GFI (ground fault) and power inverter safety

I just bought a 400W inverter and it seems to work really well but I
started thinking about safety. I plan to use it on camping trips and
other outdoor remote activities and was wondering about Ground Fault
Protection.
It doesn't have GFI built in (making it very inexpensive) so I figure
I should get a GFI module to put between the inverter and the load.
The question I have is:
Can I expect a standard GFI unit to work correctly with the inverter?

This inverter generates the AC voltage via a "modified sine" output.
I put a scope on the inverter and there seems to be power output on
both sides of the outlet. It works out to a RMS of 110v.
If the GFI unit will be perfectly happy with this sort of power output
what about correct ground fault detection? There is vehicle ground
where the battery sits and then there is earth ground outside the
vehicle. These two grounds would be isolated by rubber tires. Would
a GFI detect a short to either of these grounds?

Thanks,
BDS

 

Most GFI units are for 15A outlets, so within that loading, it would
be acceptable to use a GFI unit.
The protection it gives is relative only to the "ground" you give it.
If you can manage to test the generator to determine AC leakage from
its frame to earth ground in bad weather as well as dry weather, and
measure that the leakage is below the safety limits, then use the
generator frame for "ground".
However, if that leakage is too much or cannot be determined, then use
a grounding stake and connect the generator frame to it.
**BUT**
Some earth "ground" areas are verrrrrrrrrrrrrrry lousy on conduction
and may be worse than useless.
Possible problem places: sandy areas (deserts, some beaches), rocky
areas, some areas with a lot of dry clay.
To determine how good an area is, place two stakes a foot apart and
measure the resistance between them with an ohmmeter; i am guessing but
i would think that anything over 1K is unacceptable.
It is my understanding that PG&E specifications is a one ohm ground,
but i could be wrong.

 

A GFI outlet doesn't give a crap about ground - doesn't even require
it in fact. A GFI looks at the current flows on the hot and neutral
lines and trips if there is a difference above a certain threshold (I
don't recall the value at the moment). A difference in current means
current is leaking out somewhere is shouldn't. For example through a
person to ground.

Some cheaper GFI units are more prone to nuisance tripping. The
noiser output from an inverter may also cause nuisance trips. I vote
for trying it and seeing what happens.

-Chris

 

I beg to differ; one of the fault conditions that is monitored is
current to ground.
It is possible to have a common mode current that is not zero, and it
can kill as surely as differential current.

 

no, and its not meant to. It will detect when there are 2 paths to
ground, and thus ground current flows. You dont get electrocuted when
theres only one path to ground.


For a small portable invertor I doubt any of this is necessary. But
consult your local laws to be sure.

yes, and GFCIs would not trip in this case. They dont stop people
dying from electrocution, that is a myth, but they can improve the
odds.


Regards, NT

 

Hmmm... As I recall a ground fault circuit interrupter uses the
transformer principle to compare the current flow on the two lines, if
current 1 and current 2 differs with more than the trigger current (in
other words, if the current flowing in is more or less than the
current flowing back by more than the current the interrupter is made
for), it will break both of the live wires (but not the ground wire,
this isn't even connected to the interupter circuit). The technique
used to measure the differential current is quite simple really, if
there is no current leaking to earth the magnetic fields of the two
wires will cancel each other out(If you had a clamp ampere meter
around the two wires it would show 0mA). If a ground fault occurs, the
two currents are no longer the same, so a small magnetic field will be
created around the wires, and this field will trigger the breaker (the
sensor is a coil placed around the two wires, so that it forms a
transformer secondary when the two currents are not equal). It's worth
to notice that this arrangement will not be able to detect equal
current leaks, but as the current differential needed is so small, its
probably close to impossible to make a ground fault of that kind
without precision resistors and stuff... The trigger current
differential can be as low 5-10mA, since they are meant to cut of the
current before it becomes lethal..

 

The current on the ground pin isn't monitored. It's not even connected
to the GFCI circuitry and just passes through the plug to the wall
wiring.