Terminating unused electric cable

[JohnW2]

To put that into context, you (and I) have about a 1 in 2,700 chance of being killed or seriously injured on the UK roads in the next 12 months (killed about 1 in 37,000; seriously injured about 1 in 3,000)

You cannot in any meaningful way say what your, my, or Bernard's chances are of that. We all use different roads, types of roads, in different ways, at different times, and for different amounts of time. Population outcomes cannot be used to infer individual ones.

OK, I suppose I 'asked for that' by mentioning bernard and myself! However, as is so often the case in relation to statistical information, in the absence of any other information about all those other factors affecting the risk to individuals, population probabilities ('averages') are all one can talk about.

If it makes you more comfortable, think about absolute figures, rather than ('on average') probabilities. If the population risk of being killed or seriously injured on the UK roads in a year is about 1 in 2,700, that means that about 23,700 people would be killed or seriously injured in a year - although, as you say, without a lot more information one could not say how likely it is that you, bernard or I would be one of those 23,700. Similarly, if (as I was hypothesising) there were a 1 in 1,000,000 chance of harm coming to persons or property as a result of an "unsafe" electrical installation, then about 64 people/properties would suffer that fate, even though we can't know which 64 individuals/ properties they will be. However, in terms of the population, one can legitimately compare those 64 with the 23,700 .

Your comments obviously apply as much to the "unsafe electrical installation" as to injuries/deaths on the roads. Just as an individual who never leaves their house would have a zero probability of being injured/killed on the roads, so would a person (like my late grandmother, who wouldn't even touch a light switch!) who would never dream of touching a light fitting have a zero probability of coming to any harm as a result of it being metal and unearthed.

The more data one has, the more that one can look at 'averages' in subgroups, but any statistical/probabilistic information inevitably has to deal with 'on average' - whether within those 'averages' relate to whole populations or subgroups thereof. Such is the nature of anything probabilistic. If we knew for certain whether or not you, I or bernard were going to be injured on the roads in the next 12 months, there would be nothing 'probabilistic' to talk about - only 'known facts'.

The same concepts apply to things we often discuss here. We may 'know' from available surveys that 'in the population' ('on average') 7% (or is in 1 in 7 - I can never remember?!) of in-service RCDs are 'faulty'. However, without knowing a lot more about the make/model of the device, environmental conditions, loads to which it is subjected, duration of period in service etc. etc., we cannot take that to mean that this represents the probability of any particular individual device being faulty. However, that does not mean that population statistics are not useful.

Kind Regards, John

 

[JohnW2]

OK, so a hypothetical external influence which made all installations unsafe for a period of 86.4 seconds a day would not, in your eyes, result in that figure of "99.9%".

Mathematically it would - but, as bernard has said, it's a ludicrous hypothesis.

Do you think that if 99.9% (or thereabouts) of installations done in compliance with BS 7671 are "safe" then working to BS 7671 is a reasonably safe thing to do?

As I've been saying/implying, the answer to that is crucially dependent on what is meant by "safe" and "unsafe". At one extreme, if 99.9% of installations being "safe" means that 1 in 1,000 installations will, on average, result in a death/injury, that would clearly (hopefully!) not be regarded as a "reasonably safe" situation. On the other hand, if it means that 1 in 1,000 installations have defects which, if present, themselves carry a 1 in 1,000 risk that a death/injury will result (i.e. an overall 1 in 1,000,000 risk to individuals) then that might well be regarded as "reasonably safe".

Kind Regards, John

 

[TonyW2]

Hope the OP is following this closely and will soon be able to decide on a course of action.

 

[JohnW2]

Hope the OP is following this closely and will soon be able to decide on a course of action.

The OP got his answer early on page 1, and reported that "the job was done" early on page 2.

Kind Regards, John

 

[stillp]

We may 'know' from available surveys that 'in the population' ('on average') 7% (or is in 1 in 7 - I can never remember?!) of in-service RCDs are 'faulty'. However, without knowing a lot more about the make/model of the device, environmental conditions, loads to which it is subjected, duration of period in service etc. etc., we cannot take that to mean that this represents the probability of any particular individual device being faulty. However, that does not mean that population statistics are not useful.

