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

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: