Electrocution while swimming near a houseboat

[vince99]

My wife has a friend who died, supposedly from electrocution/drowning while swimming about 10 feet away from a houseboat in a freshwater lake. Apparently this is more common than I thought.

Can anyone explain how this happens? Obviously there is must be some mis-wiring on the boat, that part I can understand and I am not addressing that here.

My question is, how does a body that is in contact with only one conductor, the water, have a flow of electricity through it? Generally you need a path in and a path out, at different voltage potentials, in order to have a flow of current.

Thanks

 

[iwire]

My question is, how does a body that is in contact with only one conductor, the water, have a flow of electricity through it? Generally you need a path in and a path out, at different voltage potentials, in order to have a flow of current.

I will take a shot at it.

The water, even over the short distance of a persons body can be at different potentials. Think of the water as more of a resister then a 'good' conductor.

 

[don_resqcapt19]

And also think of the wet body as a resistor with a lower resistance than that of the water.
Don

 

[cadpoint]

Might I add...

Might I add...

...

Can anyone explain how this happens? Obviously there is must be some mis-wiring on the boat, that part I can understand and I am not addressing that here.

My question is, how does a body that is in contact with only one conductor, the water, have a flow of electricity through it? Generally you need a path in and a path out, at different voltage potentials, in order to have a flow of current.

Thanks

Google searching : water electrocution, various subject are covered from why fish don't get fried oppone lighting strikes to, the difference of fresh to salt water, elctrocutions

Hers's one from Mike Holts old forum Here which happens to be on page 2.

I read through most of this Here which was on page 6 of this google search that gives a great over view of safe shore power. Note, Outlets, GFCI, Safety at the bottom with respects to boats.

 

[vince99]

Thanks for the replies.

Thinking of the water as a resister rather than conductor helps in understanding.

Here's a direct link to the post referred to above:

http://forums.mikeholt.com/showthread.php?t=77828
It is useful in understanding this.

But another question comes up for me. Since the water is a resistor, then a shorter distance would imply lower resistance. And since current from an onboard generator can only flow back to the return on the same generator, which I assume on a boat in the middle of the water is most likely going to be the metal drive components underwater, then why does current from the submerged hot conductor (somewhere on the same boat, due to an error or component failure) shock someone as far away as 20 ft (from what I've read) when a lower resistance path would seem to be directly back to the drive components which are much closer?

 

[nakulak]

In one of Mike Holt's dvds (I think its grounding and bonding), there is an excellent discussion of this exact thing. my recollection is that the following things were discussed:

- more people are electrocuted in fresh water as opposed to salt water because salt water has lower resistance than the body, and is a very good conductor, while fresh water is not, and has a higher resistance than the body. so, when you are immersed in fresh water and there is current flowing through the water, you are a much better current carrying conductor.

- there are two main reasons for current flowing through the water:
1) miswired boats, houseboats, etc

2) so called "stray" current, for which the primary causes are
a) houses which have loose neutrals (usually from corroded aluminum conductor connections)
b) return current from power company lines, since they use the ground as a return path for any fault current,


- I hope I haven't paraphrased any of the above wrongly, and if I did I apologize. Mr Holt has done a lot of research in the stray current thing and I would encourage you to check it out because it is extremely informative and some of it is very scary. Its unfortunate that there is a simple solution to stray current that the power companies will not invest in - adding a grounding conductor instead of using the ground as an alternate path for fault current.

 

[iwire]

And since current from an onboard generator can only flow back to the return on the same generator,

I assume the death was caused by shore power not by an onboard generator for the very reason you brought up.

 

[cadpoint]

Ode to Physic's

Ode to Physic's

...
so, when you are immersed in fresh water and there is current flowing through the water, you are a much better current carrying conductor.

Basic Physic's Priniciple: Pure Water is not a conductor, I will state that!

My first thought is that salt water would help disturb any voltage/amperage volumes. It is the composition of other materials that are in the H2O that make it reactive.

I guess the old phrase test the waters still applies!

 

[charlie b]

Current does not take the low resistance path. It takes all available paths, regardless of their resistance. The amount of current that takes any given path is dependent on the resistance of that path, in comparison to the resistances of the other paths. But all paths will get some current. The problem is that the amount of current that can electrocute a human is not very much.

Furthermore, many such tragic deaths are not so much from electrocution, but rather from what is called "drowning by electocution." That phrase means that the amount of current flowing through the victim's body might not have been enough to cause death by itself. But it was enough to cause the victim to lose all control of their muscles. Therefore, they do not have the ability to force their arms and legs to move (i.e., to swim). So the actual cause of death is recorded as drowning.

I believe that a person's best chance of survival, once they feel this happening to them, is not to try to swim. Rather, tread water, keeping your body vertical, and keeping your hands and legs as close to your body as you can. You want to lower the resistance of your body, as seen along the path of current flow. If the current in the water is only flowing past your body for the distance between your front and your back, there is a smaller resistance than you would get if the current flowed past the length of your body, as you were attempting to swim to safety.

My heart and my prayers go out to the family of your wife's friend.

 

[charlie b]

Basic Physic's Priniciple: Pure Water is not a conductor, I will state that!

True. But no water is pure, least of all the water we encounter in our daily lives. Not even the best of the demineralized waters you can buy in the store is pure enough to prevent the flow of current.

 

[charlie b]

I assume the death was caused by shore power not by an onboard generator for the very reason you brought up.

Maybe, and maybe not. But it still may be a fault in a nearby boat that is allowing current to leak (from the shore power source) into the water.

 

[iwire]

Maybe, and maybe not. But it still may be a fault in a nearby boat that is allowing current to leak (from the shore power source) into the water.

Where is the current path if this was a generator on a boat?


BTW, when I said 'shore power' I was talking about a boat plugged into shore power.

 

[ELA]

Sorry about your wife's friend.
This is interesting as I had always wondered about this.

I did some research on the conductivity of water - converted that to resistivity-cm and then calculated the equivalent resistance (assuming a sq-cm area) over twenty foot of water.

I put this in series with 1000 ohms to model a human wet.

I found the conductivity for several types or bodies of water.

I then assumed 120V initially.
I did not know what current to pick as enough to cause the person to not be able to swim. So I just figured some numbers around 6 ma.

This is just a first approximation that I did for my own interest and may be flawed but here it goes:

Based on those assumptions it appreared to me that 20 ft of fresh water would be a stretch to cause electrocution at 120V. For lake water I found less than 1 ma to flow.
Even at 480 volts it was still less than 1 ma.

For the same exercise but in Atlantic Ocean water (salty) I found the current to be 8 ma at 120V. So that seems plausable.

For the Great Salt Lake I found 24ma at the 20 ft distance and 120V.


Back to the fresh water I then figured that at 2 ft , 480 volts would produce 6 ma.

I admit that I did not check my work real well and welcome input if my approximations seemed out of line. Or did I miss the boat completely

 

[LarryFine]

Based on those assumptions it appreared to me that 20 ft of fresh water would be a stretch to cause electrocution at 120V.

I would look at it differently, and look at the voltage gradient. 120v across 20' of water means that there will be six volts per foot of water, so the more spread out a body is, the greater the voltage across the extremities; thus the suggestion of keeping vertical and 'small.'

If one stretches out six feet, that places 36v across the farthest contact points. Now, if we assign the body to be 1000 ohms, that means there will be (ignoring differences between body and water resistance) 0.12a (120ma) across the body, which is enough to be a real hazard.

 

[vince99]

OK this is starting to make some sense now.

But then, why do you say 120v over 20 ft? Yes he is 20 ft away from the boat, but the lake water continues on past his position, so why assume the entire 120v is spread out only over the 20 ft?

Thanks

 

[ELA]

I would look at it differently, and look at the voltage gradient. 120v across 20' of water means that there will be six volts per foot of water, so the more spread out a body is, the greater the voltage across the extremities; thus the suggestion of keeping vertical and 'small.'

If one stretches out six feet, that places 36v across the farthest contact points. Now, if we assign the body to be 1000 ohms, that means there will be (ignoring differences between body and water resistance) 0.12a (120ma) across the body, which is enough to be a real hazard.

Larry,
I see what you are saying. That makes sense if you were talking about all one medium such as the water.

Based on my example with water at ~30K/ft. - compared to say (20) -30K ohm resistors in series.
One near the middle has 6 volts across it. If you now subsitute a 1K resistor in parallel with that middle 30K you would change the gradient value at that point. This due to different mediums. Would there still be 6 volts across that 1K resistor?

So we see it differently and I admit I don't know the answer for sure on this one. The big thing that I am unsure about is the effects of having current enter and leave the body over a very large area vs. a point contact like touching a live wire and ground.
Perhaps with a large current density through the body it might take much less current amplitude to prevent a person from swimming.

I would agree with your gradient comments if we were talking about salt water vs. fresh water. In that case the assumed 1000 ohm body resistance would be much greater than the per foot resistance of the water and so the combined body+water parallel resistance (over one foot) would be closer to that of the water alone and thus nearly the full (water) voltage gradient could be felt by the body.

 

[nakulak]

Mr Holt has done a lot of work in this area, the newsletter has a bunch of articles about it. here's one link I could find:
http://www.mikeholt.com/newsletters.php?action=display&letterID=36

 

[infinity]

You want to lower the resistance of your body, as seen along the path of current flow. If the current in the water is only flowing past your body for the distance between your front and your back, there is a smaller resistance than you would get if the current flowed past the length of your body, as you were attempting to swim to safety.

Could someone explain why we would want to lower the body's resistance? Am I incorrect in thinking that an increase in resistance would cause a decrease in the current flowing through the body?

 

[ELA]

I am not convinced it would make that much difference in fresh water. (at least not at 20ft).

What is more important in this discussion is the relative resistances between the medium the body is in parallel with and the body resistance.

I do not know the resistance figures for the difference between the human body from head to toe vs. from chest to back. In the water example I would think the head to toe and chest to back resistances would not be that much different but I really don't know.

You cannot ignore the resistive differences between the body and the medium it is in because that makes all the difference in determining if you get shocked-electrocuted or not.

I do not like the voltage gradient concept (while ignoring the difference in mediums) for understanding shock potential.
For example:

If you walk under a power line at 4160 Volts (assume 40 ft off the ground) your body may experience an Electric Field of (4160v/40' x 6') or 600+volts.
Do you experience 600V/100,000 ohm = 6 ma current or higher to flow in your body?
No , because the air is such high resistance that it limits the current. You experience a large electric field but not the equivalent calculated current flow.

Back to the fresh water example:

Vertical:
If you assume approximately 30K ohms per foot of water and your body at only 1K ohms - over a distance of 20ft your body experiences ~ 120V/ (19 * 30K) + 1K = 0.21ma.

((Based on all the previous posting assumptions.))

Horizontal:
If your body did exhibit a higher resistance head to toe (say 5K) then
your body experiences ~ 120V / (14 * 30K) + 5K = 0.28ma

Based on this your body does experience a higher current by being horizontal. Not much but it is larger.

If you calculated the same example in salt water it would be very significant. As well as if the distance from the source to the body was shorter.

 

[dbuckley]

The other thing worth noting is that you don't need a fault to cause appreciable current to flow; because of the mutigrounded neutral system and because shore power will be connecting to a ground point that maybe hundreds of feet from the waterline, there will inevitably be a potential difference between the hull, and the soil that forms the base and sides of the lake. How big that potential could be "depends", and it'll definitely be worse under some fault conditions than when all is quiescent. I really wouldn't like to be in the water if there was a MV distribution fault!

Edited for speeling.