When selecting low-voltage electrical appliances, choose circuit breakers with different short-circuit breaking capacities to adapt to the different expected short-circuit current requirements of the circuit (when I is the same). The selection principle for circuit breakers is: the short-circuit breaking capacity of the circuit breaker ≥ the expected short-circuit current of the circuit.
1. Different types of circuit breakers should be selected for different loads. The most common loads of low-voltage electrical appliances are power distribution lines, motors, and household and similar household appliances (lighting, household appliances, etc.). Most of the DW15 series, DW17(ME) series, AH series, and DW40 and DW45 series of universal (also known as frame type) circuit breakers are type B, while the DZ5, DZ15, DZ20, TO, TG, CM1, TM30, and HSM1 series, and some specifications of the universal DW15 and DW17 series, because they only have two-stage protection (overload long-delay and short-circuit instantaneous), belong to the non-selective type A circuit breaker. Selective protection.
When a short circuit occurs at point F, only the circuit breaker QF2, which is closest to point F, will trip, while the circuit breaker QF1, located above point F, will not trip. This is selective protection (because QF1 does not trip, the unaffected branches QF3 and QF4 are kept powered). If both QF2 and QF1 are Class A circuit breakers, then when a short circuit occurs at point F and the short-circuit current reaches a certain value, QF1 and QF2 will trip simultaneously, de-energizing the entire circuit of QF1 and its downstream branches. This is no longer selective protection. The reason selective protection is possible is that QF1 is a Class B circuit breaker, which has short-circuit short-delay performance. When a short circuit occurs at point F, the short-circuit current flows through both the QF2 branch and the QF1 circuit. The instantaneous trip unit of QF2 trips (its total breaking time is typically no more than 0.02s). Due to the short-delay of QF1, QF1 will not trip within 0.02s (its short-delay is ≥0.1s or 0.2, 0.3, 0.4s). When QF2 trips and disconnects the faulty line, the entire system returns to normal. Therefore, it can be used with Class A circuit breakers (including molded case and universal types).
