Molded Case Circuit Breaker (MCCB): Types, Working, and Applications Explained

A circuit breaker is an electrical switch that protects the circuit or load against faulty currents. This circuit breaker utilizes air as the dielectric medium to break an electrical circuit. Here, air has lower dielectric strength compared to other mediums, and is thus utilized for protection within low voltage circuits. An MCCB can use different mediums to extinguish the arc, such as air, SF6, oil, or vacuum. So they detect faults with protective relays. Whenever a fault is noticed, the circuit breaker trips & shuts off the current. Unlike fuses that must be replaced, CBs can be reset manually or automatically. These electrical devices are used in offices, homes, industrial areas, etc. This article provides an overview of a Molded Case Circuit Breaker (MCCB), their working principle and applications.

What is a Molded Case Circuit Breaker (MCCB)?

The MCCB or Molded Case Circuit Breaker is a significant component within electrical systems that provides short-circuit protection and overload protection to a circuit. Generally, these are installed within the major power distribution boards, which allow the system to be simply shut down whenever required. These are available in different sizes and ratings based on the electrical system size. These circuit breakers stop the current automatically whenever the current goes beyond the trip setting. These circuit breakers usually include thermal magnetic trip units, whereas large molded case CBs are equipped with solid-state trip sensors.

MCCBs are generally operated electrically or manually and with large capacity. These can be classified into different types based on electronic over-current tripper applications, such as Class A and Class B. So Class B type CBs have good three-stage safety characteristics but due to the cost the market share of Class A products is higher due to their use of thermal-magnetic trippers..

A Molded case circuit breaker includes contacts, arc extinguishing chambers, trippers, and operating mechanisms in a plastic case. Generally, maintenance is not considered. It is suitable for protective switches used as a bypass. Overcurrent trippers are of a thermal-magnetic type and electronic type.

Construction

The construction of a molded case circuit breaker can be done with different components which include an Arc chute, Contacts, operating mechanism, terminal connector, thermal trip unit, magnetic trip unit, handle / trip-free mechanism, and trip button. So the molded case circuit breaker diagram is shown below.

Arc Chute

Arc chute includes a set of parallel metal plates made with ferromagnetic material. These are insulated mutually from each other. So this circuit breaker helps in quenching the arc by dividing the arc & lengthening it which is called an arc splitter or arc divider.

Contacts

Metallic conductors, like contacts are responsible for carrying the current supply to the electric load. So these are available in two types fixed & moving contact which are made from arc resistance material which have corrosion & low resistivity. The circuit breaker’s lifetime is decided by the material’s quality.

Operating Mechanism

The MCCB’s mechanism is responsible for opening & closing the current-carrying contacts. So it is simply connected through a trip unit that activates the operating mechanism. Here the trip unit works on a magnetic & thermal mechanism.

Terminal Connector

This connector helps in connecting the circuit breaker to the outside circuit. So the higher terminals are connected to the load whereas the bottom terminals are to the supply. Even though they are bi-directional, the designation of input & output is because of their physical installation.

Trip Unit

The MCCB’s trip unit activates the operating mechanism. So it comprises a thermal mechanism mainly for overload and magnetic tripping for a test button and short circuits for testing.

Thermal Trip Unit

It utilizes a thermal mechanism like a bimetallic strip that opens the contacts whenever the temperature increases because of the overloading.

Magnetic Trip Unit

This trip unit includes a relay that produces a magnetic field when high current flows through the solenoid due to a short circuit, it trips the CB. When the test button is utilized to replicate the above mechanisms & test the circuit breaker response.

Handle

It is used to control the circuit breaker manually by opening or closing. The handle can also be called a trip-free mechanism as it trips even if the handle is in the ON position. Generally, the handle of this CB could be in any three positions upward, downward, or middle. If it is in an upward place it will be in place. Similarly, if it is in the middle position, it indicates the breaker has tripped, while the downward position indicates OFF.

Trip Button

It is a red-colored button, used to test the circuit breaker. When it is pushed then it trips the operating mechanism.

How do Molded Case Circuit Breakers Work?

A molded case circuit breaker works by protecting a circuit against fault current. It uses both magnetic and thermal mechanisms. The magnetic mechanism is used to protect short circuits, whereas the thermal mechanism is to protect overload.

Overload Protection

Overload happens whenever the current surpasses a limit for an extended period. This circuit breaker includes a thermal mechanism that includes a bimetallic contact to defend from overload. A bimetallic strip is designed with two different kinds of metal with different thermal expansion rates. The main current flows through the bimetallic strip, which bends or contracts on temperature change.

If the current supply exceeds a certain limit, then bimetallic contact heats up & expands. The strip bends and trips the circuit because of the different expansion rates.

The current in electrical devices can overload for short time durations; it is normal & should not be measured as fault current. So, this circuit breaker has a time delay that permits the overload current for a short time duration before circuit tripping.

Short Circuit Protection

MCCB protects from a short circuit with a solenoid that generates electromagnetic force. So the main current supplies throughout the solenoid that attracts & repels a plunger accountable for circuit breaker tripping. If the current remains in the threshold, then the solenoid produces a weak magnetic force that cannot attract the plunger. During a short circuit, very high current supplies throughout the solenoid generate a very strong magnetic force. So it attracts the plunger that trips the circuit.

MCCB Specifications:

The MCCB specifications include the following.

• The rated current (In) is the maximum current that the MCCB is designed to hold constantly without any tripping. Generally, MCCB’s current ratings range from 10A to 2,500A.
• The rated Voltage or Ue is the voltage supply where the MCCB is designed to function. These circuit breakers are generally used for both high and low-voltage applications with up to 600V or 690V typical ratings.
• Rated insulation voltage or Ui is the highest voltage that the MCCB can resist in insulation stress.
• So it is always higher as compared to the rated operating voltage to give a security margin.
• Short-circuit breaking capacity or Ics is the highest fault current that the MCCB can disrupt in normal service conditions. The ultimate short-circuit breaking capacity or Icu is the highest fault current that the MCCB can disrupt without harm.
• Its interrupting capacity typically ranges from 10kA to 200kA.

MCCB Wiring Diagram

Here the proper MCCB wiring diagram is shown below. MCCB is a protective device that protects the electrical equipment or electrical circuit from two main electrical faults like short circuits and over currents. As compared to MCB, this circuit breaker is designed to provide almost high to very high currents. MCCBs are used mostly for industrial-based high-current applications like motor circuits, used as incoming breakers within LT panels, elevators, CNC machines, electric cranes, and many more.

The wiring connections of this circuit breaker follow as;

• Initially, choose the MCCB with the correct rating based on the load.
• Connect all the power supply phases at the input side ensuring there are no loose connections.
• Connect all the load phases at the output side without a loose connection.

If you observe the above wiring diagram, this circuit breaker includes a total of six terminals where three are input terminals & remaining are output terminals. Usually, most of the circuit breakers are designed to connect the input supply at the base and the output at the top.

Thus, you can observe that all three R, Y, and B phases of the input power supply are connected to the base side of the circuit breaker, while the outgoing terminals are connected to the upper side. These outgoing terminals are then connected to a busbar, which distributes power to multiple loads connected to the same circuit. The busbar helps simplify wiring and ensures balanced load distribution across connected devices.

MCCB Types

Molded case circuit breaker types are explained below.

Type B MCCBs

Type B MCCBs are designed to trip whenever the current flowing through them exceeds 3 to 5 times their rated current. The tripping time for this circuit breaker ranges from 0.04 to 13 seconds. Generally, these types of MCCBs are installed in industrial and commercial building distribution boards with lower fault currents. Therefore, they are frequently used with resistive loads like heating and lighting.

Type-C MCCBs

These MCCBs provide more protection compared to Type B MCCBs. They trip at 5 to 10 times the rated current with a delay time of 0.04 to 5 seconds. Type C MCCBs offer a balance between short-circuit and overload protection. They are suitable for industrial and commercial applications involving equipment with moderate inrush currents, such as transformers and motors.

Type D MCCBs

Type D MCCBs are designed for high-inrush current applications such as large industrial machinery. These breakers trip at 10 to 20 times the rated current, and their response time ranges from 0.04 to 3 seconds. They are used in environments where substantial equipment frequently starts and stops. Their high tolerance to inrush currents makes them ideal for heavy-duty operations, such as those involving compressors, lifts, engines, etc.

 

Type K MCCBs

Type K MCCBs are mainly effective in protecting circuits that use two-core cables. They trip at 8 to 12 times the rated current and are commonly used in installations with sensitive equipment. These MCCBs provide excellent feeder protection and can operate for up to 5 seconds, making them ideal in environments where surge currents are common.

 

Type Z MCCBs

Type Z MCCBs trip at just 2 to 3 times the rated current. They are used to protect sensitive electronic equipment in applications such as data centers and telecommunications. Although less versatile than other MCCB types, they respond to small overloads and are essential in sensitive circuits. These highly sensitive MCCBs can tolerate only 1.5 to 3 times the rated current and are best suited for electronic loads requiring high-speed tripping.

Molded Case Circuit Breaker vs Air Circuit Breaker

The difference between the molded case circuit breaker and the air circuit breaker is shown below.

