Bathroom Light Switch

[JohnW2]

What are the figures for the number of people killed or injured by using normal light switches in bathrooms?

What are the figures for the number of people killed or injured by electricity by any process? It does not seem that a very low level of risk stops people (and regulators) considering ways of further minimising those risks. In terms of your actual question, normal light switches in bathrooms are relatively uncommon, so, forgetting all the other problems, reliable 'figures' would be very hard to come by.

How hard is it to not use such a switch until you've dried your hands?

How hard would it be to not touch exposed bare live conductors, if such were allowed?

If the rest of your body being wet makes it hazardous, what kind of switch fault(s) are you positing, and/or what body part(s) do you envisage being used to operate the switch?

I'm not envisaging body parts other than the hands being used to operate the switch but a shock requires two 'points of contact'.

There are plenty of examples you could cite in which doing something because "that's the way it's always been done ..." is clearly not a good reason, and may well be silly/thoughtless, but there clearly are some fairly 'rational' reasons in the specific case you're discussing.

Kind Regards, John

 

[PBC_1966]

As I've said, I wouldn't personally go that far, because (despite the theoretical potential for RCDs to prevents deaths/injuries) I just don't know enough facts (in terms of injuries and deaths) to be able to know.

Perhaps I shouldn't have gone as far as saying "in many cases" but should have restricted it to "some cases." Logic suggests that, however few in number, there must be some instances in which an RCD or GFCI tripping as a result of shock current has saved a life, or will do so at some point, even if we have to go to the extreme of somebody using a faulty power tool (say a cord so battered that the ground wire has separated from its connection and in use contacts the live) while standing ankle deep in muddy water. Of course, the primary cause of the fatality would still be the poor state of repair and thus if the user had taken care to inspect his equipment and keep it in good condition it would never have happened anyway, but the GFCI could still be a life-saver in such a case.

But I certainly agree that the low statistics to begin with coupled with the fact that often we really don't know the precise details of the incident mean that they're likely nowhere near as responsible for saving lives as some proponents would have us believe. Which, of course, in view of the other recent thread, is why I really can't get worked up and run around shouting that the sky is going to fall if somebody adds one more socket without 30mA RCD protection in a house that's already full of them.

AC current does not tend to cause one not to let go, so it is unlikely one would ever be in contact for more than 40 ms.

While we probably generally think of d.c. as being more liable to cause the muscles to contract and "lock on" to whatever it one is holding, I'm sure that the research done during the development of the GFCI in the 1960's demonstrated that a.c. at certain frequencies could also have a similar effect in some people. Again, it's been years since I read the relevant paper, but I believe there was something to the effect that around 60Hz is just the right (or wrong, depending upon the viewpoint!) frequency for the effect.

 

[ban-all-sheds]

There are plenty of examples you could cite in which doing something because "that's the way it's always been done ..." is clearly not a good reason, and may well be silly/thoughtless, but there clearly are some fairly 'rational' reasons in the specific case you're discussing.

Let's have them then.

 

[JohnW2]

Logic suggests that, however few in number, there must be some instances in which an RCD or GFCI tripping as a result of shock current has saved a life, or will do so at some point ....

Indeed so - that's the 'qualifier' I usually add when talking about the lack of information about how many lives ("if any") have been 'saved' by RCDs.

But I certainly agree that the low statistics to begin with coupled with the fact that often we really don't know the precise details of the incident mean that they're likely nowhere near as responsible for saving lives as some proponents would have us believe.

Quite. I often note that to put two RCDs into every UK domestic dwelling will probably cost well in excess of £1billion and that, given not much more than 20 domestic electrocutions per year, I can't help but wonder whether the same amount of money 'invested' in other ways (road safety? even medical research) wouldn't have the potential to save a lot more lives.

While we probably generally think of d.c. as being more liable to cause the muscles to contract and "lock on" to whatever it one is holding, I'm sure that the research done during the development of the GFCI in the 1960's demonstrated that a.c. at certain frequencies could also have a similar effect in some people.

