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Old 07-01-2013, 10:53   #1
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Amp Leak

due to a reported diver being shocked when he touched prop shaft while cleaning the bottom of a friends boat. I checked the shore power cord with a clamp on amp meter, I get 1.5 amps reading. I have tried a different power cord as well as a different shore power source with the same reading.
The boat has a galvanic isolator installed. could this be a cause of the leak.
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Old 07-01-2013, 11:09   #2
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Re: amp leak

No, the galvanic isolator is on the ground side of your shore power circuit to stop stray low voltage DC currents that would corrode away your anodes and other metals and yet allows the ground to work as it is supposed to to protect you when you have a short.

The 1.5 amps that you are measuring is on the hot side of your shore power, right? Where you start tracing the problem is at your AC panel where your shore power connects to the panel. You then start turning off circuits until you are no longer drawing 1.5 amps. You then trace that circuit by turning off each individual load on that circuit until you find it.

Clamp ammeters (or current meters) are not necessarily sensitive enough to measure 1.5 amps of AC current. They can but you want to test it on a known amount of a small amount of current. If you test it on a 120 volt lamp with a 60 watt bulb you should see a current draw of 0.5 amps. Current meters are generally meant for measuring larger amounts of current which generate a larger field for the meters coils to have inducted into them. If your current meters reading is wrong then use a good quality digital multmeter to measure this small an amount of AC current. Most multimeters can measure up to 10 amps of AC current.

http://www.yandina.com/galvanicIsolator.htm
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Old 07-01-2013, 11:55   #3
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Re: amp leak

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Originally Posted by boeing1 View Post
due to a reported diver being shocked when he touched prop shaft while cleaning the bottom of a friends boat.
Is this really possible? This would mean that the current would have to go though the body of a diver then into the water to ground? I would think the current would always just go though the water to the ground and that the body of a diver would be higher resistance.

And just think of how high the potential would have to be to shock the diver.

Someone talk me out of it as my BS meter is going off.
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Old 07-01-2013, 12:01   #4
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Re: amp leak

People have died when swimming by docks with bad wiring.
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Old 07-01-2013, 12:18   #5
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Re: amp leak

I would REALLY like to hear Fastbottoms (sp) on this. Serious.
I refuse to participate in marina work when divers are changing zincs.
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Old 07-01-2013, 12:30   #6
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Re: amp leak

This is how I think it works:

The boat that shocked the diver has a fault from the hot AC to the boat's ground and bonding system. But he has an open ground to the shorepower so that the circuit breaker won't trip. The boat's bonding system including all of its exterior metal is then hot, which is very, very dangerous.

Water is not a good conductor, so that there is only a few amps dissapated from the boat's exterior metal to earth through the seawater and not enough to trip the breaker. The resistance is roughly proportional to the area exposed to seawater.

A diver's body probably has several times the surface area than the exterior metal. So he touches the metal and lots of amps pass through his body and sometimes he dies.

If I were the marina operator I would kick that boat out of my marina.

David
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Old 07-01-2013, 12:50   #7
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Re: amp leak

I did a quick google search and found some reports of people dying from electrocution. But all were in fresh water.

When I was on submarine duty we used to ask people how come it was safe for us to be in a steel boat surrounded by water with an ungrounded 480V AC electrical system. It was because our bodies had enough resistance that in order to get the current to pass though it you had to be holding on to the short while standing waist deep in seawater.

I just can not see the resistance of a human body to be less than that of of seawater. So unless you really got directly between a couple of wires I think the current would just bypass the body and travel though the water.

But "I ain't no expert"
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Old 07-01-2013, 12:56   #8
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Re: amp leak

We understand how serious this problem is. and the boat in question does not have shore power connected to it untill we can get this problem fixed.
I have turned off everything on the boat A/c and D/C and the power cord still reads 1.5 amps with the clamp on meter,
any suggestions where to start.
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Old 07-01-2013, 13:58   #9
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Re: amp leak

Don, a swimmer or diver can be electrocuted by a nearby lightning strike simply because of the difference in potential from one end of the body to the other. I don't know the details of the physics or mechanics, but this is part of all SCUBA training and water safety courses. I'd be surprised if they were all wrong.
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Old 07-01-2013, 14:21   #10
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Re: amp leak

Have you checked the wiring of the pedestal to ensure that the problem is with the boat? If so, then I would switch to using an ohmmeter to look for shorts between the 110VAC hot line coming in and boat ground. Of course the shore power cord will be disconnected.
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Old 07-01-2013, 14:55   #11
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Re: amp leak

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Originally Posted by boeing1 View Post
We understand how serious this problem is. and the boat in question does not have shore power connected to it untill we can get this problem fixed.
I have turned off everything on the boat A/c and D/C and the power cord still reads 1.5 amps with the clamp on meter,
any suggestions where to start.
How are you checking the power cord? Did you test all 3 leads individually?
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Old 07-01-2013, 15:51   #12
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Re: amp leak

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How are you checking the power cord? Did you test all 3 leads individually?
Good question! If you have a 120 volt shore power cord, and you test the black (power) and white (neutral) together you will read leakage that occurs on your boat. The current onto the boat differs from the current off the boat. If you read zero amps in this condition, but find current flow when testing just the green (ground) you have current in the water from another source.

I once encountered this condition. It was caused by an adjacent boat with an electric heater which was mis-wired with the white and green wirkes reversed. It was putting 3500 mA into the water.

A Fluke 360 clamp meter will measure down to fractions of milliamps for this purpose. It is costly, ~$600.
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Old 07-01-2013, 16:08   #13
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Re: amp leak

possible I have not checked it correctly. we checked it by placing a clamp on amp meter around the power cord going from the pedestal to the boat. it read 1.5 amps. I thought it should read zero, if there was no leak.
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Old 07-01-2013, 16:26   #14
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Re: amp leak

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possible I have not checked it correctly. we checked it by placing a clamp on amp meter around the power cord going from the pedestal to the boat. it read 1.5 amps. I thought it should read zero, if there was no leak.
You are correct, if there is no leakage on your boat and no leakage from elsewhere, you would not find 1.5 amps while clamping all three together. If you get to a place where you can clamp the wires separately, you can determine where the problem exists. If the black and white clamped together shows current, look for the problem on your boat or shore connection. If that is zero, and the green has current the problem is likely from another boat nearby. If it's on your boat, try turning things off to see what makes it go away. If it's the green wire disconnect the power to adjacent boats to see what makes it go away.

If it is 1.5 amps through the water, it is in the lethal range. The reason for using a meter capable of detecting a few milliamps is that more than ten milliamps is a serious risk. Since you are reading 1500 milliamps, your existing meter is sufficient to find such a big leak.
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Old 07-01-2013, 17:04   #15
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Re: amp leak

A simple trick to make the clamp meter more accurate is making more wingdings. Simply make sure the wire passes lets say 5 times trough the clamp and divide the value with 5. You just made a simple high amp clamp meter 5 times more accurate for low amperage's.
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