MCCB	ACB
The term MCCB stands for molded case circuit breaker.	ACB stands for air circuit breaker.
These are compact and fit in small distribution panels.	These circuit breakers are larger because of their high current ratings,
They have magnetic, electronic, and thermal-magnetic trip units.	They have electronic or thermal-magnetic trip units.
MCCBs have low current ratings up to 3200Amps.	ACBs have high current ratings up to 6300 amps.
It uses a combination of different technologies to quench the arc, frequently depending on a plastic housing mainly for insulation.	It uses air to quench the arc by operating at atmospheric force.
These are normally not designed for simple repair & may need whole unit replacement.	ACBs are frequently designed to be stronger and may be simple to repair.
These are generally not as expensive as compared to ACBs.	These are more expensive.

Characteristics

The characteristics of MCCB include the following.

• MCCBs utilize thermal trip mechanisms to notice and react to prolonged overloads by avoiding damage to apparatus & circuits.
• These circuit breakers use magnetic trip mechanisms to disrupt circuits fast throughout short circuits, ensuring security & avoiding fires.
• They have in-built GND fault protection.
• These have high breaking capacity, so they can securely interrupt huge fault currents to make them suitable for commercial and industrial applications.
• MCCBs are mainly designed for particular voltage ratings for indicating the highest voltage at which point they can work securely.
• These are mainly designed to have less contact resistance by ensuring a very efficient current supply and avoiding overheating.
• Its high insulation resistance avoids electrical leakage & ensures secure operation.
• They can be switched ON/OFF manually by allowing for simple operation & maintenance.
• These are available in a variety of pole configurations to suit different electrical system necessities.
• They can be operated remotely by allowing centralized monitoring & control.
• They protect capacitor banks within commercial and industrial electrical systems, avoiding harm from high currents.
• They can be inspected visually to recognize potential issues, cracks, or contacts within the casing.

Advantages and Disadvantages

The advantages of MCCB include the following.

• The molded case circuit breaker handles higher currents than fuses or MCBs.
• They allow for tripping characteristics adjustments like time and current settings to match particular load requirements.
• These are designed with improved security features.
• MCCBs protect property, people, and equipment against overvoltage or overcurrent.
• They have a durable and compact design.
• These can be reusable & simple to reset.
• Not like fuses, they can be returned after tripping by removing the need for substitution.
• These are appropriate for a broad range of applications.

The disadvantages of MCCB include the following.

• MCCBs are usually more costly.
• These are not appropriate for higher voltages.
• This circuit breaker is susceptible to corrosion and dust.
• These are larger as compared to MCBs.
• They have limited adjustability/
• MCCBs have a minimum higher current rating
• These are sensitive to ecological factors.
• These MCBs are fixed and thus cannot be simply reconfigured or moved for different applications.
• MCCBs may need to be changed completely when a fault occurs.
• They have restricted short-circuit withstand ability as compared to other kinds of CBs.

Applications

The applications of MCCB include the following.

• MCCBs generate an electromagnetic field by responding to short-circuit faults.
• These can be adjusted to defend motors without tripping in the inrush current.
• They stop overheating and wiring of electrical equipment.
• They guard electric feeder circuits that provide power to big distribution boards.
• MCCBs defend welding machines by simply connecting the appliance to its own CB.
• This CB is used for high current ratings, which range upto 1600Amps with upto 55kA breaking capacity.
• These are used in commercial as well as industrial applications.
• They handles higher currents thus widely used in heavy-duty based applications like adjustable trip settings mainly for applications with low currents, defending motors, welding machines, capacitor banks, electric feeders & generators.

MCQs

Where are molded case circuit breakers used?

MCBs are used in large commercial and industrial settings for high-current circuits, so they protect short circuits, equipment failures, overloads, etc.

Does a molded case switch trip?

Yes, an MCCB does trip to defend against short circuits & overloads.

How does a molded case breaker protect against sustained overloads and short circuits?

MCCBs defend from short circuits and sustained overloads with a blend of magnetic trip and thermal mechanisms. Magnetic protection reacts to immediate high currents and thermal protection notices prolonged over currents.

What two elements make up a molded-case circuit breaker?

A thermal element is used for overload protection, whereas a magnetic element is used for short-circuit protection.

Thus, this is an overview of the molded case circuit breaker (MCCB), pinout, features, specifications, circuit, working & its applications. These are types of circuit breakers designed to defend electrical circuits against short circuits and overloads. So these are essential circuit breakers that protect electrical systems & ensure security by providing safety from short circuits and overloads. MCCBs are significant components in both industrial and residential applications. These are important devices due to their safety, versatility, and reliability. Here is a question for you: What is MCB?