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Thread: Using MV Cable Shield for Equipment Ground Conductor

  1. #1
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    Using MV Cable Shield for Equipment Ground Conductor

    Is there anywhere in the NEC or elsewhere that permits the use of the internal cable shield on an MV cable to be used as an equipment ground conductor and satisfy the requirements of 250.122?

    A project I am involved with has an MV feeder (4 sets of 750 MCM) between an feeder breaker and the downstream equipment with no separate EGC's run with the feeders. It appears that they are using the internal cable shield for any grounding or ECG requirements.

    This is a 4.16kV LRG system (400A) so perhaps this is permissible on an LRG system with reduced fault current as opposed to a solidly grounded system?

  2. #2
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    Read 250.190(C)

    Roger
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  3. #3
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    do you have single individually shielded conductors or a cable assembly?

    you a 3 ph mv cable without a ground? (other than the shields)
    don't think I've ever seen one
    do you have a mfg/pn?

    looks acceptable if the lrg limited current can be carried without damage for the requied trip time

    what is your ngr ohmic value?
    whatgf trip value? any time delay?
    The difference between genius and stupidity is that genius has its limits.

  4. #4
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    Quote Originally Posted by roger View Post
    Read 250.190(C)

    Roger
    OK I see now that this section permits the shields to be used as the EGC on an impedance grounded system provided that the ground fault current and duration is less than the damage point of the shield.

  5. #5
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    Quote Originally Posted by Ingenieur View Post
    do you have single individually shielded conductors or a cable assembly?

    you a 3 ph mv cable without a ground? (other than the shields)
    don't think I've ever seen one
    do you have a mfg/pn?

    looks acceptable if the lrg limited current can be carried without damage for the requied trip time

    what is your ngr ohmic value?
    whatgf trip value? any time delay?
    Yes the feeder is 1/C cables with shielsds but no separate EGC's. I don't have a manufacturer or part #.

    Ohmic value is 5.98ohms limiting ground fault current to 400A.

    I need to look into relay settings for delay setting.

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