The circuit breaker ratings are given according to the duties performed by it. Circuit breakers are required to perform 3 major duties under short circuit condition.
Breaking capacity: A circuit breaker should be able to break the circuit (breaking capacity of the circuit breaker) and isolate the faulty section of the power system.
Making capacity: Circuit breaker must be capable of making the current in the greatest asymmetrical peak in the current wave. Making capacity of the circuit breaker will be always greater than (2.55 times) the breaking capacity of the breaker.
Short time Capacity: In power systems several circuit breakers are provided for operation. Each set of circuit breakers are provided to protect certain zone during fault condition. If these breakers fail to operate backup circuit breakers or circuit breakers protecting the other zone should operate after certain time lag. Therefore the circuit breaker should able to carry the short circuit current safely for certain short duration of the time while the other circuit breaker (in series) is clearing the fault. This referred to short circuit rating of the circuit breaker
Rated Voltage of Circuit Breaker:
During normal of the power system, voltage at any point will not be constant. Due to this manufacturer guarantees perfect operation of the circuit breaker at rated maximum voltage which is higher than the rated nominal voltage.
The rated maximum voltage of the circuit breaker is the highest rms voltage, above nominal system voltage , for which the circuit breaker is designed and is upper limit of the operation. The rated voltage is expressed in kVrms and refers phase to phase voltage for three phase current
Rated Current of Circuit breaker:
The rated normal current of the circuit breaker is the rms value of the current which the circuit breaker shall be able to carry at rated frequency and at rated voltage continuously under specified conditions. Under the specific conditions, important is the temperature rise of various components of the circuit breaker under normal load. The important condition for working of an oil circuit breaker is that the temperature of the oil should not be more than 40o and that of the contacts should not exceed 35oC
Rated Frequency of Circuit Breaker:
The rated frequency of the circuit breaker is the frequency at which it is designed to operate. Standard frequency is 60Hz or 50Hz. Applications at other frequencies needed special considerations