Reverse Polarity Problem

[c07chrisjohnson]

Aloha,

I recently moved to a new marina and started having reverse polarity problems. At my prior marina I left my shore power plugged in all the time charging the batteries on the DC side and running a de-humidifier on the AC system. These ran without problems indefinitely. I have a 1985 Hunter 34.

After moving to a new marina it seems as though after about 5 days my AC systems senses a reverse polarity and kicks everything off. The AC switches are left in their on position, but the reverse polarity light is on and everything is turned off including the battery charger, leaving the batteries dead.

I'm going to purchase a testing unit to try and figure out what is going on.

If anyone is knowledgeable about reverse polarity or has encountered a similar problem I would appreciate any advice.

Thanks!

 

[Benny17441]

Check that the prongs on the plug of your shore power cable are not fitting loosely on the outlet at the Marina power pedestal. A loose contact can trigger the "reverse polarity" warning. I usually carry a plug in tester to check the polarity of the power at Marinas before I plug in the boat. (nothing more that a regular household 15A outlet tester plugged in to a 30A to 15A adapter). The problem with reversed polarity is the danger for electrical shock from an ungrounded wire becoming hot. Alternate Current switches direction and it should not affect the operation of the appliances. Now some smart battery chargers may sense the reverse polarity on the supply side and turn themselves off as a safety feature. There are now too many smart fixtures in electrical systems that bring up faults that in years past would go unnoticed. There is no way we can tell whether the fault lies at the Marina or at the boat's end without testing both ends. Reverse polarity should not turn everything off, have you installed any GFCI outlets that could be tripping?

 

[Brian D]

Inside the boat, if you have a DVM (Digital Volt Meter) or even an analog meter, you can test any outlet to determine which side is hot and which side is neutral. You can do this also outside, but inside the boat is where you really are concerned about.

As you are looking that any outlet there are three plugins. If the ground plugin is on the bottom then the (might be) long plugin on the upper left is neutral. The hot plugin is on the upper right. With your volt meter, check between the hot and ground. You should get 120VAC. Checking between the neutral and ground will result in 0VAC. Checking between neutral and hot will result in 120VAC.

You can also check to see if your neutral and ground are connected together. Check with an ohm meter. Between neutral and ground you should read 0Ω. If you get something like OL, or no movement of the meter then there is no connection.

I am sure you already know this and checking at the dock connection will tell you right away. But there is the possibility that the boat is not wired correctly and may have been "corrected" to work with your last marina.

Good luck.

 

[c07chrisjohnson]

Benny17441,

I think the problem might be how the cable fits into the marina pedestal, it is definitely loose, and probably takes a couple of days to slide out to the point where it causes a reverse polarity issue. The plug mated to my old marina's power pedestal much better, there is a circular plastic piece that screwed into the former marina pedestal and held it in place, but the new marina doesn't have this same receptacle. I may need to research different types of power cords.

Brian D

I have a DVM and I'll have to narrow down the outlets, but I'm thinking the loose connection might be to blame since everything works fine for a couple of days then at some point starts the reverse polarity.

 

[jssailem]

A simple tester will help you identify if the wiring is the issue.

 

[jssailem]

My first thought went to loose plug. Then you stated that the plug felt loose. I’d follow up on that. It may be that the plug fitting is worn out and needs to be refit.

 

[Brian D]

So how would a loose plug cause a "reverse polarity" issue? AC is like digital: either it works or it doesn't. Power will not flow down the neutral line unless the hot line is connected. If the connection is loose then things will turn on and off depending on what is happening at the marina end. Reverse polarity means the neutral line is showing 120VAC with respect the ground and the hot line is at 0VAC with respect to ground.

I can see a loose connection causing fluctuating power issues, but not a reverse polarity issue.

JMHO

 

[Davidasailor26]

So how would a loose plug cause a "reverse polarity" issue? AC is like digital: either it works or it doesn't. Power will not flow down the neutral line unless the hot line is connected. If the connection is loose then things will turn on and off depending on what is happening at the marina end. Reverse polarity means the neutral line is showing 120VAC with respect the ground and the hot line is at 0VAC with respect to ground.

I can see a loose connection causing fluctuating power issues, but not a reverse polarity issue.

JMHO

Excerpting from Nigel Calder’s Boatowners Mechanical and Electrical Manual: “Wired as described, a reverse polarity indicator will sometimes come on even when there is no reverse polarity. This situation is most likely when some high-load AC equipment is operating in a condition where there is undersized wiring or resistance in the neutral connection to the shore. Instead of the neutral wire being held at ground potential by its shoreside connection to ground, it will experience a voltage drop from the load to the ground connection, resulting in the neutral on board being above ground potential. This may cause the reverse polarity indicator to alarm. So although you will not have a reverse polarity situation, you will have a situation that needs attention.” (Emphasis mine, as a loose connection would cause resistance.)

Basically, the reverse polarity detector works by checking for voltage between Netrual and Ground. If it sees voltage there it doesn’t know whether that’s from actual reverse polarity (Neutral carrying 120V) or because the Neutral is floating at some level above ground because of resistance in the circuit.

 

[Benny17441]

So how would a loose plug cause a "reverse polarity" issue?



JMHO

It just mimics a polarity issue for the indicator.

 

[Brian D]

Well then, I am still skeptical, but it is what it is. I truly believe if you have any voltage on the neutral side with reference to ground, then you have a serious shock hazard. I equate that to having current flowing between two ground points, or what is known as a ground loop. That too is a serious shock hazard.

If this is happening at your marina you need to disconnect and report it immediately.

 

[JamesG161]

For my own knowledge...

Could another boat near by, that has a AC Circuit problem, cause this issue?
Jim..

 

[Davidasailor26]

Well then, I am still skeptical, but it is what it is. I truly believe if you have any voltage on the neutral side with reference to ground, then you have a serious shock hazard. I equate that to having current flowing between two ground points, or what is known as a ground loop. That too is a serious shock hazard.

The shock hazard would depend on the amount of voltage, which would depend on the resistance and the load in the circuit. Voltages under 36V or so should be pretty safe, the same way you can touch the poles of a battery without being shocked.

Another explanation of this failure mode is available from Blue Seas at https://www.bluesea.com/support/articles/AC_Circuits/85/AC_Reverse_Polarity_False_Indicators

 

[c07chrisjohnson]

I ended up buying a new marinco shore power cable (it was time for a new one anyway). This one is able to connect to the marina power outlet and lock into the receptacle--solving the loose fitting issue. Since I replaced the cable on 4/20 I haven't had a reverse polarity issue so I think the problem is solved.

Thanks everyone for helping me out!

 

[Artey1]

Hopefully your issue is solved. I had a similar issue but my reverse polarity indicator light was causing me significant issues popping the pedestal over and over but not my AC main breaker inside the boat. I ended up solving this by replacing the old incandescent reverse polarity light with a led version of the from Blue Seas and the problem was solved.

 

[Tedd]

I usually carry a plug in tester to check the polarity of the power at Marinas before I plug in the boat.

Based on my experience with houses, that's an excellent idea. Every house I've ever owned has had at least one mis-wired outlet. (And one had far worse problems.) It wouldn't surprise me if a non-negligible proportion of shore power outlets are mis-wired.