Overview

All electrical installations must be protected against short-circuits every time there is an electrical connection, which is generally when there is a change in conductor cross-section. The short-circuit current value must be calculated at every stage of installation for different possible network configurations. This is done to determine the characteristics of the equipment that must withstand or switch the fault current.

In order to choose the appropriate switching devices (circuit-breakers or fuses) and set the protection functions, four short-circuit values must be known:

The root mean square value of the maximum short-circuit current (symmetrical three-phase short-circuit)

This determines:

• the breaking capacity of the circuit-breakers and fuses;

• the temperature stress that the equipment must withstand.

It corresponds to a short-circuit in the immediate vicinity of the downstream terminals of the switching device. It must be calculated to include a good safety margin (maximum value).

The peak value of the maximum short-circuit current (value of the first peak of the transient period)

This determines:

• the making capacity of the circuit-breakers and switches;

• the electrodynamic withstand of the trunkings and switchgear.

The minimum phase-to-phase short-circuit current

This must be known in order to choose the tripping curve of the circuit-breakers or fuses or set the thresholds of the overcurrent protection devices, especially when:

• the cables are long or when the source has a relatively high internal impedance (e.g. generators);

• protection of persons relies on the phase overcurrent protective devices operating. This is essentially the case in low voltage for TN or IT...