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Grounding question

Discussion in 'Electrical Engineering' started by JoeRaisin, Dec 20, 2009.

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  1. JoeRaisin

    JoeRaisin Guest

    I work (when there IS work) as a gas/oil drilling consultant. There are
    generally several work trailers on the drilling location powered from
    the generator house - a large semi-trailer itself.

    Suffice it to say the generators provide fluctuating power levels at
    best which is why most trailers utilize a couple of layers of UPS
    protection to level things out. My question is about grounding, which
    can sometimes be an issue.

    Some folks have taken to hammering in their own ground stakes outside
    their trailers. These trailers are usually 50-150 feet from the
    generator house.

    Some have taken the added measure of NOT using the ground line from the
    generator and use only the ground stake and they swear this has saved
    their equipment on numerous occasions.

    How dangerous is it to have these separate grounds that far apart? If
    they insist on their own ground stakes, would it be better to keep the
    generator ground and bond the two grounds via the ground bus on the
    trailer's panel?

    I probably won't be able to convince anyone to alter their own beliefs -
    I'm just trying to assess the level of risk I'm at when entering/exiting
    these trailers in a snow/mud mix ankle deep (ie: should I wear my thick
    rubber gloves when grabbing the metal door handles).

    Most of these trailers use a split phase as they need 220 for the
    heaters (Michigan).

    Thanks in advance.
     
  2. Bill

    Bill Guest

    This is an interesting question!

    The problem would be if the neutral became disconnected between the
    generator and the trailer (Not an unusual occurrence with high amperage
    connections!). Electricity would then want to flow through the ground (if
    the ground was bonded to neutral at the trailer main electric panel - which
    would be a necessary thing for safety of grounded appliances in the
    trailer).

    But this is the way the electric company provides electricity - no ground
    wire to a home/business - just the hots and a neutral and the neutral
    grounded at the transformer. Then the neutral and ground bonded at the main
    electric panel for the home/business and grounded there.

    I suppose you could say a generator is different because it is on the
    ground! But the electric company's "pad mount transformers" are on the
    ground as well, and no ground wire is run from this to the home/business!

    Perhaps it is because unskilled persons would not tend to touch a "pad mount
    transformer" metal case? Yet they would have their hands all over a
    generator if there was an electrical problem???

    Or perhaps because generator wiring to a trailer would tend to be temporary
    with lines strung on the ground. Whereas the wiring from the electric
    company would be in the air or underground in well protected conduit????


    "JoeRaisin" wrote in message
     
  3. A floating neutral like what you describe can be very dangerous-all metal
    frames of equipment would be electrified. Best to make sure about neutral
    return integrity- especially if N is bonded to ground.
     
  4. That would be _roughly_ equivalent to a TN-C-S earthing system
    in Europe (also called Protective Multiple Earthing).
    That's a TT earthing system in Europe (or an IT earthing
    system if the neutral also isn't grounded at the generator).
    It's regarded as safer than using a distant ground connection, but
    (in UK at least), you would have to have an RCD/GFI as ground rods
    often can't be relied upon to have a low enough impedance to handle
    a fault current (short to earth) and pass enough current to blow the
    circuit fault current protection within the required disconnect time.
    I would say local ground rod is a really good idea.

    TN-C-S requires a high integrity PEN (combined Protective
    Earth and Neutral) conductor, so there's pretty much no
    chance of it breaking with the live still connected.
    PEN conductor also has to be sized to carry other stray
    ground currents in the area which are nothing to do with
    the trailer supply.

    TT is probably safer but the mandatory RCD/GFI could be
    an issue in some circumstances.
    Sorry, I don't know the US regs in this area, so what I've said
    above may not be permitted or customary in that location.
     
  5. bud--

    bud-- Guest

    Its like deja vou all over again.

    "Grounding" has 2 major functions.

    1 To keep the voltage with respect to earth at a reasonable value.
    Connecting the system neutral to earth provides this earth reference. A
    ground rod is among the worst earth electrodes. The resistance to earth
    for a quite good rod is 10 ohms. [There might be a lot better electrode
    at the drill rig - like a metal well casing.]

    2. To provide a high current path for a short-to-ground (ground fault)
    to rapidly open protective breakers or fuses. This *requires* a metal
    path back to the power source. The National Electrical Code (NEC) does
    not allow the earth to be used as part of this path because an earth
    path is far too high a resistance to be reliable.

