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)
An article that listed all the ways in which old installs can be not upto current regs would be a pointless article. The point is to list the main ways that do constitute a safety issue, and discuss how these issues can be handled. NT 11:36, 1 November 2012 (GMT)
No, I don't see this article being just about safety. Its about old electrics and how they may differ from what one might install now. It does need to cover safety, but it also needs to convenience & changing patterns of use, as well as how things were different in the past and the implications that will have now. So undersized meter tails are significant, if for no other reason than they would cause the DNO to downsize your main fuse on the next metre change. Single insulated tails are significant, because they don't offer the mechanical robustness of a modern install etc.
(and since when has an article being pointless been a problem? We have one on making things from plastic pipe that is in the top ten page impressions!)
--John Rumm 22:40, 1 November 2012 (GMT)
If you want a pointless article, why not list the entire 17th edition. There are bits of installs out there that fail to comply on every point in them. But so what... what is relevant to diyers is lack of functionality and lack of safety. And since the average reader of a dry article doesn't read it for days on end, its most best to concentrate on the main issues. NT 13:25, 4 November 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. (John Rumm)
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)
First I've taken the bold step of moving the above 2 paragraphs up to here, as they belong together and its just impossible to address the various points in any sane layout otherwise.
What you say above very much misunderstands cable ratings. Ring circuits are designed to run at above their continuous rating at times, an do so routinely in most British households. This is not a problem, I've never seen a ring circuit cable that sustained heat damage from normal operation - (except when some external heat source melted it). Ring circuits hahve worked this way since day 1, and still do. Cables are perfectly happy about it.
Now, the same principle applies to tails. The 47A rating addresses long term cable health with 47A continuous current. Short term use at moderately above 47A is simply not an issue, this is why the tail sizes were used. If you can find me a single set of old sized meter tails that have caught fire or electrocuted someone as a result of carrying 70A on occasion, I'd be interested to see it, and would wonder what other fault was at play. NT 11:36, 1 November 2012 (GMT)
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)
That's true of most things in life, but we don't go round calling them all significant risks. NT 13:25, 4 November 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. (John Rumm)
You can take the worst/best case if you like, about 50% of properties have RCD provision. NT 13:25, 4 November 2012 (GMT)
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. (John Rumm)
The above is a straw man argument, and with imagined figures. All the risk figures are relevant - but only when they're factual. NT 13:25, 4 November 2012 (GMT)
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, (John Rumm)
The same is true of pretty much all potentially fatal risks. We compare death rates because they're relatively firm figures, and by far the biggest problem. NT 13:25, 4 November 2012 (GMT)
... but at what point do you say that an injury level is minor enough to make the cost of an RCD not worth it? (John Rumm)
a) IIRC the NHS spending limit per qaly is about £20k now. b) A look at the top 10 causes of death should clarify where the main spending on safety is best spent. The safety benefit provided by RCDs is a good 4 orders of magnitude smaller than life's real problems. NT 13:25, 4 November 2012 (GMT)
[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)
That is a significantly different logic error. NT 13:25, 4 November 2012 (GMT)