I think you're probably understating the situation, since I don't think that there is much basis for the 'belief' that DC is more dangerous in that respect. IIRC, the usual 'let-go threshold' is about 60 mA with DC, but only about 15mA with 50-60Hz AC. As you go on to say, 50-60Hz is, unfortunately, just about the worst possible in terms of both skeletal muscle (hence 'let go') and heart muscle (hence risk of ventricular fibrillation). Ventricular fibrillation threshold currents are, again, much higher (IIRC around 5 times higher) for DC than for 50-60Hz AC.

Kind Regards, John

 

[PBC_1966]

IIRC, the usual 'let-go threshold' is about 60 mA with DC, but only about 15mA with 50-60Hz AC.

Did that 15mA figure come from research suggesting that as the level at which the majority of people had trouble with "letting go" of a.c? That would probably tie in with the selection of 5mA for GFCI protection here as being the level at which the majority of people have no trouble, with perhaps around 10mA being the 50/50 point. I wish I could find a link to the paper I remember reading years ago, if it's even scanned and available online anywhere.

By the way, this article has a good history of the NEC requirements for GFCI protection over the years, if you're interested:

http://www.necconnect.org/resources/gfcis/

It started off reasonably enough in the sort of locations you'd expect, but I really can't help feeling that after a few years they started getting carried away somewhat. There were also some instances in which problems with nuisance tripping then caused them to write a specific exception into the next code edition.

Though I'm sure you'll also note the following in the closing paragraph:

After 50 years, it is impossible to know how many lives have been saved or injuries avoided since the introduction of GFCI requirements in the NEC.

 

[JohnW2]

IIRC, the usual 'let-go threshold' is about 60 mA with DC, but only about 15mA with 50-60Hz AC.

Did that 15mA figure come from research suggesting that as the level at which the majority of people had trouble with "letting go" of a.c?

I'm sure it must have done, but I am relying on decades-old dusty memory - so the figure may not be totally correct!

That would probably tie in with the selection of 5mA for GFCI protection here as being the level at which the majority of people have no trouble, with perhaps around 10mA being the 50/50 point.

That would make some sense.

The fact that 50-60 Hz AC is much 'worse' in this respect than DC has a very sound physiological basis. If one applies "DC" to a muscle, or the nerve supplying a muscle, one gets just one 'twitch' of the muscle, lasting milliseconds. To get a sustained contraction of the muscle, a continuous train of pulses has to be applied (directly to the muscle or, physiologically, via a nerve), typically at the rate of 50-100 per second - the resulting summation of 'overlapping twitches' then results in a sustained contraction of the muscle, which persists so long as the train of pulses continues. It is ironic and/or a co-incidence that this usual ('optimum') frequency of nerve impulses is very similar to the frequency of mains electricity supplies. At appreciably lower frequencies, the 'summation' does not occur, so one just gets a series of individual 'twitches', whilst at appreciably higher frequencies, the muscle tends to get 'overwhelmed'.

Kind Regards, John

 

[EFLImpudence]

After 50 years, it is impossible to know how many lives have been saved or injuries avoided since the introduction of GFCI requirements in the NEC.

I presume that is literal; i.e. no one knows if any.

After all, the same wording could be used for anti-biotics; in that case far too numerous to count.

 

[stillp]

I wish I could find a link to the paper I remember reading years ago, if it's even scanned and available online anywhere.

There's some information in IEC/TR 60479: https://webstore.iec.ch/searchform&q=60479

 

[JohnW2]

I wish I could find a link to the paper I remember reading years ago, if it's even scanned and available online anywhere.

There's some information in IEC/TR 60479: https://webstore.iec.ch/searchform&q=60479

They appear to accept American Express, Visa and Mastercard - do you happen to have one of those we could borrow? The seemingly most potentially relevant ones appear to add up to 790 Swiss Francs - what's that in English?

Kind Regards, John

 

[stillp]

This might be more use: http://shop.bsigroup.com/SearchResults/?q=60479
Some larger public libraries (and university libraries) have online access, usually read-only.

 

[JohnW2]

This might be more use: http://shop.bsigroup.com/SearchResults/?q=60479

Well, yes, "more use" in the sense that it gives the prices in GBP

Some larger public libraries (and university libraries) have online access, usually read-only.

That could be more useful - and I wouldn't envisage wanting "write" access!

Kind Regards, John

 

[ban-all-sheds]

Ooh - imagine if you could overwrite parts of various BSs.....

Although I suspect he means no making, or making off with, copies.