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Air-insulated frame circuit breakers, molded case circuit breakers, and air switches are common low-voltage circuit breakers used in industrial process control. Changhui Instruments provides a detailed explanation of the structure of low-voltage circuit breakers, the working principle of trip devices, the classification of their applications, as well as their various functions, thereby facilitating electricians and instrument technicians in the proper use and selection of such circuit breakers. Uses and functions of low-voltage circuit breakers: A circuit breaker is a switching device that can connect to, carry, and disconnect the current in circuits operating under normal conditions; it can also, in circuits operating abnormally (due to overload or short circuits), connect to the current under specified conditions, carry it for a certain period of time, and then disconnect it. Low-voltage circuit breakers are often used as power switches in low-voltage distribution boxes and cabinets, and they can automatically cut off the circuit to provide protection when faults such as overcurrent (overload and short circuits), phase loss, and electric leakage occur in the circuit. The **standard** followed in the manufacturing and use of low-voltage circuit breakers is GB14048.2-2008, whereas the standard for circuit breakers used in households is IEC60898. Low-voltage circuit breakers can be classified into three types based on their structural design. The first type is the air-insulated frame circuit breaker; its international common name is Air Circuit Breaker, abbreviated as ACB circuit breaker ； The second type is the molded case circuit breaker; its international common name is Moulded case circuit breaker, abbreviated as MCCB circuit breaker ； The third type is the miniature circuit breaker; its internationally recognized name is Micro Circuit Breaker, abbreviated as MCB. The circuit breakers used for home power distribution are MCBs, and are often referred to as air switches. Low-voltage circuit breakers can also be divided into distribution-type circuit breakers and motor protection circuit breakers, as well as circuit breakers specifically designed for lighting circuits, and residual current circuit breakers, among others. The categories of applications for low-voltage circuit breakers are shown in Table 1. Table 1: Applications and classifications of low-voltage circuit breakers: http://yunrun.com.cn/upload/202001/30/202001302136569593.png As a fully functional low-voltage circuit breaker, its basic functions are listed in Table 2. Table 2: Various functions of circuit breakers: http://yunrun.com.cn/upload/202001/30/202001302147372080.png The structure of low-voltage circuit breakers and the working principle of their trip units: When a circuit breaker is used to connect or disconnect circuits, it does so by operating the handle of the manual operating mechanism (referred to as manual operation), or by using an electric operating mechanism (referred to as electric operation), thereby closing or opening the circuit breaker’s moving and stationary contacts. When the circuit in which the circuit breaker is installed experiences overload, the bimetallic element in the circuit breaker’s thermal trip device heats up (either directly or as a result of heating elements located near it), which causes it to deform and bend; this in turn releases the latch and causes the circuit breaker to trip. Thermal trip devices are generally used for overload protection. When a short circuit occurs in the circuit to which the circuit breaker is connected, the short-circuit current attracts the moving armature of the magnetic release, which in turn activates the pulling mechanism to cause the circuit breaker to trip. Magnetic release devices are generally used for short-circuit protection. When the voltage in the circuit to which the circuit breaker is connected falls below 70% of Un (the rated voltage), the under-voltage release will trigger the circuit breaker to perform a tripping operation. This type of tripping is known as under-voltage tripping ； When the operator needs to operate the circuit breaker to trip from a distance, a release relay can be used. A shunt release allows for remote operation of the circuit breaker. The trip unit of a circuit breaker includes sensing elements for temperature, current, and voltage, transmission elements, monitoring and control elements, and actuating elements. Based on the measurement and control method, circuit breaker trip units can be divided into thermal-magnetic trip units and electronic trip units, as shown in Figures 1 and 2. Figure 1: Schematic diagram of the structure of a circuit breaker with a thermomagnetic trip unit. Figure 2: Schematic diagram of the structure of a circuit breaker with an electronic trip unit. From Figures 1 and 2, we can see that the main contacts and auxiliary contacts are connected by a drive rod; when the operating handle is pushed counterclockwise, the closing force is transmitted to the drive rod through the free-trip mechanism, causing the contacts to close. Finally, the latch locks the free-breaking mechanism, and the protected circuit is connected. Let’s first look at the thermal overload relay in Figure 1: To provide overload protection, the thermal overload relay is equipped with a bimetallic strip that measures the overload current. When the overcurrent is not high, the thermal bimetallic strip bends slowly (in inverse proportion to the current level); after a certain delay, it pushes the tripping shaft, causing the mechanism to trigger tripping (thermomagnetic type). Let’s take a look at the magnetic release in Figure 2: when a short-circuit current occurs, and the current is high enough to generate an electromotive force in the air gap of the release core that is sufficient to overcome the force exerted by the return spring, the core moves upward rapidly, pushing the release shaft and causing the mechanism to trip instantly. Looking at the measurement system in Figure 2, when an overcurrent occurs, the Rogowski coil in the overcurrent release device processes the overcurrent signal and triggers the mechanism to release. It can achieve protection characteristics of long delay for overload, short delay for short circuits, and instantaneous operation for high short-circuit currents. Transmission mechanisms can be either manually operated or electrically operated. Electric operation is further divided into electromagnet operation and motor operation. The principle behind using a motor-driven actuator is as follows: the motor reduces the speed of rotation through a gear system, which in turn compresses the energy-storing spring until sufficient energy is stored; this energy is then released to drive the actuator to close rapidly. The under-voltage release in Figures 1 and 2 enables the circuit breaker to provide under-voltage protection, while the auxiliary release allows the circuit breaker to be controlled remotely. When the above components are placed inside a plastic enclosure, it becomes a molded case circuit breaker MCCB; when all the components are installed in a frame made of metal, it becomes a frame-type circuit breaker ACB. The rated current of frame-type circuit breakers is much higher than that of molded case circuit breakers. Figure 3 Schematic diagram of the principle of an electronic circuit breaker trip unit. An microprocessor is installed in the electronic trip unit, and electronic technology based on this microprocessor is used to measure and protect against overload and short-circuit currents. In Figures 2 and 3, the current sampling signal is obtained through a hollow current transformer, namely a Rogowski coil. Hollow current transformers are used to avoid the flux saturation effect of ferromagnetic current transformers when measuring overload and short-circuit currents. The voltage acquisition device of the circuit breaker is used to collect three-phase current information in order to provide under-voltage and over-voltage protection. The operating power supply for the circuit breaker comes from the energy obtained by the quick protection current transformer. The purpose of using fast-acting protection current transformers is to prevent destructive impacts on the power supply system when large currents flow in the primary circuit of the circuit breaker. In Figure 3, analog quantities such as current and voltage are input into the CPU through analog acquisition electronic switches; the CPU issues gating controls to these switches in order to enable the high-speed sequential input of various analog quantities ； The various digital signals of the circuit breaker are input to the CPU from the digital signal acquisition electronic switches. The CPU’s outputs include: an LED display that shows measurement and control information as well as analog data; a keyboard and encoding circuit for enabling human-machine interaction; drive circuits for output relays to carry out various tripping operations; and an RS485 drive circuit for exchanging information with higher-level systems. Source: Changhui Instruments http://yunrun.com.cn/