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Old 13-08-2016, 01:06   #1
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Temporary reverse polarity?

Hi all. I'm sitting at a transient marina plugged in to shore power. The fan is blowing. The fridge is running. All of a sudden everything stops. I check the panel and it has a red indicator that says Reverse Polarity. I flip the breaker a couple times, still says reverse polarity and no electricity to the boat. (I didn't even think to check it when we plugged in last night). After a little looking online without much success, I try again and all is now working. No light indicating reverse polarity anymore. Can anyone tell me why? And what I can do to prevent it again? It's really hot here... Must have the fan!


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Old 13-08-2016, 02:18   #2
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Re: Temporary reverse polarity???

plug into shore power that doesn't cut out. Check your manual to see when the reverse polarity warning appears (maybe open circuit on neutral or earth)
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Old 13-08-2016, 15:19   #3
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Re: Temporary reverse polarity???

Sounds like an intermittently bad neutral connection to the dock. Check the shore cord for signs of burns on the plugs.
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Old 14-08-2016, 12:48   #4
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Re: Temporary reverse polarity???

I think it's also possible that there is an intermittent neutral connector in your panel. I had that happen where a neutral that was poorly crimped came undone, and the reverse polarity alarm sounded.
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Old 14-08-2016, 13:51   #5
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Re: Temporary reverse polarity???

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I think it's also possible that there is an intermittent neutral connector in your panel. I had that happen where a neutral that was poorly crimped came undone, and the reverse polarity alarm sounded.
That's possible. Basically if the neutral becomes disconnected it will rise to almost the same potential as the hot wire, which is why the reverse polarity light illuminates.
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Old 15-08-2016, 08:45   #6
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Re: Temporary reverse polarity???

A lot of things, probably most, will work with reverse polarity. It's just not a safe thing. So it sounds like you lost power along with the reverse polarity.

As noted above, check the shore power cord (both ends) and the dock pylon outlet for signs of corrosion or black from heat. Replace your cord if you see any. At least replace the end(s) and cut back to perfect wire. Better to just replace since the ends aren't cheap either and don't seal well.

As noted, check ALL your wiring from the shorepower inlet (including the wires connected to the inlet) to your panel. Check for loose connections, burnt parts, at the master breaker close to your inlet (per standards although you may not have this) and the main breaker at the panel. Also check all the wiring on the panel itself while you are at it. Make sure to turn off all power (unplug the shore power and disconnect the inverter if you have one).

The PO of my last boat had had an actual fire behind the main AC panel. This didn't show up on the survey since it "not responsible for inspecting any parts requiring tools to access". This was not conducive to good electrics.

I have found many surprises working on other boats too from questionable wiring practices by prior techs and/or owners. Good idea to check all wiring over time as soon as you can get to it.
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Old 16-08-2016, 12:21   #7
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Re: Temporary reverse polarity???

Thanks for all of the replies. There was no sign of burnt wire or damage to my cord or boat. I haven't gotten behind the breaker panel yet. I suspect it was a problem with the marina pedestal power. No recurrence of any problems since getting home to my slip. Fortunately, I'm going to be redoing my console and will be getting deeper in the wiring then for a better look. Thanks again for all of your thoughts!


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Old 16-08-2016, 19:03   #8
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Re: Temporary reverse polarity???

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Thanks for all of the replies. There was no sign of burnt wire or damage to my cord or boat. I haven't gotten behind the breaker panel yet. I suspect it was a problem with the marina pedestal power. No recurrence of any problems since getting home to my slip. Fortunately, I'm going to be redoing my console and will be getting deeper in the wiring then for a better look. Thanks again for all of your thoughts!


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Could well have been a pedestal problem, or maybe just the shore cord not fully inserted and locked. (Those 30A plugs are notorious for not locking properly due to the receptacle being recessed slightly behind the cover.)
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Old 17-08-2016, 16:36   #9
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Re: Temporary reverse polarity???

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Originally Posted by sgumedic View Post
Thanks for all of the replies. There was no sign of burnt wire or damage to my cord or boat. I haven't gotten behind the breaker panel yet. I suspect it was a problem with the marina pedestal power. No recurrence of any problems since getting home to my slip. Fortunately, I'm going to be redoing my console and will be getting deeper in the wiring then for a better look. Thanks again for all of your thoughts!


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Most often the cause of reverse polarity is the marina dock pedestal. And given your description of not having it at your own slip makes it seem even more likely. That's the reason to check the polarity indicator before flipping on the AC master breaker at the panel. The polarity indicator should be wired on the incoming side of the master breaker. Have fun tearing in to your wiring. I'm completely replacing my AC and DC panels and straightening out the wiring while I am doing it. It was a mess before.
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Old 17-08-2016, 17:47   #10
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Re: Temporary reverse polarity?

A loose "neutral" connection somewhere will create this exact problem. As boats age things loosen up. We had a loose neutral wire to our fridge and did not find it until I noticed a spark behind the breaker panel when the fridge (AC powered) kicked in. Check all the wires leading up to the polarity indicator including the shore power cord connectors. The standard 30A connector is notorious for going bad at hot and/or neutral terminals. Look for signs of arcing or heating on the boat side prongs and where the cord plugs into the pedestal.
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