A circuit breaker is a critical 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 and carry current under normal operating conditions. Circuit breakers are typically categorized into high-voltage and low-voltage types based on their application range. Although the boundary between high and low voltage is not strictly defined, equipment rated at 3kV or above is generally considered high-voltage.
Circuit breakers serve multiple purposes, including distributing electrical energy, starting motors infrequently, and protecting power lines and motors from damage caused by overloads, short circuits, or undervoltage. Their function is similar to that of a fuse switch, but unlike fuses, they do not require replacement after a fault. Instead, they can be reset and reused, making them more cost-effective and practical for modern applications.
Power generation is a fundamental part of the electricity supply chain, and the power distribution system includes transformers and various high- and low-voltage electrical devices. Low-voltage circuit breakers are among the most commonly used electrical components in residential and industrial settings due to their versatility and reliability.
**Working Principle of a Circuit Breaker**
Most circuit breakers consist of several key components: contact systems, arc suppression systems, operating mechanisms, trip units, and a protective casing. When a short circuit occurs, a large current generates a strong magnetic field that overcomes the spring’s resistance, triggering the trip unit to activate the operating mechanism and open the circuit instantly. In case of overload, the increased current heats up a bimetallic strip, which then deforms and pushes the mechanism to disconnect the circuit. The time it takes for this to happen depends on the severity of the overload.
For electronic circuit breakers, current from each phase is measured using current transformers and compared with a pre-set value. If an anomaly is detected, the microprocessor sends a signal to trigger the electronic trip unit, which then activates the operating mechanism.
The primary function of a circuit breaker is to safely isolate faulty circuits, preventing further damage and ensuring system integrity. High-voltage circuit breakers must manage large arcs—often reaching 2 meters in length—which require advanced arc-quenching techniques. These include cooling the arc, stretching it to promote recombination of charged particles, and rapidly restoring the dielectric strength of the medium.
Low-voltage circuit breakers, also known as automatic air switches, are widely used in electrical systems for switching loads and controlling motors. They combine the functions of knife switches, overcurrent relays, under-voltage relays, thermal relays, and residual current devices. This makes them essential for safety and protection in low-voltage distribution networks.
Low-voltage circuit breakers are popular due to their wide range of protective functions (such as overload, short-circuit, and under-voltage protection), adjustable settings, high breaking capacity, ease of use, and enhanced safety. They consist of an operating mechanism, contacts, protective devices (trip units), and an arc suppression system.
The main contacts of a low-voltage circuit breaker can be manually or electrically closed. Once closed, the free-trip mechanism locks the contacts in place. The overcurrent release coil and thermal element are connected in series with the main circuit, while the under-voltage release coil is connected in parallel. During a short circuit or severe overload, the overcurrent release pulls the armature, activating the free-trip mechanism and opening the circuit. Overload conditions cause the thermal element to heat up, bending the bimetal and triggering the same mechanism. Under-voltage situations release the armature, also causing the free-trip mechanism to act. A shunt release allows remote control, and when activated, it energizes the coil to open the circuit.
*The images above show different representations of circuit breaker symbols.*
**Meaning of Circuit Breaker Symbols**
- **In**: Rated current of the circuit breaker, indicating the current value of the thermal release selected.
- **Ir1**: Long-time delay setting current, representing the overload protection setting.
- **Ir2**: Short-time delay setting current, used for short-circuit protection, typically 2–12 times Ir1.
- **Ir3**: Instantaneous setting current, used for immediate tripping, usually 5–10 times Ir1 for power distribution and 12 times for motor protection.
- **Ir4**: Single-phase ground fault setting current, used for grounding protection, adjustable between 0.2–0.6 times Ir1.
- **Ire**: Leakage current setting, used for residual current protection, typically 0.03–0.5 A.
- **Ir0**: Pre-alarm current, signaling when the load exceeds a preset threshold.
- **tIr2**: Short-time delay trip time, adjustable between 0.05–0.45 seconds.
These parameters allow precise customization of the circuit breaker's performance based on specific application needs, ensuring both safety and efficiency in electrical systems.
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