Stray current questions

[John R]

I'm just asking this out of curiosity:

I've read that it's unsafe to swim at a marina because of the danger of stray current. In fact, I read somewhere of somebody being electrocuted that way. My question is: How about people who make a living cleaning boat hulls? They go in at marinas all the time.

I think I also read that stray current is a greater danger is lakes than in salt water. I know that salt water conducts electricity better, but why does that make it less of a danger?

 

[Sailvayu]

Check this link out http://www.huffingtonpost.com/terry-gardner/esd-hidden-danger-in-fres_b_693454.html

It should answer most of your questions.

Capt. Wayne Canning, AMS
www.projectboatzen.com

 

[JohnShannon]

You are basically a bag of salt water, in fresh water your body presents the path of least resistance for the electric current. So more current flows through your body, enough current and something goes wrong. In salt water, the water is much more conductive and current flows around, not through you. Molten copper would be even more protective but by that point you would have other problems.

 

[walt]

If you listen to this http://www.youtube.com/watch?v=O7-s_mdEPb0&feature=youtu.be it describes this partiuclar issue in detail.

One thing of interest in there is that it takes a "voltage gradient" in the water to be dangerous to a swimmer and the numbers are surprisingly low and I think the reason is that it’s not really electrocuting you but its messing with your heart and muscle control.

A voltage gradient means "what is the voltage over some distance". That video gives some number that are dangerous (can’t remember the number) but the units are in volts per distance..

A couple of things to note about how to run into a higher voltage gradient. If we put a voltage point source into the water and the current is returning by ground, the volume of water that the current see's expands dramatically as you move away from the point source. That water has some resistance so the highest voltage gradients are seen at close distance to the voltage source where the water volume is still lower. The voltage gradient falls off with the square or cube of distance (I believe..) from the source likely proportional to the volume of water as the current expands away from the point source. This is why someone can swim up near a dangerous source and almost not even know it’s dangerous until they get too close.

Also, the resistance of the water would also affect the voltage gradient. The lower the resistance, the lower the gradient (or volts per distance). So fresher water has higher resistance and also higher voltage gradients = potentially more dangerous. Salt water would result in lower voltage gradients.

 

[fstbttms]

I'm just asking this out of curiosity:

I've read that it's unsafe to swim at a marina because of the danger of stray current. In fact, I read somewhere of somebody being electrocuted that way. My question is: How about people who make a living cleaning boat hulls? They go in at marinas all the time.

I think I also read that stray current is a greater danger is lakes than in salt water. I know that salt water conducts electricity better, but why does that make it less of a danger?

Almost all Electric Shock Drownings (ESD) occur in freshwater. It is very rare in saltwater. I have performed over 20,000 in-water hull cleanings in 18+ years here in the Bay Area and have never once had a problem with electricity in the water. Further, during those 18+ years, well over 1,000,000 in-water hull cleaning events have taken place in the Bay Area, and to my knowledge, there has never been a death or injury to a hull cleaner due to stray current in the water. I think it's pretty clear that swimming in saltwater marinas is not particularly dangerous, as far as electricity goes.

 

[John R]

Hi, Matt

Almost all Electric Shock Drownings (ESD) occur in freshwater. It is very rare in saltwater. I have performed over 20,000 in-water hull cleanings in 18+ years here in the Bay Area and have never once had a problem with electricity in the water. Further, during those 18+ years, well over 1,000,000 in-water hull cleaning events have taken place in the Bay Area, and to my knowledge, there has never been a death or injury to a hull cleaner due to stray current in the water. I think it's pretty clear that swimming in saltwater marinas is not particularly dangerous, as far as electricity goes.

Yes, and mine is one of the hulls you clean.

Thanks to everybody for the explanations. I take it that cleaning a hull in fresh water is different.

 

[fstbttms]

Yes, and mine is one of the hulls you clean.

 

[Maine Sail]

Almost all Electric Shock Drownings (ESD) occur in freshwater. It is very rare in saltwater. I have performed over 20,000 in-water hull cleanings in 18+ years here in the Bay Area and have never once had a problem with electricity in the water. Further, during those 18+ years, well over 1,000,000 in-water hull cleaning events have taken place in the Bay Area, and to my knowledge, there has never been a death or injury to a hull cleaner due to stray current in the water. I think it's pretty clear that swimming in saltwater marinas is not particularly dangerous, as far as electricity goes.

I am going to take a stab on this and guess you at least unplug the shore cord for the boat you are cleaning? Still won't prevent an auto transfer inverter from firing up but better than nothing.

 

[fstbttms]

I am going to take a stab on this and guess you at least unplug the shore cord for the boat you are cleaning?

Of course I do. As unlikely as it may be, electrocution can ruin your day.

 

[rpwillia]

Amp Meter

This discussion came up at an answer and question time at Deltaville Yacht Center. Lew Grimm the owner suggested a good way to check is to get a clamp on ammeter. If you have nothing running you should see nothing, but if you do see a current flow, you or one of your dock neighbors (and therefore you) has a problem.

www.youtube.com/watch?v=z-KfZvbjyBY

I may just try this next time I am at my boat just to see what it does.

Mainsail - your $0.02?

 

[tkanzler]

If you have nothing running you should see nothing, but if you do see a current flow, you or one of your dock neighbors (and therefore you) has a problem.

I think you mean 'if you do not have leakage current through the water you should see nothing', and yes, if you read a current flow when clamped around the entire shore power cord, you have leakage current into the water or via some other pathway back to the source.

 

[walt]

In the ABYC video referenced earlier, they also use the clamp on current meter to test for leakage. If you clamp the current meter around the entire power cord going to the boat, it should read zero amps even if the boat is using AC power. The current meter works by measuring magnetic field and if the AC current going out on the hot is exactly the same as what is coming back on the neutral, the magnetic field cancels and the meter will read zero. If any of the current does not return on the power cord, it went into the water and the meter will now detect that.

I mentioned before that the ABYC video brought up that it is the voltage gradient (volts per distance) in the water that ends up being dangerous and they mentioned a number like 2 volts per foot as a threshold. It’s interesting to note that the actual leakage currents for a given fault would be a lot higher in salt water vs. fresh water because of the higher conductivity of salt water. But we never see any of the drownings in salt water. However the voltage gradients would be a lot higher in fresh water vs. salt water because of the lower conductivity of fresh water. So the danger is not really the amount of current (generally higher in salt water), it’s the amplitude of the voltage gradient (higher in fresh water).

Also in the ABYC video, those guys went to a fresh water marina I believe in Portland (OR?) and measured a bunch of power cords at slips and found a fairly high number of power cords that did show a dangerous level of leakage into the water. It was interesting to me living in Colorado where our water should be particularly fresh to note that the fresher the water, the lower the leakage current level would be in order to be dangerous.