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Zone ground fault protection. Blocking and tripping?

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    Zone ground fault protection. Blocking and tripping?

    A substation has zone ground fault protection. Which statement is true?

    a. The main will trip before the feeder breakers
    b. The feeder breakers send a block to the main and tie so they donít trip
    c. The feeder breakers send a block the main and tie so they donít trip until their designated tripping time.
    d. The main will trip and block tripping to the other side of the bus

    It is my understanding that when zone interlocking is not defeated the breaker will trip at its minimum time no matter what. Defateating zone interlock will allow the breaker to trip at its programmed time delay. My answer would be "C". Can someone help verify this answer?

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    Quote Originally Posted by marvinray View Post
    A substation has zone ground fault protection. Which statement is true?

    a. The main will trip before the feeder breakers
    b. The feeder breakers send a block to the main and tie so they donít trip
    c. The feeder breakers send a block the main and tie so they donít trip until their designated tripping time.
    d. The main will trip and block tripping to the other side of the bus

    It is my understanding that when zone interlocking is not defeated the breaker will trip at its minimum time no matter what. Defateating zone interlock will allow the breaker to trip at its programmed time delay. My answer would be "C". Can someone help verify this answer?
    From what I've researched you are correct. For ground fault zone protection, the closest protective device to the fault will trip instantaneously and send a signal upstream to start a time delay on the upstream device. Though each device can trip instantaneously, depending on the location of the fault. Hope this helps.

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    I agree that the answer is C. Option B is close but wrong because it implies that the main and tie will never trip.

    When a device equipped with Zone-Selective Interlocking (ZSI) senses a short circuit or ground fault, it sends a restraint signal (via the restraint wires) to the ZSI device just upstream from it. This activates the preset time delay on the upstream device.

    When a device equipped with ZSI senses a short circuit or ground fault and does not receive a restraint signal, its preset time delay will not be activated and it will trip with no intentional delay.

    The system is designed to limit fault stress on the electrical distribution equipment by reducing the time it takes to clear the fault while maintaining system coordination between overcurrent and ground-fault protective devices.

    Click image for larger version. 

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    Here are a couple documents that explain Zone-Selective Interlocking schemes in great detail:
    http://www2.schneider-electric.com/r...0600DB0001.pdf
    http://es.eaton.com/protectiverelays...P02602002E.PDF

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    Ground Fault Zone Interlocking

    Quote Originally Posted by marvinray View Post
    A substation has zone ground fault protection. Which statement is true?

    a. The main will trip before the feeder breakers
    b. The feeder breakers send a block to the main and tie so they donít trip
    c. The feeder breakers send a block the main and tie so they donít trip until their designated tripping time.
    d. The main will trip and block tripping to the other side of the bus

    It is my understanding that when zone interlocking is not defeated the breaker will trip at its minimum time no matter what. Defateating zone interlock will allow the breaker to trip at its programmed time delay. My answer would be "C". Can someone help verify this answer?
    Yes, you are correct!

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