ZIG ZAG UnSAFE zone

[stillp]

If one makes a wild guess at 20 million domestic electrical installations in the UK, each with around 80 bits of sleeved CPC, my arithmetic says that amounts to around £128 million in 'labour costs'.

Irrelevant. They are not all being rewired.

 

[PBC_1966]

However, I am aware of the fact that there are still some remaining ('legacy'!) unsleeved CPCs dotted around my house (I occasionally come across them!), but I have never lost a second's sleep over that!

Somehow I never thought you would! And although my casual observation for comparison with how things are done elsewhere was dismissed as "irrelevant" by a certain person earlier, I'm sitting in a house full of switches and receptacles which have no sleeving on the ground wires anywhere and I'm not about to lose a second's sleep over it either.

It's difficult to make any probabilistic statements without being accused of being irresponsible, callous, or worse.

Join the club.....

However, the probability of (a) such an L-CPC short happening (as a result of no sleeving) AND (b) someone managing to touch two things which were, as a consequence, at dangerously different potentials AND (c) that someone being unusually susceptible to electric shock AND (d) that touching to happen within the few tens of milliseconds it would take for the device to operate would surely be as close as makes no difference to the oft-mentioned 'vaninshingly small'!

Precisely. Really, the chances of all these things happening at just the split second it would take are tiny, far smaller than the chances of many other things which nobody seems at all concerned about.

At risk of leaving myself open to more accusations from some quarters, I can't help feeling that sometimes some of those who are too wrapped up in BS7671 and the current generally accepted method of wiring homes in the U.K. seem to lose the ability to think clearly for themselves, and just follow the "party line" that if BS7671 says it must be done, then it must be an important safety issue and failure to comply will result in untold death and destruction (even, as in our other example debated above, if BS7671 didn't require it just a few years ago and everybody seemed quite happy with it then).

 

[PBC_1966]

If one makes a wild guess at 20 million domestic electrical installations in the UK, each with around 80 bits of sleeved CPC, my arithmetic says that amounts to around £128 million in 'labour costs'.

Irrelevant. They are not all being rewired.

But many have been rewired since the sleeving requirement was introduced part way into the life of the 14th edition. And gradually, over time, the remaining wiring from the 1960's and earlier is likely to be replaced.

 

[JohnW2]

If one makes a wild guess at 20 million domestic electrical installations in the UK, each with around 80 bits of sleeved CPC, my arithmetic says that amounts to around £128 million in 'labour costs'.

Irrelevant. They are not all being rewired.

Yes, but that's not the point. Nor are all RCDs being fitted 'now' etc. - as you know, the real world just does not work that simply. All one can really do is estimate what would be the total cost of sleeving all CPCs, or installing RCDs in all domestic dwellings, over however long (decades) that takes to achieve, and then see how that 'investment' relates to the resultant number of 'lives saved' (per year, decade, century or whatever!).

Kind Regards, John

 

[stillp]

Only if one is interested in improvements that show a benefit in financial terms, rather then reducing a risk because, although small, it can easily and cheaply be reduced.

 

[JohnW2]

Only if one is interested in improvements that show a benefit in financial terms, rather then reducing a risk because, although small, it can easily and cheaply be reduced.

I essentially agree with that, although everything comes down to money in the end (as witness the fact that you talk of "cheaply" even in your "...rather than (financial terms) ..." clause!). So it's really a question of what one regards as cheap" - and that was the reason I presented some figures to contemplate. If (as I suspect might well be the case with this example), mathematically, the 'cost' could be measured in tens or hundreds of £millions to save one life per year, then someone (thankfully not me!) has to decide, on behalf of 'us' (society), whether that is "cheap" and/or a reasonable/acceptable price to pay for that one saved life per year.

Looked at slightly different, given the 20-30 domestic electrocutions per year in UK one would presumably have to wait ('on average') very many years, probably many decades, before even one death was avoided because of CPC sleeving.

But again, I reiterate that I personally always do use sleeving, and I wouldn't suggest that anyone else didn't - even if I might be hard-pressed to 'justify' doing it.

Kind Regards, John

 

[ban-all-sheds]

And everything else being equal, the difference in safety between the socket installed now without earth sleeving and the socket installed 45 years or so ago without it is........ What exactly?

I will stop telling you that there is a difference between assessing what is already installed and installing something new shortly after you stop pretending that there isn't.

 

[stillp]

If (as I suspect might well be the case with this example), mathematically, the 'cost' could be measured in tens or hundreds of £millions to save one life per year, then someone (thankfully not me!) has to decide, on behalf of 'us' (society), whether that is "cheap" and/or a reasonable/acceptable price to pay for that one saved life per year.

No they don't. They have only to decide if it is reasonable to reduce the already small risk by sleeving the CPC.

 

[JohnW2]

No they don't. They have only to decide if it is reasonable to reduce the already small risk by sleeving the CPC.

In such a trivial case as this, that may be all they would do - but, in general, I do not think that it is reasonable, realistic or necessarily even 'responsible' to consider only the 'benefit' side of a requirement without also considering the 'cost'.

However, in this case, if we don't consider 'cost', I'm not sure how one would go about deciding whether it was "reasonable to reduce the already small risk", given the magnitude of that 'already small risk'. Given the current 20-30 UK domestic electrocutions per year, it would seem probable that the number of deaths which would result from unsleeved CPCs would be extremely small, probably less than one per year on average. In deciding whether measures/requirements to reduce that are 'reasonable', one might want/need to consider, for example, the fact that (IIRC) 3-4 people die in the UK every year as a result of being struck by lightning.

