Electrocution in a pool
December 21, 2006 10:12 AM   Subscribe

Is it possible to be electrocuted by jumping into a pool that has a light that has short circuited?

Saw Syriana last night and I thought the situation depicted was impossible because of the bulb design?
posted by Keith Talent to Home & Garden (51 answers total) 1 user marked this as a favorite
 


I don't know what Syriana is. Depending what you mean by short circuited, yes. All you need is the water in the pool able to make connection with a hot leg of the wiring. This is why GFCI breakers are mandated by most code for any wet locations.
posted by Mitheral at 10:34 AM on December 21, 2006


Its also -extremely- rare.
There have been 60 deaths and nearly 50 serious shocks reported over the past 13 years involving electrical hazards in and around swimming pools.
60 deaths in 13 years. I would think most pools would just trip the breaker if such a thing happened and the deaths come from pools made without breakers, bad ground, electrocuted by exterior lights, shocks from pumps or maintenece equipment, etc. The syriana accident can happen but is very rare.
posted by damn dirty ape at 10:35 AM on December 21, 2006


Wouldn't the bulb break/crack and immediately short circuit? Even without a GFCI?
posted by Keith Talent at 10:37 AM on December 21, 2006


I wonder how small of a body of water it would have to be. What if someone dropped the end of an extension cord in a lake? What if it was a pond or a river?
posted by teg at 10:38 AM on December 21, 2006


It is possible, but I wouldn't say that it is fairly common. The U.S. Consumer Product Safety Commission (CPSC) reports 60 deaths and nearly 50 serious shocks reported over the past 13 years involving electrical hazards in and around swimming pools. Note that this includes "around swimming pools" so the number of electrocutions actually in a pool is less.

Most electrocutions occur while touching an electrical device while in a pool. In this case your body forms an electrical path directly from the device to the ground. There are cases of touching the metal housing of the light while in a pool, but just floating in the water is unlikely to lead to electrocution. Electrocution requires a flow of current and current requires an electropotential difference in the direction of flow. It is unlikely that there will be a difference in voltage, say, between you head and your feet in a pool of water.

Touching or nearly touching an underwater light, a metal ladder, the bottom and side of the pool at the same time or the stereo boom box on the deck, maybe, but just floating in the pool, I think not.

Most cases of so-called pool electrocutions involve touching or handling an electrical device while in or near a pool.
posted by JackFlash at 10:42 AM on December 21, 2006


This grim day at Ohio's Kings Island amusement park claimed 4 people, three of which were electrocuted in a pond. The pond was decorative, however, and not designed for bathing, so it was a little bit different than innocently stepping into a pool. This archived article doesn't say it, but IIRC, these fellows were intoxicated.
posted by M.C. Lo-Carb! at 10:44 AM on December 21, 2006


It appears to be less likely but more fatal than swimming pool suction entrapment. (second link is not safe for anyone)
posted by b1tr0t at 10:45 AM on December 21, 2006 [1 favorite]


(Opps. I meant to say UN-common)
More from my link up-thread:
The CPSC is most concerned about faulty underwater lighting; aging electrical wiring that hasn't been inspected in years; the use of sump pumps, power washers, and vacuums that are not grounded; and electrical appliances (such as radios and TVs) and extension cords falling or being pulled into the water. All of these hazards present an even greater risk if the lighting, circuits, and nearby receptacles are not protected by Ground-Fault Circuit-Interrupters (GFCIs) - the best safety device to prevent electrocution.

"The best protection for families is inspection, detection, and correction of electrical hazards in and around swimming pools, hot tubs and spas," says CPSC Chairman Hal Stratton. "CPSC strongly encourages residential and commercial pool owners and operators to upgrade protection of the lights, receptacles, and switches with GFCIs. Older pools are the biggest concern, as underwater lighting fixtures may have degraded with age and may not be protected by GFCIs."

The CPSC and the American Red Cross are also warning swimmers that electrical hazards around a pool, hot tub or spa can lead to multiple deaths or injuries. This occurs when an individual becomes incapacitated by stray current in the water and one or more persons jump in or reach out to save the victim, resulting in multiple electrocutions or serious shocks.

