In the design of civil buildings, low-voltage circuit breakers are primarily utilized for the protection and operation of circuits against overload, short circuits, overcurrent, voltage loss, undervoltage, ground faults, and leakage currents, as well as for automatic switching between dual power sources and for the infrequent starting of electric motors. In addition to adhering to fundamental principles regarding the environmental characteristics of low-voltage electrical equipment (refer to the *Design Manual for Industrial and Civil Power Distribution*), the selection of circuit breakers must also take the following conditions into account:
1) The rated voltage of the circuit breaker shall not be less than the rated voltage of the circuit;
2) The rated current of the circuit breaker-as well as the rated current of its overcurrent trip unit-shall not be less than the calculated current of the circuit;
3) The rated short-circuit breaking capacity of the circuit breaker shall not be less than the maximum short-circuit current present in the circuit;
4) When selecting distribution circuit breakers with selective coordination capabilities, consideration must be given to the short-time short-circuit making and breaking capacity, as well as the coordination of time-delay protection between protection levels;
5) The rated voltage of the circuit breaker's undervoltage trip unit shall be equal to the rated voltage of the circuit;
6) When utilized for motor protection, the selection of the circuit breaker must account for the motor's starting current to ensure that the breaker does not trip during the motor's starting period; design calculations for this application should refer to the *Design Manual for Industrial and Civil Power Distribution*;
7) The selection of circuit breakers must also take into consideration the selective coordination between circuit breakers themselves, as well as between circuit breakers and fuses.