Talk:Old electrical installations

Meter tails

Are old sized meter tails in some way an issue? I thought even old sizes were heavily overspecced. NT 11:01, 28 October 2012 (GMT)

It is not uncommon to come across 10mm tails (and I have seen 6mm tails) and VIR meter tails. I also often see single insulated meter tails that pass through a wall between the meter and the CU. I know that swapping the tails is not a DIY job but the tails are part of the electrical installation and worth a mention. --ARWadsworth 17:56, 29 October 2012 (GMT)

6mm^2 is 47A rated continuous. Greater loads will get drawn, but I don't think for long enough to cause a problem. NT 21:20, 29 October 2012 (GMT)

6mm or 10mm meter tails are not up to current standards. The article is about older installations and not just fires and electrocution.--ARWadsworth 19:23, 31 October 2012 (GMT)

Much depends on the nature of the appliances in use. In some households 47A is easily swallowed by one big load like a shower. The normal load on top of that (even if only 20A) would be enough extra to become a moderate medium term overload, and hence the risk of cable damage is fairly high.

Re: RCDs; NT, your change of wording to includ "slight" has totally changed the meaning of the statement. Inclusion of a RCD is probably the single most significant contribution you can make to reducing risk of injury from shocks - hence omission of one is a *significant* increase in the risk. That has nothing to do with the likelihood of receiving a shock in the first place.

I think it is irresponsible to dismiss any shock at mains voltage as a "slight" risk. If the circumstances are sufficiently unfavourable, then a mains shock will kill you. --John Rumm 00:45, 31 October 2012 (GMT)

I forget the exact figure but something like half a million A&E visits following shocks a year (so many more shocks), and around 20 deaths, so it is a low risk. NT 01:31, 31 October 2012 (GMT)

I am not sure how you draw that conclusion. Firstly you don't know how many injuries were prevented by working RCDs - and hence A&E admissions prevented.

Secondly you are falling into a common statistical trap of choosing the wrong sample set[1]. Looking at the grand scheme of things, the risk of being killed by an electric shock is indeed low. However we are not talking about the grand scheme of things. We are talking about the outcome for subset of the population who do receive a shock. So out of those 100% who do get a shock, the risk of being one of the 20 killed then rises dramatically since you are no longer talking about 20 deaths out of 65m people, but 20 deaths out of (say) 1.5m people - still not a huge risk (1 in 75000) but much less attractive odds.

Its like the difference between the odds of dying in a road accident in general, compared with that for people involved in a road accident dying. i.e. if not are not in the accident in the first place, your chances of dying from it are close to zero, if you are then the odds are much higher than the general average for the whole population.

Also I believe it is wrong to fixate on death as the only bad outcome. Some fairly significant number (high hundreds, low thousands) of those hospital admissions will require treatment for a serious injury that will likely have life changing and ongoing effects (e.g. serious burns, loss of sight, cardiac injury, neurological damage, fell off a ladder and broke bones etc). That alters the picture dramatically. Say that number is 2500 people/year. Out of the population as a whole that is still a 1 in 26000 chance - which many will be quite happy with. But, looking our actual class of people we are talking about, the odds of serious injury are now 1 in 60 (if we go with the 2500/1.5m ratio). That now sounds somewhat more menacing. A larger number still will have unpleasant level of injury that they will make a full recovery from, but at what point do you say that an injury level is minor enough to make the cost of an RCD not worth it?

[1] it was an error of that sort that resulted in various parents being convicted of killing their children following cot deaths, as a result of flawed expert testimony from a doctor who created the impression that the cumulative odds of two or more cot deaths were unrealistically high to be "accidental" --John Rumm 18:44, 31 October 2012 (GMT)

Re moderate tail cable overloads, again its the result that counts, ie the number of fires & shock deaths. Running cable insulation at above 70C now and then is quite a different thing to a fire or shock. NT 01:33, 31 October 2012 (GMT)

And the chances of the tails failing in a bad way when they are adequately sized is pretty much zero, and when undersized is obviously greater. Its a bit of a moot point these days since most DNOs will downgrade the main fuse when they see undersize tails these days. (they did here - reduced from 100 to 80A because the tails out of the meter are only 16mm). --John Rumm 18:44, 31 October 2012 (GMT)