It is necessary to consider limiting the fault of the system to the minimum range, so as to prevent expanding the blackout area in case of fault. Therefore, the following selection principles should be added:

(1) the setting value of long delay action current of MCCB is less than or equal to the allowable current carrying capacity of conductor. For the case of wire and cable, 80% of the allowable current carrying capacity of wire and cable is recommended.

(2) the return time of the setting value of 3 times long delay action current is greater than or equal to the starting time of the motor with the maximum starting current in the line.

(3) the setting value of short delay action current is selected according to the formula

In the formula, ISD -- short delay action current, a;

ID -- calculated load current of line, a;

KS -- starting current multiple of motor

In -- rated current of motor, a.

(4) The setting value of instantaneous current is selected according to the following formula

In the formula, iin -- instantaneous current, a;

KP -- impact coefficient of motor starting current, generally KP = 1.7-2; INM -- rated current of the largest motor, a.

(5)The time step of short delay is determined according to the section of distribution system, and the general time step is 2-3. The short delay time difference between each stage is 0.1-0.2s, and its return time should ensure the selective action of each stage. After the short delay step is selected, it is better to check the thermal stability of the protected object.