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post #1 of 81 Old 12-28-2016 Thread Starter
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Stray AC current testing

Just before hauling, my marina claimed to have done random clamp tests on shore power cables around the marina and found mine was intermittently showing stray current (ie, not net zero amps, when testing the cables). I don't recall how much they claimed, but they did say they went back and it wasn't consistent. I have two suspect appliances: my invertor/charger, which has been acting funny, and my ice maker, both of which may cycle on/off.

Anyone know if I can I effectively test these with the boat plugged in on the hard, or does it have to be in the water? My thru-hulls are not bonded, if that matters.

I'm heading out this weekend to take my own clamp readings, while plugged in. I can energize all AC appliances, except the air conditioning systems. Although, the air also cycles and is suspect.

I would rather identify the issue myself, even if I have to pay the electrician to repair it.


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post #2 of 81 Old 12-28-2016
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Re: Stray AC current testing

The easiest way I know of while on the hard is to attach a length of wire to the engine block, which is usually the best ground system you have available for this test. Next, using the multi-meter, set the AC voltage on the lowest scale, then connect the black test lead to the engine block ground. Then with the equipment fired up, touch the red test to any unpainted metal part of the individual appliances and look for a reading. Usually, floating ground items are the likely culprits.

Good luck,

Gary
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Re: Stray AC current testing

I can give that a whirl, Gary. My plan was to recreate the test that the marina used, by using a clamp meter around the shore power cables themselves. With both the pos and neg wires in the same cable, the meter should read zero, unless something is leaking AC current. They claimed to have measured a reading this way, but I don't recall how badly. The manager did say it was enough they would have required a power down, if I was in fresh water. Salt water is not as much a concern, other than for anodes, and our marina does not permit divers.

As I think about it, I'm not having any anode issues, so this is perplexing. One set lasted the season (7 months), but were at their limits.

I also wonder if this could be the shore power pedestal itself.


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Re: Stray AC current testing

Yes, it could very well be the shore pedestal, or even a poor grounding system with the shore power itself, which would not be unusual. And, saltwater is a better conductor of electricity than freshwater, so the marina guy had no idea what he was talking about, which is not at all unusual based on my experiences.

Good luck,

Gary
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Re: Stray AC current testing

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Originally Posted by travlin-easy View Post
...saltwater is a better conductor of electricity than freshwater, so the marina guy had no idea what he was talking about, which is not at all unusual based on my experiences.
Correct, saltwater is more conductive, but that's why it's more dangerous to have stray current in freshwater.

Saltwater is more conductive than a human body, freshwater is less conductive than a human body. Thus in freshwater the fastest way for stray current to get to ground is through you.
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Re: Stray AC current testing

I'm not sure what your marina guy was measuring and calling stray current, since a clamp meter placed around a 3-wire grounded AC power cable wouldn't "know" which lines were unbalanced, they're all in there together.

Not making any assumptions, but it would be necessary, not optional, for someone to actually test the circuit AT the dockside power post, with your cable disconnected, in order to first determine that there was no "leakage" (i.e. potential difference) between the neutral and ground lines, no? And it sounds like they didn't do that, so there's no way to be sure of where the problem was.

The odds are your icemaker just has a 2-lead power cord and isn't doing anything with the ground leg. That like most appliances it is just connecting to the hot and neutral sides, in which case it cannot be involved in any leakage problem. And you can ignore it.

Which leaves the flaky charger--and if it is flaky that's reason enough to look into it. You'd have to start by checking the power socket on the hard, and then the wiring on the boat. Which is not always a clear thing, since "ground" is sometimes tied back to neutral on boats (in gensets, etc.) because there's no real ground on the boat. Sometimes they are wired up like houses, sometimes not, and neither is "wrong".

If there is a GFI in the power post, or hopefully you have one installed where the AC comes into your boat, that should be tripping out if there's any significant problem, shouldn't it? If you don't have one, this might be a good time to add one.
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Re: Stray AC current testing

I do have GFI circuits, but no trips. I can't say that every 110v appliance is on a gfi circuit, but all receptacles are. The ice maker is plugged into a receptacle.

I have read up on checking net amps through a shore power cable, with a clamp meter. Pretty sure it's a legit measurement, but there doesn't seem to be an easy way to help identify the casual problem.


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Re: Stray AC current testing

If you have a ground fault that results in current flow in the safety ground (green) wire a clamp-on meter on the shore power cord will not show it. If you have a fault that puts current in the water it will.
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Re: Stray AC current testing

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Originally Posted by Minnesail View Post
Correct, saltwater is more conductive, but that's why it's more dangerous to have stray current in freshwater.

Saltwater is more conductive than a human body, freshwater is less conductive than a human body. Thus in freshwater the fastest way for stray current to get to ground is through you.
So, your hypothesis is that that electricity that barely conducts at all in freshwater, will leap through the nearly non-conductive substance, then pass through human skin (which is not very conductive either), subcutaneous tissue (fat), muscle mass, bone and rapidly go to earthen ground? While in saltwater, which is highly conductive, it would leap directly to ground and not effect the human body standing in the water near the current source? OK - got it!

All the best,

Gary
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