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Generator Earthing

Discussion in 'Electrical Engineering' started by Barry, Feb 26, 2004.

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  1. Barry

    Barry Guest

    I have purchased a "top shelf brand" single phase 240volt portable
    generator. The manufacturer's instructions instruct me to use an earth
    stake..

    Why?

    I have seen many other people with generators who do not use earth stakes.

    I don't understand.... Am I at risk if I don't use one?

    What causes the risk?

    Any help would be appreciated
     
  2. To cover there own backsides.

    The risk is a piece of metalwork becomes live, and is not grounded
    to the same point as the supply thats making it live.
    sQuick IEng MIIE (elec)
     
  3. Barry

    Barry Guest

    Thanks. however I am still a little confused why an earth stake is
    necessary. How can the connection of a wire from ground to the frame of the
    generator guarantee that the peice of metalwork you refer to is connected to
    frame as well?

    Sorry for appearing dumb.
     
  4. SQLit

    SQLit Guest

    When ever you create electricity there must be a path to earth. With out the
    earth ground you run the risk that you circuit breaker will not trip. Ever
    notice that every electrical service in your neighbor hood has an earth
    ground? If the utility could get away from using them do you not think that
    they would quit spending the money? Not earthing the neutral can cause
    voltage imbalances to be created. A lot of people just rely on the frame of
    the genset to be in contact with the earth... close enough. It takes no time
    to drive an minimum 8 foot ground rod flush with the earth and connect it to
    the generators frame to the lug provided. (usually)

    Make it safe and make your SO's safe. It only takes about 5 milliamps to
    kill. Less in some conditions.

    I worked for a university and the city folks set 10 20 kw generators on
    tires for temp Christmas lighting not one was grounded. I shut them off and
    locked them out. The mayor and the president of the university were in the
    electric shop the next morning. I explained the safety issues for the public
    and the mayor immediately called his people and they installed the required
    ground rods. Christmas lights were dark one night. No big deal compared
    with making the headlines all over the world. Personally I do not want to
    be on TV.
     
  5. SQLit

    SQLit Guest

    Electrical grounding is always done at a single point at the point of
    service or generator. It get a little more complicated when you are working
    with a transfer switch. Assuming single phase residential USA. Transfer
    switches come in 2 pole solid neutral and 3 pole where even the neutral is
    switched. I prefer the 3 pole switches because then the neutral is directed
    to the source and is not shared with the other source. Which can be a
    bugger to figure out some times.

    The ground rod for the generator can be driven local to the generator. Then
    you will carry the ground to the other source and tie it together. You will
    be creating a supplementary ground system. When either source has a fault
    the path will take it back to the ground. Tripping the breaker. If you do
    not carry the ground you can have a situation where is can become dangerous.
    Try to locate the Soars Book on Grounding at the library,
    http://www.ecmweb.com/ar/electric_earth_not_bonding/

    Might be a better picture than my words. I just moved, and still have not
    unpacked my books.

    Section 230 of the NEC is written for grade 22. I have submitted several
    times to the NFPA to change the wording so it is not so hard for people to
    understand. They have nixed it each time. Oh well I like tilting at
    windmills.
     
  6. Barry

    Barry Guest

    Thanks SQLit,

    I can see that a 1phase unit needs an earth stake now. Is that because one
    end of the winding is connected to the frame of the genset and the other end
    is the active?

    If this is the case do I need to use an earth stake with a 3 phase gen set
    connected in Star if the earth and neutral are internally bonded ie
    earth -neutral link.

    Is the star point on the 3 ph genny always at 0 potential?
     
  7. Mark or Sue

    Mark or Sue Guest

    I disagree with what you say here. In many ways, systems would be safer if they weren't grounded.
    You'd have to touch two wires to get shocked instead of just one. The reason utility wires are
    grounded is because of lightning and the possibility of a very high voltage wire falling onto a
    lower voltage one. A breaker won't trip on an earth ground fault at normal voltages because the
    resistance of the earth is too high -- you must bond the grounding system to the neutral in order to
    trip a breaker through the equipment grounding wire. However, once a system is grounded, then you
    need to keep grounding everything connected to it or else you could still be shocked. An isolated
    generator generally is immune from most of these risks. A bunch of long extension cords or christmas
    lights could be susceptable to lightning, but that is all. An equipment grounding (really bonding)
    wire that goes back to the generator neutral would also be very helpful if metal equipment is
    plugged in.
    Weren't these portable generators (even though they are big they were temporary weren't they)?
    Aren't these exempt from ground electrode requirements?

