Short circuit current

Short Circuit Current

Due to large current passing through the network during faults, there are many effects of short circuit currents as following:

1. At fault location:

Damage Insulations for cables & bars.
Welding of conductors.
Fire & danger to life.

2. On fault circuit:

Electrodynamics forces result in deformation of bus bars & disconnect cables.

3. On other circuits:

Voltage Dip during the time.
Shut Down of a part of the network
Dynamic instability and /or the loss of machine synchronization.
Disturbances in control/monitoring circuits.

So, the power systems should be designed to stand short circuit currents for a short period of time before the trip process takes place.
While the types of trips performed by a circuit breaker are:

Thermal trip: Responsible for protection against over load currents.

Magnetic trip: Responsible for protection against short circuit currents
Fig.A is a Circuit Breakers Characteristics, from a/m fig. :

Ir is normal breaker current that described as operation current.

Im is a current that break trip become by magnetic part.

Isc is a maximum short circuit current or maximum current that breaker can stand for a short trip time.

There is another parameter that effect in breakers selection, this parameter is making current which defined as maximum current break can stand during making