When the F point is short-circuited, only the QF2 circuit breaker near point F is active while the QF1 circuit breaker is not active at the upper position. This is the selective protection (because QF1 does not operate, the failed QF3.QF4 branch remains powered ). If QF2 and QF1 are all Class A circuit breakers, short-circuit occurs at point F and when the short-circuit current reaches a certain value, QF1.QF2 will operate at the same time. The QF1 circuit breaker circuit and all the branches below it will not be selectively protected .
The reason for the selective protection is that QF1 is a Class B circuit breaker that has short-circuit short-time performance. When F is short-circuited, the short-circuit current flows through the QF2 branch and also through the QF1 circuit, and the QF2 instantaneous trip (Usually less than 0.02s), QF1 will not operate within 0.02s due to QF1's short delay (its short delay is ≥0.1s or 0.2.0.3.0.4s). The QF2 action cut off the fault line, the entire system returned to normal. Visible, that is to say it can choose Class A circuit breakers (including molded case and universal)