In the present low-voltage electrical industry, the plastic molded case Circuit Breaker and the need is becoming more, this has led to choose a suitable circuit breaker also becomes more important. However, in some electrical design program, the molded case circuit breakers of the correct and rational use and unsatisfactory, often overlooked selected manufacturers specifications, models, accessories and other electrical parameters, especially for some new MCCB The electrical parameters not thoroughly understood, marked incomplete, application type, the use of such ill-considered occasions and purposes. Selection of an inappropriate MCCB, leading to difficulties in complete factory orders, selective protection of variation, sensitivity, rationality does not meet the design specifications, not only the molded case circuit breakers do not make the best use, but resulted in waste, reduced with electric system reliability, affect the production of industrial and mining enterprises and people's lives. For this, the main reason I refer to some of its basic parameters, to discuss how to use molded case circuit breakers.
　　Common basic relevant symbol:
　　Inm - breaker frame degree current  A, it is the meaning of meaning that can be installed within the circuit breaker switch and release the maximum current value.
　　In - rated current of the circuit breaker  A, meaning it is referred to within the choice of the circuit breaker rated thermal type trip current value, without adjustable fixed heat release in the In = Ir1.
　　Ir1 - long delay circuit breaker setting current  A, meaning it is referred to overload the circuit breaker trip setting of the current value.
　　Ir2 - a short time delay circuit breaker setting current  A, meaning it refers to a short delay of the circuit breaker trip setting current, its value in the electronic trip unit adjustable for  2 ~ 12Irl so adjustable.
　　Ir3 - instantaneous breaker setting current  A, meaning it is referred to the circuit breaker instantaneous trip setting of the current, its value is not adjustable fixed in release, the distribution type is 5Irl, 10Irl two species, motor protection as 12Ir1, electronic adjustable, for  4 ~ 16Irl so adjustable.
　　Ir4 - single-phase circuit breaker setting current  A, it means that the circuit breaker within the meaning of the line or equipment to protect single-phase ground fault, ground fault protection trip setting current value, its value is 0.2 ~ 0.6Irl so adjustable.
　　Ire - Circuit Breaker of the residual operating current  A, it means that the circuit breaker within the meaning of the line or equipment to protect abnormal leakage current, leakage protection trip setting current value. Its value is 0.03/0.1/0.3/0.5A several.
　　Ir0 - breaker pre-alarm operating current  A, meaning it refers to the load current of the circuit breaker exceeds pre-set current, pre-alarm indication signal alarm device, its value is about adjustable  0.5 ~ lIr1 .
　　Ir2 - short trip time delay trip setting value  s, adjustable time of 0.05 ~ 0.45s.
　　Rated breaking capacity  Icu, Ics:
　　According to IEC947-2 "Low-voltage switchgear and control equipment, low voltage circuit breakers"  GBl4048.2 specification, low breaking capacity circuit breaker has two standards, one is the ultimate short-circuit breaking capacity  Icu that means it is a circuit breaker by breaking the current test cycle, do not assess the ability to carry current, that is no longer able to continue to use. Another is the short-circuit breaking capacity  Ics, it means that breaking the current circuit breaker, the need to continue to carry the rated current. That is, after troubleshooting can continue to use the line. These two values ??are the periodic component of the RMS. Manufacturers and the general design information in the% Icu Ics with the number indicated. Manufacturing practice at the circuit breaker has Ics =  25%, 50%, 75%, 100% Icu standard fourth gear ratio, and of the whole to the nearest integer. Breaking capacity of these two data is in the selection of MCCB, breaking capacity check when necessary. Circuit breaker manufacturers in the sample, in general, is provided in accordance with design specifications Icu, low-voltage circuit breaker breaking current  Ifd load-side short-circuit current should be larger than the maximum  Id. This creates a problem, in the end is the choice of Icu, Ics or as Ifd. I believe that the more important when the circuit breaker for power distribution lines or equipment, excluding short-circuit fault, called for the speedy restoration of electricity, it shall be considered by Ics value. If a general power distribution circuit, you can press the Icu value to the assessment. Time for action is less than 0.02s by Icu MCCB also to assess the value can be. In fact, introduced in the design manual for low-voltage circuit breaker at this time breaking capacity test requirement is that the circuit breaker breaking capacity  the impact of current RMS, or peak short circuit current should be larger than the first cycle started all current RMS or peak.
　　Short-time withstand current Icw
　　 protection features that have three sections of the MCCB, there is an electrical parameter is rated short time withstand current Icw, it means that we have often said that the dynamic thermal stability of the current concept that resistance in the 1s time by the current value, tolerance does not affect the continued use of the circuit breaker. It is mainly a short delay in the assessment of the state circuit breakers, circuit breaker to continue to ensure their electrical parameters unchanged, housing and components without deformation. Protection features in Sec MCCB, none of this value.
　　These are molded case Circuit Breaker on how to choose a part, of course, there are other factors and reference parameters for the appropriate selection.

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