I. Introduction Low-voltage circuit breakers are divided into two main categories: universal circuit breakers and molded case circuit breakers. Currently, China mainly produces universal circuit breakers such as the DW15, DW16, DW17 (ME), and DW45 series, and molded case circuit breakers such as the DZ20, CMI, and TM30 series. Circuit breakers consist of the main body and accessories. The main body, without any accessories, ensures smooth circuit closing and opening, and automatically disconnects faults in the event of overload or short circuit. Accessories, as a derivative supplement to the circuit breaker's function, add control methods and expand protection functions, making the circuit breaker more widely applicable, with more complete protection functions, and offering more operation and installation methods. Currently, circuit breaker accessories have become an integral and important part of the circuit breaker. However, more accessories are not necessarily better. Accessories should be used appropriately based on the specific control and protection circuits to avoid unnecessary waste. It's also crucial to distinguish the voltage level, AC or DC, and the number of auxiliary contact pairs. Improper application can not only fail to provide protection but also cause significant economic losses. The following analysis of the functions and applications of circuit breaker accessories will help users when using them. II. Internal Accessories 1. Auxiliary Contacts: These are contacts mechanically linked to the circuit breaker's main circuit opening and closing mechanism. They are mainly used to indicate the circuit breaker's open/closed status. Connected to the circuit breaker's control circuit, they control or interlock related electrical components through the circuit breaker's opening and closing, such as outputting signals to indicator lights and relays. Universal circuit breakers have six pairs of contacts (three normally open and three normally closed), while the DW45 has eight pairs (four normally open and four normally closed). Molded case circuit breakers with a frame rating of 100A use single-break changeover contacts, while those of 225A and above use a bridge contact structure, with a specified thermal current of 3A. Circuit breakers with a frame rating of 400A and above can be equipped with two normally open and two normally closed contacts, with a specified thermal current of 6A. The number of operating cycles is the same as the total number of operating cycles of the circuit breaker. 2. Alarm Contacts: These are used to detect circuit breaker malfunctions. They only operate after the circuit breaker has tripped. They are mainly used when the circuit breaker trips due to overload, short circuit, or undervoltage faults. The alarm contact changes from its normally open position to the closed position, activating indicator lights, bells, buzzers, etc., in the auxiliary circuit to display or remind the circuit breaker of its fault tripping status. Because the probability of a circuit breaker tripping due to a load fault is not high, the lifespan of the alarm contact is 1/10 of the circuit breaker's lifespan. The operating current of the alarm contact generally does not exceed 1A. 3. Shunt trip unit: This is a trip unit excited by a voltage source, and its voltage can be independent of the main circuit voltage. The shunt trip unit is an accessory for remotely controlling the tripping of circuit breakers. It can reliably disconnect the circuit breaker when the power supply voltage is equal to any voltage between 70% and 110% of the rated control power supply voltage. The shunt trip unit operates on a short-time basis; the coil energization time generally cannot exceed 1 second, otherwise the coil will burn out. To prevent coil burnout, a microswitch is connected in series with the shunt trip coil in molded case circuit breakers. When the shunt trip unit is attracted by the armature, the microswitch changes from a normally closed state to a normally open state. Because the control circuit of the shunt trip unit's power supply is cut off, even if the button is manually pressed, the shunt coil will never be energized, thus preventing coil burnout. When the circuit breaker recloses, the microswitch returns to the normally closed position. However, for the universal DW45 product, the user must connect a set of normally open contacts in series before the shunt trip coil during use. 4. Undervoltage release: An undervoltage release is a type of trip unit that trips the circuit breaker with or without a delay when its terminal voltage drops to a specified range. When the power supply voltage drops (even slowly) to within 35% of the rated operating voltage, the undervoltage release should operate. When the power supply voltage is equal to 35% of the release's rated operating voltage, the undervoltage release should prevent the circuit breaker from closing completely. When the power supply voltage is equal to or greater than 85% of the undervoltage release's rated operating voltage, under hot conditions, it should ensure reliable circuit breaker closure. Therefore, when a certain voltage drop occurs in the protected circuit, the circuit breaker can automatically trip to disconnect the power supply, protecting the load appliances or electrical equipment downstream of the circuit breaker from undervoltage damage. In use, the undervoltage release coil is connected to the circuit breaker's power supply side. The circuit breaker can only close after the undervoltage release is energized; otherwise, the circuit breaker cannot close. III. External Accessories 1. An electric operating mechanism is an accessory used for remote automatic opening and closing of circuit breakers. There are two types: motor-operated and electromagnet-operated. Motor-operated mechanisms are used for molded case circuit breakers with a frame rating of 400A and above, and universal circuit breakers. Electromagnet-operated mechanisms are suitable for molded case circuit breakers with a frame rating of 225A and below. Whether using an electromagnet or a motor, their attraction and rotation directions are the same; opening and closing are achieved solely by the position of the cam inside the electric operating mechanism. When operated by an electric mechanism, the circuit breaker should be able to reliably close at any voltage between 85% and 110% of the rated control voltage. 