Fault: Short Circuit

•Fault Current flows in any part of the power system due to different types of short circuit conditions  in the system.

•There are essentially four types of short circuit faults that may occur in the Power System:

1.Three-phase and Three-Phase-to- Ground

2.Single line-to-ground,

3.Double line-to-ground, and

4.Line-to-line.

 Each of these types of faults can result in different magnitudes of fault current 

In all types short circuit fault, however, there is a common element: an abnormally low-impedance path or shorted path for current to flow, hence the name short circuit current. Such a condition can lead to extremely high currents. 

 Phase to Ground Fault: In this type of Electrical fault all the three sequence components (positive, negative and zero sequence components ) are present and are equal to each other. In case of isolated neutral connection to the generator, there will be no return path for the current. So for such fault, fault current is zero.

• Phase to Phase fault: These are unsymmetrical faults as these faults give rise to unsymmetrical currents (Current differ in magnitude and phase in the three phases of power system).In case of Phase to Phase fault positive and negative sequence component of current are present, they are equal in magnitude but opposition in phase. zero sequence components are absent  .

Three Phase Fault: This type of faults are called symmetrical fault. This type of faults occur very rarely but more severe compared to other faults. In this faults negative and zero sequence component currents are absent and positive sequence currents are present .

Phase - Phase to Ground Fault: These faults are of unsymmetrical nature. In this type of faults negative and zero sequence faults are in opposition with positve sequence cmponents.

Three Phase Faults :

Three Phase Fault In a three phase fault, all three phases (L1, L2 and L3) are shorted together.

To find the fault current at any point in the network, a sum is made of the impedances in the network between the source of supply (including the source impedance) and the point at which the fault occurs

Earth Faults :

In an earth fault, one phase is directly connected to earth (L1 to earth for example). 

To find the value of earth fault current at any point in a network, a sum is made of the earth fault impedances in the network between the source of supply (including source impedance) and the return path impedances .

•Phase to Phase Faults :

• Phase to Phase Fault In a phase to phase fault (L1 to L2 for example), two phases are connected together. 

The fault current is again, the nominal applied voltage divided by the summed impedance.