Question

How do you calculate the symmetrical fault current in a circuit?

Answer 1

Calculating the symmetrical fault current in a circuit involves understanding the fault conditions and the circuit parameters. A symmetrical fault refers to a fault where the fault impedance is the same in all three phases of the electrical system, resulting in balanced currents. This is typical in three-phase power systems.

The formula to calculate the symmetrical fault current is:


f,sym
=

LL

f,sym
I
f,sym
    ​

=
Z
f,sym
    ​

V
LL
    ​

    ​


where:


f,sym
I
f,sym
    ​

 = Symmetrical fault current (in Amperes)


LL
V
LL
    ​

 = Line-to-line voltage (in Volts)


f,sym
Z
f,sym
    ​

 = Symmetrical fault impedance (in Ohms)

To calculate the symmetrical fault impedance (

f,sym
Z
f,sym
    ​

), you'll need to consider the impedance contributions from various components in the fault path, such as cables, transformers, generators, and other equipment.

Transformer Impedance: For transformers, the fault impedance is typically given in percentage values (per-unit) of the transformer's rated impedance. You can convert this percentage impedance to Ohms using the formula:


f,trans
=

trans
100
×

base
Z
f,trans
    ​

=
100
Z
trans
    ​

    ​

×Z
base
    ​


where

f,trans
Z
f,trans
    ​

 = Fault impedance of the transformer (in Ohms),


trans
Z
trans
    ​

 = Percentage impedance of the transformer (usually provided by the manufacturer), and


base
Z
base
    ​

 = Transformer's rated impedance in Ohms.

Cable Impedance: The fault impedance for cables can be calculated using their per-unit impedance and the base impedance of the cable: