a 60A main fuse with a 1mm² CPC which sprang out of my hand and touched the incoming line screw at the main switch.
It's obviously possible that even just 'passing contact' might result in a low enough contact resistance, for long enough, for an OPD to operate almost immediately.

Kind Regards, John
Edit: Format mess tidied up
 
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It seems we have scared him off, he has not returned to say exactly what happened. It has however made me change my thoughts, if a manufacturer does not give the maximum size of a protective device, then it is up to him to internally protect it. Never looked at it like that before.
I looked at the instructions for my mothers Zanussi oven, it clearly states max overload size 20 amp. It uses less than 13A but clearly states 20A max.
It is fed from a 13A FCU so fitted correctly. Since I don't have a stand alone oven over 13A I don't know what they state.
What do they say for cookers which are the same inside?
 

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