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Old 01-11-2016, 08:37   #1
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Faulty reverse polarity light or not?

Here's my situation, my boat has two, 30 A inlets going to a shore power main breaker panel with a breaker for each leg and indicator lights for reverse polarity. Shore power #1 will indicate reverse polarity with a red light sometimes, but not always. Shore power #2 is fine, green light, all normal. I have a plug-in tester and each of my outlets read correct polarity, both port and starboard. I have tested the marina's plug and that reads correct. Where am I getting this reverse polarity signal? My initial guess is that the water heater or battery charger may have the hot and neutral reversed but I haven't had time to check it.
Another thing that may be an issue is that there is a jumper breaker that ties shore power #1 to shore power #2 when only using one cord to plug in.
Thoughts? I know there could be many issues and I am new to this boat, so still learning its secrets
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Old 01-11-2016, 10:36   #2
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Re: Faulty reverse polarity light or not?

My main breaker panel isn't too dissimilar to this one from Paneltronics, just the top half, I have a large distribution panel that is separate for both the AC and the DC. No generator installed
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Old 01-11-2016, 10:39   #3
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Re: Faulty reverse polarity light or not?

A possible cause of polarity flicker is corrosion or poor contact on the neutral wire between the polarity light and the shore side ground.
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Old 01-11-2016, 10:44   #4
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Re: Faulty reverse polarity light or not?

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A possible cause of polarity flicker is corrosion or poor contact on the neutral wire between the polarity light and the shore side ground.
Thanks. I will be opening things up this weekend and will troubleshoot for corrosion at those points, as well as the water heater.
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Old 01-11-2016, 10:57   #5
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Re: Faulty reverse polarity light or not?

A reverse polarity light is simply a lightbulb between the neutral and the ground wires. If it lights, there is significant voltage between neutral and ground (which shouldn't be there). This can be caused by reversing the hot and neutral wires somewhere in the dock or boat. It can also be caused by a floating ground wire--in this case the bulb will not be as bright and can flicker.

I'd start at the dock outlet socket and measure the voltage between the ground and neutral. The fact that it doesn't always light points to either some miswired docks or a faulty ground connection
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Old 01-11-2016, 18:30   #6
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Re: Faulty reverse polarity light or not?

You can get a weakly illuminating reverse polarity light caused by voltage drop in the neutral conductor. Typically this is the result of heavy load and a worn shore plug our boat receptacle.

It can also happen on older dock wiring, again caused by voltage drop in the neutral conductor.
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Old 01-11-2016, 21:31   #7
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Re: Faulty reverse polarity light or not?

if you swap cables on the boat side (so swap cords and dock pedistals) does it still happen only on line 1?
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Old 02-11-2016, 02:56   #8
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Re: Faulty reverse polarity light or not?

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You can get a weakly illuminating reverse polarity light caused by voltage drop in the neutral conductor. Typically this is the result of heavy load ...
Indeed ➥ https://www.bluesea.com/resources/85
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Old 02-11-2016, 03:54   #9
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Re: Faulty reverse polarity light or not?

Boatbod and Gord, thanks! I will try to determine if this is the cause or not.
Trans, It only happens on shore power #1, when I switch plugs/cable, switch #2 still shows a green light and #1 an intermittent red.
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Old 02-11-2016, 10:23   #10
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Re: Faulty reverse polarity light or not?

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A possible cause of polarity flicker is corrosion or poor contact on the neutral wire between the polarity light and the shore side ground.
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Old 15-11-2016, 20:46   #11
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Re: Faulty reverse polarity light or not?

My reverse polarity light came on today as well. From the information I read it was either a dock wiring problem or a contact somewhere in my boat between the neutral and ground wire. The light first came on while I was cleaning the exterior with fresh water. I reset breaker at dock and boat. System still indicated reverse polarity and none of the boat outlets work when the warning light is on. In my troubleshooting I noticed the plug was a little loose at the dock (it does not have the threads to tighten my cord down with). I wedged a triangle cut board in there to get it nice and flush. Reset system and now light is off. Everything works good again. Marine hand told me they will be upgrading outlets in a few years to threaded type. I feel this is only part of my issue though. Somewhere I must have reversed polarity for the light to ever come on, correct?
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Old 16-11-2016, 01:25   #12
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Re: Faulty reverse polarity light or not?

No, if the problem is in the neutral connection then that is all it takes to light the light and stop your outlets from working.

The screw ring doesn't help make the connection. It just helps keep the plug from falling out. I would check the prongs of your shore plug for signs of heating (blackened rubber or pit marks in the prongs). An intermittent connection in the neutral can lead to over hearing and a fire. If your prongs are in good shape ask the marina to replace the socket on the dock. These 30A sockets are notorious for wearing out over time and starting fires.
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Old 16-11-2016, 02:45   #13
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Re: Faulty reverse polarity light or not?

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.....Shore power #2 is fine, green light, all normal.....
What loads are on shore power#2? If you load that up heavily that might show a dull red light too.

As others have said its usually down to corrosion or even badly sized cables causing a voltage drop on the neutral wire.

The reverse polarity can light up with only a 5v drop on the neutral. You can be on the start of the pontoon and have no problems. On the end of the pontoon with all the other boats with aircon running the red light may be on dimly.

Because no current flows through the earth this will shows as a small voltage difference between Neutral and Earth - which lights the light.

Elecktrickery is very confusing!!!!
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Old 16-11-2016, 03:40   #14
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Re: Faulty reverse polarity light or not?

It doesn't take much voltage to get a LED to glow.

I had a similar problem tracked down to a cheap extension cord that was left plugged in and when the receptacle end got wet, it lit the reverse polarity light.

The marina receptacles could be OK, but any cord plugged into them could also be an issue.

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Old 16-11-2016, 03:45   #15
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Re: Faulty reverse polarity light or not?

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What loads are on shore power#2? If you load that up heavily that might show a dull red light too.

Elecktrickery is very confusing!!!!
At the moment, there isn't much of a load on shore power #2, mainly just port side outlets at the moment. In the future, the air conditioning load will be on this circuit. Shore power #1 has the water heater, starboard outlets and microwave and we will usually plug in a small ceramic space heater when the temps drop.
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