In May 2002, a 14-year-old girl from Arlington, Texas, was electrocuted when wiring problems in an apartment swimming pool's underwater lights charged the water with electricity. A 16-year-old boy was seriously shocked when he jumped in the pool to try to save the young girl. Another teenager used a fiberglass shepherd's hook (a non-conductive device) to pull both victims from the water.


The Statistics
Following are the causes of electrical deaths in the United States associated with pools in the years 1990-2002:

28 - Plugged-in radios or stereos, extension cords or power tools

13 - Underwater pool lights

10 - Pool pumps

9 - Sump pumps, pool vacuums or pressure washers


Source: U.S. CPSC

In August 1997, Yasmin Paleso’o was splashing in the pool of her apartment complex in Daly City, Calif., when electricity began surging through the 13-year-old’s body. Five other girls playing in the pool started screaming as the electrical shocks began to sting them.

Spc. Chase R. Whitman, 21, of Oregon, died May 8, in Mosul, Iraq, while he was in a swimming pool when an electrical current charged the water. Whitman was assigned to the 296th Forward Support Battalion (Brigade Support Battalion), 3rd Brigade, 2nd Infantry Division, Fort Lewis, Washington.
posted by Floydd at 10:48 AM on December 21, 2006


You have to think about the mechanism here.

The only way it could kill you would be if your body provided a route to ground for the mains voltage.

Just floating in a swimming pool, not touching anything, is not going to kill you even if the water is at 110V.

However, if you are touching something metal (eg a tap, a pool ladder) which is grounded, the moment you hit 'live' water you are liable to become part of a circuit, with electricity flowing through your hand towards whatever part of your body is in the water.

If this route goes through your heart area (eg left hand to right hand, left hand to right leg, maybe even left hand to left leg) it may be enough to kill you.

People die in baths because they use ungrounded equipment or equipment without a GFI, using wet hands, which makes a very nice connection, thank you. At the same time they are grounded via their butt or legs to the water, which is itself grounded via the outlet pipe. Again they become part of the circuit and buh-bye.

In the SYRIANA context it would depend on the victim's body forming the path of least resistance for the power to go to ground. Not impossible but not certain either.

(by the way I've been electrocuted by 110v and 220v and the latter is a whole different experience)
posted by unSane at 10:56 AM on December 21, 2006


It would depend on the victim's body forming the path of least resistance for the power to go to ground.

Not exactly -- least resistance is not required. Current can follow many paths simultaneously. It only requires 60 to 100 mA of AC current to cause fatal electrocution. For a 10 amp source, that means only 1% of the current needs to flow through your body. The other 99% can flow through another, lower resistance path.
posted by JackFlash at 11:06 AM on December 21, 2006


I was thinking about what unsane wrote.

Consider the following:

In a large body of water, would it be possible to have differently charged regions? Would chlorine or some other chemical in the water assist this? Not all water conducts equally well.

Then all you'd need would be a sufficient differential between non adjacent regions, and then you jump in, bridge the regions, and zap, no? Much like lightning within a cloud?
posted by Pastabagel at 11:09 AM on December 21, 2006


I'm with unSane... float all damn day in the water at kiloVolt potentials and nothing happens. Provide a current path through your body on the way out... different story.

I get the impression people think 'electrified' water is like acid or something. It's not. Jumping or falling in is not lethal.

It's more like a wire. Voltage does not electrocute, current does. You have to bridge a potential with your body to have current flow. It will flow in a path of least resistance to the two points. If it's say, on one arm, you'll get an injured arm. If it's between two arms, the current path through your heart may kill you (and may not... specific circumstances rule.)
posted by FauxScot at 11:11 AM on December 21, 2006


Pastabagel,

Perhaps there could be minor local discontinuities in conductivity. I'd presume that a constantly circulating pool had a fair degree of homogeneity.
posted by FauxScot at 11:14 AM on December 21, 2006


I'd presume that a constantly circulating pool had a fair degree of homogeneity.

what if you'd just peed? (serious question.)
posted by sonofslim at 11:21 AM on December 21, 2006




What you have is an anecdotal description. You don't know the details of the incident. Was it a shallow children's wading pool? Where the girls touching the bottom (most likely)? Were they touching the metal drain or filter fixtures on the side of the pool. There is no description of what actually caused the electrical fault other than "faulty wiring."
posted by JackFlash at 11:30 AM on December 21, 2006


floydd....

