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Plotting of relay current curves

  1. Feb 20, 2015 #1
    When Plotting relay current curves using log paper, I understand that all curves should be plotted on a common base voltage (converting currents accordingly) and the curves can be cut at the maximum fault current level that the device is likely to see (corrected to the base current). What I would like to know is where the curves should be started, e.g. at what current value should I start the curve?

  2. jcsd
  3. Feb 20, 2015 #2


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    Science Advisor

    I think we need some context. Your reference to “fault current” suggests that you are asking about over-current protection relays. I assume you are plotting fault current on the logarithmic axis. Is the other axis something like the “Time Multiplier Setting” of the relay? Are the curves "Trip Time in seconds"?
    Maybe at the maximum expected load current on the circuit being protected. That line should not cross a time / current curve.
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