In the design of Delixi power distribution system, the selective cooperation between the upper and lower levels of the circuit breaker must have "selectivity, rapidity and sensitivity".
Selectivity is related to the cooperation of the upper and lower circuit breakers, and rapidity and sensitivity are related to the maintenance of the characteristics of the appliance itself and the way the line operates.
If the upper and lower circuit breakers cooperate properly, faulty circuits can be selectively blocked, and other fault-free circuits in the distribution system can continue to operate normally. Otherwise, the reliability of the power distribution system will be affected.
Cascaded maintenance is a detailed use of the circuit breaker's current limiting feature. Its main principle is to use the current limiting effect of the upper circuit breaker, and when selecting the lower circuit breaker, the circuit breaker with lower breaking ability can be selected, so as to achieve the purpose of reducing costs and saving costs.
The higher-level current-limiting circuit breaker QF1 disconnects the maximum expected short-circuit current at its device location. Since the upper and lower circuit breakers are connected in series in the power distribution system, when the lower circuit breaker QF2 short-circuits at its outlet, the actual value of the short-circuit current is much smaller than the expected short-circuit current there due to the current limiting effect of the upper circuit breaker QF1. That is to say, the breaking capacity of the lower circuit breaker QF2 is greatly enhanced with the assistance of the upper circuit breaker QF1 and exceeds its rated breaking ability. There are also certain conditions for this kind of cascade maintenance, for example, there can be no important load on the adjacent circuit (since once QF1 trips, the QF3 circuit will also be powered off), and the instantaneous setting values of QF1 and QF2 should be properly matched. The cascade data can only be confirmed by experiments, and the matching selection of the upper and lower circuit breakers can only be confirmed by the circuit breaker manufacturer.
Sensitivity of the circuit breaker
In order to ensure that the instantaneous or short-delay overcurrent trip of the circuit breaker can operate reliably in the event of the slightest short-circuit fault within the maintenance range of the system under the minimum operation mode of the system. The sensitivity of circuit breaker maintenance must meet the requirements of the "Design Code for Low-voltage Power Distribution" (CB50054-95), and its sensitivity should not be less than 1.3, that is, Sp=lk.min/Iop≥1.3. where lop is the operating current Ik of the instantaneous or short-delay overcurrent tripper. Min is the single-phase short-circuit current or two-phase short-circuit current at the end of the circuit breaker maintained by the circuit breaker under the minimum operation mode of the system, and Sp is the sensitivity of the circuit breaker.
When selecting Delixi circuit breaker, you should also pay attention to its sensitivity verification. With regard to selective circuit breakers with short-delay and instantaneous overcurrent trippers, it is only necessary to verify the operating sensitivity of the short-delay overcurrent tripper, but not the operating sensitivity of the instantaneous overcurrent tripper.
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