Calculation of Maximum and Minimum Short-circuit Currents

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 U_n	Voltage factor c for the calculation of
Nominal voltage U_n	maximum short-circuit currents c_max¹⁾	minimum short-circuit currents c_max
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	1.10	1.00
1) c_maxU_n should not exceed the highest voltage Um for equipment of power systems. 2) If no nominal voltage is defined c_maxU_n = U_m or c_minU_n = 0.90 × U_m 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 c_max 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 Z_Q are used to represent external networks, the minimum equivalent short-circuit impedance shall be used;
motors shall be included if appropriate;
resistance R_L of lines (overhead lines and cables) at 20 °C.

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

voltage factor c_min 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 R_L of lines (overhead lines and cables, line conductors, and neutral conductors) at a higher temperature: