MCB
MCB is short for miniature circuit breaker. Not main circuit breaker. Each sub circuit in the consumer unit starts with a MCB. Protects against overcurrent, caused by both overload and short-circuit current.
Has 4 functional components:
- A thermal overload trip (bi-metal)
- An electromagnetic short-circuit trip
- A switching mechanism with contacts
- Arc exhausting system
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
Ratings
- Rated current
- Rated voltage (single phase and 3-phase)
Differences to a fuse
| MCB | Fuse |
|---|---|
| Non-destructive | Self-destructive |
| Used repeatedly | Wire must be replaced before reuse |
| Automatic or manual | Only automatic |
| High cost | Low cost |
| More accurate | Less accurate |
In MCBs, both a thermal device and an electromagnetic device are involved.
- Thermal mechanism for small overloads and time-graded operation
- Electromagnetic mechanism for high fault currents and near-instantaneous operation
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.