DESCRIPTION • MCB stands for Miniature Circuit Breaker .
• is an automatically operated electrical switch used to protect low voltage electrical circuits
from damage caused by excess current from an overload or short circuit.
• Back in the day we protected against over-current by using fuse wire (indeed, we used to sell
it!). The principle was fairly simple - an over-current would quite literally, 'blow' the fuse
wire by rapidly heating and melting it, thus breaking the electrical connection and in doing so,
protected the rest of the electrical circuit.
• MCBs improve on this functionality as they are usually not destroyed during over-current so
are reusable. They are also much easier to use, offering the convenience of 'on/off switching'
for circuit isolation and since the conductor is housed within a plastic casing, they are much
safer to use and operate.
CONSTRUCTION
Every MCB consists of the following part:-
1. External Casing:
External Casing holds all the internal components firm and protects them from dust. It is made of insulating materials such as plastic or ceramics.
2. Contacts:
A pair of contacts can be found inside an MCB. One of them is fixed and the other is movable.
3. Knob:
MCBs can be turned ON and OFF using this knob.
4. Mechanical Latch:
A latch arrangement is made inside MCBs to hold the contacts under spring tension at ON position.
5. Bimetallic strip:
The bimetallic strip offers delayed overload protection by sensing the prolonged flow of current greater than its rated current.
6. Solenoid:
Solenoid offers instantaneous protection against short circuit by releasing the mechanical latch. Solenoid gets activated when the current through
the coil exceeds a particular value, normally more than 3 times of its rated current. This solenoid is not activated by overloads.
7. Arc Chutes:
Arc chutes are used for splitting and quenching of arcs.
ADVANTAGES
• MCB is more sensitive to current than fuse. It detects any abnormality in the current flow and
automatically switches off the electrical circuit.
• In case of MCB, the faulty zone of electrical circuit can be easily identified. Faulty circuit trips to
the off position. On the other hand in case of fuse, the complete fuse wire needs to be checked by opening fuse
grip for confirming the faulty zone.
• With MCB it is very simple to resume to the supply. You just need to push the knob of MCB back
to on position. But in case of fuse, the entire fuse wire needs to be replaced.
• MCB provides a better interface with the help of knob than a fuse. In case of fuse the complete handle
needs to be taken care out.
• Handling MCB is electrically safer than handling a fuse.
• MCB is reusable and hence has less maintenance and replacement cost. Whereas a fuse needs to be
replaced whenever it goes faulty.
The working principle of MCB
Whenever continuous overcurrent flows through MCB, the bimetallic strip is heated and deflects by bending. This deflection of bi-metallic strip releases a mechanical latch. As this mechanical latch is attached with the operating mechanism, it causes to open the miniature circuit breaker contacts, and the MCB turns off thereby stopping the current to flow in the circuit. To restart the flow of current the MCB must be manually turned ON. This mechanism protects from the faults arising due to overcurrent or overload.
But during short circuit condition, the current rises suddenly, causing electromechanical displacement of plunger associated with a tripping coil or solenoid. The plunger strikes the trip lever causing immediate release of latch mechanism consequently open the circuit breaker contacts. This was a simple explanation of a miniature circuit breaker working principle.
An MCB is very simple, easy to use and is not generally repaired. It is just easier to replace. The trip unit is the main part, responsible for its proper working. There are two main types of trip mechanism. A bi-metal provides protection against overload current and an electromagnet provides protection against short-circuit current.
MCB operation
If the circuit is overloaded for a long time, the bi-metallic strip becomes overheated and deformed. This deformation of Bi-metallic strip causes, displacement of latch point. The moving contact of the MCB is arranged by means of spring pressure, with this latch point, that a little displacement of latch causes, release of spring and makes the moving contact to move for opening the MCB.
The current coil or trip coil is placed so that during short circuit fault the magneto-motive force (mmf) of the coil causes its plunger to hit the same latch point and make the latch to be displaced. Again, when operating lever of the miniature circuit breaker is operated by hand, that means when MCB goes off position manually, the same latch point is displaced as a result moving contact separated from fixed contact in the same manner.
It may be due to deformation of a bi-metallic strip, or increased mmf of a trip coil or maybe a manual operation, the same latch point is displaced and same deformed spring is released, which ultimately responsible for movement of the moving contact. When the moving contact separated from fixed contact, there may be a high chance of arc. This arc then goes up through the arc runner and enters arc splitters and is finally quenched. When we switch it on, we reset the displaced operating latch to its previous on position and the MCB is ready for another switch off or trip operation.
COMPARISON BETWEEN MCB , RCCB, ELCB AND MCCB
SUMMARY
• Miniature Circuit Breaker is an automatically operated electrical switch used to protect low
voltage electrical circuits from damage caused by excess current from an overload or short
circuit.
• Whenever continuous overcurrent flows through MCB, the bimetallic strip is heated and
deflects by bending. This deflection of bi-metallic strip releases a mechanical latch.
• Types of MCB based on Number of Poles
1.Single Pole (SP)
2.Double Pole (DP)
3.Triple Pole (TP)
