What are the general principles for selecting circuit breakers?

What are the General principles for selecting circuit breakers ?

First, select the type and level of the circuit breaker according to the purpose; select the rated current of the circuit breaker according to the maximum working current; select the type of release, the type and specification of accessories according to the needs. The specific requirements are as follows:

1.  The rated working voltage of the circuit breaker ≥ the rated voltage of the line
2.  The rated short-circuit breaking capacity of the circuit breaker ≥ the calculated load current of the line
3.  The rated short-circuit breaking capacity of the circuit breaker ≥ the maximum short-circuit current that may occur in the line (generally calculated as effective value).
4. The single-phase to ground short-circuit current at the end of the line ≥ 1.25 times the instantaneous (or short-delay) tripping setting current of the circuit breaker.
5.  The rated voltage of the circuit breaker undervoltage release is equal to the rated voltage of the line.
6.  The rated voltage of the circuit breaker's shunt release is equal to the control power supply voltage.
7.  The rated working voltage of the electric transmission mechanism is equal to the control power supply voltage.
8.  When the circuit breaker is used in a lighting circuit, the instantaneous setting current of the electromagnetic release is generally taken as 6 times the load current.
9.  When a circuit breaker is used as short-circuit protection for a single motor, the setting current of the instantaneous release is 1.35 times (DW series circuit breakers) or 1.7 times (DZ series circuit breakers) the starting current of the motor.
10. 10.When a circuit breaker is used as short-circuit protection for multiple motors, the setting current of the instantaneous release is 1.3 times the starting current of the largest motor plus the working current of the remaining motors.
11. When a circuit breaker is used as the main switch on the low-voltage side of the distribution transformer, its breaking capacity should be greater than the short-circuit current value on the low-voltage side of the transformer, and the rated current of the release should not be less than the rated current of the transformer. The setting current of the short-circuit protection is generally 6-10 times the rated current of the transformer; the setting current of the overload protection is equal to the rated current of the transformer.
12. After the type and level of the circuit breaker are initially selected, it is also necessary to cooperate with the protection characteristics of the upper and lower switches to avoid over-tripping and expand the scope of the accident.