One other issue which has not been mentioned is a 'downside' of sleeving CPCs, particularly for DIYers (most of whom will not undertake 'all required testing'). G/Y sleeving is commonly quite loose on the conductor - and I doubt that I am the only person who, as a consequence, has on a number of occasions tightened a terminal screw onto the sleeving, rather than the conductor. I hope that my testing has always detected those errors, but I feel sure that such occurrences will sometimes go undetected (with obvious safety consequences), particular by non-electricians.

Kind Regards, John

 

[stillp]

How would you go about making a decision on whether or not to reduce a risk in the absence of any historical data?

 

[JohnW2]

How would you go about making a decision on whether or not to reduce a risk in the absence of any historical data?

One would have to do one's best to make an assessment on the basis of whatever data, theoretical arguments or even anecdotes could be found. One really cannot assume, 'by default', that anything which is theoretically possible (but may 'never' actually happen in practice) deserves measures to reduce the risk of it happening simply because one has not got enough hard data to properly quantify the risk.

In the case we are discussing, there will be some 'historical data' (changes in death rate following introduction of sleeving) and information from countries which do not routinely sleeve CPCs - but in all cases it would probably difficult/impossible to separate out any effects of sleeving CPCs from all the many other factors resulting in differences/changes in death rates.

Kind Regards, John

 

[PBC_1966]

And everything else being equal, the difference in safety between the socket installed now without earth sleeving and the socket installed 45 years or so ago without it is........ What exactly?

I will stop telling you that there is a difference between assessing what is already installed and installing something new shortly after you stop pretending that there isn't.

So no answer to the actual question then, not surprisingly.

it would seem probable that the number of deaths which would result from unsleeved CPCs would be extremely small, probably less than one per year on average.

Given the level of coincident occurences needed, as you outlined above, coupled with the low figures for electrocution per annum overall (not just now, but over a very long period of time) most of which can be attributed to many other things anyway, I'd be willing to hazard a guess that the number is not just less than one per year on average, but could well be nil in total, ever since earths started to be run to sockets and switches. It's really such an unlikely combination of events that, as you say, you probably have more chance of being struck by lightning.

I've never seen any explanation as to why the sleeving amendment was made - Was it really based upon some desire to guard against the possibility of an extremely tiny risk given evidence of such risk, or did it just "seem like a good idea" at the time?

I wonder how many other electrical codes around the world require it? Much of Europe seems to favor insulated singles in conduit or cables which have insulated earth wires anyway; Australia & New Zealand have a T&E-type cable which also has an insulated earth, although I'm not sure when that was introduced, if they used British-style T&E before, and if so whether or not they used sleeving. North American NM-type cable has a bare ground wire which - at least in the U.S. & Canada - is just left bare when terminating.

One other issue which has not been mentioned is a 'downside' of sleeving CPCs, particularly for DIYers (most of whom will not undertake 'all required testing').

That's an interesting point. Given the lack of dexterity of some very casual electrical DIYers, coupled with no testing, I'd hazard a further guess that the chances of that happening are quite probably significantly greater than the chances of somebody ever being electrocuted through the extremely unlikely combination of circumstances outlined for a bare earth.

 

[EFLImpudence]

In view the single reply to my question (purpose of sleeving) and subsequent comments - i.e. the CPC may touch a live conductor (as opposed to a live conductor may touch it) - do people think it would therefore be comparable and advisable to insulate the back box?

 

[JohnW2]

... I'd be willing to hazard a guess that the number is not just less than one per year on average, but could well be nil in total, ever since earths started to be run to sockets and switches.

Yes, that's almost certainly true. I thought I might get less criticised if I stuck with the very conservative 'less than one per year' (rather than 'probably never'!!), particularly since that was more than adequate to make my point ....

It's really such an unlikely combination of events that, as you say, you probably have more chance of being struck by lightning.

... and that was my point. I would say that it's all but certain that the 3-4 UK deaths per year due to people being struck by lightning is appreciably greater than the risk of being electrocuted because of the absence of CPC sleeving.

I've never seen any explanation as to why the sleeving amendment was made - Was it really based upon some desire to guard against the possibility of an extremely tiny risk given evidence of such risk, or did it just "seem like a good idea" at the time?

I have no idea, but would very much suspect the latter.

That's an interesting point. Given the lack of dexterity of some very casual electrical DIYers, coupled with no testing, I'd hazard a further guess that the chances of that happening are quite probably significantly greater than the chances of somebody ever being electrocuted through the extremely unlikely combination of circumstances outlined for a bare earth.

I agree, but I think it may be a more common problem that you imply. I would probably not regard myself as "very casual electrical DIYer who lacked dexterity" but, as I said, I've certainly done it a few times (that I know of - possibly more!).

In fact, it's one situation in which I often still do 'twist together' the ends of conductors - IME, if there are two or three CPCs, each with loose-fitting G/Y sleeving going, into the same terminal, it can be next-to-impossible to be sure that all of the conductors (rather than sleeving) have been properly gripped by the terminal screw. At least twisting together the ends of the conductors ensures that this problem does not arise.

Kind Regards, John

 

[stillp]

How would you go about making a decision on whether or not to reduce a risk in the absence of any historical data?

One would have to do one's best to make an assessment on the basis of whatever data, theoretical arguments or even anecdotes could be found. One really cannot assume, 'by default', that anything which is theoretically possible (but may 'never' actually happen in practice) deserves measures to reduce the risk of it happening simply because one has not got enough hard data to properly quantify the risk.

The risk doesn't have to be quantified. When a risk is identified, there is a duty to reduce that risk to as low as reasonably practicable. It is clearly a trivial cost and inconvenience to add sleeving, therefore there is no need to justify its use.