Electrical faults can occur in power systems for various reasons, such as equipment failure or lightning strikes. When a fault occurs, a large current flows through the system, and it is important to determine the maximum current that can flow to protect the system from damage. This is known as the fault current.

The fault current can be calculated using Ohm's law, which states that the current through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance between them. In the case of a fault current, the voltage is typically constant, and the resistance is low, so the current is very high.

The fault current can be calculated using the following formula:

$I_{sc}=\frac{V}{Z}$

I_sc         is the fault current, A
V         is the voltage, V
Z         is the impedance of the system, Ω

The system's impedance measures its resistance to the flow of current. It is determined by the resistance of the conductors, the reactance of the transformers, and the capacitance of the cables.

In power systems, the fault current is typically classified into three types: three-phase, phase-to-phase, and single-phase-to-ground. The fault current in each case can be calculated using different equations.

Three-Phase Fault

In a three-phase fault, all three phases of the system are shorted together. The fault current, in this case, can be calculated using the following formula:

$I_{sc}=\frac{V_{LL}}{\sqrt{3} Z_p}$

V _LL      is the line-line voltage, V
Z_p         is the phase impedance of the system seen from the point of fault, Ω

Phase-to-Phase Fault

In a phase-to-phase fault, two phases of the system are shorted together. The fault current, in this case, can be calculated using the following formula:

$I_{sc}=\frac{V_{LL}}{2Z_p}$

Single-Phase-to-Ground Fault

In a single-phase-to-ground fault, one phase of the system is shorted to ground. The fault current, in this case, can be calculated using the following formula:

$I_{sc}=\frac{V_{LL}}{\sqrt{3} Z_g}$

Zg        is the grounding impedance of the system, Ω 

In the case of a fault, it is essential to determine the maximum fault current that can flow to ensure that the system is protected from damage. Using the equations described above, it is possible to calculate the fault current for different types of faults in power systems.

In conclusion, the fault current is an essential parameter in power systems determining the maximum current that can flow during a fault. The fault current can be calculated using Ohm's law and the system's impedance.