MCB
Nowadays we use more commonly miniature circuit breaker
or MCB in low voltage electrical network instead of fuse.
The MCB has some advantages compared to fuse.
1.It
automatically switches off the electrical circuit during abnormal condition of
the network means in over load condition as well as faulty condition. The fuse
does not sense but miniature circuit breaker does it in more reliable way. MCB
is much more sensitive to over current than fuse.
2.Another
advantage is, as the switch operating knob comes at its off position during
tripping, the faulty zone of the electrical circuit can easily be identified.
But in case of fuse, fuse wire should be checked by opening fuse grip or cutout
from fuse base, for confirming the blow of fuse wire.
3.Quick
restoration of supply can not be possible in case of fuse as because fuses have
to be rewirable or replaced for restoring the supply. But in the case of MCB,
quick restoration is possible by just switching on operation.
4.Handling
MCB is more electrically safe than fuse.
Because
of to many advantages of MCB over fuse units, in modern low voltage electrical
network, miniature circuit breaker is mostly used instead of backdated fuse
unit.
Working Principle Miniature Circuit Breaker
There are two arrangement of operation of
miniature circuit breaker. One due to thermal effect of over current and other
due to electromagnetic effect of over current. The thermal operation of
miniature circuit breaker is achieved with a bimetallic strip whenever
continuous over current flows through MCB, the bimetallic strip is heated and
deflects by bending. This deflection of bimetallic strip releases mechanical
latch. As this mechanical latch is attached with operating mechanism, it causes
to open the miniature circuit breaker contacts. But during short circuit
condition, sudden rising of current, causes electromechanical displacement of
plunger associated with tripping coil or solenoid of MCB. The plunger strikes
the trip lever causing immediate release of latchmechanism consequently open the circuit breaker
contacts. This was a simple explanation of miniature circuit breaker working
principle.
Miniature Circuit Breaker Construction
The trip unit is the main part, responsible for
proper working of miniature circuit breaker. Two main types of trip mechanism
are provided in MCB. A bimetal provides protectionagainst over load current and an electromagnet
provides protection against short-circuit current.
Operation of Miniature Circuit Breaker
There are three mechanisms provided in a single miniature
circuit breaker to make it switched off. If we carefully observe the picture
beside, we will find there are mainly one bi - metallic strip, one trip coil
and one hand operated on - off lever. Electric current carrying path of a
miniature circuit breaker shown in the picture is like follows. First left hand
side power terminal - then bimetallic strip - then current coil or trip coil -
then moving contact - then fixed contact and - lastly right had side power
terminal. All are arranged in series.
If circuit is overloaded for long time, the bi - metallic strip becomes over heated and deformed. This deformation of bi metallic strip causes, displacement of latch point. The moving contact of the MCB is so 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 such a manner, that during short circuit fault the mmf of that coil causes its plunger to hit the same latch point and make the latch to be displaced. Hence the MCB will open in same manner. Again when operating lever of the miniature circuit breaker is operated by hand, that means when we make the MCB at off position manually, the same latch point is displaced as a result moving contact separated from fixed contact in same manner. So, whatever may be the operating mechanism, that means, may be due to deformation of bi - metallic strip, due to increased mmf of trip coil or may due to manual operation, actually the same latch point is displaced and same deformed spring is released, which ultimately responsible for movement of the moving contact. When the 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 into arc splitters and is finally quenched. When we switch on an MCB, we actually reset the displaced operating latch to its previous on position and make the MCB ready for another switch off or trip operation.
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