What is a Miniature Circuit Breaker (MCB)?

What is a Miniature Circuit Breaker (MCB)?


What is Miniature Circuit Breaker (MCB)?

To begin with a short answer: Miniature Circuit Breaker, aka MCB, is a smart device that helps out in faulty situations.

And who wouldn’t love something that saves the day!

This comprehensive guide will walk you through everything you need to know about this savior device: what a miniature circuit breaker is, its construction, how it works, its types, and some interesting things.

What is a Miniature Circuit Breaker?

By now, you already know it’s something that helps in unorderly situations, situations that can result in potential damage.

When talking about electricity, the two faulty situations are:

●    Excess current due to overload:

This is a slow and small overcurrent situation that causes the temperature and ampacity (load) of the circuit to increase gradually with time.

This happens when too many devices/appliances are put on one circuit. A simple example can be taken from a home. If appliances like washing machine, microwave oven, blender, dishwasher, kettles are being used at the same time, this may put a load on the circuit resulting in drawing more current than usual. This overload can cause potential damage to the appliances.

●    Short circuit

It is an intense and rapid current situation that causes an increase in the ampacity of the circuit. 

This happens when there is a direct connection between the live conductors and neutrals. The magnitude (speed and scale) of current is pretty elevated than that of the overload, making it potentially more dangerous than overload.

These faults can cause electrical circuits to damage resulting in equipment failure, and sometimes even account for serious explosions.

This is where a miniature circuit breaker (MCB) comes in handy.

It is an automatic electric switch that interrupts the current flow as soon as it detects the fault. The MCB automatically trips in case of short circuit or overload and protects against equipment failure and other electrical damages.

And the best part is:

A miniature circuit breaker is reusable. Unlike fuses, they can be reset after they have performed their task, i.e., tripping. The circuit then closes back, and the flow of current once again starts.

So, it saves your equipment by tripping and breaking the current flow and also saves your time and visits to the market to get a new MCB.

Pretty amazing, right?

MCB Ratings

Ratings, well, these are the maximum current limit in ampere that a circuit breaker can carry without operation (tripping). For example, if an MCB is rated 6kA, then the current flow above this rating will cause the MCB to trip.

The MCB ratings for domestic applications and industrial applications are different.

The domestic installation MCBs are typically rated 6000A or 6kA. The relationship between the rating of a domestic appliance and the usual voltage (240 volts) ensures that the overcurrent in the case of a short circuit does not exceed 6kA.

On the other hand, industrial machinery and commercial applications use 415v. The MCB rating required for such use is 10,000A or 10kA.

Pole Versions of MCB

MCBs come in several different pole versions. The pole version of an MCB helps decide how many lines and switches to be connected to the pole version. The pole versions of the miniature circuit breaker are:

  • Single Pole
  • Double Pole
  • Triple Pole
  • Four Pole

Construction of Miniature Circuit Breaker

The construction of a typical MCB uses two technologies:

●    Thermal magnetic technology

This technology uses a bi-metallic strip as its main element. The strip heats when the current is passed through it, resulting in bending and biasing onto the main latch. When current exceeds the designated limit, the latch is released, and the contact breaks.

●    Hydraulic magnetic technology

In this technology, a ‘dashpot’ is used instead of a bi-metallic strip. It’s a tube filled with oil and contains a spring-loaded iron core. As the current passes through the coil around the tube, an iron pole piece, an iron frame, and a spring-loaded armature complete the magnetic circuit. When the current exceeds the predefined safe value, the core attracts to the pole causing the armature to get attracted to it, resulting in the latch to trip, and the contact then breaks.

Irrespective of the technology used, the MCB has three main elements in its internal construction:

  1. A switching mechanism
  2. An overload control mechanism
  3. A  short circuit trip mechanism

We’ll discuss the internal construction of a typical MCB using thermal-magnetic technology.

The construction of a miniature circuit breaker consists of both magnetic tripping and thermal tripping mechanisms. The magnetic tripping mechanism is sensitive to current, while the thermal tripping mechanism is temperature-sensitive.

Both mechanisms are independent of each other, and they trip as per the respective fault. The thermal tripping mechanism trips to protect from overload situations, and the magnetic tripping mechanism will keep your circuits safe against short circuits.

It consists of the following parts:

  1. Housing/External Casing

All the components of the MCB are enclosed in an insulated material casing. The casing is super strong and has fire-retardant and non-hygroscopic properties. The casing has a high melting point, low coefficient of linear thermal expansion, high-dielectric electric strength, high deflection temperature under overload, and low water absorption at saturation, making it safe in case any fault occurs.

  • Contacts

An MCB has a pair of contacts that allows the current to flow when in contact and interrupts the current when disconnected. One of these contacts is fixed while the other is movable.

  • Knob

The knob is used to turn the miniature circuit breaker ON and OFF.

  • Mechanical Latch

The mechanical latch arrangement inside the MCB makes sure that the contacts are held together when the spring tension is ON.

  • Bimetallic Strip

The thermal tripping mechanism of an MCB consists of a bimetallic strip that is made up of two distinct metals, mostly steel, and brass. The metals are welded together along their lengths.

For high current MCBs, low-resistance bimetal is used, while for lower current MCBs, high-resistance bimetal is used.

When the current greater than a specified limit flows for a longer period, the bimetallic strip senses it and bends to open the breaker’s contacts and interrupts the current.

  • Arc Chutes/Arc Runners

The arc chutes (metallic splitter plates) limit and dissipate the arc energy during faulty situations (overload and short circuits). The plates are stacked up and held together by an insulating material. In some cases, where the arc chute stack does not surround the contacts, arc runners are provided to force the arc into arc chutes.

  • Solenoid

Solenoid falls in the magnetic tripping components of the MCB. It works under a short circuit and releases the latch when the current increases the predefined value.

Functions of an MCB

Following are the important functions that an MCB effectively performs:

1.    Short Circuit Protection

In the case of short circuits, the magnetic tripping mechanism of the MCB works. The magnetomotive force produced as a result of rising in current projects the plunder towards the latch resulting in contact break.

2.   Overload Protection

The thermal tripping mechanism of the MCB gets activated in case of overload. When too many devices are operated on one circuit, it draws more current, resulting in a rise in temperature. The increase in temperature heats up the bimetallic strip and it bends knocking down the latch – the contact breaks.

3.   Switching Mechanism

The switching mechanism is highly functional and comes in handy in case the circuit breaker needs to be switched ON and OFF manually especially during maintenance. When the knob is pushed to ON, the internal latch holds the fixed contact. And when the knob is turned to OFF position, the latch is released and the contact breaks.

4.   Arc Quenching

When the movable and fixed contacts break apart under load or short circuit, an arc is formed between them. This arc moves through the arc runners and reaches the arc chutes or arc splitters. The splitters are designed to extract arc energy and cool it.

Working Principle of Miniature Circuit Breaker

The main operation of an MCB is to release the latch to break the contact and interrupt current flow in case of overload and short circuits to protect your electrical appliances and electrical circuits from potential damage.

These mechanisms, as discussed previously, fall into two categories, both of which are independent of each other.

●    Working of The Thermal Tripping Mechanism

The thermal tripping mechanism saves the equipment from constant overload. The main component of this tripping mechanism is a bimetallic strip that deflects when the temperature rises.

The strip is located behind the circuit breaker trip bar. When the temperature due to the overload rises, it deforms the bimetallic strip. It moves the trip bar releasing the latch mechanism and the contact breaks resulting in current interruption. When the operation is completed, the MCBs are reset and the contacts are restored.

IMPORTANT: The overload protection is mandatory in case of low-magnitude but long time overcurrents. Such currents are potentially dangerous and reduce and risk the life of electrical installations, components, and conductors. They can also result in a fire if left ungoverned.  

●    Working of the Magnetic Tripping Mechanism

The magnetic tripping mechanism works under short circuits. During a short circuit, the solenoid energizes and the magnetic field generated in the solenoid is strong enough to force the plunger towards the latch. This electromagnetic operation instantly displaces the latch and breaks the contacts.

Both the working mechanisms have one aim: to displace the latch and break contacts to interrupt the flow of current. As a result of which an arc is produced in each of the operations. The arc passes through the arc runners to the arc chutes where it is broken into smaller segments and the overall energy is dissipated.

If the fault current is too high, the MCB works at a rapid speed and activates in just fractions of seconds. But if the current is near to the safe limit, the MCB automatically responds slowly.

Once the MCB completes its operation, it is easy to get it back to its original position by just turning ON the knob. The reusability of MCB makes them preferable to use.

The working principle of an MCB highly depends on external factors such as environmental temperature and especially affects the deformation of the bimetallic strip. Choosing an appropriate MCB based on its ambient temperature is the key factor to make sure your MCB works effectively.

