IMPORTANCE OF SHORT CIRCUIT CURRENTS

Knowledge of short circuit current values is necessary for the following reasons.

1. Fault currents which are several times larger than the normal operating currents produce large electro-magnetic forces and torques which may adversely affect the stator end windings. The forces on the end windings depend on both the dc and ac components of stator currents.

2. The electro dynamic forces on the stator end windings may result in displacement of the coils against one another. This may result in loosening of the support or damage to the insulation of the windings.

3. Following a short circuit, it is always recommended that the mechanical bracing of the end windings to checked for any possible loosening.

4. The electrical and mechanical forces that develop due to a sudden three phase short circuit are generally severe when the machine is operating under loaded condition.

5. As the fault is cleared within 3 cycles generally the heating efforts are not considerable.

Short circuits may occur in power systems due to system over voltages caused by lightning or switching surges or due to equipment insulation failure or even due to insulator contamination. Sometimes even mechanical causes may create short circuits. Other well-known reasons include line-to-line, line-to-ground, or line-to-line faults on overhead lines. The resultant short circuit has to the interrupted within few cycles by the circuit breaker.

It is absolutely necessary to select a circuit breaker that is capable of operating successfully when maximum fault current flows at the circuit voltage that prevails at that instant. An insight can be gained when we consider an R-L circuit connected to an alternating voltage source, the circuit being switched on through a switch.