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Old 27-08-2006, 07:53   #1
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Reverse Polarity Light

I completed the complete re-wiring of my boat a few months ago and just recently completed the AC. However, under certain circumstances, I am getting the "Reverse Polarity" light on when I connect the shore power at my marina.

My Ac is run into a Blu Sea Systems AC/DC panel and before it gets there it goes through a Galvanic isolator and a breaker box.

Now the first time I plugged it in, NO Reverse polarity light came on until my son was running an electric drill. I noticed the RP light came on when he stopped. I turned off the AC at the panel, flipped the breakers, and when I turned all back on the light was still there. Then I went out to the Marina breakers and flipped those on and off, that did the trick. It happened again yesterday and it is intermittent. I am %100 sure the wiring is correct but I am re-checking it.

Does anyone have any ideas what may be causing this? Or how to fix it.???
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Old 27-08-2006, 10:26   #2
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Check the ground/common to see if there is any feedback (voltage), starting at the dock.

If someone in the marina has a dead short it will feedback into the ground/common.

If not on the dock, then check your boats ground/common while disconnected from the dock, assuming you have an inverter.
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Old 27-08-2006, 13:49   #3
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Do you also have an Earth Leakage sensing Device??? We call them RCD (residual current device) Ummmm, I think you guys call them something else.
If you have a leakage, I would ahve expected that device to have tripped. The galvanic Isolator has basicaly removed you from the shore Earthing system. But there could be some stray current finding it's way back to the shore system somehow. It may not necessarily be a fault at your end. However, I would expect a ground fault interuptor to trigger if it detected a stray current back.
If there is a stray current returning via the water, then you are also going to have anodes disapearing quickly.
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Old 27-08-2006, 14:49   #4
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In the US, I think what you are talking about is called a "Ground Fault Interrupt" or GFI device. Now under ten dollars and available in many forms (inline, outboard, in extension cord, etc.) from many sources, they will trip out a breaker and shut power if there is leakage current to the ground side of the circuit.

But if Alan P. is going through a galvanic isolator...isn't that a transformer which is *totally* isolating his hot and neutral lines, so that a reverse polarity in the marina side shouldn't be any concern for him? (Although worth warning the marina about.)
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Old 27-08-2006, 18:12   #5
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I checked all my AC wiring and it is all connected correctly at every device, ie shore power receptacle. outlets (all GFI Interrupt protected), glavanic isolator and the breaker box and the Blue Sea panel.

However, my glavanic isolator has a place for a 12V connection and I had not connected that yet. I may be stupid but I didn't think it would make any difference as all the isolator does is pass thru the ground and separate the neutral AC. I figured the 12VDC was just to run the LED's. So I hooked that up and it seemed to solve the problem! Why? I dont get it.

Also even though the RP light was coming on my GFI protected outlets were not tripping and they trip with 5ma I think.???

So I am still confused and my Galvanic Isolator has a steady green LED now.

Alan
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Old 27-08-2006, 20:20   #6
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I'm not certain about this, but AFAIK the GFI will only trip when there is a GROUND FAULT, that is, a connection to the THIRD WIRE, ground. Reverse polarity would not affect that. On the AC side you've got hot, neutral, and ground, and RP should mean the hot and neutral are reversed--without indicating anything about ground.
That could just mean someone connected those two wires to the wrong sides of an outlet someplace, which can be dangerous but also go unnoticed and without any harm for years.

Could it be your galvanic isolator needs 12V to power the electronics in it which are lighting the lights and making the decisions? So the readings without 12VDC were bogus? (Got a manual? Or try calling the maker?)
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Old 27-08-2006, 22:11   #7
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To understand the problem you have to understand what the "reverse polarity" light is and how it is hooked up.

It is connected between the netural and ground. If there was a true reverse polarity at the shore connection, the netural would be at 110 volts compared to ground, instead of at the same potential.

What you are seeing is a volatage between netural and ground enough to light the LED. This is most likely caused by a high resistance connection somewhere in the netural line anywhere between the panel and the earth ground at the marina. These can be a nightmare to track down, because they only show up under load. Put a voltmeter between your netural and ground, you might see a few volts (<4) because of the galvanic isolator, but no more than that.

If you can get into the shore power box, test there next with as much power draw into your boat as you can pull. If you see the same issue there, it is the marina wiring. If it is OK there, it is somewhere on your side. cable, plug, connection inside the boat.

The problem is NOT in the wiring in the boat AFTER the panel.

Good Luck!
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Old 28-08-2006, 02:27   #8
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From Blue Sea Systems:

Reverse-Polarity Indicators:
http://www.bluesea.com/Article_detai..._ID=290&id=301

AC Reverse Polarity False Indicators:
http://www.bluesea.com/Article_detai..._ID=290&id=296

HTH,
Gord
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Old 28-08-2006, 15:23   #9
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Alan;
You may be interested in the article on shore power by Nigel Calder in Professional Boatbuilder. Go to

http://www.proboat-digital.com/proboat/200604/

then at the bottom of the page select the "Shore Power Safety Tests". I was particularly struck by the comment in the article about GFCI towards the end relating to trying to protect the whole boat with a 5 ma GFCI will almost certainly result in nusiance trips due to humidity.

hth,
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Old 29-08-2006, 20:07   #10
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Quote:
Originally Posted by hellosailor
Wheels-
In the US, I think what you are talking about is called a "Ground Fault Interrupt" or GFI device. Now under ten dollars and available in many forms (inline, outboard, in extension cord, etc.) from many sources, they will trip out a breaker and shut power if there is leakage current to the ground side of the circuit.

