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.


  • A Filipino Engineer, Registered Professional Engineer of Queensland (RPEQ) - Australia, and Professional Electrical Engineer (PEE 2574 - 1st Place April 1991) - Philippines with extensive experience in concept selection, front-end engineering, HV & LV detail design, construction, and commissioning of Hazardous and Non-Hazardous Area electrical installations in water and wastewater pipeline and pumping facilities, offshore platforms, hydrocarbon process plants and pipelines including related facilities. Hazardous area classification and design certification (UEENEEM015B, UEENEEM016B, UEENEEM017B).


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