Understanding Miniature Circuit Breakers (MCB)

Introduction

• MCBs are used for protection against overload and short circuits.
• Distinct mechanisms for operation.

MCB Specifications

• Example: MCB labeled C 16.
  • Class C with an overload current of 16 Amps.

MCB Components

• Input Side: Phase wire connection.
• Bimetallic Strip:
  • Composed of two different metals.
  • Metals expand at different rates when heated.
  • Bending occurs due to thermal expansion when current flows.
• Moveable Contact:
  • Mechanism to turn on/off the MCB.
• Fixed Contact:
  • Current flows from here to the solenoid coil.
• Solenoid Coil:
  • Creates a magnetic field that pushes a plunger with a pin.
• Output:
  • Current goes out to appliances.
• Spark Divider:
  • Extinguishes sparks generated during contact separation.

Operation Mechanism

Overload Situation:

1. MCB is on; current flows through the bimetallic strip, begins bending.
2. The bending depends on the current and strip manufacturing.
3. Graph: Time to trip vs. current through MCB.
4. As the current increases, the strip bends and pulls a plastic part to trip the MCB.
5. Springs help break the circuit quickly.
6. Spark generated is extinguished by the spark divider.
   • Metal plates with insulators spaced apart.
   • Guides direct the spark towards the divider, increasing distance breaks the spark.
7. Ceramic plates protect the plastic casing from heat damage.

Short Circuit Situation:

1. Large current flows through MCB.
2. Solenoid creates a strong magnetic field, pushing the pin to separate contacts and trip MCB.
3. Spark generated is also dissipated by the spark divider.

Need for Two Mechanisms

• Bimetallic Strip:
  • Triggers on prolonged small overloads or brief large overloads.
• Solenoid Trigger:
  • Provides instant tripping during short circuits for immediate protection.
  • Bimetallic strip takes time to heat and expand; solenoid trips instantly to protect against short circuits.

Conclusion

• Understanding MCBs helps in using electricity safely.
• Suggested follow-up: Video on how mobile chargers convert AC to DC.