Inverters with ungrounded 120 volt receptacles

E

Eddie702

Licensed Electrician
Location
Western Massachusetts
Occupation
Electrician
I had this question asked of me this morning. My cousin has a van they use for camping. It can be plugged in at an RV park and when that is the case the receptacles in the van are grounded 3 prong receptacles which the ground pin is grounded by the EGC that they are plugged into and everything is fine.

He also has a battery and an inverter they can use to power the receptacles when they are stand alone or not plugged into a power supply. He happened to plug in his receptacle tester, and it shows open ground at the receptacle and with a meter hot to ground is like 30 volts while running on the inverter

This got me wondering as I have a tiny inverter in my truck that plugs into the cig lighter, and I use the inverter which has 3 wire receptacles to charge tool batteries etc

I would assume that the receptacle ground pin should be connected to the vehicle chassis but maybe I am wrong.

What should be done in this situation? Feed it with a GFCI receptacle?? Or is there supposed to be a N-G bond?
 
Hard to know for sure from here but 30V hot-to-ground kinda suggests some electronics referencing ground to neutral. Hopefully serves a GFCI function or similar ground fault protection function, but who knows. Is the inverter UL listed? Does it have any documentation?

If you want GFCI protection then certainly running the output through a GFCI device is prudent and I think shouldn't cause a problem.

My guess is that such devices have electronic circuit breakers and that they won't feed a large amount of current into a hot-ground fault. But that is not the same as meeting a GFCI standard for personell protection. And also you never know with these types of things what kind of product standards they're designed to.
 
Eddie702 said:
Or is there supposed to be a N-G bond?
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When I opened one and put on a scope its a 60/120V system so definitely dont add a N-G bond, the choppy sine wave tricks your meter into reading 30V. There is a code article with guidance on 60/120 v systems 647 that indicates a standard GFCI is sufficient protection.
 
Eddie702 said:
Seems to be conflicting information about this. I gues protecting it with a GFCI would be a good idea. Any reason not to?
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Unless the inverter comes with GFCI, adding it probably won't work. The modified sine wave will almost certainly cause tripping. The same goes for UPS outputs too; GFCI's do not placy nice with synthesized AC output.


SceneryDriver
 
A lot depends on whether it’s a modified sine wave or pure sine wave inverter.

Modified sine wave inverters will show some odd stuff with standard meters. Most will only show 82-85 volts between hot and neutral. You’ll need a true rms meter to get an accurate reading.

As far as the grounding, I’m not sure. I always thought it was done internally through the dc ground. I do know that many of the higher end, pure sine wave inverters have a GFCI receptacle. The pure sine wave unit in my truck has one.
 
Unless, at the inverter, there is a N to G bond and the G is bonded to the vehicle frame how would there be a shock hazard for a GFCI to work with? It’s a “floating” system.

No more of a shock hazard than a battery operated drill. Picture a battery powering an inverter with a 120v drill plugged into it.

-Hal
 
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hbiss said:
Unless, at the inverter, there is a N to G bond and the G is bonded to the vehicle frame how would there be a shock hazard for a GFCI to work with? It’s a “floating” system.

No more of a shock hazard than a battery operated drill. Picture a battery powering an inverter with a 120v drill plugged into it.

-Hal
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Last vehicle mounted inverter I tinkered around with was a floating system I believe. Egc may have been bonded to chassis of inverter but it was a 60v/60v L to N I believe.
 
SceneryDriver said:
Unless the inverter comes with GFCI, adding it probably won't work. The modified sine wave will almost certainly cause tripping. The same goes for UPS outputs too; GFCI's do not placy nice with synthesized AC output.


SceneryDriver
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My point is that even if the inverter or generator comes with a GFCI it can’t protect you from shock unless you are in contact with the hot conductor of something plugged into the GFCI and the neutral BEFORE the GFCI to create at least a 15ma imbalance. There is no way for that to happen unless you contact the hot of something plugged into the GFCI and the ground- which IS connected to the neutral ahead of the GFCI. So, as far as I’m concerned, forget the GFCI receptacle and leave the ground pin UNCONNECTED. Now, the only way to get a shock is to contact both conductors which wouldn’t trip a GFCI anyway.

-Hal
 
jaggedben said:
Why would that be?
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Not really sure, but the one time we tried feeding a GFCI receptacle with the 120VAC output of a rack-mount UPS, it tripped immediately, even with no load attached. I didn't have time to put a scope on it or troubleshoot further. We sub-ed in a standard receptacle, and moved on.


SceneryDriver
 
SceneryDriver said:
Not really sure, but the one time we tried feeding a GFCI receptacle with the 120VAC output of a rack-mount UPS, it tripped immediately, even with no load attached. I imagine it had something to do with the not-really-a-sine-wave-but-close-enough that angered the GFCI sensing electronics. I didn't have time to put a scope on it or troubleshoot further. We sub-ed in a standard receptacle, and moved on.


SceneryDriver
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Eddie702 said:
He happened to plug in his receptacle tester, and it shows open ground at the receptacle and with a meter hot to ground is like 30 volts while running on the inverter
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With the typical electronic high impedance meter that 30 volts could be a "ghost" voltage that goes away when you put any type of load across it.
 
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