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oft 06-01-2007 04:38 AM

Shorepower and Reverse Polarity
Perhaps someone can enlighten us about reverse polarity. Don Casey's tip (copied below) is clear enough, but still fails to explain our 'current' situation.

Earlier today we went for the first sail of the season. When we arrived at the dock all was well with the shorepower that we had been connected to over the winter. Upon our return from sailing I plugged us back in and the AC panel indicated reverse polarity so I disconnected shorepower. The reverse polarity indicator lit up once last year when one of the batteries died, but it has since been replaced and this time both batteries measured 13V. The only thing that changed was the boat that has the other outlet/meter on our electrical post had plugged in and was charging. A few questions:

1) Any ideas why after 1 year a reverse polarity problem would appear? Could it involve the old boat charging next door or are marine meters/outlets usually adequately isolated from one another?

2) What implications does reverse polarity have for charging batteries?

3) How can one adequately protect themselves from bad shorepower? Is an older Protech 4 enough or???


Don Casey Tip #40

Alternating Current/Reverse Polarity

Since alternating current, by definition, flows in one direction then the other, what is meant by polarity when applied to an AC shorepower connection and why is polarity so important on a boat? Even though the current flow reverses, the "hot" wire is connected to the generator at the power plant and the "neutral" wire is connected to ground there. That means the electricity flows to us through the hot wire. All switches and circuit breakers must be in this side of the circuit to disconnect the load from the power.

Now suppose connections to the dockside receptacle are reversed. That puts all the AC breakers on the boat in the neutral side of the circuit. An overload might still trip the breaker, but since the breaker is in the neutral side, the circuit is unprotected from a short. Current will continue to flow until the circuit burns open. A fire aboard is the likely consequence.

Reversed polarity also presents a serious shock risk. Turning off a breaker appears to remove power from the circuit because it turns off all appliances connected to that circuit. But with reversed polarity you have disconnected the appliance from ground, not from power. The circuit is stll live!

If your AC switch panel does not have a polarity tester, buy a plug-in tester and use it. Most also detect an open grounding wire and other dangerous conditions.

sailingdog 06-01-2007 07:21 AM


1) Any ideas why after 1 year a reverse polarity problem would appear? Could it involve the old boat charging next door or are marine meters/outlets usually adequately isolated from one another?
They may have re-wired something over the winter. That could have reversed the polarity of the wiring at the dockside, even though nothing appears to have changed.


2) What implications does reverse polarity have for charging batteries?
Maybe none/maybe a lot—the batteries will still charge normally in most cases. It depends on the AC-based charger you have. Some will not work if the polarity is reversed.


3) How can one adequately protect themselves from bad shorepower? Is an older Protech 4 enough or???
The real danger with reversed polarity is that the "neutral" wire is now "HOT" and on the side without the breakers. So if you decide to shut off the shore-power, but leave the cable connected, the "neutral" terminals at all of the outlets are still going to be live and electrocution is a good possibility.

Valiente 06-01-2007 12:03 PM

So that's what that little red light on the AC panel means...

donradclife 06-02-2007 10:12 AM

The little red light is hooked up between the neutral and the ground wire, and as the other posts indicate, it means somewhere in the shore power system the neutral and load wires have been switched. Stick your mutimeter (carefully) in the marina socket--Assuming you are using 110V, if you measure 110V between the neutral and the ground, talk to them about their wiring.

On a 15 amp North American circuit, the neutral is the wider blade and the ground is the round plug.

On a 30 amp circuit, the ground is the blade with the extra bend. Looking at the socket, if the ground is at 6 o'clock, the neutral should be at 10 and the load should be at 2.

USCGRET1990 06-02-2007 10:41 AM

If that other boat is not wired correctly, your panel sensor may be detecting it. I'd start by unplugging the other boat momentarily and see if your red light goes out. If that doesn't change anything, get a portable polarity tester and test the marina's outlet. If it shows it's reversed, then it's a problem they will need to correct.

Valiente 06-02-2007 04:47 PM

Guys, I was joking. I am new to the boat, but not new to the topic. My little red light is currently off (no pun intended) and I hope it stays that way.

sailaway21 06-03-2007 01:03 PM

And all it takes is one outlet with the hot wired to the neutral terminal to cause a problem. This condition can "backfeed" the entire neutral bar, energizing it, and in essence negating the breakers. Bears investigation soonest.

wlcoxe 06-03-2007 04:29 PM

Just had a similar problem. I installed a galvanic isolator during the spring prep period and it worked great, including at the dock after launch. Went for a short sail, came back, and the RP sounded every time I lit off the inverter. Dock-side sages said that blowing the isolator would do that. Took the isolator out of the circuit, no more alarm, will monitor zincs closely. Heard tell that there was a surge at the dock at some point, but it would have been over unless it occurred in the short time between plug-in and turn on. FWIW.
Bill Coxe, O40 Kukulcán, New London, CT

sailingdog 06-03-2007 06:46 PM

USCGRET has a good point. It very well could be a wiring problem on the other boat causing problems with yours. Unplug it and see what your boat reports. ;) If no change, ask the marina what they changed over the winter??

oft 06-07-2007 11:16 PM

Thank you for all of the input.

We managed to get back down to the boat and alas the problem has had the nerve not to self-correct (and since the boat next door was no longer plugged in we'll have a hard time continuing to blame it on them). When we briefly tried plugging our boat in again (she was left not plugged in), we found that not only did the reverse polarity indicator light up but also that we had no power at any of our 120V outlets. Attempting to throw and reset the circuit breaker and the GFI outlets had no effect. I'll explore further with the marina any changes they have made, but any answers to a couple more questions would be greatly appreciated:

1) What impact could it have on a boat if the marina wiring (30A) was changed so that when the boat was plugged in the ground, neutral, and hot wires got switched (e.g., if hot went to the neutral or ground wire on the boat)? Could these switched wires have taken out my 120V system (from what I could find there were no obvious burn marks, triggered breakers, etc.) and how?

2) Given all that can happen, is it the best policy to always leave one's boat connected to shorepower?

Thanks again.

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