Miniature Circuit Breaker (MCB) - Principle of operation

Miniature Circuit breaker

Miniature Circuit Breaker (MCB) - Principle of operation

Miniature circuit breakers normally known as MCBs, are mechanically operated switches cum electro-mechanically operated automatic circuit protection devices. They are used to interrupt a circuit during overload and short circuits. They are used as an alternative of fuses. The advantage of MCBs over fuses is that MCBs are reusable even after interruption of the circuit after an overload or short circuit. Moreover, MCBs are more sensitive to faults than fuses.

Working Principle of MCBs

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 bimetallic stip.
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.

Constructional Features of MCB:

Each MCB consists of the following part:

Miniature Circuit breaker1.       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 other is movable.

3.       Knob:

MCBs can be turned ON and OFF using this know.

4.       Mechanical Latch:

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

5.       Bimetallic strip:

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

6.       Solenoid:

Solenoid offers instantaneous protection against short circuit by releasing the mechanical latch.

7.       Arc Chutes:

Arc chutes are used for splitting and quenching of arcs.

Operation of MCBs:

Miniature circuit breakers performs 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 an 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 of 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 any one of the poles will open all the poles together.

MCB 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 ambient temperature of MCB. Miniature circuit breakers are available in the following ratings:

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

Miniature Circuit breakerMiniature Circuit breaker
Miniature Circuit breaker

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 as 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 curve

 Trip curves defines the trip current ratings of an MCB. Trip current is the minimum current at which an MCB will trip instantaneously.

Above  3 to 5 times rated current
Above  5 to 10 times the rated current
Above  10(excluding 10) to 20 times the rated current
Above 8 to 12 times the rated current
Above 2 to 3 times the rated current.

Know more about Trip curves by clicking here.

7. Ambient temperature

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

Application of MCBs

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. MCBs can offer better overload protection than fuses.
3. MCBs can be reset after the clearance of fault. But fuses needs to be rewired or replaced.
4. Safer interruption of short circuit current and arc quenching.
5. Knob makes operation of MCBs much easier than a fuse.
6. MCBs can be turned off whenever we want. Therefore, Circuit isolation during maintenance is much easier compared to fuses.

Tripping of MCB and possible faults

MCBs may trip whenever there is a short circuit, overload or even when they are being under rated, in the sense ampere rating of the MCB is not properly chosen. The reason for tripping should be identified and faults must be cleared before turning ON them again. If an MCB trips, first of all we have to check whether there is any visible faults such as short circuits or overloads. Even fused LED lamps can trip MCBs. If there is no visible fault in the circuit protected by the MCB, wait till the MCB 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.

Please note:

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

Related Topics:

1. Trip Curve in MCBs2. Difference between MCB and MCCB3. Difference between MCB and RCBO