Electrical Problems at a Boat Dock

Electrical Problems at a Boat Dock


By Chuck Newcombe (aka Bish)


During the eight years that I’ve been writing the Beyond the Basics column for Fluke News Plus, I’ve discussed wiring and grounding problems in industrial plants, homes, and dairy farms. This one adds grounding issues at boat docks.


Consider what can happen when an unsuspecting swimmer comes near a boat with stray ac leakage current flowing between the boat hull and grounded dock components or the bottom of a lake.


The grim reality is that ac leakage current, often less than 100 milliamps, may be flowing undetected in the water. That’s far from the amount required to trip a 20 amp breaker, but enough to paralyze a swimmer. You wouldn’t see any thrashing in the water, or hear a call for help. Later, a drowned person would be pulled from the water with no clue provided as to the cause of the drowning.


How can one detect the likely presence of ac leakage currents in the water?


My choice is the Fluke model 360 Leakage Clamp Meter. This meter can measure up to 60 amps ac in normal use but has two special features that make it appropriate for leakage current testing.

1. The current measurement sensitivity extends down to a 3 mA range with 1 μA resolution.

2. The jaw size is 1.5 inches, allowing the jaws to be placed around thick cables and conduit.


Where to measure


Text Box:    The 1 2 3 test for ac leakage  Kirchoff’s current law states that the sum of all currents flowing into and out of a node must equal zero. We can use this information to devise a test procedure for a dock. In the diagram below, the node we’ll use is marked “N.” Arrows with solid lines show where current should flow, and the dotted lines show where leakage can occur.


If you want to measure the current flowing from the source to the load, you would place the clamp around a single conductor (1). But, using Kirchoff’s current law you can place the clamp around multiple conductors as shown at positions 2 and 3.


In a properly wired circuit, the clamp meter reading should be zero for positions 2 and 3. Any leakage current flowing in the green safety ground wire would be seen as a non-zero reading at position 2, and any leakage current from the load to earth would show a non-zero reading at positions 2 or 3.

Text Box:

Since the green wire is included in the test at position 3, only the earth current, is registered as leakage in that case. I’ve made an accessory to allow convenient measurements in the three test positions shown in the diagram above. I bought a short extension cord and modified it as shown here to make the three conductors individually accessible.


You can read additional details in my original article here.


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  • It only takes around 12ma going through a human to cause paralysis which results in drowning. 100ma through a human will stop the heart (fibrillation).

    When you have a 100ma leak into the water it's virtually impossible for a person to have all that current pass through them.

    In a USCG Safety Study completed in 2008 we found that if less than 100 ma was leaking into the water around a boat it wasn't even close to the level required to produce a lethal voltage gradient (2v/ft).

    This 100 ma goes off in all directions heading back to it's source. When you get into the several AMPS category, then you can produce that lethal gradient which can paralyze and kill.

    Send me an email and I will send you a copy of the study if interested. Also, always interested in good products for testing. My colleague and I pioneered the use of clamping a whole cord for determining water leakage. Always interested in evaluating and promoting good tools for this purpose.


    David Rifkin  July 24 2014, 3:48 pm EDT
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  • Reply from: R.O. Bowling   August 2 2014, 7:01 am EDT
    Please send a copy of the study.

    Randy O. Bowling Arkansas M-4312, J-2836 World Safety Organization. "Who's Who in the Safety Profession" 1990

    Reply to R.O. Bowling

    Reply from: David   August 2 2014, 11:09 am EDT
    Send an email to qualitymarinesvcs@comcast.net and I'll send you a copy of the Study, David
    Reply to David

  • I have been using this method to find Ground Loops in isolated telephone central office switching equipment for years. Also used on DC current systems for ground loops but used a DC current meter. Works great.

    Curtis Leary  July 22 2014, 9:09 am EDT
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  • This is a very interesting article. I live on a canal and my grand kids as well as I swim in the canal right alongside my boat. This newsletter leads me to conduct the test as indicated. I will follow up with comments.

    Mike Albarran   July 22 2014, 9:02 am EDT
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  • I think I have the Kirchoff's Law part but you still didn't explain how to test for leak current at a boat dock. What I have done in the past is run a Fluke DVM lead from a known ground at a boat dock or on a boat itself that has either shore power or a generator with the other lead in the water set on AC and see if I register any voltage that way. Tell us where you measure leak current in a marine situation.

    Mark Key   July 22 2014, 7:33 am EDT
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  • Reply from: Chuck Newcombe   July 22 2014, 10:55 am EDT
    My use of a generalized diagram for leakage testing might be more clearly understood in the marina environment by considering the dashed line around the load to be the boat hull, and the ac source and wiring to be the dock receptacle and extension cord providing ac power to a battery charger on the boat, for example. Test #3 around the extension cord will reveal any leakage from the boat back to the grounding system on the dock.
    Reply to Chuck Newcombe

    Reply from: David Rifkin   July 24 2014, 3:57 pm EDT
    The probe in the water doesn't tell you much unless the voltage is very high (10s of volts). It's normal to see a few volts in a saltwater marina when everyone is running their A/C units. Our grounded neutral system results in ground currents everywhere. Ideally there shouldn't be any, but there are many parallel return paths since pole transformers and pad transformers are all grounded to the earth.

    To find the water leakage. Clamp the cord on the boat with AC loads running. Ideally it will read zero. Any current you read is current that is either leaking from the boat you are clamping or coming back from another source via the cord you are clamping. After getting that reading, turn off the pedestal breaker for the boat you are clamping. Any current change is the result of a ground fault on that boat.

    Example: clamp reads 500ma. open breaker and it now reads 100ma. This means that 100ma is coming back on the ground wire from another source (maybe the boat next door for example) and 400ma is going in the water from a fault on the boat you are clamping.

    At this point you can use an adapter to split out the conductors and measure the currents individually. In many cases you will see even more current on the bonding conductor (green wire in the cord). Add this to what's going into the water and you have the total fault current from the fault. Remember, only takes about 60w to start a fire. That's half an amp at 120v.

    Contact me if you have more questions.
    Reply to David Rifkin

  • Question. I'm working in a 3 stories home. Flood level is 8'. electric meter will set at 6' of the finish ground. Switches and receptacles in the first floor got to be at 8'? Second question. how much do you charge to prepare me for the state exam in South Carolina?

    Evodio s  July 21 2014, 8:52 pm EDT
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