The following section will lead you through the types of MCBs and will further talk to you about selecting the best ones for your needs.

Hang on for a little more!

Types of Miniature Circuit Breaker

The types of an MCB are classified according to their trip current. Trip current is the least amount of current at which the breaker will trip instantly.

MCBs are of multiple types like type A, B, C, D, K, and Z. The primary types are B, C, and D. These types are used both for domestic and commercial purposes.

An explanation to these types is listed down below:

  • Type B – This type of MCBs is the most sensitive one and is used for domestic purposes, low-voltage commercial applications, or any applications where current is likely to be small. These circuit breakers trip as soon as the current hit three to five times the rated value of the load.
  • Type C – These MCBs are applicable for powerful electrical equipment where the current surges are higher. They are mostly used in industries and for heavy-duty commercial purposes where they are used for fluorescent lamps and smaller electric motors. These MCBs trip when the current exceeds five to ten times the rated load.
  • Type D – This type of MCBs trip at ten to twenty times the rated current. They are used for the least sensitive situations. These MCBs are designed for heavy-duty systems. The most common applications are X-ray machines, welding equipment, UPS systems, transformers, battery changing systems, large motors, and uninterrupted power supply units.
  • Type K – These MCBs are great for motors and trip when the rated value exceeds ten to twelve times.
  • Type Z – This is a specialized type of MCB which is highly sensitive and trips when the current exceeds the predefined value by just two to three times. These MCBs are used for semiconductors that are too prone to short circuits.

Which One to Select?

The selection of the type of MCB depends on many factors. The following key factors are essential to look into before getting an MCB:

●    Application:

The essential factor on which the selection of the MCB depends is application. Once their use is known (domestic, commercial, or type of equipment), then looking into the other factors comes in secondary.

●    Tripping Characteristics:

The ‘trip curve’ or tripping characteristic is the least value of current at which tripping occurs. The types of MCBs based on their tripping characteristics are already discussed beforehand.

●    Operating Voltage:

The voltage of equipment that needs to be protected against current faults is mandatory to be kept in consideration.

●    Rated Current:

The ambient temperature and the load current of the equipment should be kept into account before deciding the rated current of the MCBs. The current ratings of MCBs are 6A, 10A, 16A, 20A, 25A, 32A, 45A, 50A, 63A, 80A, 100A, 125A.

●    Breaking Capacity:

The breaking capacity of an MCB is the maximum current that an MCB can handle and trip safely.

●    Rated Frequency

Rated frequency is the frequency at which the miniature circuit breaker is designed to function.

●    Number of Poles

MCBs are available in different pole versions such as single, double, triple, or four-pole versions. These pole versions are the number of switches that can be connected to the MCBs.

●    Ambient Temperature:

The ambient temperature affects the trip current and rated current of the MCBs. It is also another key factor to be kept in mind before getting an MCB.

Fuse or Miniature Circuit Breaker?

Now, if you are aware of fuses (which we consider you are), you must be wondering that they are used for a similar purpose – to save electrical circuits from damage. Then what is the difference between the two and which one to go for?

Yeah, we’ll cover that too for you!

To start with, it is necessary to understand a fuse.

Fuse – similar to the miniature circuit breaker, a fuse is a safety device that interrupts current flow (in case of overload) and protects the electrical circuit from damage. They work on a simple principle. Their main component is a strip or a metallic wire that melts as soon it detects the fault resulting in interrupting or stopping the current flow. 

So, where does the difference lie?

The following table will elaborate on the basis of the differences between the two.

