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Old 07-21-2019, 05:55 PM   #1
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Using a clamp meter

I got in the clamp meter which I ordered, and plan to go down to the marina tomorrow and do some troubleshooting on the electrical issues on the boat.

My first time to use a clamp meter (I only learned about them the other day, when I posted about my electrical problem). So I looked on the internet to see how to use it to measure amperage. And thought that I understood it.

But some postings which I have read on here recently have me puzzled, so I thought someone might take pity and enlighten me.

It was my impression that in order to measure amperage, the meter has to be clamped around only one conductor. But if I understand the posting which I read yesterday, someone was measuring the amperage on the shore power cable by just clamping around the entire cable. I may have misread that, but don't think I did.

So I am puzzled, which is nothing new.
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Old 07-21-2019, 06:14 PM   #2
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No, if you clamp around a two conductor cable (or 3 with ground), the positive current that the meter sees on one wire is offset by the negative current on the other wire so it nets to zero.


This is true for AC or DC cables.



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Old 07-21-2019, 06:20 PM   #3
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2 different tests.


One conductor test looking for amperage....whole shore power cable looking for leakage to the water.
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Old 07-21-2019, 06:47 PM   #4
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There is actually a use for clamping around the entire cable. Good for the typical 120V, 30A cables many of us use.

If there is a current showing on the meter then you have electrical leakage to the water. It means that there is a difference in the current between the black and the white so the field is not cancelled out. It takes a GOOD meter to do this reliably though.
Otherwise the above is dead on as what goes in must come out.

So yes, one must clamp one wire/conductor at a time for normal useage which is usually what we are looking for when checking the draw of a piece of equipment.
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Old 07-21-2019, 07:22 PM   #5
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However if you clamp the entire cable, hot, neutral and ground, even if the meter shows zero, you still could trip GFI equipped docks. You could have current leaking onto the green ground wire just not into the water. You would not kill someone in the water but the GFIs will trip if the current leakage goes over their limit. Because some current could be coming back on ground the hot current going to the boat and the return neutral current would not be equal. The better way to test is to make a breakout cable adapter with the three wires exposed so you can clamp the hot and neutral only. If they are equal the you donít have leakage. I made a simple tester, put a GFI outlet (household type) on the end of an extension cord. Plug the boat into the GFI and it will trip if the boat is leaking over 6 mAmps. You canít test the whole 30 amps at once but you can test circuits up to 15 amps total. My boat would trip GFI equipped docks just by plugging it in without even turning anything on. By using the extension cord we were able to find all the faults. We just spent 8 nights in other marinas that all have the new wiring without any problems.
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Old 07-21-2019, 07:39 PM   #6
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Yes, there are other possibilities.

In this case though IF the meter shows a current flowing it means the missing current is NOT returning via the cable but rather through leakage to the water.

That's all it shows. For almost anything else you must devise other testing means.

I should have been clearer that the whole cable clamp test is intended to check for a possible leak into the seawater. TO confirm there is truly a problem and find the source and the pathway there is more work to be done.
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Old 07-22-2019, 05:48 AM   #7
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To find out what current is being drawn the wires are usually easy to get to on board behind the shore power inlet.

For a 120-240 system measure the two 120 legs individually.
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Old 07-22-2019, 06:51 AM   #8
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To all the above, keep in mind that if the objective is to test for leakage, a simple clamp meter probably isn't going to cut it. It may show if there's a significant leak, but to find the <100 milliamp leaks that can trip a GFI or ELCI breaker (usually set to trip at 30ma), you'll need a meter that's designed for leakage testing and has the sensitivity to be accurate at that low range. They're not inexpensive (like $400-$1200), and probably not the range of cost that one would expend unless it was a tool employed in earning a living. To thoroughly test for shore power leakage, you'll also need a breakout pigtail to separate the leads on the shore power cord in order to check the ground as well as the conductors.

That said, a clamp ammeter, particularly a hall effect meter that can clamp measure DC current is a very useful addition to the toolbox.
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Old 07-22-2019, 07:58 AM   #9
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OK. I understand now. Thanks.
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