3 Phase Earthing?

Discussion in 'The Projects Forum' started by Cerkit, Mar 22, 2012.

  1. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
    275
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    Hi. I need to size the earth cable from a grid tied inverter and from what I have seen the earth conductor is sized the same as the phase/neutral conductors. Wouldn't the earth conductor need to be thicker in order to be able withstand the three phase current under fault conditions??
     
  2. jimkeith

    Active Member

    Oct 26, 2011
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    99
    Actually the fault condition draws as much current in the line as in the ground conductor--if more than one line gets grounded simultaneously, those two lines are a bolted fault condition to eachother, but the resulting earth current is no greater than one line due to the vector addition of the currents.

    Such fault currents are very temporary in nature as the protective circuitry interrupts the power before the conductors are damaged. Also note that the ground conductor normally runs cool at zero current while the phase (line) conductors may run warm at full rated current, so during a ground fault, the ground wire is already at an advantage.
     
  3. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    It'll be sized according to the NEC tables, based on the ampacity of the current carrying conductors.
     
  4. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
    275
    3
    Is it right to say that if theoritcally there was a mechanical fault and all three phase wires were in contact with each other and the body of the inverter (therefore in contact with the earth cable as well) there would still be 0A going through the earth cable??
     
  5. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    Yes!

    Adding a little boilerplate because the above line is too short to post:
    Short circuit testing for UL/CSA requires that the electrical enclosure be isolated from ground via a fuse and the 3phase control output be bolted together. When they throw the switch to supply the perhaps 50kA @ line source voltage, the fuse is not allowed to blow. Blowing of the fuse indicates arcing to the enclosure and test failure--not allowed.

    Grounding at the service panel makes a good deal more sense if the utility providing the power also grounds their supply as they do--in your case the source is/may be isolated from ground (I do not know the code requirement here)--except in the case of a lightening strike...
     
    Last edited: Mar 23, 2012
  6. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    Adding a human interest story to the best of my recollection--it appeared in the Readers Digest about 20years ago.

    A rural family woke up in the middle of the night with everyones' hair standing on end--all was dark except that corona coming from all light fixtures and outlets etc--verrry eeerie. Someone turned on the bathroom fluorescent light and then found that it would not turn off. Wanting to get out of the house, they opened the door at which time the family dog ran down the steps from the front porch--as soon at its feet touched the ground, it keeled over dead! The dog saved their lives, because they were not going to repeat that one.

    This is what caused it--not totally sure of all details though. During a wind storm a downed 10kV utility line came to rest upon a rock. The resulting ground resistance to the nearest ground (the next pole some 200ft away) wire was high enough to limit the current sufficiently to prevent blowing the high voltage fuse. The house happened to lie between the two poles and it had a faulty ground connection (or perhaps in this case it did not really matter?). This raised the potential of all wiring within the house to perhaps 5kV thus causing the corona discharge. The voltage gradient between the two poles (surface of the soil) worked out to several hundred volts per foot--this is what killed the dog.

    Eventually the fuse blew and everything returned to 'normal'. The utility fixed the line--no big deal... Care to go through that one?
     
    Last edited: Mar 23, 2012
  7. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
    275
    3
    So is it because the current phasors cancel each other out that the resultant current through the earth cable would be 0A ??
     
  8. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    You've indicated a neutral, which will be grounded at source by code. Even if your neutral was included in the short, line impedance would create a voltage on the bonding line, inducing some current. Line impedance of both source and bond will dictate the amount of current flowing through the bond.
     
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