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What Are Molded Case Circuit Breakers ?

Molded case circuit breakers are the circuit breakers which 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.

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.

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

 A molded case circuit breakers manufactured, all are made up of five main components. of These are:

  • Molded Case or Frame
  • Operating Mechanism
  • Arc Extinguishers
  • Contacts
  • Trip Units

Figure 1. Five Main Components of an MCCB



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.

Figure 2. Motor Circuit Protector Components

OTHER COMPONENTS    were discussed infundamental module 

●How It Operates

Motor circuit protectors disconnect the motor load from an electrical supply under three conditions. They are:

  • When the handle is switched to OFF.
  • When an automatic trip operation occurs.
  • When a manual trip is initiated with a push-to-trip button.

Its design provides an increased air gap between the stationary and moveable contacts when in the tripped position. This air gap results in greater arc extinguishing during contact opening and provides higher interrupt ratings.



Motor circuit protectors can be used in combination starter units within a motor control center. They allow for protection against both low and high level fault currents. They can also be applied in standalone combination starters.

Operational Devices

A shunt trip provides remote controlled tripping of a molded case circuit breaker. A solenoid coil is remotely energized using a pilot device, such as a pushbutton. That moves the plunger to activate the trip bar of the breaker. At the same time, a cutoff switch operates which disconnects power to the solenoid so the coil doesn't burn out. Often, pigtail leads are supplied for connecting the shunt trip to either an AC or DC control power source.

Accessories and Modifications

Auxiliary switch

Providing circuit breaker main contact status, an auxiliary switch is mounted in the breaker. In this diagram, the contacts are shown as "A" and "B". An "A" contact is open when the breaker is open or tripped. A "B" contact is closed when the breaker is open or tripped. The contacts are rated 120V .

Figure 3. Auxiliary Switch Contacts

If you wanted to give a visual indication that a circuit is energized, you could mount an indicating light on the panel. Using an auxiliary switch with an "A" contact would allow the light to be illuminated whenever the breaker is closed. When the breaker trips, the light goes off, letting you know the breaker has tripped or been opened.

Auxiliary switches can be used for circuit interlocking purposes. The NEC requires that motor control circuits be disconnected from all sources of supply when the disconnect means is in the open position. For starters with common control, one circuit breaker would disconnect the voltage for both the power circuit and the control circuit. If the control circuit has a separate power supply, the circuit breaker would not disconnect that supply source. You could use separate disconnect means, or simply use an auxiliary switch in the breaker. When the breaker is open, the auxiliary opens the control circuit, disconnecting it from its supply source.

Figure 4. Auxiliary Contacts Disconnect Separate Control Power Source

Alarm switches

 Alarm switches differ from auxiliary switches in that they function only when the breaker trips automatically. The normally open contact of the switch closes when the breaker trips due to a short circuit, an overload condition, or when operated by a shunt trip. An indicating light, or a bell can be placed in the circuit to provide indication that the circuit has tripped. When the breaker is reset, the alarm switch is reset. A manual opening of the circuit breaker does not affect the alarm switch contact.


Handle Operating Devices

Handle operating devices provide indirect electrical or manual operation of the circuit breaker handle.

Electrical operators provide complete remote control of a molded case circuit breaker by means of a pushbutton. When energized from a remote location, the operator mechanism moves the circuit breaker handle to either the ON or OFF position. Shunt trips and undervoltage release mechanisms can only be used to trip the breaker. Electrical operators deliver a positive switching action. In case of a power failure, means are provided for manual operation. They come in a variety of designs and, depending on the circuit breaker type, are mounted in different ways.

The newer designs are front mounted on the breaker cover and fit within the trim line of the circuit breaker. For smaller frame breakers, a solenoid is used. On larger frame sizes, a motor is used to provide the increased operational force required to move the breaker handle.

Figure 5. Front Mounted Electrical Operator (Solenoid Type)

Some designs are side mounted and use an extended arm to move the circuit breaker handle. These older motor driven operators are not usually suitable for generator synchronizing because of the time it takes for operation.

Figure 6 Side Mounted Electrical Operator (Motor Type)

modern electrical operator designs, whether solenoid or motor driven, are capable of performing a closing operation in five cycles or less. This makes them very suitable for generator synchronizing applications.  

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