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MCB (Miniature Circuit Breakers) - Types, Working and Tripping Characteristic Curves

What is MCB?

MCB or Miniature Circuit Breaker is an electromechanical device which protects an electric circuit from an overcurrent. The overcurrent, in an electrical circuit, may result from short circuit, overload or faulty design

An MCB is a better alternative to a Fuse since it does not require replacement once an overload is detected. Unlike fuse, an MCB can be easily operated and thus offers improved operational safety and greater convenience without incurring large operating cost. 

They are used to protect lower current circuits and have the following specifications 

  • Current rating - Amperes
  • Short Circuit Rating - Kilo Amperes (kA) 
  • Operating Characteristics - B, C or D Curves

Miniature Circuit Breakers are usually available in the range of 0.5A to 100A.

Their Short circuit rating is given in Kiloamps (kA), and this indicates the level of its ability to work. For example a domestic MCB would normally have a 6kA fault level, whereas one used in an industrial application may need a unit with a 10kA fault capability. 

    Working Principle of MCB

    MCB’s are protective devices that are made to break the circuit in case of overload or Short circuit. For Overload protection they have Bi-metallic strip which causes the circuit to open. For short circuit it is Electromagnetic kind of thing.

    There are two arrangement of operation of miniature circuit breaker
    1. One due to thermal effect of over electric current and 
    2. due to electromagnetic effect of over current. 

    The thermal operation of miniature circuit breaker is achieved with a bimetallic strip. Whenever continuous over electric current flows through MCB, the bimetallic strip is heated and deflects by bending. This deflection of bimetallic strip releases mechanical latch. As this mechanical latch is attached with operating mechanism, it causes to open the miniature circuit breaker contacts. 

    But during short circuit condition, sudden rising of electric current, causes electro-mechanical displacement of plunger associated with tripping coil or solenoid of MCB. The plunger strikes the trip lever causing immediate release of latch mechanism consequently open the circuit breaker contacts. This was a simple explanation of miniature circuit breaker working principle.

    This video explains the working principle of a MCB is simple words.

    Tripping Mechanism of MCB

    1. Thermal Trip Unit

    The thermal trip unit protects against overload currents. 

    The thermal unit is based on a bimetal element located behind the circuit breaker trip bar and is part of the breaker’s current carrying path. 

    When there is an overload, the increased current flow heats the bimetal causing it to bend. As the bimetal bends it pulls the trip bar which opens the breaker’s contacts. The time required for the bimetal to bend and trip the breaker varies inversely with the current. 

    2. Magnetic Trip Unit

    The Magnetic trip unit protects against a short circuit.

    The magnetic trip unit is comprised of an electromagnet and an armature.

    When there is a short circuit, a high magnitude of current passes through the coils creating a magnetic field that attracts the movable armature towards the fixed armature. The hammer trip is pushed against the movable contact and the contacts are opened. 

    Types of MCB based on Tripping Characteristics

    MCBs are classified according to tripping over range of fault current as follows:

    1. Type B MCB
    2. Type C MCB
    3. Type D MCB
    4. Type K MCB
    5. Type Z MCB

    TypeTripping CurrentOperating Time
    Type B  3 To 5 time full load current0.04 To 13 Sec
    Type C  5 To 10 times full load current0.04 To 5 Sec
    Type D  10 To 20 times full load current0.04 To 3 Sec
    Type K  8 To 12 times full load current<0.1 Sec
    Type Z  2 To 3 times full load current<0.1 Sec

    1. Type B MCB:

    This type of MCB trips between 3 and 5 times full load current. 

    Type B devices are mainly used in residential applications or light commercial applications where connected loads are primarily lighting fixtures, domestic appliances with mainly resistive elements. Also used for computers and electronic equipment with very low inrush loads (PLC wiring).

    The surge current levels in such cases are relatively low.

    Function: protection and control of the circuits against overloads and short-circuits; protection for  people and big length cables in TN and IT systems. 

    Applications: residential, commercial and industrial. 

    2. Type C MCB:

    This type of MCB trips between 5 and 10 times full load current. 

    This is used in commercial or industrial type of applications where there could be chances of higher values of short circuit currents in the circuit. 

