Time-Current Curve

The calculation of maximum and minimum short-circuit currents has been discussed in IEC 60909 - Calculation of Maximum and Minimum Short-circuit Currents. What is the significance of these values?

The maximum calculated short-circuit current values are used for

  • Selecting interrupting devices of adequate short-circuit interrupting and peak rating,
  • Checking the ability of components of the system to withstand mechanical and thermal stresses,
  • Determining the time-current coordination of protective relays.

The minimum values are used for setting the required sensitivity of protective relays.

The minimum short-circuit values are sometimes estimated as fractions of the maximum values. It is only necessary to calculate the maximum values of short-circuit current.


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