# 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.

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:

### Ver Pangonilo

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