The case circuit breaker is an overload and short-circuit maintenance appliance for low-voltage three-phase, four-wire 220/380V circuits, because its breaking is usually lower than that of the all-round circuit breaker and is therefore suitable as a second, third or fourth level maintenance device for low-voltage circuits and motors; and because of its low number of auxiliary contacts, it is not suitable for active operation and maintenance of appliances or can only be used for one-time remote switching. . Low-voltage circuit breakers are not suitable for servicing appliances with more than 4 levels from the main circuit breaker to the end, otherwise selectivity would be lost. The current level of the additional current In is normally not adjusted. The additional current Inm per frame is matched to the additional current In of the circuit breaker by a fixed level difference. The following is an example of a DZ20Y type with Inm = 100A. There are 8 levels of In, 16A, 20A, 32A, 40A, 50A, 63A, 80A and 100A, but it is important to know that Inm>In>Ib.

Low voltage tests on how to select second and third level circuit breakers also require attention to.

Short-circuit currents in distribution systems Id; overload operation of distribution lines or motors.

frequent starting and reverse braking of high power heavy load motors.

The operating ambient temperature of the circuit breaker and many other factors are not conducive to safe and reliable operation.

Usually, after determining the additional current Inm of the circuit breaker frame, In is suitable for selection one level larger than the calculated current of the line, while the time/current characteristics of the upper and lower circuit breakers and the selectivity of the overcurrent holding derailer for the additional current In of the circuit breaker should also be considered when selecting the model of the moulded case low voltage circuit breaker. The functional indicators are as follows.

1. Additional voltage: AC240V, AC400V, AC415V, AC500V, AC690V, DC250V.

2. Additional current Inm of the case frame of the moulded case circuit breaker: the additional current level of each brand series is different, you need to read the information of the brand you want to choose (e.g. Shanghai Taisi Electric) Choose later.

3. Additional current In of the circuit breaker: the additional current level varies from one brand series to another, and the skill data must be selected accordingly to the load current or motor power of the distribution line depicted and the product chosen.

4. Additional operating and breaking capacity Ics: usually the circuit breaker product level is high, i.e. the breaking capacity is large. The strongest breaking capacity is a high quality product.

5. Additional limit breaking capacity Icu: Icu is equal to 0.5-1 times of Ics, i.e. no longer repeated after breaking the limit value.

6. Additional short-time withstand current Icw: overload current that can be withstood within 1s without affecting the continuous work of the circuit breaker.

7. Number of poles: the number of poles varies from series to series, generally 2-4 poles.

8. Time/current characteristic curve of the overcurrent protection striker: the characteristic curve varies from series to series, which is a point that illustrates the need for personnel to consider the selectivity of the upper and lower circuit breakers. Important parameters. The decoupling time is inversely related to the additional current overload action multiplier. Before making a decision, it is necessary to read the technical data of the product to be selected.

9. Mechanical life: The mechanical life varies from series to series and is directly proportional to the quality class of the circuit breaker. Typically, the number of operations is between 3000 and 25000.

10. Electrical life: the electrical life of each series of products is different, inversely proportional to the additional capacity of the moulded case circuit breaker and directly proportional to the quality grade of the circuit breaker, usually 1000-5000 operations.

11. Moulded case circuit breaker installation: the circuit breaker is usually fixed straight installation.

12. Case volume scale: the volume scale of each series is completely different, and you need to check the sample material of each series. Check the number of auxiliary contacts of the circuit breaker, which should meet the needs of the distribution system.