Miniature circuit breaker (MCB) – Principle of operation

Miniature circuit breakers commonly known as MCB, are mechanically operated switches cum circuit protection devices. It is an electro-mechanically operated automatic circuit protection devices. An MCB is used to interrupt a circuit during overload and short circuits. It can be used as an alternative to fuses for domestic applications. The advantage of MCBs over fuses is that they are reusable even after interruption of the circuit after an overload or short circuit. Moreover, they are more sensitive to faults than fuses.

Constructional Features of MCB:

Every MCB consists of the following part:

MCB internal parts
MCB internal parts

1. External Casing:

External Casing holds all the internal components firm and protects them from dust. It is made of insulating materials such as plastic or ceramics.

2. Contacts:

A pair of contacts can be found inside an MCB. One of them is fixed and the other is movable.

3.  Knob:

MCBs can be turned ON and OFF using this knob.

4. Mechanical Latch:

A latch arrangement is made inside MCBs to hold the contacts under spring tension at ON position.

5. Bimetallic strip:

The bimetallic strip offers delayed overload protection by sensing the prolonged flow of current greater than its rated current.

6. Solenoid:

Solenoid offers instantaneous protection against short circuit by releasing the mechanical latch. Solenoid gets activated when the current through the coil exceeds a particular value, normally more than 3 times of its rated current. This solenoid is not activated by overloads.

7. Arc Chutes:

Arc chutes are used for splitting and quenching of arcs.

Operation of MCBs:

Miniature circuit breakers perform four important functions:

1. Switching

Miniature circuit breakers can be switched ON and OFF manually. This functionality is highly useful, especially during maintenance. An internal latch arrangement is made such that it automatically holds the fixed contact as soon as the knob is pushed to the ON position. When turned OFF, the pressure given by our finger over the knob releases the latch and opens the contacts.

2. Overcurrent protection

When a piece of equipment is overloaded it draws more current from the source. This current flows through the bimetallic strip and heats it up. Bimetallic strip that deforms on heating will knock down the latch, thereby opening the contact and isolation the equipment from the supply.

3. Short Circuit Protection

During short circuits, a sudden rise in current produces MMF powerful enough to project plunder towards the latch and releases it, thereby opening the contacts.

4. Arc Quenching

When the contacts are opened under load, an arc is formed between the fixed and moving contacts. The contacts are designed such that the arc formed between them moves outwards through arc runners and reaches the arc splitters or arc chutes. Arc splitters or arc chutes is an arrangement to increase the length of the arc, splitting and quenching it.

The operation of MCBs is affected by environmental temperature, particularly the deflection of the bimetallic strip. Hence it is very important to choose appropriate MCB based on its ambient temperature.

Miniature Circuit Breakers may be of single, two, three or four-pole version. In two, three and four-pole versions, the respective number of single poles MCBs are joined together and their knobs are combined so that fault cleared by anyone of the poles will open all the poles together.

Principle of operation

In the case of overloads, a current more than the rated current is driven through the MCB. As the current flows through the bimetallic strip, it gets heated up and deflects by bending and releases the mechanical latch. Deflection time of bimetallic strip depends on the amount of current flowing through the strip. Higher the current faster will be the deflection of the bimetallic strip.
During short circuits, a transient current flowing through the solenoid forces the plunger towards the latch. This action instantaneous releases the mechanical latch and opens the contacts immediately.

Types and Selection

We have to keep in mind the following terms while choosing an MCB.
1.       Application
2.       Rated current and number of poles
3.       Operating Voltage
4.       Rated frequency
5.       Ultimate breaking capacity
6.       Trip Curve Characteristics
7.       Ambient temperature

1. Application

The area of application is an important factor in the selection process. Once the application is known, then it is very easy for us to select the appropriate Miniature circuit breaker. After understanding the application we have to check the following.

2. Rated Current and number of poles

Rated current of the MCB to be used depends on the load current of the equipment to be protected and an ambient operational temperature. Miniature circuit breakers are available in the following ratings: 6A, 10A, 16A, 20A, 25A, 32A, 45A, 50A, 63A, 80A, 100A, 125A.

MCBs are available in single, two, three and four-pole versions.

3. Operating voltage.

Rated voltage of the operating equipment/ circuit to be protected should also be taken into account while selecting an MCB.

4.  Rated frequency

The frequency at which the breaker is designed to operate is called rated frequency.

5.  Ultimate Breaking Capacity (Icn)

Ultimate breaking capacity of an MCB is the maximum short circuit current it can interrupt safely.

6.  Trip curves

Trip curves define the trip current ratings of an MCB. Trip current is the minimum current at which a miniature circuit breaker will trip instantaneously.

Trip class Trip Current
CLASS B Above 3 to 5 times rated current. Suitable for cable protection
CLASS C Above 5 to 10 times the rated current. Suitable Domestic and residential applications and electromagnetic starting loads with medium starting currents
CLASS D Above 10(excluding 10) to 20 times the rated current. Suitable for inductive and motor loads with high starting currents.
CLASS K Above 8 to 12 times the rated current. Suitable for inductive and motor loads with high inrush currents
CLASS Z Above 2 to 3 times the rated current. These type of MCBs are highly sensitive to short circuit and are used for the protection of highly sensitive devices such as semiconductor devices.

Know more about Trip curves by clicking here.

7. Ambient temperature

The ambient temperature of operation should also be taken into consideration since it may affect the rated current and trip current of the MCB.


MCBs are used in the protection of lights, refrigerators, Air conditioners etc. as an alternative for fuses.

Advantages of MCBs over fuses

1. Miniature Circuit Breakers can act faster than fuses (2-3.5ms) during short circuits.
2. They can offer better overload protection than fuses.
3. They can be reset after the clearance of fault. But fuses need to be rewired or replaced.
4. Safer interruption of short-circuit current and arc quenching.
5. The knob makes the operation of a miniature circuit breaker much easier than a fuse.
6. They can be turned off whenever we want. Therefore, Circuit isolation during maintenance is much easier compared to fuses.

Tripping of MCB and possible faults

A miniature circuit breaker may trip whenever there is a short circuit, overload or even when they are being underrated, in the sense ampere rating of the breaker is not properly chosen. The reason for tripping should be identified and faults must be cleared before turning ON them again.

When it trips, first of all, we have to check whether there are any visible faults such as short circuits or overloads. Even fused LED lamps can trip an MCB. If there is no visible fault in the circuit protected by it, wait till it cools down and turn it ON again. If it trips again immediately, that means there is a short circuit in the circuit or equipment it protects. If it trips again after a few minutes, then there is a chance that the circuit or equipment is overloaded. Clear the fault in the equipment or circuit and turn on the MCB. Ask for expert advice if required.


An MCB is a circuit protection device. It can protect a circuit or a piece of equipment from faulty conditions such as overloads and short circuit. The purpose of using it in the circuit is not to prevent overload or short circuit but to protect the circuits from greater damages caused by them. It works on the principle of deflection of the bimetallic strip on heating. It can be used as an alternative to fuses. Unlike fuses, MCB can be reused even after it trips.

Please note:

This article explains the basic principle of operation of an MCB. Constructional features of MCBs may vary upon its manufacturers.

Related Topics:
1. Trip Curve in MCBs
2. Difference between MCB and MCCB
3. Difference between MCB and RCBO

This Post Has 12 Comments

  1. Phil HEANEY

    If I connect two 63amp MCB,s together (Electrically and mechanically) at the output would this then give me 126amp supply capability

    1. admin

      This is not possible.

  2. Electrical engineer

    can you please explain me principles of MCB protects from single phase short circuit?

    1. admin

      During short circuits, a transient current flowing through the solenoid forces the plunger towards the latch. This action instantaneous releases the mechanical latch and opens the contacts immediately and the circuit is isolated.

  3. Lamidi olalekan

    Good explanation

  4. Madhuri k

    Really useful information about MCB…thanks

  5. Sunil

    I have an MCB which is rated for 240V AC 50Hz. Can I use it in a 125V 60Hz circuit?

    1. Electrical Classroom

      Yes you can use.


    How can i identify whether it is short circuited or overloaded from Mcb tripping?

  7. کلید مینیاتوری

    Thank you it is nice explanation AND Very Easy to understand

Leave a Reply