Answer Posted / usama badran ali

el salamo aleekom

to calculate circuit breaker breaker capacity...there are
four steps :
1-indicate all actual nominal voltages and MVA of your
system
2-indicate the location of circuit breaker.
3-calculate the actual impedance between the known supply
to the point of fault at which near to circuit breaker.
4-then calculate:
a- short circuit current =
voltage of supply/((3^0.5)actual impedance)
b-MVA short citcuit=
(3^0.5)*voltage of supply*short circuit current
c c-breaking capacity=
MVA short citcuit*assymetry factor.
or
short circuit current*assymetry factor
notes:
Assymetry factor:1.5~1.6 for midium voltage
1.25 for low voltage
MVAshort circuit= fault level
breaking capacity: it is the value of peak current where
the circuit breaker is breaking.

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