The current limiting molded case circuit breaker is widely used in low voltage plastic case circuit breaker. When the load of the MCCB is short circuited, the plastic case breaker is cut off before the short circuit current reaches the expected peak value in the future, which makes the energy generated by the actual short circuit current less than expected, which makes the mechanical stress and mechanical stress of the power grid and electric appliances suffer and The heat consumption is greatly reduced. The current limiting coefficient concept of the current limiting type plastic case circuit breaker is more than that of the general MCCB, which refers to the ratio of the actual breaking current peak to the expected short circuit current peak. The general current limiting molded case circuit breaker has the characteristics of quick breaking and limiting the rise of short circuit current, especially for the network where the extra short-circuit current may occur. But because the short circuit current of the current limiting plastic case breaker is greater than or equal to the setting value of its instantaneous release, it will trip in milliseconds, so it is not suitable for selective protection under the condition of lower plastic case breaker. In fact, the use category of current limiting molded case circuit breaker is marked as "a" on the product samples and Design Manual of the manufacturer of MCCB.

12. cascade protection

Cascade protection technology is the application form of current limiting characteristics of molded case circuit breaker. It uses the characteristics of Qfl of superior MCCB whose breaking capacity is greater than or equal to the expected short circuit current at lower k-point  see Figure 1. When the rated breaking capacity of the lower level is lower than the expected short circuit current at point K, the short circuit current through the line is limited by QF1 current limiting molded case breaker, which makes the short-circuit current that QF2 can not be separated, and is broken by QF1 and QF2 at the same time, so that QF2 can be successfully broken off the short circuit current larger than its rated breaking capacity. The actual actual situation of the lower and middle-level plastic case circuit breaker is invisible Breaking capacity is greatly "enhanced".

But cascade protection also has certain requirements. The breaking capacity of QF2 must be at least 50% of the maximum expected short circuit current at K. only in this way can it meet the requirements of article 8.6.25 of the civil regulations on the coordination of the characteristics of two protective appliances, so that the energy passing through the short circuit does not cause damage to the load side protective appliances and wires. Cascade protection is to solve the problem that the breaking capacity of MCCB is not enough by sacrificing partial selective protection. It is imperative to make careful decisions. Besides, the higher the breaking capacity of low-voltage MCCB is now, the higher the breaking capacity of the low-voltage molded case breaker of the same shell structure grade can be selected. For the current general low voltage power The distribution requirements of the network should be sufficient, so the author thinks that the non - 10000 must not be, as far as possible not to use cascade protection.