Relying on loads not being able to overload

[stillp]

From the IET forum:
http://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=57216&enterthread=y

 

[THRIPSTER]

What is the relevance, StilP?

Regards

 

[stillp]

"The installation had been energised without any testing and the fault current had blown away part of the screw and CPC. This was verified by forensic examination at the HSE laboratories. This left a high resistance fault from the line conductor to the metal studwork."
That could equally well have resulted in an overcurrent.

 

[EFLImpudence]

How?

 

[THRIPSTER]

I think the original fault must have severed the CPC and left the line in contact with the studwork but the metal stud work was not sufficiently 'earthy' to pass enough fault current to trip the MCB?

Otherwise cannot picture it.

Regards

 

[JohnW2]

I must confess to having the same concerns as you, originally. However, once I understood that most motor circuits have their own overload protection control gear and that the chances of equipment such as showers and immersion heaters would have a very low risk of failing to an overload condition, then I just accepted it. But hey, what do I know?

As you imply, it's probabilistic. I am sure you are right that the probability of such things failing to an 'overload' condition (which, in this context, should include low, but not negligible, impedance L-CPC faults) is very low, probably extremely low - but that's not the same as zero.

There are plenty of other situations in which we are expected and required to take measures to reduce, or guard against, risks of extremely low probability - so, if that's the level of caution/conservatism we are expected to work to (and that's obviously debatable), I'm not sure I can see why the very low (but not zero) risk we are talking about should get 'special treatment'.

As a matter of interest, is the W2 in your username because you are located in London, W2?

No - if you look, you'll see that I live in Buckinghamshire. "JohnW2" arises because I am "JohnW", but that username was already taken, so I added the "2"! I do have family in London W2, though,if that helps

Kind Regards, John

 

[stillp]

I can think of a couple of failure mechanisms but Thripster's seems quite likely. My point was that the fault resistance could have been any value, and so could the fault current.
The incorrect certificate is another interesting factor.

 

[EFLImpudence]

I think the original fault must have severed the CPC and left the line in contact with the studwork but the metal stud work was not sufficiently 'earthy' to pass enough fault current to trip the MCB?

Yes, probably not at all 'earthy'.

 

[EFLImpudence]

It has been stated that the screw and cpc were blown away.
Therefore this would have tripped the opd.

It must have been switched on again but now the cpc is no longer in contact so it remained on with the live connected to the stud.

 

[JohnW2]

So on the one side, one can argue that it's very hard to have an overcurrent fault and one hopefully has smoke detection. One could also say given the odds there are problem statistically more significant issues in the bigger picture that one should address before this.

Exactly - that's the point I've just made in another thread. The regs, and our practices, appear to be concerned about some risks which are probably appreciably lower than we are talking about.

There again on the flip side, one could easily argue, when one has to select a breaker, why select one greater than one needs.

Quite so. Unless there is a good reason (and I don't include 'only having the wrong one to hand' in that!), it seems ridiculous to 'unnecessarily' install a breaker which is too large (or a cable which is 'too small'), no matter how small the (undoubtedly non-zero) risk.

As the owner (and I stress, not installer) of the installation that precipitated this debate, my guess is the that equestrian installer in question selected it because he'd purchased an off the shelf CU that came with a range of MCBs, so he used that one. As the old saying goes "when all one has is a B32, there's a tendency for everything to look like 4mm t&e".

That's very likely. As above, I really don't think one can justify doing it because of matters of 'convenience' such as that, given that I do not believe that the risk is actually zero. In some of the scenarios, it's a bit more 'understandable' (like EFLI's example of a metre or so of 1.5/2.5mm² flex hard-wired to a '13A oven' from a 4/6mm² 32A circuit) - but, even then, it's hard to see a truly good 'excuse' for not interposibg a FCU.

Kind Regards, John

 

[mfarrow]

IThe incorrect certificate is another interesting factor.

