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Old 03-05-2006, 04:02 PM
commasense commasense is offline
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Getting electrocuted in a pool

In a major movie out now (I'll refrain from identifying it to avoid the need for spoiler boxes) someone is killed when he jumps into a pool in which one of the underwater lights has cracked. The film makes a point of showing that the lighting circuit is energized, although none of the other lights is working.

This strikes me as highly unlikely.

Is it possible? Could a cracked light bulb put enough current into a full-size pool to kill a person? Without blowing a breaker? If so, why would all the other lights in the circuit be out?
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Old 03-05-2006, 04:15 PM
ExTank ExTank is offline
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Quote:
Originally Posted by commasense
In a major movie out now (I'll refrain from identifying it to avoid the need for spoiler boxes) someone is killed when he jumps into a pool in which one of the underwater lights has cracked. The film makes a point of showing that the lighting circuit is energized, although none of the other lights is working.

This strikes me as highly unlikely.

Is it possible? Could a cracked light bulb put enough current into a full-size pool to kill a person? Without blowing a breaker? If so, why would all the other lights in the circuit be out?
To get electrocuted, you must be in the "path" of the electricity on it's way to "ground." Furthermore, you must be the path of least resistance for the electricity to choose you as its path to ground. And finally, yes, cicuit breakers and fuses are designed just to prevent a circuit from burning up from too much current (and its resultant amperage) passing through it due to a short.

Every time I watch a sci-fi show and watch control consoles explode from overload, I wonder what kind of future it could be where peole have forgotten how to make and install circuit breakers.

Chemically pure water is not a good conductor (rather high ohmage, in fact), but you only get that in laboratories and certain manufacturing techniques, so pool water would be a decent enough conductor for a cracked light, but again we come back to circuit breakers. As soon as a the cracked light became flooded, it would trip the circuit breaker (or blow the fuse), rendering the circuit dead.

In other words, the movie is typical Hollywierd: about as much resemblance to reality as the writer's/director's grasp of reality extends.
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Old 03-05-2006, 04:27 PM
Guinastasia Guinastasia is offline
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Plus, if there were a real danger, why would they put lights in pools in the first place?
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Old 03-05-2006, 07:25 PM
Edward The Head Edward The Head is online now
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I've seen enough broken lights in pools that I've stopped thinking about them. I've seen them without lenses, with the wires hanging in the pool, and all sorts of other ways, and the only reason I even notice is because I'm trying to keep my feet out of the hole. We were talking about why it doesn't do anything the last time they were working on one but I don't remember the reason. But you can have one light on and be working on another one.
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Old 03-05-2006, 08:10 PM
danceswithcats danceswithcats is offline
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There's a huge void in between 0 (no current flow) and 15 amps, or 15,000 mA (trip the breaker). A ground fault circuit interrupter (GFCI) is supposed to trip at 5 mA to protect persons in contact with the circuit from lethal shock. It's entirely plausible that a damaged GFCI could fail to protect the user of a pool while the current flow would still be too low to activate a fuse or breaker.
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Old 03-05-2006, 10:58 PM
engineer_comp_geek engineer_comp_geek is online now
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Quote:
Originally Posted by commasense
The film makes a point of showing that the lighting circuit is energized, although none of the other lights is working.
That wouldn't happen that way. The lights are wired in parallel.

I've never gone into water that has a live wire in it, but I've read the account of someone who has. They said that far away from the wire, you couldn't feel anything, but as you got closer to the wire, they started feeling a rather uncomfortable tingling sensation. They never got closer than maybe 4 or 5 feet to where the wire was.

If the victim happens to dive in close enough to the wiring fault, and happens to dive in at just the right time during his heartbeat, it is entirely possible that his heart will go into fibrulation, which would be likely to result in death. Once you get the heartbeat out of whack, it tends not to recover on its own. Someone would need to perform CPR until the paramedics arrived and gave him a good shock with a defibrulator.

