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ContinuousWave: Small Boat Electrical
Reverse Polarity on Shore Power
|Author||Topic: Reverse Polarity on Shore Power|
posted 11-03-2005 01:35 PM ET (US)
Just bought my first boat, 28 CONQUEST. It's a 2001 with 90hrs. The port motor was a little difficult to crank on Sunday, but I did manage and moved it via water to a new marina. We plugged in the shore power and had a picnic - all systems worked. I left the shore power on overnight, and also forgot to turn off the battery power at the TABS box. No DC breakers were on though. Monday I went to check on it and the reverse polarity light was on. When only the shore power is on, the light goes off. When I turn on the batteries, whether or not the shore power is connected, the light comes back on. What's up with that?
I'm afraid to power it up and use my electronics to get it to the closest dealer (1.5hrs away by water) and I don't have a trailer. Could it just be a weak battery?
posted 11-03-2005 01:39 PM ET (US)
Reverse polarity wouldn't affect your DC system. This is entirely an AC problem, probably with the wiring on the dock. Just disconnect it and let the marina manager know that something is wrong.
posted 11-03-2005 03:03 PM ET (US)
It means that the neutral and hot are reversed. Things will still genenerally work, but it can be dangerous if you have a short in a piece of equipment as you can get an ebergized body of a tool, etc.
posted 11-03-2005 03:54 PM ET (US)
Then why does it go off when only the shore power is connected?
A reversed hot/neutral is detectable with only AC power -- I
have a little plug that does that.
And why is on with only the battery connected? No way it
I'd look at the box with reverse polarity light, get the
posted 11-03-2005 05:09 PM ET (US)
[Moved to this forum.]
It is hard to say precisely what is causing the warning light. The usual interpretation of "REVERSE POLARITY" on the shore power is to indicated the HOT and NEUTRAL conductors are mis-wired. The assumption is that this lamp measures the voltage between the NEUTRAL and GROUND. It should be 0-Volts.
Marina dockside wiring is infamous for being screwed up. There are many concerns regarding connecting to marina wiring with your boat's AC circuits. These literally can fill a whole book. It is way beyond the scope of a reply here.
Check the dockside wiring at the marina with another test device. Determine if it is properly wired. There is a good chance it may not be, or that there is a problem with grounds somewhere along the line.
A 2001 Boston Whaler is still under warranty. Call your dealer for help if there is anything which looks like a malfunction or failure aboard the Whaler.
posted 11-04-2005 11:12 AM ET (US)
For your reading enjoyment:
Given the circuit show in the first, I don't see why the
posted 11-05-2005 12:11 PM ET (US)
Thanks for the hyperlinks. They point to good information.
It is safe to say that the purpose of the reverse-polarity alarm indicator is to bring attention to potential problems (pun intended) with the shore side wiring. The first level of interpretation of a reverse-polarity alarm condition is to check the shore side wiring for problems, not the boat wiring.
posted 11-05-2005 12:23 PM ET (US)
This companion article (to the two mentioned above) explains how a reverse-polarity indicator can be falsely triggered. If our initial inquirer is still with us, he may find this applicable:
posted 11-07-2005 07:59 AM ET (US)
Interesting article on how a reverse polarity indicator LED could be flase triggered. I disagree with the conclusion that "the situation is not inherantly dangerous".
First of all the LED circuit is incomplete. There must also be a diode in series with the LED or it would burn out in an AC circuit. That being said, in order to generate even 0.1mA through the LED would require a voltage drop of more than 2.5V in the length of the neutral wire. Considering that most inexpensive LEDs used in this application would not be visable with less than 1.0mA, that would require a voltage drop of 25V in the neutral wire! A more plausable cause would be a poor neutral connection. This would elevate the voltage at point "B" and WOULD be an inherantly dangerous situation.
If the boat has an inverter circuit and the neutral bus connection is not a good one, this could be what is happening. If the shore power connection provided a good neutral conduction path the LED would be off when it was connected but when on internal power only, a high resistance neutral path would light up the LED.
posted 11-07-2005 04:40 PM ET (US)
There is a diode in the circuit: The LED.
LED = Light Emitting DIODE.
posted 11-07-2005 07:38 PM ET (US)
LEDs are diodes, but they have a low breakdown voltage and as such in an AC circuit they need to be protected with a second diode (with a sufficently high breakdown voltage) in series.
posted 11-07-2005 07:41 PM ET (US)
LED's conduct and emit light with one polarity but they cannot conduct in the reverse direction without being destroyed. They are diode junctions but have very little reverse voltage standoff capability. In a DC application this is not a problem, but in an AC application a second diode is needed to prevent reverse current from destroying the LED. Don't believe it? Hook up an LED with a series current limiting resistor directly to you battery with reverse polarity and see what happens. (Actually you won't see what happens :-))
posted 11-07-2005 07:48 PM ET (US)
Looks like I posted my response at the same time as "djahncke" who has it right.
The reason I think there could be a dangerous condition is that if the neutral bus has a high resistance path, there could be a localized hot spot in the wiring harness. This would be on the A/C side of an inverter.
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