What is the working principle of a circuit breaker?

The working principle of a circuit breaker revolves around the core logic of monitoring, judgment, and execution. Through its built-in tripping mechanism and arc-extinguishing system, it achieves normal circuit connection and disconnection control and rapid interruption of faulty circuits. Specifically, it can be divided into normal operating mode and fault protection mode. The arc-extinguishing system is a crucial auxiliary mechanism ensuring reliable tripping.

When a circuit experiences faults such as overload, short circuit, or undervoltage, the built-in trip unit triggers the locking mechanism to unlock, achieving automatic tripping. Different faults correspond to different tripping principles:

Overload Protection (Thermal Tripping Principle)

Core Component: Bimetallic strip (made of two metal sheets with significantly different coefficients of thermal expansion pressed together).

Working Logic: When the circuit is overloaded, the current continuously exceeds the rated value, causing the bimetallic strip to bend and deform due to heat. When the deformation reaches a threshold, it pushes the locking mechanism to unlock, and the moving contact breaks the circuit under the action of a spring.

Features: Delayed action; short-term overloads (such as motor starting) will not trigger tripping, avoiding false tripping.

Short Circuit Protection (Electromagnetic Tripping Principle)

Core Component: Electromagnetic coil (wound around an iron core).

Operating Logic: When a short circuit occurs, the current surges instantaneously to tens or even hundreds of times the rated value. The electromagnetic coil generates a strong electromagnetic force, attracting the iron core to strike the locking mechanism, instantly unlocking the circuit.

Features: Millisecond-level rapid action, capable of cutting off the circuit in an extremely short time, preventing arc burns to equipment or causing fires.

Undervoltage/Loss-of-Voltage Protection (Undervoltage Tripping Principle)

Core Component: Undervoltage trip coil, connected in parallel in the circuit to monitor voltage.

Operating Logic: When the voltage drops below approximately 70% of the rated value or is completely lost, the electromagnetic force of the trip coil disappears, and the iron core, under the action of the spring, triggers the locking mechanism to unlock, cutting off the circuit.

Function: Prevents equipment from abnormal operation and damage under low voltage, and avoids the safety hazard caused by sudden equipment startup when power is restored.

Leakage Protection (Residual Current Protection Principle)

Core Component: Zero-sequence current transformer.

Operating Logic: Under normal circumstances, the currents in the phase and neutral lines are equal in magnitude and opposite in direction. The magnetic flux in the transformer core cancels each other out, resulting in no induced current output. When leakage occurs (such as electric shock), some current is lost through the ground, causing an imbalance in the two-phase currents. The transformer generates an induced signal, triggering the trip unit.

Features: Low operating threshold (commonly 30mA), prioritizing personal safety.