When a short circuit occurs in an electric system, heavy current flows through all the sections of the system which are in the path between the power source and the equipment. The short circuit current is limited only by the impedance of the system.
This heavy current can damage the components of the electric system if they are not properly rated. If circuit breakers are not able to interrupt the high short circuit currents in a system, arcing and explosions may occur
.
The Rating of the components is done based on the maximum short circuit current.The short circuit current is calculated from the fault level KVA of the System. The Fault Level in a distribution system is a very important parameter. The kVA at the instant of a Fault should be correctly calculated and the components of the distribution system such as bus bars, circuit breakers, isolators, etc should be properly sized.
To calculate the fault current in a system it is first necessary to calculate the MVA during a fault.
The MVA during a fault is given by
From this, the maximum current during the fault can be deduced as
All the equipments should be rated to withstand this current. The fault level should be calculated every five years and after any modification to the system such as the addition of any load or the installation of further sources of power such as transformers and alternators.