(1)The rated current and rated voltage of low-voltage circuit breaker should be greater than or equal to the normal working voltage and current of line and equipment.

    (2) The ultimate breaking capacity of low-voltage circuit breakers should be greater than or equal to the maximum short-circuit current of the circuit.

    (3) The rated voltage of undervoltage tripper is equal to the rated voltage of the line.

    (4)The rated current of overcurrent tripper is greater than or equal to the maximum load current of the line.

    Using low-voltage circuit breaker to realize short-circuit protection is superior to fuse, because when the three-phase circuit is short-circuit, it is likely that only one-phase fuse will fuse, resulting in phase-breaking operation. For low-voltage circuit breakers, as long as the short circuit is caused, the switch will trip and the three-phase will be cut off at the same time. There are other automatic protective effects. But its structure is complex, its operation frequency is low, and its price is high, so it is suitable for the occasions with higher requirements, such as the power distribution panels.

    Low-voltage circuit breaker has over-current, voltage-loss, interruption tripping mechanism and good delay characteristics. It is a relatively perfect automatic protection device, and has been widely used in low-voltage electrical equipment and its distribution lines. Correct selection of low-voltage circuit breakers plays an important role in reliability of distribution system, industrial production and people's life.

    (1) The types and protection forms of circuit breakers should be determined according to the requirements of line protection. For example, universal or plastic case circuit breakers, plastic case circuit breakers are usually used when the current is below 600A. Of course, there are also plastic case circuit breakers whose rated current is greater than 600A.

    (2) The rated voltage UN of the circuit breaker shall be equal to or greater than the rated voltage of the protected line.

    (3) The rated voltage of undervoltage tripper of circuit breaker should be equal to the rated voltage of protected line.

    (4) The rated current of the circuit breaker and the rated current of the overcurrent trip shall be greater than or equal to the calculated current of the protected line.

    (5) The ultimate breaking capacity of the circuit breaker should be greater than the effective value of the maximum short-circuit current of the line.

    (6) The protection characteristics of upper and lower circuit breakers in distribution lines should be coordinated, and the protection characteristics of lower circuit breakers should be located below the protection characteristics of upper circuit breakers and not intersected.

    (7) The long delay trip current of the circuit breaker should be less than the permissible continuous current of the conductor.

    (8) When choosing circuit breakers, the use of circuit breakers should be considered.

    (9) In DC control circuit, the rated voltage of DC circuit breaker should be greater than that of DC line. The rated voltage of DC circuit breaker should be more than twice the DC line voltage if there are reverse braking and inversion conditions.