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GFI (ground fault) and power inverter safety

Discussion in 'General Electronics' started by BDS, Feb 22, 2004.

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  1. BDS

    BDS Guest

    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
    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?

  2. Robert Baer

    Robert Baer Guest

    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.
    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.
  3. Guest

    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.

  4. Robert Baer

    Robert Baer Guest

    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.
  5. N. Thornton

    N. Thornton Guest

    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

    Regards, NT
  6. TrAI

    TrAI Guest

    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..
  7. Guest

    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
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