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Phase Differential

  1. Jan 22, 2014 #1
    Hello experts!
    Help needed.
    I got a relay manual for translation and one of the items there is "Phase Differential".
    The context is as follows:
    "These settings program the differential element when the differential feature is in use. This
    feature consists of three instantaneous overcurrent elements for phase differential
    protection. Differential protection may be considered first line protection for phase to
    phase or phase to ground faults. In the event of such a fault, differential protection may
    limit the damage that may occur."
    "A trip occurs once the magnitude of either IaIN-IaOUT, IbIN-IbOUT, or IcIN-IcOUT (phase
    differential) exceeds the Pickup Level × Differential CT Primary for a period of time specified
    by the delay. Separate pickup levels and delays are provided for motor starting and
    running conditions."

    Is it "phase mismatch"?

    Thank you!
    Last edited: Jan 22, 2014
  2. jcsd
  3. Jan 28, 2014 #2


    User Avatar

    Staff: Mentor

    I'm no expert at this, but to make sense of it I picture the word "differential" to mean difference or loss. In an ideal system, there would be perfect insulation and there would be no stray currents or losses, and the current delivered by each phase would exactly equal the current returned by that phase (you need to bring out the star-point, to make it accessible).

    In any practical system, the current returned by each phase is slightly different from the current delivered. Small differences are acceptable, but larger differentials are indicative of major problems, most likely fault conditions, so the breakers are set to operate at some specified differential that allows normal operation but trips at fault conditions. There needs to be a sensor/breaker provided for each phase.

    Before going with this, I recommend that you await independent confirmation.
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