Circuit Breaker
A device for making and breaking a circuit. Operated by hand or automatically. Can be used to isolate part of a circuits. More accurate protection compared to semi-enclosed or fully-enclosed fuse.
Things to consider when choosing a circuit breaker:
- the normal current it will have to carry
- the amount of current it will have to interrupt
MCB
MCB is short for miniature circuit breaker. Not main circuit breaker.
Has 3 functional components:
- A thermal overload trip (bi-metal) or an electromagnetic short-circuit trip
- A switching mechanism with contacts
- Arc exhausting system
Function of the thermal device
The overload trip depends on the operation of the thermally operated bi-metal strip. When heated by the current passing through, the bi-metal strip is deflected due to the difference in expansion. The deflection thus depends on the intensity of heat dependent on the intensity of the current flow and the duration. As can be seen from the figure, after the deflection (or temperature) exceeds a predetermined amount, the tripping mechanism is activated
Function of the EM device
Electromagnetic trip consists essentially of a solenoid coil through which the load current flows. In this coil, there is a fixed iron-core with a movable armature. When the current exceeds a predetermined 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.
Advantages
Advantages of a MCB over fuses:
- Non-destructive determination of tripping characteristics
- Shorter tripping times under moderate overcurrents
- Immediate indication of faulty current
- Reclosing can be effected at once after the fault has been cleared
- No stock of fuses required
- Can be used as a circuit control switch
Protection
Earth leakage current
Can be used to detect electrical faults to earth in electrical. There are 2 types of circuit breakers to detect electrical faults to earth.
- Voltage operated protection - Earth Leakage Circuit Breaker (ELCB)
- Current operated protection - Residual Current Circuit Breaker (RCCB) or Residual Current Device (RCD)
The earth leakage protection device is called as trip switch.
ELCB
2 earth terminals are required for the proper operation of a ELCB.
- Frame earth to which all non-conducting metallic parts of equipment are connected
- The ELCB reference earth
The ELCB will normally operate when the voltage across the coil, which corresponds to the voltage of the frame earth with respect to the reference earth, exceeds about 40 V. Up to about 50V has been traditionally considered as a safe voltage.
However, it is now known that what is important is the current that may pass through the human body rather than the voltage, and that too is time dependent. Thus the RCCB is now preferred to the ELCB.
RCCB
Trips when line current and neutral current are different.
The current difference between line and neutral currrents is used to energize the solenoid, which causes the switch to open. Under normal operating conditions, two identical windings, m1 and m2, will carry the main current. Since the currents are equal and opposite through the two windings, there is mmf balance and there will be no induced emf on the detector winding. Thus the operating coil will not be energized. However, in case of a fault the line and neutral currents will not equal and the trip coil will be energized due to the induced currents in the detector winding.
Advantages of RCCB
Suppose the live wire is exposed. If somebody touches it, they may get a shock if a current passes through them.
In the case of voltage-operated ELCB, this earth current is not going through the tripping coil and will cause danger. But in the case of RCCB, the return path is going to loose part of the current, which passed through the human body, which in turn would cause a resultant flux within the ring energizing the tripping circuit.