What is MCB and working principle of MCB ?

Now a days instead of fuse miniature circuit breaker (MCB) are used in every few circuit.

M.C.B (lighting loads): it is a dual purpose unit. It serves as a fuse unit as well as a switching unit. Heat sensitive elements are fitted within the unit with promptly operate at and when short circuit and overloads occurs. After the short circuit the elements within the unit is operated automatically and indication to this is available outside the unit through a toggle which moves upwards. As the unit is designed to operate at the time of short circuit and overloads, nothing harmful within the unit occurs.

The toggle is pressed downward after removing short circuit, overloads etc.

In normal cases when it is required to disconnect the particular circuit without operating main switch.

The unit is switched to off position. It is available for rating 4 Amp., 5 Amp., 10 Amp., 30 Amp., etc. for light loads.

M.C.B (Power Loads):

It is a complete unit to handle three phase supply line for industrial as well as domestic loads. It also serves as a fuse and switching unit. It is some time known as SFU (Switch Fuse Unit).

It operates automatically to off position as and when short circuit and overload conditions occur and in such cases complete unit with three phases disconnects thereby disconnecting three phases supply.

The unit can be put to on position after removing short circuit and overload parts. The principle of operation is same as written above for single phase unit.

Operation of MCB :

The MCB is a higher fault capacity thermal and magnetic current trip free automatic switching device. There are two main operation. Thermal operation with inverse time characteristics for overload protection and hammer trip assisted magnetic operation for short circuit operation.

Connect single phase MCB in series with phase wire whereas the neutral wire is to be connected from main distribution board to plug point. Similarly the earth wire has to be taken from distribution board to plug point.

Internal Operations of MCB

The thermal operation is achieved with a bimetallic strip, which deflects when heated by any overcurrent flowing through it. The latch mechanism is released and causes the contact to open.

When short circuit fault occurs, the rising current energises the solenoid operating the plunger to strike the trip lever causing immediate release of the latch mechanism

General Ratings of MCB :

Voltage rating — 240/415 AC, 50V/11V DC.

Current rating — 0.4 to 60 Amps.

No. of poles — 1, 1 + N etc.

IS code. — 8828 - 1978, BS : 3871

Rated frequency — 50 – 60 Hz

Short circuit breaking — 9KA as per IS-8828

Installation position — Any position.

Merits :

Automatic switch OFF in case of overload or short circuit.
Quick identification of faulty circuit even in dark.
Supply restored by switching ON again – No fuses to replace or rewire.
Automatic switch back to OFF if over load persists of faulty appliance is connected.
Prevent the passage of full fault current during short circuit.
Result in opening of contacts in less than 1 m sec.
Total operating time is reduced to less than 5 m sec.

Applications :

MCB are available in a wide range for the desired applications. lighting circuit in domestic section to capacitive/inductive circuit in commercial & multi-storey buildings.
A Miniature Circuit Breakers or MCB is today's version of old outdated fuses. Today most modern homes, factories, plants, offices have installed MCB replacing the earlier Rewireable Kit-Kats/ Switches.
MCB are also used for protection of Motors, A/c Units, Mixers, Heaters, Pumps, Grinders and all Lighting Circuits.
Electricians and technician can hold and provide maximum safety and protection by helping the customer to buy the right MCB for the right application and also help them in easier identification of the Right MCB.
'L' Series MCB for Lighting Circuits.

'G' Series MCB for Motor Circuit.

'ISOLATORS' in Red, Knob.

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