From the electrical safety post! Great write up by Mr. Bean.
There is no “simple” or straightforward answer, and the explanation will probably confuse some people. Instead of giving direct advice, let me just
try to explain how inverter (UPS) and portable-generator outputs differ from your utility power. Sorry, this will be long and a bit technical, there is no way around it.
Most consumer UPS units and small generators have a
floating neutral.
That means the neutral isn’t tied to earth ground. Its only reference is the hot. In other words, the only place current can flow (in theory) is between the inverter’s HOT and its own derived NEUTRAL.
Think of a battery. The only place current can flow is between its two terminals. If you touch only the positive terminal, even standing in a puddle, nothing happens. A floating inverter output is basically the same idea (ignoring AC vs. DC). Its two output terminals can only (in theory) pass current between
each other.
With your utility supply, the NEUTRAL is
bonded (electrically connected) to EARTH GROUND at your service panel. So if you touch utility HOT while you’re grounded, current
will flow from HOT through you to GROUND, partly or fully bypassing the neutral. That’s the fault path a GFCI is designed to detect.
With a floating source, touching HOT while grounded doesn’t create that path. There
is no path to ground. So (in theory) you don’t get shocked.
“Hey Bean! That sounds great, my UPS or generator can’t shock me!”
No. It can absolutely shock you!!
If you grab HOT and NEUTRAL from the floating source, you
will get shocked. You just become the load.
In a bonded system you would also get shocked if you grab hot and neutral. The difference is crucial to understand:
if you are grounded, a GFCI would trip because some of the current leaks to ground. In a floating system, there is no neutral-to-ground reference at all, so the GFCI has nothing to compare and will not trip.
If you are grounded is a very important caveat. In a bonded system if you are insulated and touch hot and neutral, you also become the load and the GFCI can't help you.
So on one hand, the floating system can be called “safer” because the typical line-to-ground fault doesn’t exist. On the other hand, there is zero protection for a line-to-neutral fault, and the system is not GFCI protectable.
I leave it to you to decide if it is safer or more dangerous overall. I think many who understand these systems would argue that line-to-ground faults are more common (frayed hot wire, hand in water, feet on floor... or face against faulting fixture, hand in grounded tank) and these faults are far less likely in a floating system. They would likely point out that in a floating system, just as in a bonded system, the GFCI offers minimal protection for line-to-neutral contact and only helps in the line-to-neutral scenario if the person is also grounded.
“Hey Bean! What is all this ‘in theory’ nonsense?”
We say “in theory” because depending on how the source is built, some tiny amount of current
can leak to ground — transistor base drive, generator excitation, internal filtering, other complex faults, etc. So even in a floating system chances of ground fault are not mathematically zero and from a safety perspective you should not treat the floating output any differently.
"Hey Bean! What about using them anyway for when the UPS or power station is delivering mains power?"
Well that depends on how your UPS or power center is built. You MUST check the documentation. Many UPS system will pass neutral through (bonded) when on Mains Power and switch to floating when on battery. So on mains power, the GFCI behaves as expected,
However many power centers like Jakery, or Bluetti or EcoFlow derive their outputs from their inverters 100% of the time, so they are always floating.
After a lot of research, it seems a GFI, even without a ground would work on a battery back up
So am I correct they only are concerned with current flow ie difference ?
They only care about the imbalance between hot and neutral. That part is correct.
But the key point (and why this gets confusing) is that a downstream GFCI can only see an imbalance
if the system has a neutral-to-ground reference. On a bonded system, a line-to-ground fault creates that imbalance and the GFCI trips. On a floating UPS/generator output, there is no reference, so there’s no imbalance, so it
won't typically trip. But back to the "in theory" part, as depending on how the device is built, there could be enough leakage current for it to trip.
The end...
if we can save a life it’s totally worth it! We’ve already lost one and that’s too many.
