MCBs

Miniature Circuit Breaker are better than fuse because they can be reset manually to come back to normal operations. These are widely used in industrial and commercial places. MCB's are automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit

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Detailed Description for MCBs

About MCBs

Miniature circuit breakers perform the dual functions of a switch and a fuse. They can open a circuit for safety or maintenance reasons simply by switching their toggle levers to the OFF position. As substitutes for fuses, they provide automatic circuit protection and need not be replaced after a dangerous over current has passed or a short circuit has been corrected.

There are three main types of MCBs, classified according to the current range at which they trip instantly.

Type B - Trips at 3 to 5 times rated current, suitable for resistive or slightly inductive loads.

Type C - Trips at 5 to 10 times rated current, suitable for moderate inductive loads.

Type D - Trips a t 10 to 20 times rated current, suitable for loads with a high inductive component.

MCB Rating

The ampere rating defines the maximum current the circuit breaker can carry without tripping. For typical miniature circuit breakers this rating is 2 to 125 A. In residential applications, single-pole breakers protect 20V branch circuits, and two-pole breakers protect 240V branch circuits.

Applications

• Single and double-pole breakers are beside common home electrical panels most widely used also in loadcenters. 

• Single-pole units, rated for 120/240 V AC, are designed to be plugged onto a single bus stab to obtain 120 V between one of the “hot” bus legs and the neutral bus, These breakers are available in ratings from 15 to 70 A, but ratings of 15 and 20 A are most commonly used in homes.

• Single-pole units are UL listed as HACR type, for air-conditioning, heating, and refrigeration equipment service, as well as being UL listed for SWD (switching duty) for switching 120-V AC fluorescent lighting loads. Two-pole breakers are rated 120/240 or 240 V AC.

• Standard sizes are plugged onto two adjacent stabs to obtain 240 V between both parallel “hot” bus bars. They are available with 10- to 125-V ratings. These breakers have a single common trip, and many are HACR type.

• Some circuit breaker applications by current rating are:

• 15 and 20 A – Protection of baseboard heaters and pumps

• 30 A – Protection of water heaters, dryers, and air-conditioning equipment s 40 to 50 A: Protection of ranges and stoves

• 50 A or more – Protection of electric heaters

• Three-pole breakers rated for 240 V require three spaces for contact with three stabs, and  they also have common toggle trips. They are typically listed as HACR type for use with air-conditioning, heating, and refrigeration equipment.

Advantages:-

 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 cut-out from fuse base, for confirming the blow of fuse wire.

 3. Quick restoration of supply cannot 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 too many advantages of MCB over fuse units, in modern low voltage electrical network, miniature circuit breaker is mostly used instead of backdated fuse unit. Only one disadvantage of MCB over fuse is that this system is more costly than fuse unit system.

 

Working Principle Miniature Circuit Breaker

There are two arrangement of operation of miniature circuit breaker:-

 

Thermal 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.

 

Electromagnetic effect of over current: 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 latch mechanism consequently open the circuit breaker contacts. This was a simple explanation of miniature circuit breaker working principle. 

 

Miniature Circuit Breaker Construction

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

 

Frame of Miniature Circuit Breaker

The frame of miniature circuit breaker is a molded case. This is a rigid, strong, insulated housing in which the other components are mounted. 

 

Operating Mechanism of Miniature Circuit Breaker

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. 

 

Trip Unit of Miniature Circuit Breaker

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. 

 

 

 


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