Cascade protection of circuit breakers

In the design of the power distribution system, the selective coordination between the upper and lower stages of the circuit breaker must be "selective, fast and sensitive".
Selectivity is related to the coordination between the upper and lower circuit breakers, while rapidity and sensitivity are related to the characteristics of the protection appliance itself and the operation mode of the line, respectively.
If the upper and lower circuit breakers are properly coordinated, the fault circuit can be selectively removed to ensure that the other fault-free circuits of the power distribution system continue to work normally. On the contrary, it affects the reliability of the power distribution system.
Cascade protection is the specific application of the current limiting characteristics of the circuit breaker, and its main principle is to use the current limiting effect of the upper circuit breaker, and when selecting the lower circuit breaker, the circuit breaker with lower breaking capacity can be selected to achieve the purpose of reducing costs and saving costs.
The upper current-limiting circuit breaker QF1 can break the maximum expected short-circuit current at its installation place, because the circuit breakers of the upper and lower stages in the power distribution system are installed in series, when a short circuit occurs at the outlet of the lower stage and the circuit breaker QF2, the actual value of the short-circuit current is much smaller than the expected short-circuit current at the upper stage due to the current limiting effect of the upper circuit breaker QF1;
That is to say, the breaking capacity of the lower circuit breaker QF2 is greatly enhanced with the help of the upper circuit breaker QF1, which exceeds its rated breaking capacity. There are also certain conditions for this cascade protection, such as the adjacent circuits must not have important loads (because once QF1 trips the QF3 circuit, there will also be a power outage), and the instantaneous setting value of QF1 and the instantaneous setting value of QF2 must also be properly matched. The cascade data can only be determined experimentally, and the mating selection of the upper and lower circuit breakers can only be determined by the circuit breaker manufacturer.