Another thing we don't know from that survey is whether 100% of the RCDs tested failed to operate 7% of the time, or 7% of the RCDs failed to operate 100% of the time, or something in between. IIRC, the Italian survey that is most often quoted was based on quite a small sample, and the 'failures' were failures to trip when the test button was pressed, which does not necessarily mean they would have failed a ramp test, or failed to disconnect on a fault.

 

[JohnW2]

Another thing we don't know from that survey is whether 100% of the RCDs tested failed to operate 7% of the time, or 7% of the RCDs failed to operate 100% of the time, or something in between.

Indeed, and I don't think that the survey in question could answer that question since, IIRC, they simply ascertained how many failed to operate satisfactorily in response to a single test at a single point in time - so we don't know whether the ones which 'failed' would have 'passed' a test at some other point in time, or vice versa.

IIRC, the Italian survey that is most often quoted was based on quite a small sample, and the 'failures' were failures to trip when the test button was pressed, which does not necessarily mean they would have failed a ramp test, or failed to disconnect on a fault.

Again, indeed. Although widely cited, that survey was unsatisfactory in many ways, not the least being a sample size far too small to enable even remotely confident conclusions about 'the population' to be drawn. IIRC, some of them actually 'failed' because they had been 'bypassed' (admittedly probably because of prior malfunction in many cases).

However, I only mentioned that particular example to underline your point that, even if the 'population statistics' had been derived from a large and high-quality survey, the 'population probability of failure' would mean little in terms of the probability of a particular individual device failing - for the reasons which both of us have mentioned.

Kind Regards, John

 

[PBC_1966]

Lies, damned lies, and statistics.....

 

[JohnW2]

Lies, damned lies, and statistics.....

Indeed - although it has to be remembered that most of the 'numerical information' we have, on the basis of which we run our world and our lives, represents 'statistics' in one sense or another.

Kind Regards, John

 

[ban-all-sheds]

IIRC, the Italian survey that is most often quoted was based on quite a small sample, and the 'failures' were failures to trip when the test button was pressed, which does not necessarily mean they would have failed a ramp test, or failed to disconnect on a fault.

And IIRC they only looked at failure to trip (properly) on the first attempt - when they tried the buttons again a lot of the first-time failures were OK. Hence the advice to test regularly - not to find out asap that your RCD has gone bad but to stop it seizing up.

So we don't know what the failure rate is when there are real faults, and we may not even know what the actual failure rate in tests is.

I do know that I'd be surprised if the EN standard for RCDs permits a failure rate of 7%.

 

[PBC_1966]

And IIRC they only looked at failure to trip (properly) on the first attempt - when they tried the buttons again a lot of the first-time failures were OK. Hence the advice to test regularly - not to find out asap that your RCD has gone bad but to stop it seizing up.

Or it might have even been just that the contacts on the test button were oxidized and that the device would have tripped properly first time on a real fault anyway.

 

[JohnW2]

I do know that I'd be surprised if the EN standard for RCDs permits a failure rate of 7%.

Indeed - but the problem there is that a Standard cannot really impose any requirements in terms of real-world in-service failure rate. About the best they can do is to require a certain performance in specified tests, which may attempt (but will never completely achieve) some sort of approximation to long-term in-service conditions (including 'accelerated tests', if appropriate).

Kind Regards, John

 

[PBC_1966]

I fully accept that it's quite possible to do things which are reasonably safe but do not comply with BS 7671. What I cannot conceive is how one could do unsafe work which does comply.

You are saying that there is a chance that someone could comply with BS 7671, as advised in the guidance on how to comply with Part P, and then be found to have not complied with Part P.

You are saying that there is a chance that an electrical installation which complies with BS 7671 might be unacceptably dangerous.

Isn't that exactly what you were claiming could happen when you made the following statement?

I can, and have more than once, put forward a convincing case that as the law requires "reasonably safe" and does not require compliance with BS 7671, if the continued use of red/black cable is a reasonable thing to do, the law actually requires you to do that.

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