The calculation of maximum and minimum shortcircuit currents as per IEC 60909 is based on the following assumptions.
 No change in the type of short circuit involved, that is, a threephase short circuit remains threephase and a linetoearth short circuit remains linetoearth during the time of short circuit.
 No change in the network involved.
 The impedance of the transformers is referred to the tapchanger in main position.
 Arc resistances are not taken into account.
 All line capacitances, shunt admittances and nonrotating loads are neglected.
These assumptions are not strictly true for the power systems, however, the result of the calculations are generally of acceptable accuracy.
Nominal voltage U_{n}  Voltage factor c for the calculation of  

maximum shortcircuit currents c_{max}^{1)} 
minimum shortcircuit currents c_{max} 

Low voltage 100 V to 1000 V 
1.05^{3)} 1.10^{4)} 
0.95 
Medium voltage >1 kV to 35 kV 
1.10  1.00 
High voltage^{2)} >35 kV 

1) c_{max}U_{n} should not exceed the highest voltage Um for equipment of power systems. 2) If no nominal voltage is defined c_{max}U_{n} = U_{m} or c_{min}U_{n} = 0.90 × U_{m} should be applied. 3) For lowvoltage systems with a tolerance of +6 %, for example systems renamed from 380 V to 400 V. 4) For lowvoltage systems with a tolerance of +10 %. 
Maximum shortcircuit currents
When calculating maximum shortcircuit currents, the following conditions will be considered:
 voltage factor c_{max} according to Table 1 shall be applied for the calculation of maximum shortcircuit 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 shortcircuit impedance shall be used;
 motors shall be included if appropriate;
 resistance R_{L} of lines (overhead lines and cables) at 20 °C.
Minimum shortcircuit currents
When calculating minimum shortcircuit currents, the following conditions will be considered:
 voltage factor c_{min} for the calculation of minimum shortcircuit 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: