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Types of Miniature Circuit Breakers (MCBs)

    Miniature Circuit Breakers

    A Miniature Circuit Breaker, famously known as MCB, is a switch designed to interrupt a circuit automatically due to overcurrent, undervoltage, or overheating. The MCB acts as an electromechanical device triggered by overload and fault current.

    Why use Miniature Circuit Breakers when we have fuses?

    In the past, ceramic fuses were commonly used to safeguard electrical circuits. Whenever there was a high current passing through the fuse wire, it would blow out, causing the circuit to disconnect. To fix this, the wire must be replaced manually; unlike ceramic fuses, Miniature Circuit Breakers trip instead of blowing out. This means that they can be reset with the flick of a switch after identifying and resolving faults in the circuit. Additionally, MCBs can also function as isolators.

    Overload Protection – A bimetallic strip is provided in the Miniature Circuit Breaker for overload protection. When a high current flows through a circuit for a sufficiently long time, it may damage the circuit, cable insulation, etc. To avoid this situation, a bimetallic strip is provided. The bimetallic strip bends with the increased heat before the installed circuit is damaged, protecting the circuit.

    Miniature Circuit Breaker

    Short circuit protection – If there is a short circuit in any part of the installation, there is a sudden surge in the current, which may result in fire. High short circuit currents cause the tripping coil to disconnect the circuit.

    Undervoltage Protection – The same magnetic coil used for overload protection is used for undervoltage protection. When there is very low voltage, the coil does not remain energised, and MCB gets tripped.

    We can classify Miniature Circuit Breakers in two ways.

    1. According to the Number of poles

    2. According to Properties (tripping curve)

    Types of Miniature Circuit Breakers classified by Number of poles –

    Miniature Circuit Breakers

    Single pole MCB – It provides switching and protection only for single-phase supply.

    Two pole MCB – It provides switching and protection for phase and neutral

    Three-pole MCB – It provides switching and protection for three phases

    Three poles with neutral MCB – It provides switching and protection to all three phases and neutral

    Four-pole MCB – It provides protection similar to three poles MCB with neutral.

    Types of Miniature Circuit Breakers classified by Properties – 

    Miniature Circuit Breakers specifications

    Miniature Circuit Breakers are also classified by their instantaneous tripping current, commonly known as the tripping curve.

    Available types of MCBs are A, B, C, D, K and Z.

    Among these types, B, C and D are mainly used.

    Instantaneous tripping current is the minimum value of current, causing the circuit breaker to operate automatically without intentional time delay. (In) (IEC 60898-1)

    Range of instantaneous tripping –

    BAbove 3 to 5 times rated current
    CAbove 5 to 10 times rated current
    DAbove 10 to 20 times rated current

    Type B MCB – 

    Type B MCD

    The type B curve MCB is built to disconnect the circuit by tripping at 3 to 5 times its rated current. That means a B16 breaker will trip when the current is more than 48A to 80A.

    Type B MCBs are used to protect cables and conductors from overload and short circuit protections. It can be used in domestic and commercial applications like homes, apartments, offices, shops etc.

    Type B MCBs are not suitable for motors or high inductive loads. The inrush current for inductive loads is very high. (for example – An induction motor three times its starting current) Type B MCB will trip at the start only.

    Type C MCB

    Type B MCB

    The type C curve MCB is built to 5 to 10 times the rated current.

    This is the most commonly used miniature circuit breaker for general over-current protection of sub-mains and final sub-circuits.

    Type D MCB –  

    Type D MCD Tripping Curve

    Type D miniature circuit breakers trip at a current 10 to 50 times their rated current.

    Specifying the mean tripping current for these circuit breakers is essential based on the specific application. They are typically used when transient over-current could cause unnecessary tripping, such as circuits supplying motors or welders.

    Tripping curve of a Miniature Circuit Breaker

    Tripping Curve of a Miniature Circuit Breaker (MCB)

    The above image shows a tripping curve of B-type MCB. 

    The tripping curve is of two parts. The lower part of the graph shows how the circuit is protected from short circuit current. We can see from the graph that a non-transient 6A current through a B6 MCB will never trip. (orange line in the graph)

    If B6 MCB is connected to a small motor, the inrush current will be up to 18A. Meanwhile, the current graph for the short circuit trip is at 30A. So, at regular starting operation, MCB will never get tripped. However, if there is a short circuit fault, the current will be more than 30A, and then MCB will be tripped as per requirement.

    The upper part of the graph represents overload protection. As seen in the graph, If there is an overcurrent up to 9A, the circuit will get heated after some time. To avoid damage because of prolonged overcurrent, bimetallic strips will act and disconnect the circuit.

    What is the difference between Type C and Type B MCB – 

    Type B6 and type C6 MCB both the MCB protect circuit for rated current 6A. But then, what is the difference between them? Why do they have different characteristic curves? 

    The key difference between all the types of MCBs having the same rated current is their ‘Earth fault loop impedance’.

    As per the image above, the earth fault loop impedance of the D-type MCB is less than that of the C-type MCB. B-type MCB has the highest earth fault loop impedance among the three.

    Different earth fault loop impedance is the reason for different tripping curves of the MCBs. 

    We can change earth fault loop impedance by decreasing the length or increasing the cross-section area of the cable(Ohm’s law), whichever is possible and easier.

    Applications of different types of Miniature Circuit Breakers

    Type of Miniature Circuit BreakerTrips at current –Application
    A2-3 times the rated currentManufacturing of semiconductors
    B3-5 times the rated currentCommercial and residential like homes, offices, shops, etc
    C5-10 times the rated currentFluorescent lighting, small motors
    D10-20 times the rated currentUPS, transformers, X-ray machines
    K8-10 times the rated currentInductive loads

    Published on: 19/04/2024

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