    The connected loads are mainly inductive in nature (e.g. induction motors) or fluorescent lighting. Applications include small transformers, lighting, pilot devices, control circuits and coils. 

    Function: protection and control of the circuits against overloads and short-circuits; protection for resistive and inductive loads with low inrush current. 

    Applications: residential, commercial and industrial. 

    3. Type D MCB:

    This type of MCB trips between 10 and 20 times full load current. 

    These MCBs are use in specialty industrial / commercial uses where current inrush can be very high. Examples include transformers or X-ray machines, large winding motors etc.  

    D-curve devices are suitable for applications where high levels of inrush current are expected. The high magnetic trip point prevents nuisance tripping in high inductive applications such as motors, transformers and power supplies.

    Function: protection and control of the circuits against overloads and short-circuits; protection for circuits which supply loads with high inrush current at the circuit closing (LV/LV transformers, breakdown lamps). 

    Applications: residential, commercial and industrial. 

    4. Type K MCB:

    This type of MCB trips between 8 and 12 times full load current.

    These are Suitable for inductive and motor loads with high inrush currents. The K and D curve breakers are both designed for motor applications where ampacity rises quickly and momentarily during “start-up.” 

    Function: protection and control of the circuits like motors, transformer and auxiliary circuits, against overloads and short-circuits. 

    Advantages: no nuisance tripping in the case of functional peak currents up to 8xIn, depending on the series; through its highly sensitive thermostatic bimetal trip, the K-type characteristic offers protection to damageable elements in the overcurrent range; it also provides the best protection to 2 cables and lines. 

    Applications: commercial and industrial. 

    5. Type Z MCB:

    This type of MCB trips between 2 to 3 times full load current.

    These type of MCBs are highly sensitive to short circuit and are used for protection of highly sensitive devices such as semiconductor devices.

    Function: protection and control of the electronic circuits against weak and long duration overloads  and short-circuits. 

    Applications: commercial and industrial. 

    All the above types of MCBs provide tripping protection within one tenth of a second.

    Types of MCB based on Number of Poles

    Another practical way of distinguishing MCBs is by way of the number of poles supported by the circuit breaker. Based on that, following types exist:

    1. Single Pole (SP) MCB
      • A single pole MCB provides switching and protection only for one single phase of a circuit.
    2. Double Pole (DP) MCB
      • A two Pole MCB provides switching and protection both for a phase and the neutral.
    3. Triple Pole (TP) MCB
      • A triple/three phase MCB provides switching and protection only to three phases of the circuit and not to the neutral.
    4. 3 Pole with Neutral [TPN (3P+N) MCB]
      • A TPN MCB, has switching and protection to all three phases of circuit and additionally Neutral is also part of the MCB as a separate pole. 
      • However, Neutral pole is without any protection and can only be switched.
    5. 4 Pole (4P) MCB
      • A 4 pole MCB is similar to TPN but additionally it also has protective release for the neutral pole. 
      • This MCB should be used in cases where there is possibility of high neutral current flow through the circuit as in cases of an unbalanced circuit.

    Selection of Right MCB

    The decision to use Type B, C or D miniature circuit-breakers for final circuit protection in residential, commercial, industrial or public buildings can be based on a few simple rules. 

    However, an understanding of the differences between these Types of device can help the installer overcome problems of unwanted tripping or make a suitable selection where lines of demarcation are less clearly defined.

    It should be stressed that the primary purpose of circuit protection devices such as miniature circuit breakers and fuses is to protect the cable downstream of the device.

    The essential distinction between Type B, C or D devices is based on their ability to handle surge currents without tripping. These are, typically, inrush currents associated with fluorescent and other forms of discharge lighting, induction motors, battery charging equipment etc 

    Type B, C and D are used for overcurrent protection of cables in accordance with IEC/EN 60898-1
    Type K for the protecting motors and transformers and simultaneous overcurrent protection of cables with overload tripping based on IEC/EN 60947-2
    Type Z for control circuits with high impedances, voltage converter circuits and semicable protection and simultaneous overcurrent protection of cables with overload tripping based on IEC/EN 60947-2

    MCB Characteristic Curves (Type B,C & D)

    Characteristic curve is the curve between release current and tripping time (Time - Current Curve).

