Calculation of Maximum and Minimum Short-circuit Currents

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The calculation of maximum and minimum short-circuit currents as per IEC 60909 is based on the following assumptions.

  • No change in the type of short circuit involved, that is, a three-phase short circuit remains three-phase and a line-to-earth short circuit remains line-to-earth during the time of short circuit.
  • No change in the network involved.
  • The impedance of the transformers is referred to the tap-changer in main position.
  • Arc resistances are not taken into account.
  • All line capacitances, shunt admittances and non-rotating loads are neglected.

These assumptions are not strictly true for the power systems, however, the result of the calculations are generally of acceptable accuracy.

Table 1 Voltage Factor c
Nominal voltage Un Voltage factor c for the calculation of
maximum short-circuit currents
cmax1)		 minimum short-circuit currents
cmax
Low voltage
100 V to 1000 V		 1.053)
1.104)		 0.95
Medium voltage
>1 kV to 35 kV		 1.10 1.00
High voltage2)
>35 kV
1) cmaxUn should not exceed the highest voltage Um for equipment of power systems.
2) If no nominal voltage is defined cmaxUn = Um or cminUn = 0.90 × Um should be applied.
3) For low-voltage systems with a tolerance of +6 %, for example systems renamed from 380 V to 400 V.
4) For low-voltage systems with a tolerance of +10 %.

Maximum short-circuit currents
When calculating maximum short-circuit currents, the following conditions will be considered:

  • voltage factor cmax according to Table 1 shall be applied for the calculation of maximum short-circuit currents in the absence of a national standard;
  • the maximum contribution from power plants and network feeders;
  • equivalent impedance ZQ are used to represent external networks, the minimum equivalent short-circuit impedance shall be used;
  • motors shall be included if appropriate;
  • resistance RL of lines (overhead lines and cables) at 20 °C.
See also  Voltage Selection Guide for Electric Power Transmission Line as to Load and Distance

Minimum short-circuit currents
When calculating minimum short-circuit currents, the following conditions will be considered:

  • voltage factor cmin for the calculation of minimum short-circuit currents shall be applied according to Table 1;
  • minimum contribution from power stations and network feeders;
  • motors shall be neglected;
  • resistances RL of lines (overhead lines and cables, line conductors, and neutral conductors) at a higher temperature:
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