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Ben E

Measuring AC & DC leaks

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I was just asked how to measure if there's any AC or DC power leaking from a boat into salt water. Is it as easy as turning everything off, putting the negative side of a multimeter on the engine block with positive to a steel cable or chain overboard, then turning things on?

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  • If everything was shut off, then it seems to me all the downstream wiring would be removed from the test.

    If the AC was shut off just inside the boat, then none of the onboard wiring would be tested. If you left the AC on, but shut off particular devices at the panel, then faults with the devices wouldn't be revealed.

    Same thing with DC. Shut if off at the battery box, not much possibility of leaks. Shut it off at the panel, a little more wire is in play, but the wire runs to the devices are excluded.

    It seems like it would be good to have every device energized and operating so that all the wiring was in use and any leaks exposed.

    Whether putting a probe in the water will do it or not I don't know, but I'm sure some electrician will let us know.

  • Ben - strike that. I see that you're going to turn them on, presumably one at a time to find the possible offender. Good plan.

  • Can you be more specific... what exactly will you be measuring? DC Current?

  • Ben, Steve D'Antonio tested for AC leaks on our boat by putting an AC clamp meter on the shore power cord. A clamp meter normally is supposed to go around only the positive or hot wire; otherwise it sums the positive and negative current and reads zero. However, in this case you want to clamp both conductors (which is in any case the only way to do it unless you filet your shore power cord); if you are leaking current via any path other than the shore power cord (which you shouldn't be), the sum of the currents through both conductors won't be zero and you'll see a current register on your clamp meter display.

    In our case we were fine with a few microamps of leakage, but another boat in the marina had 300 mA when measured this way.

  • A core feature of the Amprobe LH41A (see Dans Boat Tools) is to measure AC and DC leakage. I don't believe you can do AC leakage measurement with one multi meter.

    Especially for AC leakage, an important feature of the LH41A is the ability to sum the total of the currents going thru multiple wires (put 2 or more wires, or the entire 3 wire power cord in the clamp at the same time). The sum should be zero, any other value is current leakage.

    The book "Advanced Marine Electics and Electronics Troubleshooting" by Ed Sherman has a testing process to follow and includes allowable values in salt water environments. (leakage is very dangerous in fresh water environments, not so much in salt water)

    With meter in hand don't immediatly jump to the conclusion that the leakage your measuring at the dock pedestal is your own or that the marinas power is faulty. Current leaking from other boats will come thru your boat and to the pedestal, and can complicate your measurements.

    I have to say leakage testing is the most complicated and confusing testing I have ever performed on my boat.

  • Thanks all. The goal of the person asking the question is to find leaks that might repel fish, so the ideal would be measure leakage away from the dock, when AC is supplied by generator or inverter.

  • Repel fish? I can't think of a way to intentionally, let along accidently, leak AC current into the water when away from the dock.

    Safe to say, no leakage would be detected.

  • I don't know about fish and electrical leaks...but leaks will eat the metal parts of your boat.

    I use one of these when testing for stray current:

  • All great points but what I should add here is the original poster was measuring voltage, which I should comment to him is not a reliable way for troubleshooting this issue.

  • Quick Question
    How do i pass an electrical DC current through water salty or not?
    The electric eel can pass a high voltage through the water that hasn’t got much chloride in it then why it so hard to do? Put it this way, the eel doesn’t it very well and no one so far has given a correct answer to this question. If someone can solve this mystery that would be great and he/she would be the first to do so as I have looked everywhere and no one so far has done so. Would appreciate a answer at [email protected]

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