    The circuit breaker’s tripping curves consist of two parts:
    1. Tripping of overload protection (thermal tripping device): The higher the current, the shorter the tripping time
    2. Tripping of short-circuit protection (magnetic tripping device): If the current exceeds the threshold of this protection device, the breaking time is less than 10 milliseconds.

    The first sloping region of the curve is a graphical representation of the tripping characteristics of the thermal trip unit. This portion of the curve is sloped due to the nature of the thermal trip unit. 

    The second region is the response time of the magnetic trip which differentiates each characteristic and for which an identifying letter (Type B,C,D,K,Z) is assigned. 

    The classification of Type B, C or D is based on fault current rating at which magnetic operation occurs to provide short time protection (typically less than 100ms) against short circuits.

    The most important MCB characteristics are
    • Type B characteristic curves.
    • Type C characteristic curves.
    • Type D characteristic curves. 
    They are shown below,
    MCB Characteristic Curves
    It is important that equipment having high inrush currents should not cause the circuit-breaker to trip unnecessarily, and yet the device should trip in the event of a short-circuit current that could damage the circuit cables. 

    1. Type B Curve

    Type B devices are generally suitable for domestic applications. They may also be used in light commercial applications where switching surges are low or non-existent. 

    Type B devices are designed to trip at fault currents of 3-5 times rated current. For example a 10A device will trip at 30-50A.

    2. Type C Curve

    Type C devices are the normal choice for commercial and industrial applications where fluorescent lighting, motors etc. are in use

    Type C devices are designed to trip at 5-10 times In (50-100A for a 10A device). 

    3. Type D Curve

    Type D devices have more limited applications, normally in industrial use where high inrush currents may be expected
    Examples include large battery charging systems, winding motors, transformers, X-ray machines and some types of discharge lighting. Type D devices are designed to trip at 10-20 times (100-200A for a 10A device). 

    Normal cable ratings relate to continuous service under specified installation conditions. Cables will, of course, carry higher currents for a short time without suffering permanent damage. 

    Type B and C circuit breakers can generally be selected to achieve tripping times that will protect the circuit conductors against normal surge currents in accordance with BS 7671. This is more difficult to achieve with Type D devices, which may require a lower earth loop impedance (Zs) to achieve tile operating times required by Regulation 413-02-08. 

    Sources of Surge currents

    Surge currents in domestic installations are generally low, so that a Type B device is adequate. 

    For example Inrush currents associated with one or two fluorescent fittings or the compressor motor in a refrigerator/freezer are unlikely to cause unwanted tripping. Fluorescent and other discharge lamps produce surge currents and while one or two fluorescent lamps are unlikely to cause a problem, the block switching of a number of fluorescent lamps

    In a shop, office or factory can produce substantial inrush currents. For this reason Type C devices are recommended for these applications. 

    The magnitude of the surge current will depend on the lamp rating, starting system and type of control gear used in the luminaires. Reputable MCB manufacturers produce tables listing the number of fittings of a particular make and type that can be used with their devices. 

    Overcoming Unwanted Tripping of MCB

    Sometimes failure of tungsten filament lamps can trip Type B circuit breakers in domestic and retail environments. 

    This is caused by high arcing currents occurring at the time of failure and is generally associated with inferior quality lamps. If possible the user should be encouraged to use better quality lamps. If the problem persists then one of the measurement listed below should be considered. 

    A Type C device may be substituted for a Type B device where unwanted tripping persists, especially in commercial applications. 

    Alternatively it may be possible to use a higher rating Type B MCB, say 10A rather than 6A. 

    Whichever solution is adopted, the installation must be in accordance with BS 7671. 

    A change from Type C to Type D devices should only be taken after careful consideration of the installation conditions, in particular the operating times required by Regulation. 

    Other Considerations

    The importance of selecting circuit-breakers from reputable manufacturers cannot be over emphasised. Some imported products, claiming to have a 6kA short-circuit capacity, had failed dramatically under test. 
    In contrast the test procedures followed in British ASCTA (Association of Short Circuit Testing Authorities) laboratories are among the most suitable in the world. 

    Type B devices should only be used in domestic situations where high inrush currents are unlikely and Type C devices should be used in all other situations.