VISHWAKARMA GOVT.

ENGINEERINGCOLLAGE

Nirmal A. LuhanaElectronics & Communication – (I)Enrollment no.-140170111050

MCB(Miniature circuit

breaker)

What is a circuit breaker?

A circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow.

Basic difference between fuse n circuit breaker ?

. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation.

Detail picture ofMiniature Circuit Breakers (MCB)

• As the name implies it is the miniature form of circuit breaker.

• It can operated as an ordinary switch in making/breaking the circuit on which it has been applied on.

• It is easy to handle and provide complete safety to human beings.

• They have vast range and small steps between rated current, allow cables to be utilized fully.

Definition

MCB Working Principle

• 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 electric current, causes electromechanical displacement of plunger associated with tripping coil or solenoid of MCB. The plunger strikes the trip lever causing immediate release of latch mechanism consequently open the circuit breaker contacts. This was a simple explanation of miniature circuit breaker working principle.

MCB Construction • Miniature circuit breaker

construction is very simple, robust and maintenance free. Generally an MCB is not repaired or maintained, it just replaced by new one when required. A miniature circuit breaker has normally three main constructional parts.

1. Frame Of MCB

• The Frame of Miniature Circuit Breaker is a molded case. This is a rigid, strong, insulated housing in which the other components are mounted.

2. Operation Mechanism Of MCB

• The Operating Mechanism of Miniature Circuit Breaker provides the means of manual opening and closing operation of miniature circuit breaker. It has three-positions "ON," "OFF," and "TRIPPED". The external switching latch can be in the "TRIPPED" position, if the MCB is tripped due to over-current. When manually switch off the MCB, the switching latch will be in "OFF" position. In close condition of MCB, the switch is positioned at "ON". By observing the positions of the switching latch one can determine the condition of MCB whether it is closed, tripped or manually switched off.

3. Trip Unit Of MCB

• 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 protection against over load current and an electromagnet provides protection against short-circuit current.

MCB Working Principle

• 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.

Overload protection

bimetal

tripping bar

bimetaltripping bar

screwcalibration

stripbraid

NC45 NC100 / NC125

•A bimetal strip is used: to protect the cable against overload symbol:• principle of the bimetal: in a Multi 9 circuitbreaker

Choosing a circuit breakerIn conclusion

A Multi 9 circuit breaker is determined by:• rating•breaking capacity•number of poles•Voltage•tripping curve•Standard•Frequency•type of load

Applications of MCBfor home electrical panel

• As with all breakers, the miniature circuit breaker is designed to protect the house from circuit overload. They are much safer than the typical fuses, because they can be reset manually and because they handle much larger amounts of power. The breaker can manage the flow of energy, distributing properly the voltage even when many devices run off the same power circuit.

Applications of MCBfor ground fault trip mechanism

• In some cases, you can use a miniature circuit breaker with ground fault, or arc fault mechanisms, because the breakers consist of a system that opens the contacts if a line to ground fault occurs.

Applications of MCBfor lights

• Using miniature circuit breakers in the lighting system of the house, because they can deal with the amount of power needed to light a house, especially if using specific types of lamps, such as fluorescent lights. When switching on the lights, there is a need for additional power which might cause problems to the circuit, especially when lights are used extensively in the entire house.

Application of MCB for industrial appliances

• There are many small scale industrial buildings that use the miniature circuit breakers instead of the old fuses. These installations require up to 30 kA capacity, therefore the miniature breakers are ideal for this type of applications and devices. Same with commercial installations and applications. Some industrial applications that use miniature breakers are large stoves, and appliances used in restaurants, bakeries and food commercial stores.

Made by:Nirmal Luhana

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