The circuit breaker rating is given on the duties performed by it. The rating of the circuit breaker should be specified in terms of
- the number of poles (the number of poles per phase of a breaker is a function of the operating voltage)
- rated voltage
- rated current
- rated frequency and
- operating duty
Circuit breaker performs the major three duties under short-circuit conditions. i.e
- Breaking capacity of the circuit breaker: A circuit breaker must be capable of breaking the circuit and isolating the faulty section in the case of fault.
- Making capacity of a circuit breaker: A circuit breaker must be capable of making a circuit in the greatest asymmetrical peak in the current wave.
- Short-time capacity of a circuit breaker: A circuit must be capable of carrying fault currents safely for a short time, while another circuit breaker(in series) is clearing the fault.
During normal operating conditions, the voltage at any point of the power system is not constant. so the manufacturer guarantees the operation of the circuit breaker at the rated maximum voltage. The rated maximum voltage of a circuit breaker is the highest rms voltage above the nominal system voltage for which the circuit breaker is designed and is the upper limit for operation.
The rated voltage is expressed in KVrms and refers to phase-to-phase voltage for a three-phase circuit.
The rated normal current of a circuit breaker is the rms value of the current at which the circuit breaker is able to carry at rated frequency and at rated voltage continuously under specified conditions.
The rated frequency of the circuit breaker is the frequency at which it is designed to operate. The standard frequency is 50 Hz.
The operating duty of a circuit breaker consists of the prescribed number of unit operations at stated intervals.
The operating sequence denotes the sequence of opening and closing operations which the circuit breaker can perform under specified conditions.
o=opening of circuit breaker
co=closing operation time which is immediately followed by an opening operation without any intentional time delay.
t’=time of operation which is necessary to restore the initial conditions.
t=0.3 sec for circuit breaker intended for first auto re-closing duty
The breaking capacity of the circuit breaker is defined as the highest rms value of short-circuit current that the circuit breaker is capable of breaking under specified conditions of transient recovery voltage and power frequency voltage. It is expressed in KA rms at contact separation.
In a particular phase, the current is maximum at the instant of fault and decays. The actual current interrupted by the circuit breaker is less than the initial value of short-circuit current Imc.
Let the instant of separation of contacts.
The AC component of short-circuit current, Iac=x
DC component of short-circuit current, Idc=y
Now symmetrical breaking current=RMS value of ac component of short-circuit current at the instant of separation of contacts
Asymmetrical breaking current=RMS value of the combined sums of ac and dc components
conventionally the breaking capacity of a circuit breaker in MVA is given by
The symmetrical breaking capacity of a circuit breaker is the value of the symmetrical breaking current that the circuit breaker is capable of breaking at a stated recovery voltage and a stated reference re-striking voltage under prescribed conditions.
The asymmetrical breaking capacity of a circuit breaker is the value of the asymmetrical breaking current that the circuit breaker is capable of breaking at a stated recovery voltage and a stated reference re-striking voltage under prescribed conditions.
Making capacity: The making capacity of the circuit breaker is the ability of the circuit breaker to withstand the effects of electromagnetic forces which are proportional to the square of the peak value of the making current. The making current of a circuit breaker when closed on a short circuit is the peak value of the maximum current wave in the first cycle of the current after the circuit is closed by the circuit breaker.
To determine making current of a circuit breaker, we must multiply the symmetrical breaking current by √2 t convert the rms value to peak value, and then by 1.8 to take into account the “doubling effect” of maximum asymmetry.
Short-Time Current Rating: The short-time current of a circuit breaker is the rms value of current that a circuit breaker can carry in a fully closed position without damage for the specified time interval under prescribed conditions. It is normally expressed in terms of KA for a period of 1 second or 4 seconds known as a one-second rating and a four-second rating.