A surprisingly tiny amount of current (on the order of tens of milliamps) can stop the heart, especially if it happens to hit the heart at exactly the right time during its cycle. Anything below 5 mA is presently thought to be safe, although for obvious reasons no one has done a lot of electrocuting of people to see exactly where the limit really is. At very low current levels, there's a risk of throwing the heart into fibrulation, which would be the likely result here. At higher current levels, the heart kind of gets stuck as all the muscle fibers contract, but once the current is removed, it usually starts beating on its own again. Of course, if the current level is high enough, you can also have the body literally cooked to death. I don't see a wiring fault in a pool being capable of generating that much current though, even if the guy stuck his finger into the socket.

I'd have to say it's plausible, but only if (1) the GFCI was busted (as previously mentioned) and (2) the guy dove in fairly close to the busted light. If he dove into the other end of the pool, forget it. An investigation into the incident would reveal the busted GFCI, and whoever was responsible for the pool's maintenance would probably be facing a lawsuit or maybe even some possible criminal charges.
  #7  
Old 03-05-2006, 11:42 PM
Q.E.D. Q.E.D. is offline
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Quote:
Originally Posted by Guinastasia
Plus, if there were a real danger, why would they put lights in pools in the first place?
Yes, no corporation would manufacture an unsafe product. Perhaps I should take up smoking again.
  #8  
Old 03-06-2006, 12:38 AM
Guinastasia Guinastasia is offline
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Quote:
Originally Posted by Q.E.D.
Yes, no corporation would manufacture an unsafe product. Perhaps I should take up smoking again.
Uh, I think that's apples and oranges here.
  #9  
Old 03-06-2006, 01:41 AM
t-bonham@scc.net t-bonham@scc.net is offline
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And the electricity would take the path of least resistance.

So even if the light is cracked, and somehow the GFCI circuit breaker has NOT worked, and it's still live, the electricity would simply flow thru the water that short distance to the other side of the light bulb filament.

It certainly seems like that's a shorter path to complete the circuit than flowing across the pool, into some guys body, and out ... where? Was the guy just floating in the pool, or was he actually holding onto a grounded metal rail or something?

This really seems quite unlikely.
But it's a movie, it doesn't have to make sense.
  #10  
Old 03-06-2006, 01:56 AM
Q.E.D. Q.E.D. is offline
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Quote:
Originally Posted by t-bonham@scc.net
And the electricity would take the path of least resistance.
I hate this expression. It gives the false impression that all of the current flow is through whichever circuit path offers the least resistance with none going anywhere else. This is wrong. This situation can be roughly modeled as a bunch of parallel resistors of increasing resistance--there will be at least some current through each of the resistors; how much depends on their values. In the pool scenario, what we'll end up with is a current density gradient, with the bulk of the current flowing through the water between the filament ends, and a steadily decreasing current density the farther out into the pool we take a reading. Electricity follows EVERY available path; it has to.
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Old 03-06-2006, 02:52 AM
Grrr! Grrr! is offline
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Here in Dallas, a few years ago, there was an incident where people were hurt (and maybe one died?) from just what the OP is talking about.,
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Old 03-06-2006, 11:12 AM
commasense commasense is offline
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For the sake of refugees from CS who may not want to have a movie spoiled, here are the details of the film and the scene:

SPOILER:
The film is Syriana. The scene takes place on the palatial estate of the emir of the mythical middle-eastern country of Syriana, where someone in a control room has been showing off the fancy remote controls for the whole compound. He turns on the lights for one of the pools, then another, but the lights of the second don't go on. He believes the remote is broken, but we see a close-up of one of the underwater lights, cracked, and with the filament of the light bulb flickering, presumably indicating that electricity is still going through the circuit.

Coincidentally, although there is a big party going on, no one is in the pool at that moment. But a bunch of kids, including the six-year-old son of Matt Damon's character, are playing a chicken game, and the son is pressured to jump into the pool. There's nothing to indicate that he is particularly close to the broken lamp. Apparently, the pool is just a seething cauldron of electricity. The kid is immediately killed.

But then his father jumps in and pulls his body out. So what happened to the dangerous electricity? Special electrocution circuit breakers that only pop after they've killed someone?

I think the scene is just a clumsy attempt to make the emir responsible for the kid's death. If he had just drowned from horsing around, it wouldn't have been the host's fault. But because someone actively powered up the faulty circuit, there is guilt, which the father uses later in the story.


Here's another problem: The close-up of the lamp shows the filament intact and pulsing on and off. Correct me if I'm wrong, but it seems to me that if you powered up a cracked bulb full of water, the filament would fail instantly, right? And if, miraculously, it didn't, wouldn't that mean that the circuit was complete and nearly no electricity would be passing through the water out in the pool? So isn't the whole scenario completely and totally wrong from a technical point of view?

It all struck me as so implausible that I had to come here and have my doubts confirmed or quelled.
  #13  
Old 03-06-2006, 11:37 AM
Q.E.D. Q.E.D. is offline
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Quote:
Originally Posted by commasense
...it seems to me that if you powered up a cracked bulb full of water, the filament would fail instantly, right?
Nothing like a chance to perform a dangerous experiment involving electricity to get your day started. It occurred to me that the reason filaments fail in air is mainly because of the oxygen. It stood to reason that, for a variety of reasons, water might not have the same detrimental effect. Water contains some dissolved oxygen, but not nearly as much as air--which is why our lungs don't work so well underwater. It also conducts heat away much faster than air does, so the filament ought to run cooler, no? I had to find out.

So, I took a spare 40 W bulb, and used a tungsten carbide scribe to etch a line around the base of the glass envelope, and tapped it to break it cleanly at the scribe line. Put the bulb in a lamp and inverted it in a sink full of water, and let the bulb fill completely with water. Tapped it a few times to get the air bubbles out. Then I plugged it in.

*poof*

It was kind of anticlimactic, really. There was a small pop, almost inaudible, accompanied by a flash of white light. That was it. The filament was burnt cleanly in half, instantly. So much for running an incandescent lamp underwater with a cracked bulb. At least with ordinary tap water. Perhaps deionized, degassed distilled water might work, but I doubt anyone would have a pool full of the stuff.
  #14  
Old 03-06-2006, 11:38 AM
lieu lieu is offline
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Quote:
Originally Posted by engineer_comp_geek
I've never gone into water that has a live wire in it, but I've read the account of someone who has. They said that far away from the wire, you couldn't feel anything, but as you got closer to the wire, they started feeling a rather uncomfortable tingling sensation. They never got closer than maybe 4 or 5 feet to where the wire was.
That's exactly what I felt years ago when I started to get close to a cracked light in a backyard pool. Took me a second to realize what was happening since the light was submerged and under the diving board I was trying to get to but the tingling did increase to an uncomfortable level within about a foot or two of the fixture. It wasn't a sudden shock but instead a logarithmic increase relative to the distance.
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Old 03-06-2006, 11:51 AM
casdave casdave is offline
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Getting an electric shock in this manner is not as obvious as it may first seem, it might be better to explain it in terms of two dimensions.

If you have an electrical system where the primary earth point is an electrode driven into the ground, if a fault current occurs, it passes throught the earth electrode and is distributed into the earth mass.

If you take the electrode as being the supply conductor in this case, what happens is that you get a potential(voltage) between it and any other point on the ground, the closer you are to the electrode, the lower the potential between them.
You can plot this on a chart and what you get is concentric rings of voltages of a particular voltage around the electrode.(imagine contours of voltages around the electrode)

So you might get a circle of voltage of perhaps 10 volts between any point ten feet away from the electrode, this is just an example.

Lets continue to imagine that for every 10 feet, the voltage drops by 10 percent from the previous reading and the electrode is at 240Volts - you'll note that the change in voltage decreases with every ten feet as this is an itirative solution.

If you were to take two points one ten feet from the electrode, the other 100 feet, you would also measure a potential between them, I'm not going to do the maths, but you can see how quite a large potential will develop between them.

If you then joined those two points with a conductor then a current would flow, following the various electrical laws, but also including the supply impedance which would tend to limit the current flow..

It happens that in some conditions, this is a real problem, for a human with only a short distance between contact points, the feet, and with footwear, the chances of standing such that the potential between those feet is even large enough to feel is very small.
However, when we are dealing with farm animals, this situation is very differant.
The supply systems to farms often entails the use of earth electrodes, and a fault on the system can drive a large current throught the earth electrode.

There is a real possiblility that if the earth electrode is situated in an area where farm animals are nearby, they are of such length, and their heart is directly in the path of current flow, that they can get a 50-70volt shock between foreleg to rear leg, and it will kill them.

You note that in such a scenario, the animal has not even touched the earth electrode, it has just formed a conducting path parallel to the ground plane.

This is the same situation when we deal with swimming pools and immersion, except we are dealing with three dimensions, you do not get a shock directly from the conductor itself, but rather, you form a lower resistance path between two points at differant distances from the source of the electricity.

The purer the water is, the wider is the spread of potential and hence the more dangerous, the more contaminated, or perhaps even salty, the smaller is the spread of electrical potential, and also it is much more likely to be a better conductor than the human body.

As for distances, well, I have personally used salt water for fire fighting purposes directly onto 440volt conductors at less than 10 feet, this was done during damage control training when I was in the Royal Navy, so it would seem to me that you have to be pretty damn close to get yourself a shock in this manner
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Old 03-06-2006, 12:07 PM
Maus Magill Maus Magill is offline
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Quote:
Originally Posted by Q.E.D.
So much for running an incandescent lamp underwater with a cracked bulb. At least with ordinary tap water. Perhaps deionized, degassed distilled water might work, but I doubt anyone would have a pool full of the stuff.
Any decent sized pool would have nearly a hundred pounds of dissolved calcium hypo-chlorite in it. On top of that you'd have several dozen pounds of soda ash to adjust the pH, and maybe 50 pounds of sodium bi-carbonate. If anything is the opposite of deionized, degassed distilled water, your average pool is it.

Quote:
Yes, no corporation would manufacture an unsafe product. Perhaps I should take up smoking again.
I wouldn't know about the fictional Emir's private pool, but any pool open to the public has a metric butt-load of health department regulations for things as varied as drain cover style and water clarity to airborne chlorine level. I don't know about the electrics, but I could reasonably guess that they'd be highly regulated and regularly tested, too.

I was relatively surprised to find out the movie (stupid black boxes and your ability to make me click on you) I would have figured such a scene would have come from a Final Destination movie.
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Old 03-06-2006, 02:08 PM
Gozu Tashoya Gozu Tashoya is offline
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WRT to one of the questions in the spoiler box:
SPOILER:
IIRC, they turned the power off about 3 frames before Matt Damon jumped into the pool.

BTW, the phrase "seething cauldron of electricity" is great.
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Old 03-06-2006, 02:33 PM
commasense commasense is offline
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Wow, Q.E.D., thanks for risking life and limb to conduct the experiment. Our own in-house Mythbuster!

On the basis of your results and lieu's real-world experience (as well as the excellent theoretical posts by engineer_comp_geek and casdave), I may be willing to cut the filmmakers a little slack. Perhaps the shot of the filament flickering was to signify that the power was on and that once the filament broke, power was passing through the water. If they had made it look like Q.E.D.'s real-life pop, people might not have gotten it.

And since lieu says he actually got a shock from a broken light in a pool, perhaps the whole thing is not so completely implausible as I first thought. It would require careless wiring and the person coincidentally jumping in right next to the power source.

So in Mythbusters terminology, perhaps this goes from Busted to Plausible. or at least, not entirely impossible.

Gozu Tashoya: Thanks for the clarification. I hadn't caught that.
  #19  
Old 03-07-2006, 05:12 PM
seattlebob seattlebob is offline
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Low Voltage

Pure Hollywood fantasy...

Worked as a life guard for many years and have never seen any pool lighting system that ran at AC mains voltage. They were always 12 volt systems to avoid any possible shock hazzard.

(To an earlier response) The only type of circuit breaker that would afford any protection from electrocution would be a GFI type. They are designed to trip at micro-amp levels low enough to save a life. A standard 15 amp breaker would let you fry like a chicken and not trip.
  #20  
Old 03-07-2006, 05:20 PM
Q.E.D. Q.E.D. is offline
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Quote:
Originally Posted by seattlebob
Worked as a life guard for many years and have never seen any pool lighting system that ran at AC mains voltage. They were always 12 volt systems to avoid any possible shock hazzard.
There are plenty of mains voltage pool lighting solutions. See here.
Quote:
Originally Posted by seattlebob
The only type of circuit breaker that would afford any protection from electrocution would be a GFI type. They are designed to trip at micro-amp levels low enough to save a life.
The standard trip current for a GFCI is 5 milliamps.
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