Minimum Phase Fault Current Calculation

The type of the circuit will determine how the minimum phase fault current will be calculated. It is done according to IET Electrical Guidance Note 1, using the factors in Table E2 to calculate the maximum impedance of the conductor.

Single phase circuits:

Three phase and Neutral circuits:

Three phase (without neutral) circuits:


the phase to neural voltage


the phase to phase voltage

the impedance of the phase conductor

the impedance of the neutral conductor

the external impedance

The voltage factor Cmin is to take into account the worst case voltage variations conditions in a low voltage installation, specified in BS EN 60909 as 0.95 (optional factor)

Adiabatic Check of the Phase Conductor During Phase Fault

The Software acts according to regulation (434.5.2), in order make the adiabatic check i.e whether or not the heat energy flowing during the time taken for the protective device to trip, exceeds the conductor withstand  .



the maximum permissible disconnection time in seconds


the cable factor


the cross sectional area of the conductor mm2


the minimum phase fault current

The software compares the maximum permissible disconnection time with the time taken from the time-current curve of the protective device at the phase fault to find if the adiabatic check stands.

Note: For disconnection times less than  0.1 sec, the software checks for available Let‐Through energy characteristics of the protective device at the calculated fault current and uses them during the adiabatic check.