Moulded case circuit breakers are widely used in civil construction, national defence, mining, industry and agriculture and transport industries, for the distribution and transmission of electricity, circuit protection and control play a large role, its is a large number of use and a wide range of products.

As users do not have a deep or comprehensive understanding of the technical and characteristic requirements of plastic case circuit breakers, some concepts are particularly easy to confuse with each other, and some misunderstandings and deviations often occur in the practical application. In this article, the user in the use and selection of molded case circuit breaker is prone to problems to do an introduction, can help the user reasonable selection of the use of molded case circuit breaker.

Case circuit breaker frame rating current rating refers to the basic size of the same frame and plastic case can be installed in the largest striker rated current.

The rated current of the molded case circuit breaker refers to the current that can be passed through the detent in the circuit breaker for a long period of time, also known as the rated current of the circuit breaker detent.

There are a variety of case rating currents in the same series of molded case circuit breakers, and a variety of rated currents in the same case rating. For example, the DZ20 series has 100, 225, 400, 630, 800, 1250 and other case frame rating currents, while the 100 case frame rating current has 16A, 20A, 25A, 32A, 40A, 50A, 63A, 80A, 100A rating currents; the 225A case frame rating current has 100A, 125A, 160A, 180A, 200A, 225A rating currents. 180A, 200A, 225A rated current. 100A rated current is available in both DZ20-100 and DZ20-225 shell frame grades, but the circuit breaker volume shape and breaking capacity is not the same, so when choosing to fill in the model number in full that is, the specific shell frame grade rated current within the circuit breaker rated current. The current rating classification is selected according to a (1.25) priority factor: on the one hand, it is in line with and meets the needs of the maximum line and electrical components rated current; on the other hand, it is for standardisation in order to obtain the best possible use of the wire and the benefits of processing. The levels it therefore specifies are: 3(6), 8, 10, 12.5, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200, 250, 315, 400A, etc. Due to this provision, when the calculated load of the line is 90A, then only 100A specifications can be selected, thus affecting its protection performance to a certain extent.

The striker current rectification value refers to the striker adjustment to the action current value. Its refers to the number of times the rated current In, is the action current value, for example: overcurrent rectification to the current of 1.2, 1.3, 5, 10 times, it is written as Ir = 1.2In, 1.3In, 5In, 10In, etc.. Now some electronic decoupler, its overload long delay rated current is adjustable, the current adjusted, in fact, is still the rated current, is the maximum current that can be passed for a long time.

Rated working current is the actual working current of the circuit breaker when the contacts are fitted with auxiliary contacts (accessories) at a certain working voltage, the current is 3A or 6A for the control and protection circuit.

The rated insulation voltage is the voltage value for the design of the circuit breaker, the electrical clearance and creepage distance should be determined with reference to this value. Some circuit breakers do not specify the rated insulation voltage, the maximum value in the rated operating voltage should be regarded as the rated insulation voltage, in any case, the maximum rated operating voltage does not exceed the rated insulation voltage, the rated insulation voltage of the circuit breaker and the frequency test voltage is shown in Table 1.

The rated working voltage refers to the interrupting capacity and the voltage value associated with the use category, the rated working voltage of the moulded case circuit breaker is mostly 50Hz, 380V, but also 50Hz, 600V, the rated working voltage of 380V, 50Hz moulded case circuit breaker, is absolutely not allowed to be used on the supply voltage of 660V or 1140V.

The rated limit short-circuit breaking capacity is the breaking capacity under the specified conditions. After acting according to the prescribed test procedure, the circuit breaker is not considered to continue carrying its rated current.

The rated operating short-circuit breaking capacity is the breaking capacity under the specified conditions. After the prescribed test procedure, the circuit breaker shall be considered to continue to carry its rated current.

The rated control supply voltage is the voltage at which the moulded case circuit breaker is fitted with a sub-excitation decoupler and motorised mechanism accessories. There are two voltages, AC and DC, which must be specified when choosing AC or DC.

At the same time in order to meet the needs of different users, many domestic circuit breaker manufacturers, for the same shell frame level rated current short-circuit breaking capacity is divided into different levels, such as Changshu switch factory CM1-100 is divided into C basic type (25 ~ 35kA), L standard type (35 ~ 50kA), M higher breaking type (50 ~ 75kA), H high breaking type (85 ~ 100kA). Users in the selection, to meet the limit of the circuit breaker short-circuit breaking capacity, the line is expected to meet the requirements of the short-circuit current, no need to artificially add the insurance factor, so as not to cause waste.