# 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 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.

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: