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Testing ground fault reduced voltage and blocking capability

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    Testing ground fault reduced voltage and blocking capability

    Blocking capability and reduced voltage tripping are two tests listed for ground fault relays in NETA-ATS, can someone explain how these two tests are performed?

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    Reduced voltage is accomplished with reduced control power voltage (we typically use 55%). This simulates what the ground fault control relay would have to function with if one of the legs feeding the control power transformer was grounded.
    The blocking is, i believe, tested to ensure that the switch does not close back in on an active ground fault, or at least on an unacknowledged one.

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    Thanks for the reply. Is this test done with a simple variable voltage source? For example, if I had 120V AC control power, at 55% my AC variable source would be set to 66V, then I would push current through CT and the relay should still trip?

    Quote Originally Posted by alexanderchaney View Post
    Reduced voltage is accomplished with reduced control power voltage (we typically use 55%). This simulates what the ground fault control relay would have to function with if one of the legs feeding the control power transformer was grounded.

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    Yep. Exactly.

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    NETA references zone-interlocking for the blocking capability test. Unless a restraint signal is used, any ground fault relay with zone interlocking will trip instantaneously regardless of the delay setting.

    The restraint signal is normally applied with a small jumper between specified terminals on the ground fault relay -- consult the relay manufacturers literature for this procedure. Perform a time test with and without this signal applied to ensure proper operation.

    Quote Originally Posted by alexanderchaney View Post
    The blocking is, i believe, tested to ensure that the switch does not close back in on an active ground fault, or at least on an unacknowledged one.

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    ahh, thanks for the clarification.

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