By that I'm assuming you mean "all too common"?

I don't see the relevance to the discussion. A fault happened which burnt out the contact between the CPC and the screw. Had it burnt out the contact to the Line conductor instead then we would not be discussing it.

What would you do if you immediately found your immersion had tripped:
- Try and switch it back on?
- Call an electrician straight away?

I know what my answer would probably be.

What we're discussing here is the likelihood of high impedance 'faults' between conductors (not conductor and an insulated wall as in this case), and what JohnW2 is asking for is opinion as to whether the regulations are correct in their assumed perception that these types of fault are unlikely to occur.

I don't have any data to answer this question, and I very much doubt anyone else on this forum does. I therefore have to concede that the IET have done their homework and perceive the risk to be small enough to not regulate fixed wiring installations around it. Either that or we're talking about something which the IET/IEE/STEE have not considered or have been blind to in the last 132 years.

 

[JohnW2]

It has been stated that the screw and cpc were blown away. Therefore this would have tripped the opd. It must have been switched on again but now the cpc is no longer in contact so it remained on with the live connected to the stud.

Yes, the OPD presumably operated at the time the circuit was initially energised. However, the circuit was obvioulsy re-energised, and the report indicates that the result was a persistent high resistance fault (such as we have been discussing) from L to (presumably earthed) metal studwork (not the CPC) - which persisted for 18 months until it eventually resulted in the tragic event. One can but presume that the circuit was not RCD protected.

The installation had been energised without any testing and the fault current had blown away part of the screw and CPC. This was verified by forensic examination at the HSE laboratories. This left a high resistance fault from the line conductor to the metal studwork.
18 months later ... she was found kneeling in the cupboard slumped forward and apparently lifeless. Subsequent examination and a Post Mortum determined that she was kneeling in the charged water and received a fatal shock when she touched the earthed stop valve.

Kind Regards, John

 

[EFLImpudence]

The studwork was not bonded/earthed (nor did it have to be) so no current was flowing until the unfortunate person touched the water stopcock.

It does not state the high resistance value.

 

[JohnW2]

What we're discussing here is the likelihood of high impedance 'faults' between conductors (not conductor and an insulated wall as in this case), and what JohnW2 is asking for is opinion as to whether the regulations are correct in their assumed perception that these types of fault are unlikely to occur.

Exactly - and it must be apparent, both from the title I gave this thread and my OP, that such was the question that I was hoping to get opinions about.

I don't have any data to answer this question, and I very much doubt anyone else on this forum does.

I think that's true. However, the one electrician who has expressed an opinion seems to be 'convinced' (I think he's used that word) that the risk is more-or-less zero - not because of what the IET does or does not believe, but his own view.

I therefore have to concede that the IET have done their homework and perceive the risk to be small enough to not regulate fixed wiring installations around it. Either that or we're talking about something which the IET/IEE/STEE have not considered or have been blind to in the last 132 years.

Maybe, but I fear that a lot of what the IET etc. have done over the years has been (probably necessarily in many cases) been based on opinions and 'gut feelings', rather than hard research or statistics (aka 'homework'). Do you think, for example, that they will have any hard evidence that a 1.5mm diameter hole in the top of an enclosure presents a measurable risk to life, limb or property?

Kind Regards, John

 

[JohnW2]

The studwork was not bonded/earthed (nor did it have to be) so no current was flowing until the unfortunate person touched the water stopcock.

In this case, that presumably was the case. However, I thought (perhaps wrongly) that the point stillp was making was that the metal in question (whatever it was) might (in a different scenario) have been earthed, in which case there would have been a persistent high resistance L-E 'fault', which may (although not low enough resistance to invoke 'fault protection') have caused an 'overload' current to flow through the wiring (and OPD) - the situation which you feel is never going to happen.

It does not state the high resistance value.

Again, I thought that stillp's point was that, in other situations, the resistance could have been 'anything'.

Kind Regards, John