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Old 05-12-2008, 03:33   #1
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Reverse Polarity in alternating current

For devices (not like motors)... such as bulbs and heating elements, what does reverse polarity mean or do? How is this "sensed"? If you have a lamp, and you reverse the hot and neutral wires the lamp lights just the same. Can reverse polarity be determined other than be looking at the wiring?
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Old 05-12-2008, 03:45   #2
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Quote:
Originally Posted by defjef View Post
For devices (not like motors)... such as bulbs and heating elements, what does reverse polarity mean or do? How is this "sensed"? If you have a lamp, and you reverse the hot and neutral wires the lamp lights just the same. Can reverse polarity be determined other than be looking at the wiring?
For heating elements, light bulbs (NOT fluoros or LEDs) and similar, polarity doesn't matter a jot. These things are basically just a piece of wire and the current running through them heats them up so it really doesn't matter which way it runs.

For more complicated things (radios, LEDs and in fact most things that run on electricity), it makes a great deal of difference. So don't do it, OK?
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Old 05-12-2008, 04:07   #3
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The most important issue is whether the power supply polarity is correct.

If reversed, the Neutral and Hot legs are switched. Then all your breakers interrupt the Neutral, not the Hot, so your whole system is energized even when the breaker is off. The breaker will turn of the equipment, but it is still energized. A fault can then start a fire, since the breaker won't sense the ground fault, and current will flow until something melts and the circuit is broken. There's also the danger of electrical shock, for the same reason.
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Old 05-12-2008, 04:11   #4
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AC polarity is easily checked with a device called a receptacle tester*. It simply plugs into one of the receptacles inside the boat and a series of LED's indicates if the polarity is correct or if the hot and neutral incoming wires are reversed, or if any of the wires in the circuit are "open" (not connected).
* Ideal Circuit Testers & Receptacle Testers

You can also test for proper AC polarity with a voltmeter:
With your Voltmeter connected between Black “Hot” and Green ‘Ground” you should get 120V; whereas Connecting your Voltmeter between White “Neutral” and Green Ground will NOT produce a 120V.

See also, Reverse Polarity Indicators (RPI):
http://www.cruisersforum.com/gallery...age.php?i=1717

And:
Reverse Polarity (AC)
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Old 05-12-2008, 05:55   #5
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I wrote this post because I have recently re wired my entire AC system including all new devices and wiring, breaker panel and transfer switch. I was very careful to keep the polarity of the wiring correct (it's not hard with the wires all color coded: black white and green.

So I was surprised when everything worked but the hot water heater showed a polarity reversal. The Blue Seas panel is equipped with a reverse polarity LED. This doesn't come on for the battery charger nor for any of the ooutlets in the circuit. I have not had the time to try and reverse the leads on the heater, but it struck me as odd that this would happen. The heater of course - works. No tripped breakers.

What's up?
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Old 05-12-2008, 06:36   #6
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... So I was surprised when everything worked but the hot water heater showed a polarity reversal.
... The heater of course - works. No tripped breakers.
What's up?
As you suspect, you've likely reversed the connections at the H.W. Heater.

Connect Black Line (Hot) wire to the terminal (or pigtail wire) coming from the High Temperature Limiter (cut-out) or Thermostat.
Connect the White Neutral wire to the unused screw terminal on the heating element, often marked with a white N.
Connect Green Ground to ground screw.
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Old 06-12-2008, 17:09   #7
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So I am about to reverse the black and white wires for the heater and before that I open the shore power receptacle and check that for polarity and that's fine and check the connections on the selector switch. All of these are not exactly "accessible" and discover the white lead was not secured. I tighten that up and lo and behold the polarity light goes off. The odd thing is that it wasn't really bright and so I am thinking that the weak contact made the polarity light come on. It's definitely off now, everything is tight and working. So there wasn't reverse polarity but perhaps some discontinuity in the neutral.

All's well that ends well. Now on to install a protective cover and tackle the next project.
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Old 06-12-2008, 18:47   #8
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It is interesting that most items still functioned, even with the poorly-connected neutral wire. I suppose the marginal connection was good enough for the low-current loads, but when the high-current heater was turned on the voltage drop across the poor neutral connection became excessive.

If the neutral wire was completely disconnected, nothing should have worked (at least not the 110V circuits -- a 220VAC load might not use the neutral wire).
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Old 06-12-2008, 20:01   #9
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The interesting thing is that several of the neutrals are wired to a buss, but this one was not so it was a but iffy. It sure had me scratching my head till I checked every connection.
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