It is understood that the most common loads of low-voltage electrical appliances are distribution lines, electric motors and household and similar households (lighting, household appliances, etc.). Correspondingly, there are distribution protection type, motor protection type and household and similar household protection type circuit breakers. The protection properties and protection characteristics of these three types of circuit breakers are different. For distribution type circuit breakers, it has a class A and a class B: Class A is non-selective and Class B is selective. The so-called selection type refers to the three-stage protection characteristic of the circuit breaker with overload long delay, short short delay and short circuit transient. Most of the DW15 series, DW17 (ME) series, AH series and DW40, DW45 series in the universal (also known as frame type) circuit breakers are type B, while DZ5, DZ15, DZ20, TO, TG, CM1, TM30 and Some specifications of HSM1 series and universal DW15.DW17 are only non-selective type A circuit breakers because they only have long delay of overload and short circuit protection of short circuit. Selective protection.
When the F point is short-circuited, only the QF2 circuit breaker near the F point acts, and the upper-direction QF1 circuit breaker does not operate. This is the selective protection (because the QF1 does not operate, the QF3.QF4 branch that has not failed remains powered. ). If QF2 and QF1 are both Class A circuit breakers, the short circuit at point F will occur. When the short-circuit current value reaches a certain value, QF1.QF2 will operate at the same time. The QF1 circuit breaker circuit and the branches below it will be powered off, which is not selective protection. .
The reason for the selective protection is that QF1 is a class B circuit breaker, which has short-circuit short-delay performance. When the F-point 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 action of the device (usually its full break time is not more than 0.02s). Due to the short delay of QF1, QF1 will not operate within 0.02s (its short delay is ≥0.1s or 0.2.0.3.0.4s). When the QF2 action cuts off the faulty line, the entire system is restored to normal.
It can be seen that if the requirements of selective protection are to be met, the circuit breaker of the upper level should use a type B circuit breaker with three sections of protection. For low-voltage electrical appliances, for motor-protected circuit breakers that directly protect the motor, it is sufficient for long-term overload and short-circuit transient two-stage protection, that is, it can choose Class A circuit breakers (including molded case and universal type). In addition to the performance of distribution protection, DZ5, DZ15, TO, TG, GM1, TM30, HSM1 and DW15 have the functions of protecting the motor.
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Label: What are the precautions when using low-voltage electrical appliances?
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