Miniature Circuit Breaker (MCB) Definition, Types, Working

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In this article we discussed miniature circuit breaker also known as MCB in detail. We explained the working principle, its types, and some frequently asked questions.

What Is An MCB?

A miniature circuit breaker (MCB) is an Electrical Switch that automatically switches off the electrical circuit during an abnormal condition of the network means an overload condition as well as a faulty condition.

Nowadays we use an MCB in a low-voltage electrical network instead of a fuse. The fuse may not sense it but the miniature circuit breaker does it in a more reliable way. MCB is much more sensitive to overcurrent than a fuse.

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This is what an MCB looks like-

Handling an MCB is electrically safer than a fuse. Quick restoration of supply is possible in case of a fuse because fuses must be rewirable or replaced for restoring the supply. Restoration is easily possible by just switching it ON. Let’s look at the working of the miniature circuit breaker.

What is Inside Miniature Circuit Breaker?

A Miniature Circuit Breaker (MCB) typically consists of the following components:

1. Incoming Terminal
2. Outgoing Terminal
3. Din Rail Holder
4. Arc Chutes Holder
5. Arc Chutes
6. Fixed Contact
7. Dynamic Contact
8. Bi-metallic Strip Carrier
9. Bi-metallic Strip
10. Latch
11. Plunger
12. Solenoid
13. Switch

Main contacts: These are the contacts that carry the load current and are connected to the incoming and outgoing wires of the circuit.

Trip Unit: This is the core component of an MCB, which monitors the current flowing through the circuit and trips the breaker in case of an over-current or short-circuit. The trip unit consists of a bimetallic strip, a magnetic actuator, and an operating mechanism.

Terminal: These are the connections for the incoming and outgoing wires.

Housing: The housing is the protective casing that houses the MCB components and provides insulation between live parts and other electrical components.

Trip Indicator: An MCB typically has a visual indicator that shows whether the breaker is in the “on” or “off” position.

Auxiliary contacts: Some MCBs have additional contacts that can be used to switch auxiliary loads or provide signaling functions.

Trip spring: This is the spring mechanism that holds the MCB contacts in the “on” position. When the trip unit operates, the trip spring releases, allowing the contacts to separate and break the circuit.

Miniature Circuit Breaker – Working

Whenever continuous overcurrent flows through MCB, the bimetallic strip is heated and deflects by bending. This deflection of the bi-metallic strip releases a mechanical latch.

As this mechanical latch is attached to the operating mechanism, it causes to open the miniature circuit breaker contacts, and the MCB turns off thereby stopping the current to flow in the circuit. To restart the flow of current the MCB must be manually turned ON.

This mechanism protects from faults arising due to overcurrent or overload and short circuits.

But during short circuit conditions, the current rises suddenly, causing electromechanical displacement of the plunger associated with a tripping coil or solenoid. The plunger strikes the trip lever causing the immediate release of the latch mechanism consequently opening the circuit breaker contacts. This was a simple explanation of a miniature circuit breaker’s working principle.

An MCB is very simple, easy to use, and is not generally repaired. It is just easier to replace. The trip unit is the main part, responsible for its proper working. There are two main types of trip mechanisms.

A bi-metal provides protection against overload current and an electromagnet provides protection against electric short-circuit current.

Miniature Circuit Breaker – Operation

If the circuit is overloaded for a long time, the bi-metallic strip becomes overheated and deformed. This deformation of the Bi-metallic strip causes displacement of the latch point.

The moving contact of the MCB is arranged by means of spring pressure, with this latch point, a little displacement of the latch causes, the release of spring and makes the moving contact move for opening the MCB.

The current coil or trip coil is placed so that during a short circuit fault the magneto-motive force (mmf) of the coil causes its plunger to hit the same latch point and make the latch to be displaced.

Again, when the operating lever of the miniature circuit breaker is operated by hand, that means when MCB goes off position manually, the same latch point is displaced as a result of moving contact separated from fixed contact in the same manner.

It may be due to the deformation of a bi-metallic strip, increased mmf of a trip coil, or maybe a manual operation, the same latch point is displaced and the same deformed spring is released, which is ultimately responsible for the movement of the moving contact. When the moving contact is separated from fixed contact, there may be a high chance of arc.

This arc then goes up through the arc runner and enters arc splitters and is finally quenched. When we switch it on, we reset the displaced operating latch to its previous on position and the MCB is ready for another switch off or trip operation.

Different Types of MCBs

There are several types of Miniature Circuit Breakers (MCBs) based on different factors such as the current rating, voltage rating, and trip characteristic. Some common types of MCBs are:

1. Thermal: This type of MCB trips based on the temperature rise caused by the current flowing through the circuit. Thermal MCBs have a bimetallic strip that bends and trips the breaker when the temperature rises above a certain threshold.
2. Magnetic: This type of MCB trips based on the magnetic force generated by the current flowing through the circuit. Magnetic MCBs have a solenoid that pulls the trip mechanism and trips the breaker when the magnetic force exceeds a certain threshold.
3. Hybrid: This type of MCB combines the features of both thermal and magnetic MCBs. Hybrid MCBs have a bimetallic strip and a solenoid, and they trip based on either the temperature rise or the magnetic force generated by the current.
4. Electronic: This type of MCB uses electronic components to monitor the current and trip the breaker. Electronic MCBs are more sensitive and provide faster and more accurate tripping compared to traditional thermal and magnetic MCBs.
5. Differential: This type of MCB is used in DC circuits and protects against earth faults and short circuits. Differential MCBs monitor the current flowing in the live and neutral wires and trip the breaker when the difference exceeds a certain threshold.
6. Residual Current Circuit Breaker (RCCB): This type of MCB is used to protect against electric shock and fire caused by earth faults. RCCBs monitor the current flowing in the live and neutral wires and trip the breaker when the difference exceeds a certain threshold.
7. Isolation: This type of MCB is used as a switch to isolate a circuit. Isolation MCBs do not have a trip mechanism and are used to switch the circuit off for maintenance or testing purposes.

Different types of MCBs used in Electrical Protection Systems

In the context of Miniature Circuit Breakers (MCBs), the terms Type A, Type B, Type C, Type D, Type E, and Type F refer to different levels of protection provided by the device.

1. Type A: Type A MCBs are designed to provide protection against over-current. They are suitable for use in circuits where the maximum expected current is known and relatively constant, such as lighting circuits.
2. Type B: Type B MCBs are designed to provide protection against over-current and short circuits. They are suitable for use in circuits where the load is variable, such as in motor circuits.
3. Type C: Type C MCBs are designed to provide protection against both over-current and earth fault currents. They are suitable for use in circuits where there is a high risk of earth fault currents, such as in circuits powered by direct current (DC) or in circuits that include sensitive electronic equipment.
4. Type D: Type D MCBs are designed to provide protection against over-current and earth fault currents, with a higher tripping threshold than Type C MCBs. They are suitable for use in circuits where there is a high risk of earth fault currents, but where the fault current is expected to be higher than what can be protected by Type C MCBs.
5. Type G: Type G MCBs are designed to provide protection against over-current and earth fault currents in residual current devices (RCDs) used in electrical systems.
6. Type H: Type H MCBs are designed to provide protection against over-current and earth fault currents in electrical systems that are powered by direct current (DC).
7. Type K: Type K MCBs are designed to provide protection against over-current and short circuits in electrical systems with high fault levels.

You can check the below video to understand more about MCB.

Video Courtesy: chrvoje engineering

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This article was first published on 2 June 2018 and was recently updated on October 2024.