    I don't see any problem with adding earthing electrodes. Probably
    required by the NEC.
    Assume there is a ground fault at the trailer. The intended path is
    through the source ground wire back to the generator trailer, through a
    required ground-to-neutral bond, and to the generator neutral. That
    gives a high current path to trip a breaker.

    If you remove the source ground wire connection, the path is through the
    local ground rod, through the earth, through the generator trailer
    earthing, through the generator ground wire-to-neutral bond and back to
    the generator. Look at only one piece of this path - the local ground
    rod. If you have 120V connected to the ground rod (via the ground
    fault), you have 120V with maybe 10 ohms (likely a lot higher) resulting
    in a fault current of 12A (likely a lot less). That is unlikely to trip
    any breaker. So the ground rod (and trailer "ground") are at 120V with
    respect to "absolute earth".

    As a rule of thumb, the 80% of the voltage drop away from a ground rod
    is in the first 3 feet from the rod. Standing on the earth over 3 feet
    from the rod and touching the trailer you have at least 96 volts - not
    exactly safe. (Part of this could be at the generator earthing - so you
    could electrocute someone there.)

    Before the 2008 NEC it was probably allowed to bond the neutral to the
    panel ground at the trailer. This would provide the metalic fault path
    that is required (via the source neutral). (There remain the possible
    hazards in Tzortzakakis's post.)
    Multiple earthing electrodes that are bonded by a source ground wire are
    fine.

    IMHO a disconnected source ground, if there is not a N-G bond at the
    trailer panel, is very hazardous.

    In Andrew's (nice) post this is a TT system and requires GFI/GFCI
    protection (RCD is the same thing in the UK). GFCI protection overcomes
    the lack of an effective ground fault path. I don't believe this is
    allowed in the NEC.
    A few manufacturers used to want an "isolated ground" receptacle for
    their data equipment, with the receptacle ground connected only its own
    ground rod. (This was not only a major NEC violation, it was dangerous
    and stupid.)
    Geez - what a bunch of wimps.
     
  6. Guest

    | This is an interesting question!
    |
    | The problem would be if the neutral became disconnected between the
    | generator and the trailer (Not an unusual occurrence with high amperage
    | connections!). Electricity would then want to flow through the ground (if
    | the ground was bonded to neutral at the trailer main electric panel - which
    | would be a necessary thing for safety of grounded appliances in the
    | trailer).
    |
    | But this is the way the electric company provides electricity - no ground
    | wire to a home/business - just the hots and a neutral and the neutral
    | grounded at the transformer. Then the neutral and ground bonded at the main
    | electric panel for the home/business and grounded there.

    So I see no difference between generator to trailer wiring as described,
    and utility to home wiring, aside from there being that upstream MV risk
    for those sticking their hands inside the pad transformer, and whatever
    difference might exist in terms of the chance of a bad neutral connection.


    | I suppose you could say a generator is different because it is on the
    | ground! But the electric company's "pad mount transformers" are on the
    | ground as well, and no ground wire is run from this to the home/business!

    I would prefer to keep the generator grounded just so there isn't a slight
    voltage rise due to the voltage drop over the neutral between the generator
    and the bonding point. But that could be just the generator frame itself
    and not actually create a bond.


    | Perhaps it is because unskilled persons would not tend to touch a "pad mount
    | transformer" metal case? Yet they would have their hands all over a
    | generator if there was an electrical problem???

    A generator is not as likely to have 7200 volts.


    | Or perhaps because generator wiring to a trailer would tend to be temporary
    | with lines strung on the ground. Whereas the wiring from the electric
    | company would be in the air or underground in well protected conduit????

    I guess that is where the risk of bad neutral comes from. If you really
    want to protect against that, put an isolation transformer at the trailer
    fed with just 2W 240V. You can get your 120/240 from that with a ground
    bond on the secondary center tap.
     
  7. JoeRaisin

    JoeRaisin Guest

    Nah - just got to keep the water in the bucket in a liquid state for
    washing the samples of the cuttings from the bottom of the hole.
    Otherwise all those clay particles from the drilling fluid makes it hard
    to see the pretty colors under the microscope...

    Without that need we'd all be like, "phht - below zero? Bring it on!"

    ....at least as far as you know...

    Thanks to everyone for all the good info.
     
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