2. Release electromagnet: This type of release electromagnet is suitable for universal circuit breakers with a motor pre-energy storage mechanism (the electric energy storage mechanism stores energy in its operating spring mechanism). When the user presses the button, the electromagnet coil is energized, the electromagnet closes, releasing the energy storage spring, and the circuit breaker closes. 3. The rotary operating handle is suitable for molded case circuit breakers. A mechanism for rotating the operating handle is installed on the circuit breaker cover. The handle's shaft is fitted into a mating hole in the mechanism, and the other end of the shaft passes through the drawer door opening. The handle is mounted on the exposed shaft end on the door of the assembly. The round or square base of the handle is fixed to the door with screws. This installation allows the operator to close or open the circuit breaker by rotating the handle clockwise or counterclockwise from outside the door. Simultaneously, rotating the handle ensures that the cabinet door cannot be opened when the circuit breaker is closed; the switch panel door can only be opened when the handle is open or re-engaged. In an emergency, if the circuit breaker is "closed" and the door needs to be opened, the red release button on the side of the rotary handle base can be pressed. 4. Extension Handle: This is an external extension handle, directly mounted on the circuit breaker handle. It is generally used on large-capacity circuit breakers of 600A and above for manual closing and opening operations. 5. Handle locking device: A clip is installed on the handle frame, and a hole is drilled on the handle and then locked with a padlock. It is mainly used to prevent others from opening the circuit breaker and causing a power outage when the circuit breaker is in the closed working state, or when the circuit breaker load side circuit needs maintenance or is not allowed to be energized, so as to prevent the circuit breaker from being accidentally closed, thereby protecting the safety of maintenance personnel or the reliable use of electrical equipment. 6. Wiring method: The wiring methods of circuit breakers include front panel, back panel, plug-in type and drawer type. Unless otherwise specified by the user, front panel is supplied. Front panel wiring is the common wiring method. (1) Back panel wiring method: The biggest feature of back panel wiring is that when replacing or repairing the circuit breaker, it is not necessary to rewire. It is only necessary to disconnect the upstream power supply. Due to the special structure, the product is equipped with a special mounting plate, mounting screws and wiring screws according to the design requirements when it leaves the factory. It is important to note that the reliability of the contact of the large capacity circuit breaker will directly affect the normal use of the circuit breaker. Therefore, it is necessary to pay attention to the installation and strictly follow the manufacturer's requirements. (2) Plug-in wiring: On the mounting plate of the complete set of equipment, first install a circuit breaker mounting base with 6 plugs on the mounting base and 6 sockets on the circuit breaker connection plate. There is a connection plate on the surface of the mounting base or bolts on the back of the mounting base. The power supply line and load line are pre-connected to the mounting base. When in use, the circuit breaker is directly plugged into the mounting base. If the circuit breaker is broken, simply unplug the broken one and replace it with a good one. Its replacement time is shorter and more convenient than front-panel or back-panel wiring. Since plugging and unplugging require a certain amount of manpower, the frame current of plug-in products in my country is currently limited to a maximum of 400A. This saves maintenance and replacement time. When installing a plug-in circuit breaker, check whether the plug of the circuit breaker is pressed tightly and securely fasten the circuit breaker to reduce contact resistance and improve reliability. (3) Drawer-type wiring: The drawer of the circuit breaker is accessed by rotating a rocker arm clockwise or counterclockwise. Both the main and secondary circuits use an insert-type structure, eliminating the isolator required for fixed circuits, achieving dual-purpose functionality, improving economic efficiency, and greatly facilitating operation and maintenance, while increasing safety and reliability. In particular, the main circuit contact base of the drawer base is compatible with the contact base of NT-type fuse circuit breakers, allowing direct insertion of fuses for power supply in emergency situations. IV. Conclusion Since the shunt trip unit, undervoltage trip unit, electric operating mechanism, and interlocking electromagnet have different voltage levels and AC/DC power supplies, users should specify this when ordering. Furthermore, users cannot use a single accessory. If electrical interlocking of two circuit breakers is required (when one is closed, the other must be open), auxiliary contacts and shunt trip units or electric operating mechanisms can be selected. When wiring before and after the circuit board, screws must be tightened to prevent damage to the circuit breaker.