For a power distribution circuit breaker, it has a class A and a class B: class A is a non-selective type, and class B is a selective type. The so-called selection type refers to the three-phase protection characteristics of the circuit breaker with long delay, short-circuit and short-circuit transient. The DW15 series, DW17(ME) series, AH series and DW40, DW45 series in the universal type (also called frame type) circuit breaker are mostly type B, and DZ5, DZ15, DZ20, TO, TG, CM1, TM30 and Some specifications of the HSM1 series and the universal DW15 and DW17 are due to only overload protection with long delays and short-circuit transients. They are non-selective type A circuit breakers. Selective protection, as shown in Figure 1.

figure 1
When the F point is short-circuited, only the QF2 circuit breaker near the F point is actuated, and the upper-order QF1 circuit breaker is not actuated. This is the selective protection (because the QF1 does not act, the failed QF3 and QF4 branches maintain power supply. ).
If both QF2 and QF1 b-curve circuit breaker are Class A circuit breakers, a short circuit occurs at point F. When the short-circuit current reaches a certain value, QF1 and QF2 operate at the same time. The QF1 circuit breaker and all the branches below it are all blacked out and are not selectively protected. .
The reason why selective protection can be realized is that QF1 is a b-curve circuit breaker, which has short-circuit short-delay performance. When the point F is short-circuited, the short-circuit current flows through the QF2 branch and also flows through the QF1 circuit, and the QF2 instantaneous action trips. The actuator action (usually its full breaking time is not greater than 0.02s), due to the short delay of QF1, QF1 b-curve circuit breaker will not act within 0.02s (its short delay ≥ 0.1s or 0.2, 0.3, 0.4s). When the QF2 action cuts off the faulty line, the entire system returns to normal.

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