The Circuit breaker ratings are the important parameters should be noted before placing it in to the power system network. First as per the network and loading, the important parameters should be calculated for normal and short circuit operations. As per the calculated parameters, right circuit breaker should be properly selected to place in power system network. the following parameters are the important circuit beaker rating need to be considered:
1) Rated Voltage of circuit breaker: This is the maximum voltage that the circuit breaker can be operated with any damage. This rating is determined by the insulation thickness and quality of the insulating material used in construction of circuit breaker.
For example a circuit breaker with 500 VAC can be operated for 400 VAC but a circuit breaker with 400VAC cannot be operated at 500 VAC.
if the circuit breaker is rated for some voltage it implicates that it can successfully quench the arc when the circuit breaker contacts open. If it is operated for higher voltage then the transient recovery voltage will be high compared to the dielectric strength of the quenching medium which results arc persisting. This arc may damage the insulation of the circuit breaker.
Normally the breaker voltage rating will be high compared to bus and load ratings in a power system network.
Rated Continuous current of circuit breaker
This is the maximum rated continuous current that the circuit breaker conducts with out over heating the contact terminals. This rating is determined by the size of the contactor conductor and in-service temperature rises in circuit breaker compartment. This rating is also called as ampere rating of circuit breaker because it is measured in amps.
Rated Interrupting current of the circuit breaker:
This is the maximum short circuit current or fault current that the breaker can with stand or interrupt with out any damage to the breaker. The rating is depend on the arc breaking capability of the arc contactors, arc chutes, It also depends on the circuit breaker cabinet ability to with stand the electromagnet stress associated with short circuit current. i.e. when large currents flows through the contacts then the attractive and repulsive forces will produce which introduces high stress on the insulating material and current carrying components.
The interrupting capacity of the circuit breaker is measured in MVA and it is calculated as follows
Interrupting capacity = √3 * Rated line voltage * Rated line current / 106 (MVA)
For example a circuit breaker of 6600 V whose interrupting capacity of 500 MVA then the rated interrupting current
= (interrupting capacity / (√3 * rated voltage)) * 106
= 500 * 106 / √3 * 6600
= 43.7 K A
The rated peak current is equal to the rated short circuit current of the circuit breaker.
Number of poles: This is also one of the parameter in circuit breaker name plate. The breaker can be single pole or multiple pole..