A circuit breaker is a crucial electrical device designed to automatically interrupt the flow of current in an electrical circuit under abnormal conditions, such as overloads or short circuits. It can also close, carry, and break current under normal operating conditions. Circuit breakers are typically categorized into high-voltage and low-voltage types based on their application range. While the boundary between high and low voltage is not strictly defined, devices rated at 3 kV or higher are generally considered high-voltage equipment.
Circuit breakers serve multiple purposes in electrical systems, including distributing power, starting motors infrequently, and protecting both power lines and motors from damage. They function similarly to fuse switches but offer the advantage of being reusable after tripping. When a fault occurs—such as severe overload, short circuit, or undervoltage—the circuit breaker automatically disconnects the circuit to prevent further damage. This makes them essential for ensuring safe and reliable electrical operations.
In power generation and distribution systems, circuit breakers play a vital role alongside transformers and various electrical components. Low-voltage circuit breakers, in particular, are widely used due to their versatility and ability to perform multiple functions, such as those of knife switches, overcurrent relays, thermal relays, and leakage protectors.
The working principle of a circuit breaker involves several key components: the contact system, arc extinguishing system, operating mechanism, trip unit, and housing. When a short circuit occurs, a strong magnetic field generated by the large current overcomes the spring force, triggering the trip unit to actuate the operating mechanism and open the circuit. In the case of overload, heat generated by the increased current causes a bimetallic strip to bend, which then activates the mechanism to trip the breaker.
For electronic circuit breakers, current sensors monitor the current in each phase and compare it with a set value. If the current exceeds the threshold, a microprocessor sends a signal to the electronic trip unit, which then triggers the operating mechanism to cut off the circuit.
Arc extinction is a critical aspect of high-voltage circuit breakers, as they must quickly extinguish arcs that can reach up to 2 meters in length. The arc is cooled and stretched to reduce its energy, allowing the dielectric strength of the medium to be restored rapidly.
Low-voltage circuit breakers, also known as automatic air switches, are commonly used in residential and industrial settings. They provide protection against overloads, short circuits, and under-voltage conditions, and can be manually or electrically operated. Their design includes an operating mechanism, contacts, protection devices, and an arc extinguishing system.
The main contacts of a low-voltage circuit breaker are usually closed manually or via an electric signal. Once closed, a free-trip mechanism locks the contacts in place. The overcurrent release and thermal element are connected in series with the main circuit, while the undervoltage release is connected in parallel with the power supply. When a fault occurs, the corresponding trip unit activates, causing the contacts to open and isolate the circuit.
The symbol for a circuit breaker varies depending on the standard, but it typically represents a switch with a specific marking indicating its function. In China's GB4728 standard, the graphical symbol for a circuit breaker is clearly defined and used in electrical diagrams to indicate the presence of this protective device.
Understanding the meaning of the circuit breaker symbols is essential for proper installation and maintenance. Key parameters include the frame current (Inm), rated current (In), long-time setting (Ir1), short-time setting (Ir2), instantaneous setting (Ir3), ground fault setting (Ir4), leakage current (Ire), pre-alarm current (Ir0), and short-time delay time (Ir2). These values determine how the circuit breaker responds to different fault conditions and ensure optimal performance in various applications.
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