1. Electromagnetic Trip (Electromagnetic Coil):
* Function: This coil is responsible for quickly interrupting the circuit when a high fault current occurs.
* How it works: The fault current creates a magnetic field in the coil, which pulls on a movable armature. The armature's movement disconnects the circuit by interrupting the contact.
* Key Features:
* Fast tripping: Designed for rapid response to short circuits.
* Doesn't depend on heat: Works solely based on magnetic force.
2. Thermal Trip (Bimetallic Strip):
* Function: This element protects against overloads, where current flows above the rated capacity for an extended period.
* How it works: The bimetallic strip is made of two metals with different thermal expansion rates. When current exceeds the limit, the strip heats up and bends. This bending mechanism triggers the circuit interruption.
* Key Features:
* Time-delayed response: Allows for brief overloads (e.g., motor starting) without tripping.
* Heat-dependent: Sensitivity is based on the amount of heat generated by the current.
Why Two Coils?
* Comprehensive Protection: The two coils provide a dual protection system:
* Electromagnetic coil: Handles sudden, high-magnitude faults.
* Thermal coil: Protects against continuous overload conditions.
* Flexibility: Circuit breakers often allow you to adjust the settings for both coils, enabling customization for specific applications.
Example:
Imagine a motor starting. It draws a high current momentarily, but the electromagnetic coil may not trip because the fault is brief. However, if the motor continues to draw excessive current, the thermal coil will eventually heat up and disconnect the circuit.
In Summary:
Two coils are commonly used in circuit breakers because they work in tandem to provide a robust and comprehensive protection system against both short circuits and overloads.
