For the distribution type circuit breakers, it has Class A and Class B: Class A is a non-selective type, Class B is an optional type. Selective type refers to the circuit breakers with overload long delay, short circuit short delay and short circuit instantaneous three protection characteristics. The DW15 series, DW17 (ME) series, AH series and DW40, DW45 series are mostly B type, and DZ5, DZ15, DZ20, TO, TG, CM1, TM30 and HSM1 series and universal DW15, DW17 some of the specifications due to only overload long delay, short circuit instantaneous two sections of protection, they are non-selective type A circuit breakers. Selective protection, as shown in Figure 1.
   

  Figure 1   When the F point is short, only the QF2 breaker near point F, and the upper QF1 circuit breaker does not operate, this is selective protection (due to QF1 does not move, so that no failure QF3, QF4 branch to maintain power supply).
  If QF2 and QF1 are class A circuit breakers, then F point short circuit, short circuit current value reaches a certain value, QF1, QF2 at the same time action, QF1 circuit breaker circuit and its all branches under the power failure, it is not selective protection The
  The reason for the selective protection is that QF1 is a Class B circuit breaker, which has short-circuit short-term performance. When the F point is short-circuited, the short-circuit current flows through the QF2 branch and flows through the QF1 , QF2 instantaneous action release action (usually its full break time is not greater than 0.02s), due to QF1 short delay, QF1 within 0.02s will not move (its short delay ≥ 0.1s or 0.2,0.3 , 0.4s). In the QF2 action cut off the fault line, the whole system to resume normal.