Short circuit current

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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.

Temperature

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