    I suppose you want me to ground my car too? And the 12V to 120V inverter I plug into it?
     
  8. You sure it is not the green wire that carries the fault current to
    circuit breaker which makes it trip and not the earth connection.

    Basically the earthing of every service entrance allows the electric
    company to have a nice low impedance path back to their ground
    reference. They provide a ground about every 1/4 mile on the pole
    line. They actually use every one of the service entrance grounds to
    help provide a lower earth impedance path.

    What amazes me if earthing was so important; and you can't get a earth
    ground rod resistance of less than 25 ohms you drive a 2nd rod; if it
    is still higher than 25 ohms its ok to stop there according to the
    NEC. So I could drive two rods 6 feet apart, end up with 100 ohms to
    ground and that would be fine with the NEC and inspectors.

    I doubt unless the voltage is high enough like over 600 volts any
    fault current is going to overcome earth resistance to blow a breaker.

    Section 250.4(A)(5) says the Earth is not an effective Fault Current
    Path.

    I work on a floating bridge, there are no ground rods there, though
    concrete is a pretty fair ground. The transformer secondary X0 is
    bonded to the equipment grounding conductor in the P1 panels.
    Otherwise the earth ground floats I have no trouble blowing breakers
    on the bridge. Now if I remove the equipment grounding conductor I'd
    bet i would have a terrible time blowing the breaker.


    Gary K8IZ
    Washington State Resident
    Registered Linux User # 312991
     
  9. Ben Miller

    Ben Miller Guest

    What breaker are you referring to? If you are talking about the facility
    breaker, how does a grounding electrode affect it?
    How? Where would the imbalance be?
    For external contact with the skin, it takes a lot more than 5 mA to kill. 5
    mA is in the "mild pain" range, but not fatal.

    Ben Miller
     
  10. SQLit

    SQLit Guest


    answer to the masses, everyone is entitled to an opinion. If you disagree
    with me, OK attack me, but do not confuse the issue for the people who are
    asking for help. I try to reduce technical questions to something that the
    guy next door can handle. A lot of the time the answer is hire a qualified
    contractor. That is another subject. This is an open forum, all can voice
    there opinion. Let us leave it at that and not attack each other, we all
    know what an ass hole is..

    Lets try to be a little more applicable to the question
    MARK,,,, your comments should be left to people that UNDERSTAND the issue.
    I have worked on and installed non-grounded electrical systems. I WOULD
    NEVER recommend them to an residential customer. Your opinion is considered
    and rejected for this application.
    Lets try and answer the questions at hand, not take the average guy into
    territory that is way un - charted for the average guy.

    We are here to help each other, I for one have learned alot in the
    newsgroups.

    my thoughts alone all negitative email should come to me
    not the group.
     
  11. Mark or Sue

    Mark or Sue Guest

    My answer wasn't meant as an attack. I'll give an opposing view to anyone, so don't take it
    personally....

    Since we're here to help each other, comments should be aired in the open. The OP asked what was
    unsafe about an ungrounded portable generator. Most of your answer was incorrect or not relevant --
    that a breaker won't trip without a ground rod and comparing it to a distribution system that is
    hundreds of miles long. I could agree that a ground rod may help a GFCI trip, but no one stated if
    GFCI's were on the generator.

    We can all have opinions, but here is what the NEC says:

    250.34 Portable and Vehicle-Mounted Generators.
    (A) Portable Generators. The frame of a portable generator shall not be required to be grounded and
    shall be permitted to serve as the grounding electrode for a system supplied by the generator under
    the following conditions:
    (1) The generator supplies only equipment mounted on the generator, cord-and-plug-connected
    equipment through
    receptacles mounted on the generator, or both, and
    (2) The non-current-carrying metal parts of equipment and the equipment grounding conductor
    terminals of the receptacles are bonded to the generator frame.

    Now if you want to power your house with this generator through a transfer switch, then you would be
    required to ground it and I would agree that it is a good thing. But I think you over reacted to
    christmas lights on trees being powered by an ungrounded portable generator.

    So for the "average guy", why is an ungrounded portable generator unsafe? I can think of a few
    answers of why it may not be good for the generator, but none dealing with safety.
     
  12. Barry

    Barry Guest

    Thank you all for your replies.I look at this newsgroup from time to time
    and must say that it is refreshing to be able to read the different
    opinions that you guys have. I don't understand most of it but it is still
    enjoyable.

    Thanks
     
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