## Protection circuit breakers (molded case circuit breakers)

Protection circuit breakers can be divided into two categories, one refers to the circuit breaker only to protect the normal operation, and the other is the circuit breaker needs for both protection and infrequent operation. After the latter type of situation need to consider the operating conditions and electrical life, the motor protection circuit breaker selection principle:

1, the long delay current setting value is equal to the motor rated current;

2, the instantaneous setting value of the protection of cage-type asynchronous motor circuit breaker is equal to (8 ~ 15) times the motor rated current; for the protection of winding-type motor circuit breaker is equal to (3 ~ 6) times the motor rated current.

The return time of the 3,6x long delay current setting is greater than or equal to the actual start time of the motor. Press the load at the start of the severity, you can choose to return to 1, 2, 3, 8, 15s in a file.

Selection of Circuit Breaker for Wire Protection

1, the long delay setting value is less than or equal to the technical load current of the line;

2, the instantaneous action setting value is equal to (6 ~ 20) times the calculated load current of the line.

Selection of distribution circuit breakers

In addition to meeting the general circuit breaker conditions, the distribution circuit breaker needs to add the following principles:

1, the circuit breaker long delay action current setting value is less than or equal to the conductor to allow the carrying capacity. Equal to the use of cable, take the wire and cable capacity to allow 80% of the current;

2,3 times the length of the delay time action current set value of the return time ≥ the maximum starting current in the line motor start time;

3, the short delay action current setting value ≥ 1.1 × (Ijx + 1.35KIed), where Ijx is the calculated load current of the line; K is the starting current multiples of the motor; Ied is the rated current of the motor.

4, instantaneous current setting value ≥ 1.1 × (Jjx + K1KIedm), where K1 is the impact coefficient of the motor starting current; general K1 = 1.7 ~ 2; Iedm is the maximum rated current of a motor;

5, the short delay time ladder, according to the distribution system will be broken. The general time ladder is 2 ~ 3, the short delay time difference between each pole is 0.1 ~ 0.2s, depending on the short-time delay mechanism of the circuit breaker, the return time should ensure the selective action at all levels The After selecting the short delay step, it is better to check the thermal stability of the object to be protected.

Post time: Nov-26-2019