Analysis of Low-Voltage Circuit Breaker Structure, Tripping Principle, and Selection Application

Low-voltage circuit breakers are core protective switching devices in low-voltage power distribution systems. They can complete normal circuit switching and automatically disconnect circuits under fault conditions such as overload, short circuit, and leakage. They are widely installed in distribution boxes and cabinets, and are manufactured according to the national standard GB14048.2-2008. Household miniature circuit breakers conform to the international standard IEC60898. The wiring terminals of these devices are generally equipped with Circuit Breaker Screw Clamp Terminals, which use a screw clamping structure to stably hold cables, preventing overheating and arcing faults caused by loose wiring, and ensuring the long-term safe operation of the power distribution circuit. Based on the enclosure structure, products are divided into three categories: ACB air-insulated frame circuit breakers, MCCB molded case circuit breakers, and MCB miniature circuit breakers. Miniature circuit breakers are mostly used in household electrical installations and are commonly known as air switches.

Based on applicable protection scenarios, low-voltage circuit breakers can be divided into four main categories: distribution protection, motor protection, lighting circuit protection, and residual current protection. Each category differs significantly in current range and tripping characteristics. Lighting single-phase circuit breakers have a rated current of 5-50A and are only equipped with overload and transient short-circuit protection. Residual current circuit breakers have a rated current of 20-200A and can complete leakage tripping within 0.1 seconds, protecting against the risk of electric shock. They typically use Zinc Alloy Screw Wire Terminals, made of oxidation-resistant zinc alloy, suitable for low-current residential lighting circuit wiring needs. Distribution circuit breakers cover a current range of 10-630A, and are divided into Class A current-limiting type and Class B selective type, supporting two-stage and three-stage stepped current protection, suitable for factory incoming lines and main feeder lines.

This complete low-voltage circuit breaker integrates multiple functions including control, protection, measurement, and human-machine interaction. Besides basic on/off isolation, it also supports remote tripping, equipment maintenance interlocking, insulation and grounding fault monitoring, and voltage acquisition and display. The contactor locations within the complete cabinet all use a standardized Contactor Screw Wire Clamp Terminal, ensuring consistent screw clamping wiring and simplifying the wiring process. Undervoltage tripping and shunt tripping are common expansion accessories. Undervoltage tripping triggers tripping when the line voltage drops below 70% of the rated voltage, while shunt tripping enables remote control tripping, adapting to the centralized management needs of automated power distribution systems.

The circuit breaker's opening and closing are driven by a manual handle or an electric operating mechanism that actuates the moving and stationary contacts. Electric mechanisms include both electromagnet-driven and motor-driven models. The motor-driven model achieves rapid closing by compressing an energy-storage spring using gears. The core protection component of the entire unit is the trip unit, which comes in two main types: thermomagnetic and electronic, integrating sensing, transmission, measurement, and actuation components. The thermal-magnetic trip unit relies on a bimetallic strip for overload delay protection. Under short-circuit conditions, the magnetic armature instantaneously engages, triggering tripping. The incoming line is equipped with an integrated screw clamp terminal, whose integrated terminal structure reduces assembly space and adapts to the compact housing layout of molded case circuit breakers.

The thermal-magnetic trip unit relies on current heating to drive the bimetallic strip to deform; the larger the current, the faster the bending speed, delaying the tripping action. The magnetic trip unit handles instantaneous large short-circuit currents; electromagnetic attraction overcomes spring resistance for rapid action, achieving millisecond-level breaking. The electronic trip unit is equipped with a microprocessor and uses a Rogowski coil to collect current signals, avoiding the magnetic flux saturation problem of traditional current transformers. It can accurately achieve three-stage protection: long-delay overload, short-delay short-circuit, and instantaneous protection against extremely large short circuits. The electronic trip unit's signal acquisition module is paired with a Steel Cage Screw Terminal Block; the cage-type screw terminal firmly fixes the sampling signal line, ensuring the accuracy of current and voltage analog signal acquisition.

The electronic tripping system integrates current, voltage, and temperature acquisition units. Its power supply is derived from a dedicated fast-saturation current transformer to prevent damage to the control circuit from high current surges. The acquired analog and digital signals are uniformly processed by the CPU. An LED display and keypad enable human-machine interaction, and an RS485 communication interface connects to a higher-level power distribution monitoring system. The fuse holders for the branch circuit breakers utilize Fuse Holder Screw Clamp Terminals, with a screw clamping structure adapted to the thin cables of the fuse holders, ensuring stable transmission of fault sampling signals.

The frame circuit breaker uses a metal frame to house all components, and its rated current is significantly higher than that of molded case circuit breakers (MCCs). ACB frame products are the preferred choice for high-current industrial power distribution scenarios. All terminals on the entire unit adhere to screw clamping design specifications. The general-purpose model uses a galvanized steel screw terminal clamp, made of wear-resistant and corrosion-resistant steel, suitable for high and low temperature industrial environments. Common electrical components are uniformly equipped with Clamp-Type Screw Terminals for Circuit Components, and standardized clamping terminals reduce wiring adaptation costs for different electrical connections. During on-site equipment maintenance and commissioning, the terminal clamping force can be manually adjusted to flexibly match different wire diameters, improving the efficiency of power distribution equipment assembly and maintenance.