inverters & ground

[ryancousins]

I have a very cheap modified sine wave inverter that I hooked up with a DPDT switch and proper fuses to be able to provide power to my camper outlets when shore power is not available. One side of the switch swaps the outlets between the shore power breaker and the inverter (with a fuse downstream of the inverter output), and the other side of the switch turns on power to the inverter as needed so it's not running when I have shore power available. The problem is neutral floats 60v above ground, which I know is common in inverters and generators. Obviously it throws the GFCI outlet in my garage that I have running to my camper as it can't deal with the floating neutral. I'd like to get a nicer inverter but when I look at specs for them I never see it listed if it has a proper neutral at ground potential. Is that even such a thing or are they all designed this way? Should I be concerned about floating neutral or is it not a big deal? What about the ground pin built in to the outlet on my inverter; do I just not use it, now? Doesn't seem safe.

 

[Gary RV_Wizard]

The problem is neutral floats 60v above ground, which I know is common in inverters and generators.

It's common for the neutral to float, but not for it to cause a problem (except for test equipment looking for that condition).  And the inverter should be wired to chassis ground on the RV.  I've had several inverters and generators that did not cause problems with the shore source.

I'm not clear how it affects the GFCI that feeds your RV.  You stated the DPDT switch takes the inverter out of the picture when connected to shore power. What is happening?

 

[Henry J Fate]

The GFCI is looking for a balance of power from hot to neutral. If the balance is thrown off, the GFCI will trip. If for example, you wired an appliance using the ground wire instead of the neutral, theoretically you would have a working appliance but the GFCI would immediately trip because of the imbalance between hot and neutral. It doesn't take much of an  Imbalance to trip the GFCI. I would suspect that somewhere you are losing current to ground.

I would first remove the switch from the circuit to see if it continues. Unplug all devices from the inverter to see if it trips. I would also double check all wiring. If all looks good then look at a bad GFCI, inverter or something other than the devices diverting power to ground. One other thing to try is to pre switch the DPDT switch to the inverter before plugging in. If that works, or if the GFCI stays good after a reset when switched to the inverter, it becomes likely a switch issue. Hope this helps.

 

[Gary RV_Wizard]

The GFCI is looking for a balance of power from hot to neutral. If the balance is thrown off, the GFCI will trip.

A GFCI is merely measuring the neutral current to assure it is identical to the hot current. Not sure how this "balance" effects that.  But more important, he states that it is the shore source GFCI that is tripping, and that should NOT be seeing any float at all. Where is the neutral current going to?

 

[Henry J Fate]

Yes it seems a little weird. I had a thought that maybe the switch is throwing it off. I think it will need a process of elimination to find it. It is not something that's easily detected. My other thought is the GFCI itself. I have seen plenty of those go bad from sitting out in the elements.

 

[John From Detroit]

Believe it or not it is really the ground that's floating

Now. this may or not work, but I assume this cheap inverter has an outlet or two
Try a "Bonding Plug"    These are commonly used with portable generators.
You will need a 3rd pole on your switch however. YOu can also do it at the switch instead of the plug

 

[Lou Schneider]

Try a "Bonding Plug"    These are commonly used with portable generators.

That's a great way to blow up a balanced power inverter (one where hot and neutral float 60 volts above ground).

Tying the neutral to ground connects it to the vehicle chassis, which also holds the negative battery terminal.  The output transistors on the neutral side won't appreciate working into a dead short.

 

[ryancousins]

I have continued to investigate and I'm at a loss. I know the GFCI outlet in my garage is working fine. When I have the switch set to shore power everything is fine. When I switch to inverter, the outlets in the camper will run fine from the inverter but it will throw the gfci outlet in my garage where my shore power is plugged in. I guess that's not the end of the world since if I'm camping where I don't have shore power, it's a moot point. I could always get a three pole switch and switch the neutrals, too, I suppose. But I'm still concerned about how ground works on these inverters. Since there is no real ground reference, how can it sense a fault and trip? Since neutral is isolated and sitting 60 volts above "ground", if either the hot or "neutral" lines in any of the outlets in the camper were to come loose and touch the camper chassis, what would cause the fuse to blow? I don't even understand how these inverters are legal. I would think all portable inverters would have to have gfci built into the outputs since there is no grounding conductor to handle fault currents. The ground pin on the output of the inverter is tied to the 12v negative. I don't see how that does anything useful.

 

[Gary RV_Wizard]

If that DPDT switch is working as you describe, there is no way anything the inverter does could affect the shore power source. It's not, and something is still connected to the shore line when you engage the inverter, so current is flowing back to the garage outlet, either on the shore hot or shore neutral. It's a good thing you have a GFCI there, cause you could severely damage the house circuits if it did not trip immediately.

 

[Tom_M]

It sounds like you are switching the 120 vac hot side and 12 volt DC. Since a GFCI has both hot and neutral going through it, you need to switch both the 120 hot and neutral, so a 3PDT switch will be required.

 

[Gary RV_Wizard]

The ground should not require switching, so DPDT should be sufficient. However, if some circuit is [improperly] completing via the 120vac ground instead of neutral, all bets are off.

 

[Tom_M]

The OP is using a DPDT switch. Apparently one pole is switching the 120 volt hot and the other pole is used to turn the inverter on and off. He needs to be switching both hot and neutral of the 120 volt. Thus he needs a 3PDT switch.