But if Alan P. is going through a galvanic isolator...isn't that a transformer which is *totally* isolating his hot and neutral lines, so that a reverse polarity in the marina side shouldn't be any concern for him? (Although worth warning the marina about.)
It is interesting how the Ground Fault Interrupter actually works. It does not sense current in the ground wire; that would not detect current flowing through a hair dryer, into water in a bathtub, and down the pipes back to ground. It compares the current in the "hot" wire with the current in the "neutral" wire. If they don't match, some of the current must be getting out somewhere else -- this is called a "ground fault" in the US. It doesn't matter where it is going, since anywhere other than where we expect it is a problem.


The galvanic isolator is not a transformer. It is made of silicon diodes. This diode only conducts current in one direction. It also does not start conducting until the voltage across the diode rises to about .7 volts. To make a galvanic isolator, you put two diodes in a series for a voltage drop of about 1.4 volts. Then, because you have AC, you put two more in parallel with the first two, but in the opposite direction. Put this device between your on-board ground and the ground pin of the shore power connector. The hot/neutral wires are connected normally.

The process of galvanic corrosion makes a battery. In this case, the battery is made of some part of your boat, the sea water, and some part of another boat or the marina. If you connect the two terminals of the battery (through the AC ground wire, because both boats are grounded), current will flow and the metal will corrode.

But with the galvanic isolator, the voltage caused by the corrosion is not high enough to make the diodes conduct, so no current flows. It is as if you have disconnected the ground wire.

But if something happens on your boat that causes a short-circuit to ground, suddenly the ground wire has 120 or 240 volts on it. 120 is much greater than 1.4, and the diodes conduct, carrying the full short circuit current. If you have a really good short, the diodes only have to conduct long enough to trigger a fuse/circuit-breaker somewhere. If you have a high resistance short, the diodes may be called on to carry a high current (but less than the fused value) indefinitely.

If you have a short to ground, your boat is not protected from corrosion for the duration of the short. This is a minor consideration under the circumstances.

The nice thing about a galvanic isolator is that it costs much less and weighs much less than an isolation transformer.
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Old 29-08-2006, 21:23   #11
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Very interesting, Coot. So...a galvanic isolator is useless if there's more than a 1.4V difference to ground?! In the real world of less than gold plate marinas, are they really of any use at all then??
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Old 30-08-2006, 01:22   #12
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No the Islolator is doing it's job. If you have a fault to ground, you WANT the Isolator to conduct, or you would have no ground path for the fault current to flow and the result would be the appliance or what have you would then be live and lethal. When all is well, there is NO current flowing in the ground conductor, so the forward voltage of the diode means no voltage path for stray galvanic currents to be able to flow. Galvanic currents are very low. Much lower than the 1.4V. Well we would really hope so, or your metal parts would be disolving away before your eyes.
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Old 30-08-2006, 10:51   #13
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" When all is well, there is NO current flowing in the ground conductor, " The thing is, I've seen home wiring with 4-7VAC difference between the neutral and the ground, and the owners and utility co each just kinda say "Yeah, sometimes that happens, its wrong but it's not our problem."

Seems like the deeper I look into "home" AC wiring, the more differences in opinion I find about it. Like our brand name electrical company says "We're only required to supply power for heat and light." (Ahuh.)

Then again, sometimes we find old cotton cambric covered housewiring in this part of the world, too. On porcelain posts, too!
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Old 30-08-2006, 13:49   #14
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Sorry I wasn't clear. I was refering to any current that was flowing in the ground as being Galvanic due to two boats tied together via a cable. Because this current is "normally" so low, the diodes effectively isolate that very small galvanic current.
Just to go back to that lug extra lug on the isoltaer that was tied to the DC system, it is possible the light was being lit due to PD in the cables within the boat. Tieing to a point on the isolator efectively cancels a path that a PD could occur via.
In the audio world it was very easy to find those little gremlins of stray currents. They showed up as hums in the system. It didn't make the cure any easier though.
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Old 30-08-2006, 14:21   #15
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"In the audio world it was very easy to find those little gremlins of stray currents." Please, don't mention audio.<G> I thought I'd do some "easy" transferring of old music to the computer, haha. First there was buzz. Then I got rid of the buzz with an attenuator in the patch cord (from the tape deck) and instead picked up...ticks and clicks that are in sync with the heartbeat light on the SCSI bus controller.

Sigh. It's always something, I don't have the hear to start gutting a computer in order to chase "well its not buzz".
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