Low voltage circuit breakers are widely used in low-voltage distribution systems in buildings. They are protective electrical components with functions such as circuit breaker protection, overload protection, control, and isolation. They are suitable for industrial and civil terminal low-voltage distribution systems. If the low-voltage circuit breaker is not designed properly in the distribution system, it will affect the normal operation of the power supply circuit. Therefore, when designing low-voltage distribution systems, attention should be paid to details such as the selectivity and cascade protection of low-voltage circuit breakers.

The selectivity of low-voltage circuit breakers. In order to ensure the reliability of low-voltage distribution systems, the selectivity of low-voltage circuit breakers has become an important aspect of terminal low-voltage distribution system design.

In a distribution system protected by circuit breakers: when an electrical fault occurs, the circuit breaker closest to the fault point will cut off the fault, while other levels of circuit breakers will not operate, thereby limiting the power outage caused by the fault to the minimum range, so that other faultless power supply circuits can still maintain normal power supply. This is the selectivity required for low-voltage circuit breakers. The selectivity of low-voltage circuit breakers plays a very important role in the design of low-voltage distribution systems. It can bring convenience to users and ensure the continuity of the power supply circuit. Without selective protection, once an electrical failure occurs in household appliances, the continuity of the distribution circuit cannot be guaranteed, causing household appliances such as refrigerators and range hoods to be in a shutdown and standby state, which affects the daily life of users.

Low voltage circuit breakers in low-voltage distribution systems can be divided into two categories based on their protective performance: selective and non selective. There are two types of selective low-voltage circuit breakers: two-stage protection and three-stage protection. The instantaneous and short delay characteristics are suitable for short-circuit action, while the long delay characteristics are suitable for overload protection. Non selective low-voltage circuit breakers generally operate instantaneously and are only used for short circuit protection, while others operate with long delay and are only used for overload protection.

In low-voltage distribution systems, if the upper level circuit breaker adopts a selective circuit breaker, the lower level circuit breaker adopts a non selective circuit breaker or selective circuit breaker, mainly by utilizing the different delay actions or delay action times of the short delay release to obtain selectivity. When using the delay action of the upper level circuit breaker, attention should be paid to the following issues: firstly, whether the lower level is a selective circuit breaker or a non selective circuit breaker, the instantaneous overcurrent release setting current of the upper level circuit breaker should generally not be less than 1.1 times the maximum three-phase short-circuit current at the output end of the lower level circuit breaker. Secondly, if the next level is a non selective circuit breaker, in order to prevent the sensitivity of the instantaneous action of this level from being insufficient when a short circuit current occurs in the circuit protected by the next level circuit breaker, causing the previous level's short delay overcurrent release to act first and lose its selectivity, the set current of the short delay overcurrent release of the previous level circuit breaker is generally not less than 1.2 times that of the next level's instantaneous overcurrent release. Thirdly, if the next level is also a selective circuit breaker, to ensure selectivity, the short delay action time of the previous level circuit breaker should be at least 0.1 seconds longer than that of the next level circuit breaker.

Cascade protection of low-voltage circuit breakers. In the design of low-voltage distribution systems, the selective coordination between the upper and lower stages of low-voltage circuit breakers must have selectivity, speed, and sensitivity. Among them, selectivity is related to the coordination between the upper and lower low-voltage circuit breakers, while speed and sensitivity are respectively related to the characteristics of the protective equipment itself and the operation mode of the circuit. If the upper and lower circuit breakers are properly coordinated, the faulty circuit can be selectively cut off to ensure the normal operation of other fault free circuits in the distribution system; On the contrary, it affects the reliability of the distribution system. Cascade protection is a specific application of the current limiting characteristics of circuit breakers. Its main principle is to utilize the current limiting effect of the higher-level circuit breaker. When selecting the lower level circuit breaker, the lower breaking capacity circuit breaker can be selected to achieve the goal of reducing costs and saving costs.

The upper level current limiting circuit breaker can break the maximum expected short-circuit current at its installation location, because the lower level low-voltage circuit breakers in the low-voltage distribution system are installed in series. When a short circuit occurs at the outlet of the lower level low-voltage circuit breaker, the actual value of the short-circuit current is much smaller than the expected short-circuit current at that location due to the current limiting function of the upper level low-voltage circuit breaker. In other words, the breaking capacity of the lower level low-voltage circuit breaker is greatly enhanced with the help of the upper level low-voltage circuit breaker, exceeding its rated breaking capacity. Cascade data can only be determined experimentally, and the selection of coordination between upper and lower low-voltage circuit breakers can only be determined by the low-voltage circuit breaker manufacturer.

Sensitivity of low-voltage circuit breakers. To ensure that the instantaneous or short delay overcurrent release of the low-voltage circuit breaker can complete reliable action in the event of the slightest short circuit fault within its protection range under the minimum operating mode of the system. The sensitivity of low-voltage circuit breaker protection must meet the requirements of the Design Specification for Low Voltage Distribution, which stipulates that its sensitivity should not be less than 1.3.

At the same time, when selecting low-voltage circuit breakers, attention should also be paid to verifying their sensitivity. For selective circuit breakers with both short delay and instantaneous overcurrent releases, only the sensitivity of the short delay overcurrent release needs to be verified, and there is no need to verify the sensitivity of the instantaneous overcurrent release.

The ambient temperature of low-voltage circuit breakers. The overload protection of low-voltage circuit breakers relies on thermal release devices. Usually, the rated current of the thermal release device of low-voltage circuit breakers is set by the manufacturer according to standards at a reference temperature of 30 degrees Celsius.

The thermal release is made of a set of bimetallic sheets. When the circuit is overloaded and the overload current flows through the heating resistance wire, causing the bimetallic sheet to heat, deform, and bend, the thermal release will push the hook open, causing the low-voltage circuit breaker contacts to open. The thermal release of low-voltage circuit breakers is directly related to the ambient temperature. If the ambient temperature changes, it will cause a change in the rated current value of the low-voltage circuit breaker.

Low voltage circuit breakers are generally arranged in an orderly manner and fixed on the distribution panel, and then installed inside the distribution box. The installation methods of distribution boxes are divided into two types: exposed and concealed. The heat dissipation effect of exposed distribution boxes is better than that of concealed distribution boxes. The air inside the concealed distribution box is not suitable for convection, and its heat dissipation effect is poor, causing the temperature rise of the surrounding air inside the distribution box due to the temperature rise of the low-voltage circuit breaker.

The actual working temperature of low-voltage circuit breakers is about 10 to 15 degrees Celsius higher than the ambient temperature. When the ambient temperature is greater than or less than the calibration temperature value, we must correct the rated current value of the low-voltage circuit breaker according to the temperature and current carrying capacity correction coefficient table provided by the manufacturer.[Kailong Electrics]