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Old 05-16-2013, 12:00 AM   #1
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Question for the electricians

Tonight I helped a friend instal an out drive on his searay. The boat is on the hard and when I bumped my arm against is other prop I felt voltage!

I got out my meter and shoved the negative in the dirt and positive to the prop and saw 50 volts ac. My buddy started turning off breakers looking to see it disappear. With everything off but the main it dropped to 22v. Turned off the main and it dropped to 6v. I'm suspecting the shore power plug but could use some guidance. I'm stumped!

Thoughts?
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Old 05-16-2013, 12:35 AM   #2
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I am not an electrician, but in addition to the shore power plug, I would check the generator/shore power switch. Also if there is an inverter, turn it off. It could bleed off AC from the batteries. Whatever you do, don't put it in the water until this is sorted out. There is enough power there to kill a swimmer or diver. Unplug the shore power. Good luck.
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Old 05-16-2013, 12:38 AM   #3
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Thanks moonstruck. No gen and no inverter.
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Old 05-16-2013, 12:47 AM   #4
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It could also be the marine or yard that is the problem. years ago a friend had a similar problem and realized it was the supply from the yard. He got sipped when crawling around beneath the boat and bumped the props.

When I unplugged the shore power cord and went from cord ground to real ground there was about 40 volts which meant the yard was the problem.

Don't assume it is the boat.

Do not ignore that it could be the boat. If it is the boat the consequences can be serious. As pointed out someone could be killed or leakage, real leakage, can quickly destroy metal components.
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Old 05-16-2013, 02:50 AM   #5
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Somewhere there is a short between the line and ground, probably on the boat. I say "probably" because these things can be very elusive, so it's good to never dismiss possibilities.

As a first step, I'd unplug the shore power cord and measure the impedance between the various plug pins that make up the shore power inlet.

Is this a 120V boat (i.e. a 30A inlet), or 240V (i.e. 50A inlet)?

When you measure, the line to neutral, and line to ground measurements should be open circuit. Try it with different combinations of breakers on/off as you did before.

If that checks out, then I'd take a look at the marina power, or better yet get the marina to look at the marina power.
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Old 05-16-2013, 11:55 AM   #6
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I'll pass on the info and thanks. I'm suspecting the boat because as this was happening and we were checking things out he mentioned that his zincs have been taking a beating lately.

I just real curious to solve this more from a personal perspective.. I have heard of a/c leakage before but have never actually experience it.
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Old 05-17-2013, 01:28 PM   #7
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I just real curious to solve this more from a personal perspective.. I have heard of a/c leakage before but have never actually experience it.

Use your volt meter and read the voltage between the white neutral and green ground.
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Old 05-17-2013, 05:35 PM   #8
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Thanks. I'm assuming you mean at the inlet?
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Old 05-17-2013, 06:28 PM   #9
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Just make sure that he doesn't put the boat back in the water with the problem unsolved. I'm sure you are aware that if a boat is leaking current like this it can attack and damage the props and through-hulls of nearby boats in short order unless they have an isolation transformer installed which very few recreational boats have.
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Old 05-17-2013, 06:31 PM   #10
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Yeah he's not going in the water for awhile yet! I did check out the galvanic isolator and seems ok.
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Old 05-18-2013, 07:24 AM   #11
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I have seen this quite a few times and it has been amateur electricians playing with stuff they should not or sometimes caused by chafe. It is very likely that the AC neutral is bonded to the AC ground somewhere which has a lethal potential not to mention severe stray current issues. Bonding neutral and ground might be OK in your house but not on your boat.

Plug the shore power cord into the boat but do not connect the shore end. Tuen on all AC breakers and put your meter on the ohms setting. touch the probes to the AC neutral and AC ground on the unplugged shore end of the cord. If you see any continuity there than thats what the issue is and needs to be addressed immediately. If you have G.F.C.I's onboard its a little different. You will see some continuity but as per ABYC it must be higher than 25kilohms.

This is a very serious issue and I highly recommend this article on "Electric Shock Drowning" and related issues.
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Old 05-18-2013, 04:45 PM   #12
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It's the cord. Glad we figured it out! Thanks for all your feedback.

Cheers,
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Old 05-18-2013, 08:33 PM   #13
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It's the cord? Was it wired incorrectly? Was this the cord he's been using when in the water? More info, please.
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Old 05-18-2013, 11:49 PM   #14
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Best we can tell is it must have deteriorated over the winter ( stored boat) but there was no ground connection so of course when plugged in it was looking for "ground" and found it when I put my arm against his prop. Bought a new cord, plugged it in, voila! I'm guessing this may have gone on longer then he's admitting, but the problem seems resolved.
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Old 05-19-2013, 07:50 AM   #15
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Originally Posted by jukesy View Post
...but the problem seems resolved.

The problem still exists, you have simply diverted the electrical leak to the safety ground where it is supposed to go when something has gone wrong.

The safety ground provides a better path to earth than you do, that's all.

Something is still wrong in the system and should be dealt with.
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Old 05-19-2013, 08:53 AM   #16
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^^^ +1

A good place to start.
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Old 05-19-2013, 09:34 AM   #17
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OK, so the OP's symptom was that he had voltage on his "wet" parts, In this case his props.

He replaced the shore power cord and according to him this resolved the issue.

We're now debating "how could that be?"

Here's how....

If, he has a neutral-ground short on his boat
and
If his shore power cord has a high resistance in the neutral wire
and
If his shore power cord has a high resistance in the ground wire

then

You would see voltage from the metalic parts of his boat to earth ground.

So the OP replaced the shore power cord, thats fantastic. I could see how a shore power cord could have a high resistance on both the neutral and the ground conductors (for example corrosion at a connector).

The problem remains that he probably has a neutral to ground short on his boat.

The new power cord will "hide" it because with a good connection on the neutral and ground conductors the metalic parts of the boat will be at earth ground potential through the new power cord and the shore power system.

Testing for this is not hard. Unplug the shore power cord and use an ohm meter between the neutral and the ground conductors on the boat.
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Old 05-19-2013, 10:04 AM   #18
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Quote:
Originally Posted by jukesy View Post
Best we can tell is it must have deteriorated over the winter ( stored boat) but there was no ground connection so of course when plugged in it was looking for "ground" and found it when I put my arm against his prop. Bought a new cord, plugged it in, voila! I'm guessing this may have gone on longer then he's admitting, but the problem seems resolved.

It shouldn't have found it if there wasn't still a problem on the boat. As stated, you still have an issue on the boat!
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Old 05-19-2013, 11:53 AM   #19
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The only correct way to measure these kind of problems is with use of a "Megger"......

MEGGER (for example the BMM2500 Series Premium Insulation Multimeters)

Insulation measurement up to 200 GΩ. (BMM2580) 200 mA Continuity Range Voltage, current and kΩ. resistance measurement Backlight mV Transducer inputs Result storage and Data Logging PowerSuite Compatible RS232 output Download Manager Software included Direct Printer Output Remote control switched probe IP54 Waterproof and Dustproof

Google on the words; megger BMM2500
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Old 05-19-2013, 12:48 PM   #20
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The only correct way to measure these kind of problems is with use of a "Megger"......
Yeah, right. Nothing like zapping half your installed equipment with 500 or a 1000 volts just to put a number on your electrical leak.

A megger has its place but this isn't it.

Put a voltmeter (or ammeter with a series resistor) between neutral and ground and start isolating equipment until the leak goes away. That will tell you which piece of gear is faulty.

What you do from that point is up to you. It is probably a cheap power supply or a surge protector. Get back to us when you find it ... or have a marine electrician look for it.
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