Electric open neutral

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flyguy_helo

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Aug 1, 2009
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Near Montr?al, Qu?bec
Hi all,
Had a great season this year.
After reading articles about pedestals and miswired camping plug ins, I decided to purchase a voltage checking device.
After receiving it I went in my coach to try it out. First I plugged it in while connected to my house. The polarity and voltage checked out good as expected. I then ran the generator and again  got good polarity and good voltage.
Next I ran the inverter and checked it. The voltage was good but to my surprise it indicated an open neutral.
The generator and the inverter follow the same line to the main power. There is a manual switch in between so they cannot run together.
Is this open neutral normal?
If not what are the dangers? I have been running this way for 3 years now. How do I correct it?
Any input would be appreciated.

Thanks,
Larry

 

Gary RV_Wizard

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Feb 2, 2005
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At our Silver Springs FL home
Some questions:

What is the year/make/model of the RV?

What indicated an open neutral? Where and how did you check it?

Is the inverter a factory install or add-on? I'm guessing an add-on, since you mentioned a manual transfer switch.  What inverter make and model? Can you describe how the transfer switch is wired in? That seems the most likely place where the neutral could be "lost".

The generator and the inverter both supply their own neutral when feeding power to the circuits. So does shore power.  The generator and shore power share nearly everything electrically in the RV - just the source differs. The inverter, however, usually supplies only a subset of the circuits in the RV and does not flow through all the same paths.

With an open neutral, you have no usable 120v power at the outlets. Anything you plug in won't work. So, No, it is definitely not "normal".
 

flyguy_helo

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Joined
Aug 1, 2009
Posts
51
Location
Near Montr?al, Qu?bec
Hi,
The model is a four winds hurricane 2006.

The polarity was checked with a prime products ac line meter. It indicates proper polarity through a sequence of lights. In this instance 2 red lights indicate open neutral.

The inverter is an add on, It is an aims pure sine wave 2000 watts.

I used the generator line going to the transfer box at the rear of the coach. I put in a manual transfer switch in that line. The generator goes through the transfer box and down the same route it did before. The inverter goes through the same transfer box and follows the generators line to the rear thus feeding the same things the gene does. When I use the inverter I flip a few breakers off like the ac, the converter and the microwave. It works very well and there cannot be more than one source powering the coach.

I use my inverter for our toaster, coffee maker, tv and occasionally a battery charger. They all work normally.

Today however while using the ac voltage and polarity checker (for the first time) it indicated an open neutral when using the inverter as a power source. It was showing 124 volts at the different outlets I tried.

You mentioned there would be no voltage at the outlets but I do have power to each outlet and everywhere else. Is there danger of shoc?

I would also (as you suggest) suspect the added transfer box connection. What would be the proper way to make the connections in the box.

If I had a diagram I could verify if I missed something. I had followed the original paperwork that came with the box and all appeared to be working normally. It is only by chance while using this test device that I came across this open neutral indication.

Larry
 

Lou Schneider

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Mar 14, 2005
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The open neutral indication says the neutral is not bonded to ground.  This is normal with inverters that supply balanced power - i.e. neither side is referenced to ground as opposed to normal power with one line hot and the neutral line grounded.

Usually it's the cheaper inverters that do this.  It has to do with the negative side of the battery being connected to AC service ground because both of them are tied to the vehicle chassis.  Instead of having one line hot and the other grounded like normal power the inverter sends voltage out on both sides of the plug.  You get the two light indication because the neutral is not at the same voltage as the chassis ground.

When you're plugged into campground power, neutral is tied to ground at the campground's main power panel.  When you're connected to the generator the neutral and ground connection takes place at the generator.

The inverter puts out balanced power and it's output is isolated from the negative side of the battery, which also makes it isolated from the AC ground.  There's nothing wrong with this except it drives the 3 light testers nuts.  And no, you can't fix it by tying the inverter's neutral line to ground.  Doing this will short out half of the inverter's output stage.

In other words, there's nothing to worry about.
 

Just Lou

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Dec 25, 2005
Posts
8,105
Your inverter is designed to feed appliances that are plugged directly into the inverter output receptacles via their factory supplied power cords.  It is not meant to be integrated into your coach wiring.

The condition you observed, and the explanation and reasoning that Lou provided, is exactly why the warning and recommendation usually accompanies all inverters with  a receptacle output (rather than a hard wired interface) to NOT feed the inverter output into wiring generally fed by external power sources.

Using an inverter that is designed to be an alternate source of house power (usually one containing it's own transfer switch) will likely give you the option to bond the neutral to ground, or at least passthru the bonding, if you so desire.

You likely won't have a problem as long as you use it only as you described (and have verified), but future use, and future users, need to be aware of your installation choice.  JMHO
 

Gary RV_Wizard

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Feb 2, 2005
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73,979
Location
At our Silver Springs FL home
Look at the inverter install instructions - it usually says the inverter should have a chassis ground. If it doesn't, that may be the root cause.

Once again Lou S. gave a truly excellent overview reason behind what  is happening. We are really lucky to have him around!
 
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