A Miniature Circuit Breaker, often known as an MCB, is an electromechanical device that protects an electric circuit against overcurrent. Overcurrent in an electrical circuit can occur as a result of a short circuit, overload, or defective design.
Circuit breakers are mechanical devices designed to close or open contact. It closes or opens electrical circuits under normal or abnormal conditions. A circuit breaker is also a switching and current-interrupting device. It consists of fixed and moving contacts that are touching each other and carry the current under normal conditions. When the circuit breaker is closed the current-carrying contacts called electrodes engage each other under the pressure of a spring.
The miniature circuit breaker has a breaking capacity of up to 10KA and is small in physical size. It has a thermal as well as magnetic tripping device. Therefore it provides overload as well as short-circuit protection. Because of their small size, they are suitable for mounting on the distribution board to protect the lightning circuit, motor circuit, and heating circuit. It can be used again after clearing the fault in the circuit. The switching knob provides a direct indication of a faulty circuit and is more reliable than a rewirable fuse.
The standard rating are 6A, 10A, 16A, 32A, 40A, 50A, 63A, 80A, 100A, 125A etc.
Functions of Miniature Circuit Breakers
- SENSE when an over current occurs.
- MEASURE the amount of over current.
- ACT by tripping the circuit breaker in a time frame necessary to prevent damage to itself and the associated load cables.
Miniature Circuit Breakers
- Miniature circuit breakers are electromechanical devices which protect an electrical circuit from an over current.
- The over current in an electrical circuit may result from short circuit or overload
- Miniature circuit breaker is a better alternative to Fuse since it does not require replacement once an overload is occurred.
Principle of operation
- A Miniature circuit breaker functions by interrupting the continuity of electrical flow through the circuit once a fault is detected. In simple terms, MCB is a switch that automatically turns off when the current flowing through it passes the maximum allowable limit.
- There are two contacts one is fixed and the other movable.
- When the current exceeds the predefined limit, a solenoid forces the movable contact to open and the MCB turns off
- In order to restart the flow of current the Miniature circuit breaker is manually turned on.
- To protect against over current or increase in temperature, a bi-metallic strip is used.
- MCBs are generally designed to trip within 2.5 millisecond when a fault occurs.
- In case of over current or temperature rise, it may take 2 seconds to 2 minutes for the MCB to trip.
Internal View of Miniature circuit breaker
- Top terminal
- Arc chamber
- operating Knob
- Fixed contact
- moving contact
- Blow-out magnet
- Bi metal
- Bottom terminal
- Din rail clip
Tripping mechanism of Miniature circuit breaker
- The Miniature circuit breaker contacts and the position of the knob can easily be seen in both, ON and OFF, states. In the ON state, the moveable contact touches the fixed contact as shown in the image.
The path of the current in the ON state
The path of the current in the ON state is shown in the image below. The live wire is screwed at the input terminal. From the input terminal the current flows to the fixed contact which is touching the input terminal. The current then moves to the moving contact (since in the ON state it is touching the fixed contact). The moving contact is connected to the electromagnetic coil of the solenoid utilizing a thick wire. The current from the moving contacts enter the electromagnetic coil. The current from the coil goes to the bimetallic strip using another thick wire and finally to the output terminal
Connections of the coil
The following images clearly show the connection of the endpoints of the coil with the other parts of the circuit and how the current enters and exits the coil.
Switching or the tripping mechanism
Role of Plunger
The arc consists of a column of ionized gas having molecules that have lost one or more electrons. The electron that is negatively charged is attracted towards the positive contact with a high velocity and on the way they detach more electrons by impact. The positive ions are attracted towards the negative contact and they move towards it slowly and cause the flow of current due to the movement of electrons.
When an overcurrent is interrupted by the circuit breaker by opening its contacts, current tries to bridge the gap. In an attempt to maintain the circuit, the air heats up and becomes a conductor. As a result an arc forms.In general, when air and gases are heated, they become electric conductors. The hotter they get, the better they conduct.
Factors for quenching the arc
- Speed: When the contacts separate rapidly, there is less time for the arc to form and maintain itself.
- Distance: When the distance between opened contacts is more, the arc has to stretch more to maintain the current flow which requires more voltage.
- Cooling: When the arc is forced against a cold material, it absorbs and dissipates the heat.
The image shows the arc chute used in the MCB for arc quenching purposes. The fixed contact is extended to the arc chute. This is done to transfer the arc to the interior of the breaker. The arc is broken into smaller arcs by the arc chute. This segmented smaller arc has a very small potential and hence gets naturally dissipated.
Over heat protection
The bimetallic strip is made up of two metals with different temperature coefficients.When the current flows through the bimetallic strip, it gets heated. The heating results in bending of the bimetallic strip. The more is the heating, the more is the bending.When the bimetallic strip bends it forces the metal strip to pull the plastic flap which in turn triggers the tripping mechanism after certain allowed temperature limit for which the bimetallic strip is designed.
B Type: light, heaters etc.
C type: inductor, motors etc.
D type: heavy inrush current (transformers, x-ray machines)
Range (varies company wide)
6A to 40A – ‘B’ Curve
0.5A to 100A – ‘C’ Curve
0.5A to 100A – ‘D’ Curve
0.5A to 63A for DC Application
Breaking capacity up to 10 kA (high enough for most of the applications)
Current Limiting Design
Single Pole (1P)
Single Pole & Neutral (1P+N)
Double Pole (2P)
Three Pole (3P)
Three Pole & Neutral (3P+N)
Four Pole (4P)
Advantages of Miniature circuit breaker
- It is more reliable than re-wirable fuse.
- Easy installation.
- NO need of replacement(i.e can be re-used)
- Non-deterioable characteristics.
Selection Criteria of Circuit Breaker
- Service voltage
- Number of poles i.e. single, double, three pole.
- Current rating.
- Rupturing capacity in KA or MVA.
- Mounting, i.e. floor or panel mounted.
- Size of cable boxes required.
- Type of protection required.