Low voltage apparatus shall select different types of circuit breakers with short-circuit breaking capacity to meet the needs of different expected short-circuit currents of the line (when I is in the same situation). The selection principle of circuit breaker is: the short-circuit breaking capacity of circuit breaker ≥ the expected short-circuit current of the line.

1. Different types of circuit breakers shall be selected for low-voltage apparatus with different loads

The most common loads of low-voltage appliances are distribution lines, motors and household and similar household (lighting, household appliances, etc.). Most of DW15 series, dw17 (me) series, ah series, DW40 and DW45 series of universal (also known as frame type) circuit breakers are type B, while DZ5, dz15, DZ20, to, TG, CM1, tm30 and hsm1 series and some specifications of universal dw15.dw17 are non selective type a circuit breakers because they only have two-stage protection of overload long delay and short circuit instantaneous. Selective protection.

When point F is short circuited, only the QF2 circuit breaker close to point F acts, while the QF1 circuit breaker in the upper direction does not act, which is selective protection (because QF1 does not act, the qf3.qf4 branch without fault remains powered). If both QF2 and QF1 are class a circuit breakers, there is a short circuit at point F. when the short-circuit current reaches a certain value, QF1 and QF2 act at the same time, and all QF1 circuit breaker circuits and their branches are powered off, so it is not selective protection. The reason why selective protection can be realized is that QF1 is a class B circuit breaker, which has short circuit and short delay.

When point F is short circuited, the short-circuit current flows through QF2 branch and QF1 circuit. The instantaneous action release of QF2 acts (usually its full breaking time is not greater than 0.02s). Due to the short delay of QF1, QF1 will not act within 0.02s (its short delay is ≥ 0.1s or 0.2.0.3.0.4s). When QF2 acts to cut off the fault line, the whole system returns to normal. It can be seen that it can select class a circuit breaker (including molded case type and universal type)