Notes on Miniature Circuit Breaker (MCB) Lecture

Introduction to MCB

• MCB protects against electrical mishaps.
• Two main fault conditions it addresses:
  • Short circuit
  • Overload conditions

Short Circuit Protection

• MCB trips in less than three milliseconds.
• Isolates internal connections.
• Comparison with Fuses:
  • Fuses had low melting point wires that melted under fault conditions, requiring manual replacement.
  • Circuit breakers (like MCB) automate this process.

Circuit Breaker Mechanism

• Circuit breakers keep one side of the wire moving and the other fixed during faults.
• After a fault, the MCB can be manually reset.
• Current Direction in MCB:
  • MCB operates faster than the half cycle time of alternating current.
  • Current flow can be simplified in analysis as one direction.

Detection of Electrical Faults

• MCB has sensing elements that activate a triggering mechanism.
• Triggering Mechanism Design:
  • Involves a lever with a rectangular ring connected by two springs.
  • The mechanism detects movement and torque changes:
    • Clockwise and counterclockwise torque based on lever position.
• Mechanism allows for quick opening of the circuit.

Electromagnet and Fault Detection

• Electromagnet generates magnetic fields proportional to current.
• Short Circuit Scenario:
  • Current spikes up to 1000 times, creating strong magnetic field.
  • Iron cylinder and pin inside the electromagnet get triggered to open the circuit.

Arc Extinguishing Mechanism

• Fault currents can create hazardous arcs.
• Arc Runner/Chute:
  • Composed of parallel plates to extinguish the arc.
  • Increases resistance by maintaining distance between contact points.
  • Arc is divided into small pieces to reduce area and extinguish it.

Overload Condition Protection

• Overload occurs with multiple appliances connected in parallel.
• Current in overload rises 2-5 times the rated current.
• Challenges:
  • Lower-rated electromagnets trigger unnecessarily due to in-rush currents.
• Solution:
  • Bi-metallic strip senses overload:
    • Bends due to heat over time, activating the switch after two seconds.
    • Adjusted by manufacturers to calibrate the trip point.

Conclusion

• MCB effectively protects against electrical disasters.
• Other breaker types include RCD, ELCB, and MCCB.