MCCB (Moulded Case Circuit Breaker) Working, Types, Rating

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Moulded Case Circuit Breaker (MCCB) :

  • Moulded case circuit breakers are tested and rated according to the UL489 Standard. Their current carrying parts, mechanisms and trip devices are completely contained within a molded case of insulating material. MCCBs are available in various frame sizes with various interrupting ratings for each frame size.

Moulded Case Circuit Breakers Explained - GSES

  • Molded case circuit breakers are designed to provide circuit protection for low voltage distribution systems. They protect connected devices against overloads and/or short circuits. They are used primarily in panel boards and switchboards where they are Fixed Mounted. Some of the larger MCCBs are available in Drawout Mount design.

  • Molded case circuit breakers are available with special features making them suitable for the protection of motor circuits when used in conjunction with separate overload protection device. In the applications, they are often referred to as motor circuit protectors (MCPs).

  • Although there are many types of molded case circuit breakers manufactured, all are made up of five main components. These are :

    1. Molded Case or Frame

    2. Operating Mechanism.

    3. Arc Extinguishers.

    4. Contacts

    5. Trip Units

  • The function of the frame is to provide an insulated housing to mount all of the circuit breaker components. The frame is often of a glass-polyester material or thermoset composite resin that combines ruggedness and high dielectric strength in a compact design. The frame is also known as a molded case.

  • A frame designation is assigned for each different type and size of molded case. This designation is used to describe the breaker’s characteristics such as maximum voltage and current ratings. However, each manufacturer has their own identification system to account for the differences between breaker characteristics.

Moulded Case Circuit Breaker

Operating Mechanism :

  • The operating mechanism is the means to ope close the contacts. The speed with which the con open or close is independent of how fast the handle is moved. This is known as Quick-Make, Quick-Break. The breaker cannot be prevented from tripping by holding the handle in the ON position. This is know as Trip-Free. The handle position indicates the statue don of the contacts-closed, open or tripped. When the contacts are in the tripped position, the handle is in a midway position.

  • To restore service after the breaker trips, the handle must be moved first to the OFF position from its center tripped position. Then the handle must be moved to the ON position. When breakers are mounted in a group, as in a panel board, the distinct handle position clearly indicates the faulted circuit. Some breaker designs also incorporate a push-to-trip mechanism. This allows a manual means to trip the breaker and test the mechanism.

Arc Extinguisher :

  • Whenever a circuit breaker interrupts current flow, an arc is created. The function of the are extinguisher is to confine and divide that arc, thereby extinguishing it. Each arc extinguisher is made up of a stack of steel plates held together by two insulator plates.

  • When an interruption occurs and the contacts separate the current flow through the ionized region contacts induces a magnetic field around the arc and the arc extinguisher. (Module 5, “Fundamentals of Circuit Breakers,” covers this topic in detail.

  • The lines of magnetic flux created around the arc and its force drives the arc into the steel plates. The gas goes through Deionization and the arc divides, allowing it to cool.

  • Standard molded case circuit breakers use a linear current flow through the contacts. Under short circuit conditions, a small blow-apart force is created, which helps open the contacts. The majority of the opening action comes from mechanical energy stored in the trip mechanism itself. This is because the current in both contacts are going in the same direction and attract each other.

  • Newer design breakers use a reverse loop of current flowing in essentially opposite paths. This creates a repulsion action and results in a greater blow-apart force. This force assists with rapid arc extinguishing by causing the contact to open faster. The force is directly Proportional to the size of the Fault Current. The greater the fault, the greater the force, and the faster the contacts open.

Advantage

  • Occupy less space in the electrical panel

  • Cheaper than ACBs

  • Adjustable current settings

  • Maintenance-free

  • Fault clearing times are faster than the fuse units.

  • No need to keep the spares, hence the inventory will be reduced.

  • Suitable for industrial and commercial applications

Disadvantages

  • LISG fault needs to tailor it.

  • Needs additional add-on for getting feedback from the MCCB.

  • Not suitable domestic application.

  • Life span is less as compared with ACBs

  • Not suitable for high voltage applications.

Applications:

  • I have used it for many induction motor’s starters such as star-delta, DOL starter, Soft starter etc,

  • Panel incomer applications,

  • Distribution feeder

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