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Old 20-03-2010, 21:57   #1
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Two Amp Feedback to Shore Power Cable - Help!

I can't believe this is uncommon but I searched the archives and could not find a similar problem. The wharfinger tells me I have a 2 amp feedback through my shore power cable when I am just sitting at the dock with nothing connected. So I am barred from using power till I get it fixed.
When I touch the ground terminal of my shore power cable to the aluminum shore power box, a little arc occurs between the two. I have a galvanic isolator and I followed the troubleshooting directions and it is fine.The ground from the shore power goes DIRECTLY to the isolator, I can see it (with my head all the way under the electric panel and just my feet sticking out!).
I can isolate the house batteries with a switch and the feedback is still there. The electrical system is in very good shape other than this. My zincs do not seem to get used up very fast at all.
I was going to do a bunch of spring work on the boat today, but had no power and so spent the day trying to figure it out.
Ideas anyone?
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Old 20-03-2010, 22:07   #2
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Does the feedback stop when you throw off the shorepower circuit breaker(s) on your boat?
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Old 20-03-2010, 22:10   #3
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Measure the voltage between the seawater and your ground wire pin, unconnected. Do the same with the dock ground. That will tell you who has the problem. If it's you, start disconnecting things from that ground circuit until the voltage goes to some small value. You can switch to more reliable current measurements when the 'arc' goes away.
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Old 20-03-2010, 22:15   #4
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No- feedback is still there with it in gen position, off position or shore position.
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Old 20-03-2010, 22:24   #5
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okay. that narrows it down quite a bit. a ground leakage or stray current problem exists.

daddle's advice is the next step in locating the source.
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Old 21-03-2010, 07:00   #6
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Just finally install that isolation transformer and be done with it. No-one can expect you to troubleshoot all those shore power troubles and most don't even like you when you report a problem that you found. An isolation transformer allows you to use it anyway without a problem for you.

cheers,
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Old 21-03-2010, 07:40   #7
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Again, though, you should check to see if the problem really exists boat side as Daddle suggests.

If it does exist boat side, Nick has a point in that if you install the isolation transformer it will decouple the ground and should stop the feedback. However, the ground fault will still exist on the boat side. So you're not truely resolving the root cause of the problem.

That may or may not cause you problems down the road. Depends on how much work vs $ you want to put into it, I guess.
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Old 21-03-2010, 07:46   #8
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Actually, scratch that. The galvanic isolator should be doing the job of arresting the ground current loop, so swapping in the isolation transformer will likely only solve the problem if the galvanic isolator has failed - an expensive fix.
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Old 21-03-2010, 08:15   #9
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Also, can you verify for us if the off/gen/shore switch switches both the black (hot) and white (neutral) AC leads OR does it switch just the black, and all the whites are tied (bussed) together?
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Old 21-03-2010, 09:20   #10
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Hi Scottie2,

It's true that I assume some parameters to the equation but I have learned that these are valid assumptions in 99% of all cases. I assumed that the problem is shore side simply because the differential still exist when the boat is completely powered down as demonstrated by the spark. Even when the boat has ground shorted to both neutral and hot... if the boat is powered down and the spark occurs, it is caused by a shore ground that is faulty.
I also assumed that the boat is indeed powered down so no genset running and batteries disconnected.

I often see a voltage of between 2- and 80V in shore power sockets between the neutral and ground leads (should be zero). The problem is often that the wiring is too small diameter and too long for the power transfer it is used for... leading to all that nastyness.

An isolation transformer deals with every possible AC grounding problem between boat and shore. It also deals with inverse polarity. The only thing left to check is measuring insulation value between ship ground points (preferable at every outlet and direct connection) and the water exposed grounding plate/metal; and between hot and neutral wires and the ground wire aboard.

A galvanic isolator doesn't protect against AC differentials and neither against big enough DC differentials (diodes go to conducting state and protection is void). This means that galvanic protection is also lost with an AC differential. It also doesn't improve safety aboard for the crew. I strongly disagree with the view that the isolation transformer is just an expensive replacement for a galvanic isolator. It's the other way around: a galvanic isolator is a cheap but flawed replacement for an isolation transformer ;-)

There was an extensive thread about this in CF before; the search function should come up with it.

In short: from here I diagnose the problem as a voltage differential between the ground supplied by the shore power console and the real ground, which is common. The marina blames the boat... what's new.

cheers,
Nick.
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Old 21-03-2010, 11:07   #11
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Yup, I didn't mean / don't want to turn this into a galvanic isolater vs isolation transformer thread again. That has already been discussed ad neaseum on this board.
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Old 21-03-2010, 14:27   #12
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Of course. Question remains if the OP's boat was indeed completely powered down when he noticed the spark between the shore ground and the ground pin of the plug on his shore power cable.
Let's wait and see what he comes back with ;-)

ciao!
Nick.
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Old 21-03-2010, 16:59   #13
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Well, I was there today but without the computer so could not verify the switch item. The marina electric is fairly new - 3 years so would not suspect it has poor wiring but maybe it does.
Yes, the boat was completely powered down and I only have a genset that was off - no inverter. So, tell me if I'm wrong but if I am feeding 2 bloody amps to the dock and my batteries are not running down, have I not invented the marine equivalent of a perpetual motion machine? The batteries are not losing their charge according to the built-in meter. I am feeding power out without losing any power onboard!
I blew a fuse on my meter accidentally in the wrong position so could not check ground to ocean, etc.
Thanks for all the replies. Will take a look at the switch next time I'm at the boat, later this week. Should I start a new thread or will you guys check in on me later this week?
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Old 21-03-2010, 17:19   #14
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A simple ammeter won't tell you 'which way' the power is going. You only know that 2A is flowing. There is a potential difference between your ground wire and that of the marina. You need to determine which side has the potential (voltage) difference from actual ground (seawater). That party has a problem and it very likely could be the marina.
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Old 21-03-2010, 17:27   #15
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Where would I start disconnecting things? I have a very well laid out and labeled wiring set-up. I am assuming, if it is me, that I have something wrong with the DC side. Wouldn't the breakers disconnect that specific unit from the batteries and hence the feedback? All my breakers are off. So again, if it is me, does this mean that it is something that isn't fused?
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