Reverse Polarity

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Dec 16, 2007
Vessel Name
Old School
Vessel Make
38' Trawler custom built by Hike Metal Products
Greetings to all, especially those shivering in the PNW. Went to the boat yesterday and plugged it in to charge batts (NewMar charger). Came back today to unplug and it looks like nothing happened, at least according to the batt meter. So, looked at the panel (NewMar) and a little red light was on saying "If this light is on you have reverse polarity, unplug immediately", which I did 24 hours after the fact. I'm not an electricity guy, I swore years ago never to fool w/ electricity or brakes, and I haven't. Don't know if the fault is shore power or the boat; what is "reverse polarity"? I'm sure there is a check in my wallet that is earmarked for an electrician. Any thoughts out there?
Baton Rouge
Hey Mike -

I looked over Newmar's web site and can't find any unit with a reverse polarity indicator.* Can you give a model # or picture or something?

The two possible meanings of "reverse polarity":

- the positive and negative battery connections are reversed (that is, negative charger going to positive battery and visa-versa).* If this is a permanently installed unit, it's unlikely.

- hot and neutral are reversed on shore power.* This is more likely.*

Assuming you have a standard 120 volt 30 amp hookup, there are three leads:* hot, neutral, and ground.* Neutral and ground should have no voltage between them:* the neutral lead is intended as the power return, and the ground as a safety return.* The hot lead has the voltage on it relative to the other two.

Things will work with the hot and neutral leads reversed (that is, voltage on neutral and the hot at ground), but you have a serious safety hazard.* Since circuit breakers and switches are installed on the hot lead, they won't remove voltage from circuits when shut off.

So:* did the installation work at one time, and now doesn't work?* If you haven't had anything done to the boat's electrical system or shore power cord, something must be goobered up on the marina's electrical.

But the first question is, can you get more information on what the charger's definition of "reverse polarity" is?
Mike - I read your post that you have both a Newmar charger and that the circuit breaker panel is made by Newmar. If I am correct I believe you are saying that the "Reverse Polarity" indicator on the PANEL was lit - BAD !!! This means that your incoming power from the marina has a reversed hot and neutral wiring and you may not have any power to the devices on board. If you do not have one or cannot borrow one - go buy a simple "polarity checker" from your local hardware / box store. They are under 10 bux and you really should have one for when you are cruising anyway. They are just a small plug in unit that looks much like a "cube tap" but has three lights in the end along with engraved instructions on its use and what the lights mean. You should check back at the dock outlet and see if the checker gives it a clear bill of health - my guess is that someone changed the outlet recently and got the wires on wrong.
You should NOT use shore power until you are sure the polarity is correct and I always take my "checker" and plug it into the dock outlet whenever I am at a new facility.
I have seen boats burn due to this fault to say nothing of the potential for severe electrolysis.
Good luck
John Tones MV Penta
Sidney, BC
Chris & John, thanks for the info. Yes, the indicator light is on the NewMar panel, when I switched the panel on the little red warning light came on. I will go and get the polarity instrument that John mentioned and give the shore hookup a test. I'll let you guys know what I find. There was power to the AC appliances; the charger came on as did the refridgerator and GFCI outlet.
At* my last insurance survey, one of the items noted was that I didn't have a reverse polarity indicator. Had my marine electrician install one and it was less than $100... most labor. Have never had it "indicate" thank goodness. Matter of fact, the only time I had one go off was on my last boat, when I plugged in to the shore power outlet at a friend's dock. Oops. Oh yea, be sure to hook up a buzzer with the light, in case you don't notice it.

-- Edited by Keith at 06:35, 2009-01-14
Shouldn't be an electrolosys issue, but very definitely a safety issue in any case, since it does render circuit breakers useless against a short-to-ground fault.

I've not seen a polarity indicator for the twistlock plugs that we use for marina power in the states.** But you can get the ones that will plug into a standard outlet aboard the boat, so that's your cheapest solution (and so happens to be the one that I use).*
"Shouldn't be an electrolosys issue,"

DEPENDS , "reverse polarity" can be almost any remix of the wiring.

A useful boat reverse polarity unit will have an ALARM that you can hear as you plug into the miss-wired socket on the dock..

Quick check is with a 30A plug -15A adaptor with a hardware store reverse polarity tester shoved in.

Many times the problems come from a crappy electrical ends selection. The Charles and Marinco crap has SS fittings that have very low ampacity, NOT for rated loads* 30A.

The Hubbel may still use Monel, a nickle copper mix that will operate at its 30A rating.

Tho even IF you have good equippment the marina power pole may have junk, and full rated amperage not avilable.


-- Edited by FF at 05:50, 2009-01-16
"Nickel-plated phosphor bronze contact blades are manufactured to meet exacting standards and provide maximum corrosion resistance,"

Guess they got really tired of replacing stuff under warentee , and spent the extra 2c for a better product.

During the 22+ years I lived aboard , the only stuff that would last a winter (take the usual live aboard overloads till the dock breaker blew) was Hubbel.

No lab , just simple eyeballs watching neighbors running back to Defender (then only 5 mi away) with melted off power terminals on brand new plugs that didn't make a night.

Is this the same as "dielectric grease?" This is what we were advised to apply to the contacts in our shorepower connectors to combat corrosion and to eliminate the tiny arcing that can occur across corroded connectors which erodes the connectors and causes to fail even faster (I think I got that last part right).
*Is this the same as "dielectric grease?"

Yes.* A couple of precautions, though.

Dielectric means "insulating."* So do not put it between surfaces that have only a pressure fit (like a spade lug or battery terminal).* Plugs and connectors that have a wiping action when they mate are OK - they "bite" through the grease and make metal-to-metal contact with each other, then the grease protects them from corrosion.

There are also conductive greases - they're used extensively for terminating aluminum conductors in electrical wiring.* Same idea, except that you put it on the wire before crimping the connector on, and the thin film both conducts and protects.

You wouldn't want the conductive grease on a twistlock power connector since it would ultimately smear out and conduct electricity where you don't want it after the plug is inserted and removed several times.* But it would be a good thing to use on the wires that connect to the shorepower plug on the boat.
I've made lots of crimp connections filled with dielectric grease and they all work fine.

Also, I use Corrosion Block spray on shore power connectors, the back of my electrical panel, light bulb connectors, etc. Never any corrosion, overheating, etc. It's made for electrical stuff and does it's job as advertised.
Thank you all for the help. Bought the reverse polarity checker at Lowe's for about $5, checked the shore power hookup* and outlets etc. and all's well on Old School, except I still don't think my NewMar batt charger is working. Put my car charger on the batts and they came right up on the meter to 12.8v after a 3 hour charge.
I think I should run over to PMM's website and post a note under every discussion suggesting they come over here for help.
Dang Keith if you've got that much time to wait while each page loads, I've got some maintenence items I need you to do on my boat.

And if you have time to read a 4 page article that says "you'll have to decide", I know you have time to help me out on my little projects. Those folks can take a simple subject, like, should I upgrade my electronics, write 4 pages and in the end say, it all depends.

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