MCB

Short for miniature circuit breaker. Not main circuit breaker. Each sub circuit in the consumer unit starts with a MCB. Protects against overcurrent.

Advantages

Advantages of a MCB over fuses:

• Non-destructive
• Fast
  • Shorter tripping times under moderate overcurrents
  • Immediate indication of faulty current
• Manual operation: can be used as a circuit control switch
• Easy to use
  • No stock of fuses required
  • Reclosing can be effected at once after the fault has been cleared
• More accurate

MCBs are more expensive than fuses.

Ratings

• Rated current
• Rated voltage (single phase and 3-phase)

Mechanism

Has 4 functional components:

• A thermal overload trip (bi-metal)
  For small overloads and time-graded operation
• An electromagnetic short-circuit trip
  For high fault currents and near-instantaneous operation
• A switching mechanism with contacts
• Arc exhausting system

Function of the thermal device

Consists of a bi-metal strip. When overheated from overload current, the bi-metal strip is deflected. The deflection depends on the heat which depends on:

• Intensity of current flow
• Duration

Function of the EM device

Consists of a solenoid coil. Load current is set to flow through the coil.

In this coil, there is a fixed iron-core with a movable armature. When the current exceeds the rated value, the coil exerts sufficient electromagnetic force to attract the armature against the force of the spring. A switch mechanism is activated by the lever, to open the contacts.