Key Points for Selecting Frame Circuit Breakers and Engineering Design

In civil building design, frame circuit breakers are mainly used for overload, short circuit, overcurrent, voltage loss, under-voltage, grounding, leakage, automatic switching between dual power sources, and protection and operation during infrequent motor startups. In addition to following the basic principle of observing the characteristics of the operating environment of low-voltage electrical equipment, the following conditions should also be considered:
The rated voltage of the frame circuit breaker must not be lower than the rated voltage of the line.
2) The rated current of the frame circuit breaker and the rated current of the overcurrent trip device should not be less than the calculated current of the line.
3) The rated short-circuit breaking capacity of the frame circuit breaker shall not be less than the maximum short-circuit current in the line.
4) The selective frame circuit breaker should take into account the short-time delay short-circuit opening and closing capability as well as the inter-stage coordination of the delay protection.
5) The rated voltage of the under-voltage trip unit of the frame circuit breaker is equal to the rated voltage of the line.
6) When using it for motor protection, when selecting the circuit breaker, the starting current of the motor should be taken into account. The circuit breaker should not operate during the starting period; the design calculation can be found in “Industrial and Civil Distribution Design Manual”.
7) The selection of circuit breakers should also take into account the selective coordination between the circuit breakers and the fuses.
When circuit breakers are operated in conjunction, the instantaneous tripping action value of the upper-level circuit breaker should be taken into account. It should be greater than the expected short-circuit current at the output end of the lower-level circuit breaker. If the short-circuit impedance values of the circuit components at the two-level frame circuit breakers are similar, the upper-level circuit breaker can choose to trip with a short delay.
(2) When the short-circuit current of the current-limiting circuit breaker is greater than or equal to its instantaneous tripping setting value, it will trip within a few milliseconds. Therefore, the lower-level protective devices should not rely on the circuit breaker to meet the requirements of selective protection.
(3) When the time limit of the short-delay circuit breaker is set at the maximum delay time, its switching capability will decrease. Therefore, in the selective protection circuit, the short-delay switching capability of the frame circuit breaker should meet the requirements.
(4) It is also necessary to consider that the short-circuit delay return characteristic of the upper-level circuit breaker should not intersect with the action characteristic time curve of the lower-level frame circuit breaker, and the short-delay characteristic curve should not intersect with the instantaneous characteristic curve.
(5) When circuit breakers and fuses are used together, the coordination between the upper and lower levels should be considered. The ampere-second characteristic curves of the frame circuit breaker and the fuse should be compared to ensure the selective protection during short-circuit current.