Hysteria, perhaps? Not unheard of for little girls to squeal when one gets hurt legitimately.

Physics rule.

sonofslim...

Pee would make a local ionic gradient, and but still, it's just an extension of your body which is just pee in a bag, more or less.. with sticks. ... somewhere there has to be a potential and a path for current to flow.
posted by FauxScot at 11:35 AM on December 21, 2006


Please stop using the phrase "path of least resistance." This could give the misleading belief that all you have to do is make sure that you have, say, rubber soles to prevent electrocution. Take the example of an electric drill. If there is a leakage path in the housing, even a very high resistance path, you can be electrocuted by the 1% of current going through your body even while the other 99% goes through the much lower resistance path of the motor. The drill continues to operate without a hitch as almost all of the current continues to flow normally.
posted by JackFlash at 11:37 AM on December 21, 2006


Point is... can you really trust a journalist to explain something about which most people don't even have a hint of comprehension?

To the degree that these warnings counsel people to avoid bringing dangerous electrical appliances near their pool, to use ground fault interrupters on all pool circuits, and to maintain their assets, they are useful.

As factual explanations of what REALLY happened, they are sorely lacking.
posted by FauxScot at 11:38 AM on December 21, 2006


I agree, JackFlash. Point taken. It only takes milliAmps to kill and leakage current is enough.
posted by FauxScot at 11:39 AM on December 21, 2006


what if you'd just peed? (serious question.)
posted by sonofslim at 2:21 PM EST on December 21


Or you were sweating before or while you jumped in.

The ironic thing is that water sucks as a conductor. It's the things in water that make it conductive, those things usually being ones that produce electrolytes or otherwise have a n ionizing effect. Sweat and urine have them. So does the chlorine or whatever they put into pools. If the pool is relatively still, it's possible that different electrolytes will settle into different regions based on their densities (I'm thinking of those old stationary cells where you pour copper sulfate on the bottom and the pour zinc sulfate on the top. they settle out like oil and vinegar)

The fact is that this happens in reality, so something must be going on. Add to this that pools actually have lots of paths to ground (the circulation pipes, and the grounding in lights and things in the walls of the pool.

Furthermore, there's you. The human body has its field densites and charge distributions like any other bag of various and sundry electrolytes.

There's a lot of things that could be going on. Could it be that a standing pool can store charge like a capacitor?

In conclusion, electricity is weird.
posted by Pastabagel at 11:43 AM on December 21, 2006


This is frustrating - we're basically degenerating into the same argument I had with my friends walking out of the Syriana movie.
posted by allkindsoftime at 11:44 AM on December 21, 2006


Sorry, I had gotten derailed and forgot the original question.

The lamp falls in, dangling in to the surface of the pool. The bottom of the pool has lights, which are grounded. It has recirculation pipes, which also are grounded. In you go. It is quite possible that your body forms a path for the current to flow form the lamp to a grounded pipe. It doesn't have to be the ONLY path, just enough of one to kill you.

In reality however, the short circuited lamp would more than likely have thrown a breaker the moment it shorted. So by the time guy falls in, there wouldn't have been any current anyway.

I didn't see the movie, btw.
posted by Pastabagel at 11:50 AM on December 21, 2006


Pastabagel:
In a large body of water, would it be possible to have differently charged regions? Would chlorine or some other chemical in the water assist this? Not all water conducts equally well.

Then all you'd need would be a sufficient differential between non adjacent regions, and then you jump in, bridge the regions, and zap, no? Much like lightning within a cloud?


