What is a miniature circuit breaker and what are its types

What is a miniature circuit breaker and what is the use of it

A miniature circuit breaker is a small electromagnetic circuit breaker and it does the protection of electrical circuits from overcurrents. An MCB can protect the circuit from overcurrents and short circuits. This device can operate automatically so in case of an overcurrent or short circuit the MCB will automatically protect the circuit, MCB will react to the faults very quickly it can detect the fault easily and react to it within milliseconds. The MCB will switch the circuit to open state if the current passing through it exceeds the value it is set for. So the major purpose of an MCB is to determine the fault conditions and interrupt the flow of the current.

What are the functions and features of an MCB

  • The design of MCB is current limiting – low let-through energy
  • Silver graphite contact for weld-free operations
  • The terminals on both sides of the MCB’s are shrouded and it would make the MCB finger touch-proof
  • MCB has a breaking capacity of 10KA for 2 positions and 20KA, for three positions
  • Small in size so there is no trouble for installation
  • Longer life and less maintenance
  • Fast response to faults
  • It can protect the cables from overloads and short circuit
  • Protection against insulation faults

What are the parts of an MCB and how is it constructed

The MCB casing is made up of a molded thermoplastic polyester. So this casing is fire resistant and non-hygroscopic. The MCB contacts are mostly made up of silver-copper alloy and because of this long life.

The MCB consist of three main construction parts they are

MCB frame, a trip unit of MCB and operation mechanism of MCB. The MCB frame is made up of molded thermoplastic polyester and this housing is very rigid and strong and the components are mounted in this. The trip unit is the part that is responsible for the proper working of a miniature circuit breaker. There are two types of trip mechanisms they are overload current protection by bimetal and short circuit protection by an electromagnet. The operation mechanism provides manual operation by which manual opening and closing can be done.

Parts of MCB

1. Supply terminal

This part is mostly made up of silver or copper alloy, and the incoming phase is connected to this part

2. Arc chute

Arc chute are metal plates that are mutually insulated and this part would cool the arc.

3. Bimetallic strip

The bimetallic strip is for the thermal tripping process and this strip is wounded by a heater coil and it would produce heat according to the current flow.

4. Magnetic coil

This is the major part of the magnetic tripping process, a strong magnetic field will be created in the field during short circuits or overloads.

5. Copper braid

This part of the MCB will connect the moving element with the static element

6. ON/OFF switch

By using this part we can turn on the electric circuit

7. Outgoing terminal

By using this terminal we can connect the outgoing phase from circuit breaker

How does MCB work

A miniature circuit breaker operates in two ways, one type is due to the thermal effect of overcurrent and the other type would be of electromagnetic effect of the over current. During normal conditions, the MCB would operate as a switch so that the circuits can be made ON or OFF. But in case of overload or short circuit conditions, this device will operate automatically which means it trips automatically and because of this current interruption will take place in the load circuit. The automatic operation or tripping of MCB is done in two ways they are magnetic tripping and thermal tripping.

Thermal operation

Thermal operation is done with the help of a bimetallic strip, in case of overcurrent flow through an MCB, the metallic strip would deflect and the mechanical latch would be released and the MCB contact would open.

Magnetic operation

This operation takes place with the help of a magnet and an armature if there is a short circuit a huge amount of current would travel through the coil and this would produce a magnetic field, and because of this the armature will move and the MCB contacts will be open.

What are the types of MCB

Type B MCB

The B type MCB’s are mostly used for cable protection, this type of MCB’s are used where the switching surges are very small. The B type MCB will trip quickly if the current is three to five times than its rated current

Type C MCB

This type of MCB will trip quickly if the current is five to ten times than its rated current. This type is mostly used for high inductive loads, like small motors and fluorescent lighting. Type C MCB’s can handle higher value of short circuit currents and because of this, they are used in highly inductive commercial and industrial applications.

Type D MCB

This type of MCB will quickly trip if the current is ten to twenty-five times than its rated current and they are used for high inductive loads where high inrush currents are very frequent. This type of MCB is used for specific industrial and commercial applications like industrial welding equipment, large winding motors, etc.

How to select an MCB

  • Maximum operating voltage
  • Insulation voltage
  • Number of poles
  • Tripping curve
  • Frequency
  • Mechanical operation (durability & endurance)
  • Breaking capacity for each operating voltage
  • It must be selected according to the overload characteristics, its major purpose to prevent the accidental overloading of the cable in the no-fault situation. According to the degree of the overload, the speed of MCB tripping would vary.
  • It must be selected according to the characteristics of magnetic fault protection, which would operate when the fault reaches a predetermined level and to trip the MCB within milliseconds. So it must be selected according to the level of a magnetic trip.

What is the difference between MCB and fuse

The fuse will operate only once and it must be replaced, but in the case of MCB it doesn’t need to be replaced we can use it again, after the tripping of an MCB it can be reset. The MCB is costlier than a fuse, during the tripping of an MCB there won’t be any power but in the case of a fuse, we would only lose the power in that phase. The MCB’s are highly sensitive to overcurrent than the fuse. After the tripping quick restoration of power is not possible in the case of fuse but in the case of MCB, it is possible.

What are the advantages of MCB

  • Stable tripping characteristics
  • Common tripping for all the phases of a motor
  • After clearing the faults the circuit will be reclosed very quickly
  • For circuit isolation, this device has safety disconnect features
  • The terminals are insulated so that the operators will be safe
  • This device can be reused so maintenance and replacement costs will be less
  • Power loss is less
  • Small in size so easy installation
  • Arc interruption is stable

What are the disadvantages of MCB

  • MCB’s are costlier than a fuse
  • It releases a large amount of energy when compared to a fuse
  • If a circuit breaker trips then we would lose all the power but in case of a fuse we would only lose the power at that phase

What are the applications of MCB’s

MCB’s are widely used for residential, commercial, and industrial applications some of their applications are

  • Power supplies
  • Lightning circuits
  • Solenoids
  • Relay/contactor coils
  • Appliances
  • Control circuits
  • Motor circuits
  • Programmable logic controller input/output points


post-graduate in Electronics & communication.

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