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Earthing a generator

Discussion in 'Electrical Engineering' started by Livewire, Aug 30, 2005.

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  1. Livewire

    Livewire Guest

    I've just bought a cheapo Honda clone generator off eBay. It's a great
    machine.. . . up to a point!

    But I wonder whether it should be earthed/grounded and if so, how? The
    supplier is playing silly beggers and won't give a straight answer.

    I think I need to find a proper earth relay on the machine. But how do I
    recognise it?
     
  2. SQLit

    SQLit Guest

    Small portable generators have several possible solutions.
    Sometimes there is a lug on the frame
    Sometimes the is a terminal near the neutral/ground connection usually near
    the overcurrent devices or the outlets.

    I recommend that you accuire a manual and see what it says.
     
  3. Bob Rahe

    Bob Rahe Guest

    Hmmm. My news feed doesn't have the original article so I don't have
    the full context but... generators don't always need a local ground. As
    in here, which explains it wrt 'separately derived systems'.

    http://www.imsasafety.org/journal/marapr/ma5.htm

    Bob

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  4. I am not surprised that the manufacturer will not give you a straight
    answer. To earth a small generator ( ground) or not ...................it
    depends on the situation and the application. Many systems operate on
    'floating' earths especially overhead systems in 3rd world countries and
    also military applications.
     
  5. J. B. Wood

    J. B. Wood Guest

    Hello, and many years ago when I was with a US Army signal battalion
    (501st of the 101st Airborne/Air-assauit Division) it was standard
    procedure to always ground (via a driven ground rod and mesh copper) our
    3kW and 5kW units. If you asked any of the EM or NCOs why the answer was
    always "safety" but they could not explain it beyond that. The metal
    enclosures on truck-mounted comm equipment shelters were also earthed
    (probably more for lightning protection).

    What a lot of the discussion in this area seems to overlook is that by
    earthing a generator you can introduce a potentially fatal shock hazard
    that would otherwise not exist (except under fault conditions). If the
    neutral of the generator is bonded to earth ground via a relatively low
    resistance connection a potential difference (capable of causing a
    potentially fatal current flow) will exist between a "hot" wire at the
    generator output and the earth. Of course the generator may have a metal
    frame with the neutral connected internally to the frame. If the metal
    frame is not insulated from earth ground you can also a have path to
    ground for the neutral (but most likely not as good as with a driven
    ground rod).

    Unlike commercial power distribution systems where voltage stabilization
    and tripping overcurrent prodcution devices (fuses or circuit breakers) is
    desirable under fault conditions (e.g. a hot wire touching a conducting
    object that is in contact with earth), this is generally not an issue with
    portable generator sets intended primarily for local consumption such as
    by power tools, etc. and that are not connected to some extensive power
    distribution system/grid.

    US Navy ships in particular do have extensive A.C. power distribution
    systems in which no current-carrying wires are intentionally connected to
    the steel hull. This is done to promote operation of electrical equipment
    under fault ("battle short") conditions (e.g. a current-carrying wire
    touches the hull). The ability to have a fault blow an overcurrent
    protection device is not desirable in this environment. You still find
    the standard "U" ground outlets with the ground prong in electrical
    contact with the hull, however. If you were to measure the voltage from
    the + or - prong of a shipboard 120 VAC outlet to the ground prong you
    would see about 60 VAC due to capacitive coupling to ground/hull.




    Sincerely,

    John Wood (Code 5550) e-mail:
    Naval Research Laboratory
    4555 Overlook Avenue, SW
    Washington, DC 20375-5337
     
  6. John Gilmer

    John Gilmer Guest

    There are two separate issues here.

    One is whether to GROUND the generator frame.

    The other is whether to intentionally GROUND one of the current carrying
    conductors.

    A good case can be made to NOT deliberately ground a current carrying
    conductor.

    BUT the generator frame should be grounded. If there is an internal
    connection between a current carrying conductior and the generator frame
    when grounding the generator frame will ground the "neutral." Better that
    than risk the possibility of a fault somewhere in the system turning your
    generator into a bobby trap.
     
  7. J. B. Wood

    J. B. Wood Guest

    Hello, and if the neutral is already bonded to the frame you would be
    setting up the condition that I commented on in my previous posting, viz.,
    creating a hot-to-earth ground potential where one previously had not
    existed (or at least not capable of delivering lethal current). The
    internal neutral/frame bond will still allow the generator's fuse/circuit
    breaker to blow if the generator's hot wire(s) contacts the generator
    case/frame. But it wouldn't necessarily electrocute you if you were to
    grab a hot wire while standing on earth ground. The probability of
    electrocution increases in this scenario if the frame is metallically
    grounded to the earth.

    It's probably easiest to picture the generator as a voltage source.
    Ground one side of that source to earth and the other end becomes hot with
    respect to earth ground. Sincerely,

    John Wood (Code 5550) e-mail:
    Naval Research Laboratory
    4555 Overlook Avenue, SW
    Washington, DC 20375-5337
     
  8. daestrom

    daestrom Guest

    Couldn't have said it better myself ;-)

    Former EMC(SS)
     
  9. John Gilmer

    John Gilmer Guest

    Maybe so. But the HOT piece of metal will not be the generator.

    If you ground the generator, there is a chance that a ground fault will trip
    the circuit breaker. If you don't, that same fault makes the generator a
    bobby trap.

    The
    I say it would be about the same.
     
  10. John Gilmer

    John Gilmer Guest

    I would have NEVER considered that!

    I just figured you have a good sized metal box with all kinds of "lectricity
    running about. I want that box GROUNDED.
     
  11. J. B. Wood

    J. B. Wood Guest

    Hello, and to give another scenario to the one I recently provided:

    The generator set has a metal frame that is bonded to the generator's
    neutral wire. The generator is on a rolling cart with rubber wheels and
    is presumed to be isolated from earth ground. The generator is
    distributing power to appliances located within a metal equipment shelter
    located a short distance away. The shelter is connected to earth ground
    via a ground rod. An appliance in the shelter develops a fault resulting
    in the connection of a hot wire to earth ground. It is further assumed
    that no bonding exists between the neutral wire and the shelter's ground
    connection. Since the generator is ungrounded its frame is now hot with
    respect to earth ground. In this scenario the frame should be bonded to
    earth ground. Whether or not sufficient fault current will flow to trip
    the generator's breaker/fuse depends on the resistance of the ground rod
    interfaces and the soil between the generator and equipment shelter.

    Suppose, on the other hand, that the grounded frame/neutral generator is
    only connected to a metal-shell power tool that is supposed to be fed by a
    3-wire cord but the ground prong on the tool's cord has been cut off. An
    internal hot-to-shell fault occurs in the tool. Since the neutral on the
    generator has been connected to earth ground the tool's shell is now hot
    with respect to earth. Had the generator been floating off ground this
    situation would be more benign. Of course the primary mistake was
    operation off of two wires when three are required for safety.

    There are two safety considerations here: Protection of personnel at the
    source (generator) and at the sink/load (actually three considerations if
    you include the distribution system). From the perspective of the source,
    if I had a metal-frame single-phase portable generator that was to suppiy
    power via two wires (designated hot and neutral) to something(s) hidden
    behind a curtain I would ground the generator frame to earth ground. I
    suspect this is why we did it in the Army signal battalion since equipment
    and tactical power distrubution wires were often being reconfigured.

    I don't think grounding is necessary if you are providing connection via a
    3-wire (hot/black, neutral/white, ground/green) cord between the generator
    and an appliance such as a power tool. A two-wire connection is also OK
    if the appliance has a doubly-insulated shell.

    The need for grounding of a portable generator therefore is dependent upon
    how the generator is internally wired and the external infrastructure to
    which the generator is connected. The NEC covers the most likely
    scenarios. Sincerely,

    John Wood (Code 5550) e-mail:
    Naval Research Laboratory
    4555 Overlook Avenue, SW
    Washington, DC 20375-5337
     
  12. SQLit

    SQLit Guest


    Excellent response, John.
     
  13. John Gilmer

    John Gilmer Guest

    But if you are running a
    In that case, it would be just as important to ensure that the generator
    frame is connected to the frame of the camper.

    The only time is would be completely unnecessary is where the generator is
    just running a single load that a power drill.

    If you are running strings of lights it's a very good idea to ground as
    faults can easily occur.
     
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