Or indeed lightning striking ground or a lake. If you find yourself on part of the voltage gradient from the center of the strike to wherever it is discharging to ground (basically, a hemisphere of earth or water centered on the ground strike) then you may find yourself being electrocuted even though the 'strike' missed you. I think this has been described on AskMe within the last year.
posted by unSane at 11:55 AM on December 21, 2006


More fuel for the fire.

And 35-year-old Victor Dario Avila, a cousin of Rivera's wife Clara and his 14-year-old son also named Victor were electrocuted in the pool.
The Daily News reports the pool was wired by a caretaker to keep two pet rotweilers from jumping in. The water was electrified by a metal rod.
The boy was killed when he jumped in to cool off and then his father died trying to save him.

So if you're on part of the voltage gradient from the source of the current to wherever it is discharging to ground you run the risk of electrocution. So, yes, it would be possible. And it has happened.
posted by Floydd at 12:03 PM on December 21, 2006


pastabagel, I think you're being ridiculous. Even if different sections of water had different resistance levels, current would still flow between them just fine.
posted by delmoi at 12:07 PM on December 21, 2006


delmoi, I realize that, I was just thinking out loud, so to speak.

If the different regions, one of which is near a ground, and one of which is near a high voltage source, were separated by the distance of the pool and the pool as filled with various impurties, the current would flow but not always along the same path. Again think lightning within a cloud or between cloud layers.

Furthermore, when you jump in, some of that current will go through you. The question is how much.

My point is that you can't think of the pool as homogenous. It's going to have a charge distribution that changes over time, and there are going to be current and voltage gradients. You have to consider the problem as complex and not assume these things away because you may well assume away the mechanism that makes it possible.
posted by Pastabagel at 12:23 PM on December 21, 2006


Syriana Spoiler warning.

For those who did not see the movie Syriana, this is basically what happened (from memory). Kids are playing in swimming pool all day long. Unknown to anybody, one of the built in lights on the side of the pool (under water) is damaged. Night falls, and the kids are not in the pool except for one who was either a straggler or was pushed in at the last minute (I forget). Since it has gotten dark, the underwater pool lights are turned on, zip bam boom, kid is electrocuted.
posted by JigSawMan at 12:56 PM on December 21, 2006


When I was taking lifeguard training, we often did scenarios - four students would be the guard team, and the rest of the class would be the swimming public. While the guard team was waiting in the changeroom, the instructor would come up with a scenario (ok, paul, about 2 minutes in you're going to have a heart attack... and jenny, when they start giving him CPR I want you to get all hysterical because you're his wife).

One of his most imaginative scenarios: a few minutes after the start, he turned the lights off, so that only the emergency lights were on (indoor pool), and had everyone who was in the water just collapse and float face-down. in other words, we all got electrocuted :). this was to test the guards ability to assess the environment. of the four environmental hazards we are taught to check for - "fire, wire, gas, glass" - this would be "wire", and hopefully the guards would not jump into the pool and start grabbing people. The threat though certainly rare is real enough that guards have to know how to respond to it. also I believe the long poles with scoops on the end are insulated against electricity for this reason.
posted by PercussivePaul at 1:20 PM on December 21, 2006


unSane writes "The only way it could kill you would be if your body provided a route to ground for the mains voltage.

"Just floating in a swimming pool, not touching anything, is not going to kill you even if the water is at 110V."


This is wrong. All you need is a voltage gradient across the width of your body. It's how cows get killed by lightning strikes to the ground and it's why you should either stand on one foot or with your feet together if you can't find shelter in a lightning storm.
posted by Mitheral at 1:27 PM on December 21, 2006


A friend's dad died this way, from a faulty underwater light fixture.
I haven't been morbid enough to ask any details.
posted by signal at 1:46 PM on December 21, 2006


The only way it could kill you would be if your body provided a route to ground for the mains voltage.

True, if it was pure water, which isn't a particularly good conductor. (Large installations dealing with heat and current issues use LCW -- low conductivity water -- to cool electronics.)

Water that isn't pure, say, one with chloride ions in it, is a very different story.

