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Old 06-19-2012, 04:19 PM   #1
Lt. JG
Join Date: May 2010
Posts: 43
Default Reverse Polarity

I will try to give you all the information you need to give me an informed solution. If I leave something out please ask. I have a 2003 2700 SCR with a 5.7 Bravo 3. I have been away for two weeks and stop by the marina to check on the boat last night. After removing the cover I flipped the blower switch on to clear the bilge to start motor and realize I did not hear the blower on. I checked the meter on the master control panel and I had shore power, I then looked in the bilge and the ProMatic 30-3 meter read zero. I checked the breaker and it was on and so was the battery switch to BOTH. I put a meter on the starter and deep cell batteries and they were both dead.

I checked and the boat battery charger was getting 110. I made sure the battery switch was turned to the OFF position then I removed the battery charge and found that the internal glass fuse was blown. It was late so I plugged my home battery charger to the 110 on the boat (shore power still connected) and connected it to the starter battery.

Returned the next morning with a new fuse. Upon inspecting the home charger the red light was flashing. Per the manual that means replace the battery. I immediately unplugged the home charger. I then installed the new fuse in the boat charger. I flipped the battery charge breaker on the panel and nothing. I checked the meter on the boat battery charge and it still read zero. I removed boat battery charger and the fuse had blown again.

I ordered a new boat charger. I installed a new starter battery. Started the boat up and everything seemed to work fine.

After I shut the engine off I noticed the needle on the main panel was bouncing and a red light was on stating that their was reverse polarity. I shut off the breaker for the shore power. Shut the breaker off on the dock and checked both ends of the shore power cord and both were in all the way. Turned all breakers back on and still getting red warning light of reverse polarity. I then turn the battery switch to OFF and disconnected the batteries completely. Flipped all breakers for shore power back on and red warning light still on for reverse polarity.

Presently no batteries are connected, battery switch in off position and all shore power turned off. I plan on installing new boat battery charger tomorrow.

I also called the marina's manager and asked if they did any electrical work in the past two weeks. He said no. I told him what was happening and he is going to look into the dock power.

SO why am I getting the red reverse polarity light and what do you suggest I look at to fix this problem.

I thank you in advance for your help!

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Old 06-19-2012, 04:35 PM   #2

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Join Date: Nov 2005
Location: Everett Wa
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Reverse polarity is the opposite of normal polarity. Normal polarity in electronics is when you have the positive hooked up to the positive terminal and the negative to the negative terminal. Reverse polarity would be having the positive hooked up to the negative terminal and the negative to the positive terminal. The same concept can be used with magnets.

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.

To test it all out, you can use a multimeter capable of displaying AC voltage and test the prongs along the way. Start by testing near your reverse polarity indicator. Make sure you are reading a voltage between the green and white (ground and neutral) wires. This voltage between green and white indicates a reverse polarity situation. Keep on tracing this voltage back to the genset and/or the shore power in the marina yard. Trace your way back through the power cord, any adapters, and finally to the genset itself. (or start with the genset and work in the other direction if you really suspect the genset)

Also, before any of that, just plug the suspect cord into the marina power and see if you get the reversal. If you don't, it's the genset. You can confirm by testing the genset's neutral and ground to see if there is a voltage across them.

hope this helps..


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Old 06-19-2012, 07:34 PM   #3
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Join Date: Nov 2010
Location: Massapequa, New York, United States
Posts: 762

i would guess you have your shore power cable screwed in wrong.

If your panel is RED inside the boat, they your cable is bad. A little water will make it arch.

Try a different extension cord first, The RED panel is first in line from the shore, everything behind it should be fine.

If you have two 110v try one at a time.

everything else sounds like a symptom, not the problem.

the fuses are doing their job.
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