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Old 04-02-2009, 14:33   #1
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Reverse Polarity Indicator.

I was wondering if anybody can help me. I have a 2006 Bayliner 245 Cruiser. It has 110v shore power connection but I am in Ireland which is 220v. I run my mains cable from dock(220v) to my 110v transformer and then from that to the 110v connection on the side of the boat. When i do this the reverse polarity indicator lights up on my AC panel. Most appliances work ok( water heater, lights etc ) but the battery charger fried one of the batteries and I'm worried I will do more damage.
I have tried reversing the 2 wires in the plug top but the light still comes on. When I change around the 2 wires on the dock side it trips the circuit breaker. Has any body come across this? Is there any connections I need to switch inside the boat??? Any help would be great.


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Old 04-02-2009, 20:11   #2
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This is going to take some working out, but let's get started.

1. There are 3 wires in both your boat's 110v circuit and the dock's 220v circuit--a load wire, a neutral wire, and an earth wire.

2. In the boat circuit, the reverse polarity light goes between the neutral wire and the earth wire. Under normal conditions, the neutral wire should be within a few volts of the earth wire, and the indicator will not light. If the neutral and load wires have been switched, the light will see 110v, and will come on.

3. Now introduce a stepdown transformer into the circuit. Note that there are two types of stepdown transformers--one where the two sides are completely isolated from each other, and one with a common neutral wire, called an autotransformer. We don't know yet which type you have, but autotransformers are cheaper, and if you disonnect all the wires and find continuity between the input and output windings, you have an autotransformer, and an open circuit indicates an isolated transformer.

4. If you have the isolated type of transformer and you haven't grounded one side of the output, the voltages on both the 110v neutral and the load wires will float around, and both of them could show significant voltage differences from the earth wire, making the light come on no matter which way you have the wires (in the plug top??) connected.

5. If you haven't hooked up the earth wire from the dock, the boat earth could be also floating around, making the reverse polarity light come on. Being careful, check the voltages between all three wires on the dock side and on the boat side. If you don't feel comfortable doing that, its time to call an electrician.

6. The dock circuit breaker could be tripping due to a ground fault, which is more complicated to deal with, especially if you have an inverter/charger on board. You should also hook up a 220v pilot light between the earth and the neutral input to the transformer, which will tell you if your dock power has reversed polarity.

That's a start, but it will take some doing to sort everything out. The average Irish electrician may have a hard time with this one.

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Old 05-02-2009, 15:36   #3
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Reverse Polarity

Thank you for your reply.

I wont be back to the boat for a week or so. Assuming I have an isolation transformer, I dont understand what you mean when you say I have to ground one side of the output. How do I do that??

On the cover of the transformer I have it says 2ph+ Earth on the output.

I've already checked the voltage at the main panel inside the boat and between the Live and Earth I have 110v and between Neutral and Earth I have 110v, which is why as you said the reverse polarity light is on. I've had an electrician look at it but he couldn't solve the problem.

Thanks for your help
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Old 05-02-2009, 16:33   #4
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It would be the autotransformer which is a tapped winding which results in dividing the 220v into 2X110v supply rails. This will result in the indicator light not being able to do the normal indications as it will see either 220V or 110V. I think you could forget the light. You could confirm if you have an autotransformer or one with insulated primary & secondary windings by checking with the transformer maker.

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Old 06-02-2009, 05:02   #5
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Here is a link with a diagram showing how to ground isolation transformers--including stepdown isolation transformers--
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Old 06-02-2009, 13:36   #6
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Reverse Polarity

It turns out I have an isolated transformer and when i grounded it like in the diagram you showed me the light went off.

Thanks for ye're replys, U probably just saved me a fortune in electricians bills.

Regards Peter
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Old 06-02-2009, 14:52   #7
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As Donradclife points out (his #4 condition, floating ground) a digital voltmeter will measure 110V between neutral and ground because of the DVM high input impedance. If you us a 1000 Ohm resistor across the DVM leads it will not read 110V with a floating ground, but much closer to zero. If the voltage is more than a few hundred millivolts using the resistor then there may be too much current flowing in the ground lead.
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Old 13-12-2011, 22:18   #8
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Re: Reverse Polarity

Hi Peter,

I have the same boat as you which I have recently imported to Australia which is 240V. I would like to use an isolated transformer like you. Can you please confirm how you connected the output earth. Did you simply connect Earth to Neutral at the output of the Transformer? Also can you confirm what power rating you used? I was looking at a 300W unit as in this link eBay Australia: Buy new & used fashion, electronics & home d

Thanks for your help,

(SmartGauge Electronics - Isolation Transformers or Galvanic Isolator? 1/2).
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Old 14-12-2011, 03:36   #9
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Re: Reverse Polarity Indicator.

Greetings and welcome aboard the CF, Pablo.
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

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Old 14-12-2011, 08:34   #10
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Re: Reverse Polarity Indicator.

AC Neutral and AC earth are always connected at the power generation source. IN the case of dock side power with no transformer, this is done at the marina site or on the transformer pole, with any onboard AC generation, ( ie Transformer, generator, invertor, ) the ground needs to be connected to the neutral in the boat,

Unfortunately for items like generators and invertors , this leaves a quandary, as when the shore power is reconnected the neutral earth connection in the boat should be broken,

In the OP cases , in fact in the earth was originally connected through the shore lead, and a transformer was then inserted in the live and neutral only , the earth is no doing nothing, the common -mode voltage of the transformer would be enough to strike a neon and as the neon needs virtually no current, it would glow. In this case its not an indication of a fault.


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