What is Icu:
Icu is nothing but unlimited break Capacity of the circuit breaker Which means the circuit breaker can break the maximum fault current without damage. Typically for MCB the maximum Icu will be 6KA to 10 KA for MCCB Icu may be 200kAmps
What is Ics:
Ics means rated service breaking capacity or service short circuit breaking capacity. It means an electrical installation does not require to operate at its rated designed short circuit breaking capacity. Since after completion of testing, the new characteristics Icu will be introduced and it is called Ics. It is a percentage of Icu. It may be varied from 20 %, 30%, 40 %, 60 % and 70%, 100% according to the industrial applications.
Icw: short circuit withstand capacity:
The flow of rated short circuit current, that the circuit breaker should withstand upto 0.1 sec to 3 sec continuously without changing its temperature integrity. It means the physical state of the circuit breaker should not be changed.
All the circuit breaker will take some time to open faulty circuit typically ACB takes 20 to 30 msec. It this short time, the fault current repeats its cycle two to three times. Since the circuit breaker should be designed or tested that particular current.
Icw for class A MCCB < class B MCCB < ACB
Rated making capacity (Icm):
The maximum instantaneous value of current that the circuit breaker is designed at rated voltage in specific conditions. In AC power system Icm is having the relationship with Icu (Ultimate breaking capacity) by the factor k, which depends on the power factor (cos φ) of the short-circuit current loop
|Icu||cosφ||Icm = kIcu|
|6 kA < Icu ≤ 10 kA||0.5||1.7 x Icu|
|10 kA < Icu ≤ 20 kA||0.3||2 x Icu|
|20 kA < Icu ≤ 50 kA||0.25||2.1 x Icu|
|50 kA ≤ Icu||0.2||2.2 x Icu|
Example: A Masterpact NW08H2 circuit-breaker has a rated breaking capacity Icu of 100 kA. The peak value of its rated making capacity Icm will be 100 x 2.2 = 220 kA