If you're floating in a chlorinated pool, you're at ground potential with a very low resistance connection. If just a little bit of salt gets in the water, the pool water is basically a conductor at any reasonable voltage -- and very firmly tied to ground. Thus, you, in the pool, are very firmly tied to ground.

Grab a live conductor, and you're in real trouble -- you're the fastest path to ground, given what you are swimming in.

The fix? GFCI.
posted by eriko at 1:54 PM on December 21, 2006


Folks... the difference between a lightning strike and a floor lamp is astronomical.

Floor lamp = 120 V.. circuit capacity typically 20 Amps.

Lightning strike = MEGA volts, possibly hundreds of MEGA VOLTS and mega Amperes current.

The physics of the two are day and night. So jump in a pool at the instant of a lightning strike and yep, you're dead. Don't jump in and your are just as likely to die.

Floor lamp? Probably not.

High voltage plays by different rules.
posted by FauxScot at 2:25 PM on December 21, 2006


Pastabagel: In a large body of water, would it be possible to have differently charged regions? Would chlorine or some other chemical in the water assist this? Not all water conducts equally well.

FauxScot: float all damn day in the water at kiloVolt potentials and nothing happens.


Mitheral: All you need is a voltage gradient across the width of your body.

Mitheral's point is, if current is flowing through the water, then voltage is dropping. If you are positioned in that current flow, you will form part of the conduction path, and you may get shocked.
That is current flow through, literally. Think about the little electrons moving through the pool and you. Voltage is the force pushing the electrons, resistance is the friction/drag holding those electrons back, current is the number of electrons per second that make it through.

For example, one end of a 5 meter pool is at 120V, the other is at ground. You are in the pool, aligned with the current flow. The voltage difference from head to toe will be around 48V. This situation is probably lethal. Shorten the pool, and the situation gets worse.
Gets worse for a while, that is.. Once it gets so small that the distance between the 120V source and the ground connection is much shorter than your body length (radio falls in the tub near the grounded end) the situation changes a lot again.

Holding a 48V wire with dry hands is not dangerous because the resistance of the connection between your fingers and the tip of the wire is very high. A lot of the 48V drops right there, and there is very little voltage left to push currernt through your body. In the pool, there is very little resistance between you and the water, because the contact surface area is hundreds of times larger, and because the wet (and ionized) surface contact is much more conductive.

So anyway, I would have been on the risk is extremely low side, until I did the calculation. Looks like ground fault protection around your pool really is very very important!

Here are a couple of articles, both called The Fatal Current, which are very informative. I find that electrocution is fairly poorly understood (by me, and in general) and I'd be interested in more references.
posted by Chuckles at 2:36 PM on December 21, 2006


Err, "is probably lethal" might be a little strong.. There are a lot of things to consider, like why doesn't the breaker blow when the wire hits the water.

The total resistance from one end of the pool to the other has to be below about 8 ohms for a typical breaker to trip. That seems like it would be firmly in "maybe, maybe-not" territory, but I don't know.
posted by Chuckles at 2:46 PM on December 21, 2006


unSane writes "The only way it could kill you would be if your body provided a route to ground for the mains voltage.

"Just floating in a swimming pool, not touching anything, is not going to kill you even if the water is at 110V."

This is wrong. A pool that is "charged" by a faulty water light is not some dormant battery. The water is grounded already grounded by feed pipes, light housings, method of construction. The faulty light is a "leak" to ground or "ground fault", a very minor one usually via an intermitant arc from loose splices but a current is already flowing from source to ground and the water is inbetween. When you jump into a pool with a faulty light leak you just enter the path to ground the mA are taking and mA will flow through you. Like jflash says you only need to take up between 0.05 - 0.1% of the current to die. Get GFI breakers and outlets they are calibrated to trip at up to 7 mA. unSane; I'll rig up a trickle ground fault in a pool, you go first!
posted by Kensational at 4:10 PM on December 21, 2006


Good information here, thanks. I apologise for my faulty understanding. Perhaps Brain Fault Interruptor would be useful too.
posted by unSane at 4:20 PM on December 21, 2006


I said:
Once it gets so small that the distance between the 120V source and the ground connection is much shorter than your body length (radio falls in the tub near the grounded end) the situation changes a lot again.
But that is, more or less, falling for the "path of least resistance" line of thinking, which is wrong. The current will take every possible path from high voltage to low, the amount of current being limited by the resistance along that path. From one end of the tub to the other, and then all the way back again, is a valid path.

So, brain faults all around :P
posted by Chuckles at 5:31 PM on December 21, 2006


What if you always go swimming inside a faraday cage?
posted by b1tr0t at 5:48 PM on December 21, 2006


Just floating in a swimming pool, not touching anything, is not going to kill you even if the water is at 110V.

Wow. Way, way wrong. Water conducts electricity very well, thank you.
posted by frogan at 6:39 PM on December 21, 2006


Water conducts electricity very well, thank you.

It depends on what you call "well." By most measures water is not considered a very good conductor. Conductivity of water is usually measured in siemens which is the reciprocal of ohms. Typical tap water has a conductivity of about 100 micro-siemens per centimeter which is 10,000 ohms/cm. That is not very conductive. What that means is that you could throw the end of an extension cord into a bathtub and it wouldn't blow a fuse. It would only leak a few hundred milliamps. However this is enough to kill you if you touch it.

The fact of the low conductivity of water is what makes it so dangerous. It is low enough to prevent blowing a fuse but high enough to allow electrocution.

Water, however, is more than 10 times as conductive as dry skin which is the reason for warnings about water and electricity. Water is conductive enough to enable electricity to more easily enter the body.

By the way, salt water is more conductive than tap water, by not by a lot. This is because the charges are carried by large, heavy ions and therefore are much less mobile than electrons in a wire. Pure, distilled water can practically be considered an insulator.

Many, many years ago I worked with an old electrician that would occasionally test circuits in a breaker panel to determine if the circuit was hot. He would do this by sticking one hand in his back pocket and then drying his finger tips on the other hand by wiping his shirt. He would then delicately touch the hot and neutral bars with his index and middle fingers. If the circuit was hot he would get a mild tingle from the path down one finger and up the other. (Don't try this at home, kids.) Dry skin is a very good insulator.
posted by JackFlash at 7:51 PM on December 21, 2006


frogan: Water conducts electricity very well, thank you.

JackFlash: It depends on what you call "well." By most measures water is not considered a very good conductor. Conductivity of water is usually measured in siemens which is the reciprocal of ohms. Typical tap water has a conductivity of about 100 micro-siemens per centimeter which is 10,000 ohms/cm.

Well.. I'm not well versed in this, but.. The cm-1 unit is not the length of the current path.

To measure the conductivity of water, we have to consider the cross sectional area of the current path as well as its length. The figures from that site:
pure water: r = 2.5 x 10^5 ohm meters
saturated NaCl in water = 0.044 ohm meters
Which is a pretty big range.. JackFlash's 100 micro-siemens/cm is 100 ohm·m, right in the middle.

Lets consider a pool that is 2m deep, 5m wide, and 10m long (pool dimensions have changed from my previous comment), with a fault at one end, and the only ground connection at the other. The total resistance of the pool is about 100 ohms, for 1.2A, and no typical household fuse or breaker will trip. If it is a 120V circuit, the voltage gradient along the length of the pool will be 12V/m, which makes the voltage across a swimmer from head to toe about 24V.

From the second Fatal Current article:
The actual resistance of the body varies depending upon the points of contact and the skin condition (moist or dry). Between the ears, for example, the internal resistance (less the skin resistance) is only 100 ohms, while from hand to foot is closer to 500 ohms.
Which means the swimmer is subject to about 50mA of current.. From the same article again:
For currents above 10 milliamps, muscular contractions are so strong that the victim cannot let go of the wire that is shocking him. At values as low as 20 milliamps, breathing becomes labored, finally ceasing completely even at values below 75 milliamps.
The swimmer is in a bad way, but it will be death by drowning rather than ventricular fibrillation, I guess.
posted by Chuckles at 10:01 PM on December 21, 2006


Note that using the 500 ohm figure for the resistance from head (or extended arm, if you like) to toe completely ignores the issues of current flow in 3-D again. It seems like the actual resistance will be somewhat lower. Details, details.. :)
posted by Chuckles at 10:10 PM on December 21, 2006


Sorry, I should have inverted the dimensional units when I converted from siemens/cm to ohm-cm. Your calculation for the resistance of the pool, however, would be true only if the electrodes on either end consisted of metal plates covering the entire wall of the pool. If you just stick the ends of a couple of wires into a pool as your electrodes, the resistance will be much, much higher because you only have a fraction of a square centimeter in contact with the water. In this case the dimensions and resistance of the electrode/water interface completely swamps consideration of the pool dimensions.

The point was that the conductivity of water, even salt water, is relatively poor, more than a million times less than that of copper. You can demonstrate this for yourself. Just take a glass of tap water or salt water and stick your multimeter probes into it. You will notice that the resistance changes a lot when you vary the depth of the probe in contact with the water but very little when you vary the distance between the probes. Depending on the size of your probes, tap water will measure in the neighborhood of 1 Mohm and salt water a few Kohm. You need a large contact area to shove much current through water.
posted by JackFlash at 11:31 PM on December 21, 2006


I thought there was something about the inverse square law and electricity in water. I remember my physics instructor talking about fishing with an electrical current and the difficulties associated with it.

I also remember him not being a very good instructor, so that may be way off base. He was the one that told me I was worthless and should go dig ditches for a living or something. (Passed his puny class with a B btw once I stopped goofing off.)
posted by Chickenjack at 10:50 AM on December 22, 2006


true only if the electrodes on either end consisted of metal plates covering the entire wall of the pool.

Yes, that part of my comment got lost in editing. Thinking about it though, it is pretty easy to improve the estimate.. What is the resistance between a small point of contact and the pool water in general?
Assume that current flows from the fault into the water in every direction evenly. The fault is on the wall, and it is approximated a hemisphere of radius rfault. The cross sectional area of the current path will be hemispherical shells of progressively increasing diameter, from a radius of rfault until it fills the depth of the body of water, for the pool that is a radius of 1m. That still leaves the contact resistance at the actual interface between fault and water..

The area of a hemispherical shell is 4πr2, so the resistance contribution of a shell is (100Ω·m / 4πr2)·dr, integrating from rfault to 1m we get:

16 x (1/rfault - 1) Ω
Back to the larger problem, we can think of a few scenarios.. Along with the point fault, the ground connection could be either distributed or a point. Also, the point fault (or ground) could be small, like a test probe, or large, like the entire casing of a light fixture.

If the fault is about 1cm in diameter, the resistance contribution in the dispersing region is over 3kΩ, which is pretty much safe (the total current flowing in the entire pool would be under 40mA, and a swimmer would be subject to a small fraction of that). That resistance drops fast though, with larger faults. If the pool ladder somehow got electrified, the resistance of the dispersing region would be ~10Ω..
posted by Chuckles at 11:46 AM on December 22, 2006


Chickenjack, I think I just answered your physics problem :)
posted by Chuckles at 11:48 AM on December 22, 2006


JFlash: My grandfather and father both taught me the finger test method. Again noting the cautions of insulated boots, body parts free of touching metal (e.g. open distribution switch covers) and dry fingers. My father freaks people out by grabbing an entire live bus-bar phase without the finger on ground/neutral.
posted by Kensational at 5:43 PM on December 22, 2006


Pool lighting in properly constructed/inspected pools is 12 Volt. A transformer by the equipment converts the 120V to 12 V. No possible way for 12V to harm you.

Now, if an idiot builds his own pool, or a pool builder wants to go bankrupt quick by saving $100 on the transformer all bets are off. However I'm quite sure you can't even order a non-12V pool light.
posted by HyperBlue at 1:48 PM on April 17, 2007


Additionally, it would even be hard to get a 12V shock since most reputable pool builders ground the light to the pools structural steel and a ground point prior to gunite/plaster finishing.
posted by HyperBlue at 1:50 PM on April 17, 2007


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