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Old 10-05-2019, 17:55   #1
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Join Date: Sep 2011
Posts: 22
Red Reverse Polarity Light

I never saw my 120 volt red reverse polarity light on my circuit breaker panal go on in all the years I've been living on board.
When we connected to shore power in a upscale marina the red light was glowing dimmly. Everything was running fine on my end so I didn't think must about it. The next day I mentioned to the dockmaster that my reverse light was on. He investigated and found the boat next to us had a 50 to 30 amp defective connector.
Two days later the light went on again in the evening while it was raining heavily. The dockmaster came to my boat at 1 AM to inform me that I was causing the reverse polarity on his boat and tripping his main circuit breaker.
I found my 50 to 30 amp Y-adapter was shorting out in the center. Eliminated the connector and everything was fine. Three days later the reverse light went on again, this time a boat four slips over had a receptacle that was nearly melting. This went on two more times.
Odd that my boat seems to be the troubleshooting tool for the marina and that I have possibly prevented a fire.
Has anyone else experienced this?
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Old 10-05-2019, 19:01   #2
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Re: Red Reverse Polarity Light

The way a reverse polarity light works, is it connects connects a light across the netural to the ground wire. It a properly working system, the neutral and ground are always at the same polarity, because they are connected together back at the marinas sub-panel.

The purpose of the reverse polarity light is to show a mix up in the wiring where the hot and neutral wires have be crossed somewhere upstream of your boat's panel.

But... the light will also show other defects. The neutral and ground SHOULD be at the same potential, but if there is high resistance somewhere down the line, then the voltage drop down the netural wire can be high enough to make your light glow. I would bet that the marina actually has a problem with resistance on their neural line.

Check the voltage reading from neutral to ground at the power post. It should be less than 5 volts or so. Anything higher is a problem with THEIR wiring.

There is no way that any problem in your boat should be able to trip another boat's circuit breaker!
billknny is offline   Reply With Quote
Old 11-05-2019, 09:24   #3
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Re: Red Reverse Polarity Light

Thank you for your thorough reply. I did check the voltage between neutral and ground and only found one and a half volts. The red polarity light was not on at that time. I will check again
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