(1) The rated working voltage of MCCB is greater than or equal to the rated voltage of line.  

(2) The rated current of MCCB is greater than or equal to the calculated load current of line.

(3) The rated short-circuit making and breaking capacity of MCCB is greater than or equal to the maximum short-circuit current that may occur in the line, and it is generally calculated according to the effective value.

If the rated current of MCCB is consistent with the requirements, but the rated short-circuit breaking capacity is less than the maximum short-circuit current of the line at the installation point of MCCB, the rated current of MCCB must be increased, and the rated current of overcurrent release shall be selected according to the load current calculated by the line. If this can not meet the requirements, the following three solutions can be considered:

1) Cascaded protection (or cascade protection) is adopted to improve the short-circuit breaking capacity by using the upper MCCB and the MCCB acting together. When adopting this scheme, the instantaneous current of the release of the upper MCCB should be set at about 80% of the rated short-circuit breaking capacity of the lower MCCB.

2) Current limiting moulded case circuit breaker is adopted.

3) Molded case circuit breaker and backup fuse are adopted.

(4) The single phase to ground short-circuit current at the end of the line is greater than or equal to 1.25 times the setting current of instantaneous (or short delay) release of MCCB. This is especially important when the load current is small and the distribution line is long. Because when the line is long, the terminal short-circuit current is smaller, and the single phase to ground short-circuit current is smaller. In the three-phase four wire system, when the phase zero short circuit occurs, the short-circuit current to the ground is even smaller, sometimes smaller than the current set by the channel current release, so the over-current release can not act, so it loses protection in the case of single phase to ground. In this case, it is considered to install a current transformer on the zero line (secondary current relay, when short circuit to ground, the relay acts to break the MCCB), or use the line with zero sequence current transformer (or leakage relay) to solve the problem. When using these methods, the neutral point of transformer should be grounded.

(5) The rated voltage of under voltage release of MCCB is equal to the rated voltage of line.

Whether undervoltage protection is needed or not depends on the use requirements. Not all moulded case circuit breakers need undervoltage release. In some systems with poor power supply quality, choosing moulded case circuit breaker with undervoltage protection will cause unwanted power failure due to voltage fluctuation. If undervoltage release is necessary, appropriate time delay shall be considered.

(6) For moulded case circuit breaker with short delay, if it is equipped with undervoltage release, the undervoltage release must be delayed, and its delay time shall be greater than or equal to the short circuit delay time.

(7) Rated voltage of shunt release of MCCB is equal to control power supply voltage.

(8)The rated working voltage of the electric drive mechanism is equal to the control power supply voltage.