Basis of DifferenceFuseMiniature Circuit Breaker (MCB)
ConstructionA fuse is constructed with a piece of metallic wire (copper, zinc, aluminum, silver, or alloys) which melts when a current flow of more than the specified range is detected.The construction of a miniature circuit breaker is done by a relay mechanism and electromagnetic switch that trips in case of short circuit or current overload.
Working PrincipleThe working of a fuse depends on the electrical and thermal properties of conducting material.Miniature circuit breaker works on the principle of switching and electromagnetism.
FunctionThe function of the fuse is both to detect the fault and interrupt the current.MCB performs only one job: to interrupt the current. The detection is done through the relay system which is specifically attached to detect a fault.
Operating ModeFuse totally operates automatically.MCB can operate both manually and automatically. 
Switching OperationFuse cannot be alternatively used as an ON/OFF switch.A circuit breaker is used as an ON/OFF switch.
Reusability and Post ReplacementFuse can be used only once because of its self-destructive nature. They need to be replaced after the operation is performed.MCBs are reusable. They can perform well multiple times, and there is no need to replace them after each operation which makes them more reliable as compared to fuse. They can be reset in no time after the operation.
Breaking CapacityIn comparison to circuit breakers, fuses have a lower breaking capacity.MCBs have a higher breaking capacity.
Ambient TemperatureThe operation of the fuse does not depend on the ambient temperature.The operation of a miniature circuit breaker depends on the ambient temperature.
ProtectionThe fuse protects the electrical systems only against power overload.Circuit breaker comes in handy when protection against both overload and short circuit is required.
Characteristic CurveBecause of the aging effect, the characteristic curve of the fuse shifts, resulting in tripping and nuisance.Circuit Breaker’s characteristic curve does not shift.
Operating TimeThe operating time of the fuse is faster than the MCB. They operate in 2 ms.Miniature circuit breakers operate slightly slower than the fuse. Their operating time is 20 ms.
Auxiliary ContactThere is no need for auxiliary contacts in the fuse.Auxiliary contact is required in a miniature circuit breaker.
Pole VersionFuse comes with a single-pole version only.In MCB, both single and multiple pole versions are available.
ApplicationsFuses come in a wide range for different voltages and current ratings. They can be used to protect almost all electrical equipment, from small electric circuits to heavy-duty motors and home appliances. They can also be used in home wiring.MCBs are low voltage circuit protection devices. They are used in transformers, industrial applications, residential equipment, heavy machinery, and home wiring.
CostFuses are pocket-friendly devices, and they are more economical than MCBs.MCBs are more costly than fuses. Their cost, however, depends on the application and can vary accordingly.

Advantages of Fuse over Miniature Circuit Breaker (MCB)

Fuses have some great benefits over miniature circuit breakers. The following points discuss those advantages:

  • As compared to miniature circuit breakers, fuses are super cost-effective.
  • Another great benefit of fuses over MCBs is they are low maintenance. They do not deteriorate with time and stay stable in the same position if no fault occurs. Furthermore, they are reliable, function at high speed, and make no noise.
  • Circuit breakers have moving parts within them which make them high-maintenance. They need to be tested to be at operating conditions. In contrast, fuses are static devices and require no maintenance.
  • Rewirable fuses are easy to reuse. In case if a fuse is blown due to overload, which results in the meltdown of the fuse elements, it is simple to rewire and replace the fuse elements.

Advantages of Miniature Circuit Breaker (MCB) over Fuse

Circuit breakers come with their own set of advantages over fuse.

  • Like fuses, circuit breakers are used as protective devices to save electrical circuits and equipment from potential damage from short circuits and overload, as well as, they can be used as ON/OFF switches which is not the case with fuses.
  • If you’re looking for something that protects multiple lines, then MCBs should be your call. They enjoy a great benefit of being multiple poles over fuses which are single poles and can protect a single line.
  • Another plus point that comes with circuit breakers is their reusability. Unlike fuses, you don’t need to keep them changing every time they trip. They can simply be put to reset like an ON/OFF switch.
  • Checking and tracking the fault is accessible in circuit breakers. Whereas, in fuses, it’s not easy to indicate the fault because the fuse element is confined inside the fuse.

Regardless of their benefits over one another, fuses and MCBs aren’t always interchangeable. Their applications are different and specialized people should be called to decide which one to install based on the particular electrical scenario.

Difference Between Miniature Circuit Breaker and Other Circuit Breakers

  • MCB and MCCB (Moulded Case Circuit Breakers) – MCCBs function in a similar way as MCBs do, except that capacities are higher and their trip curves are adjustable, unlike MCBs whose trip characteristics can’t be adjusted.
  • MCB and ELCB (Earth-Leaking Circuit Breakers) – The operation of ELCBs is different from that of MCBs. They use earthing to control current and electrocution. They detect stray voltage on the device’s enclosure and then interrupt the circuit when it surpasses the rated value. 
  • MCB and RCD (Residual Current Devices) – RCDs are different from MCBs. While MCBs are for general purposes, RCDs are specifically designed devices that are meant to protect against fatal electrocution from incorrectly earthed cables and touching exposed wires.

Summing Up:

MCBs are protection devices that save electrical equipment from potential harm that can occur due to short circuits and overloads. They come in different ratings and pole versions as per their use. Their working principle is based on thermal tripping mechanism and magnetic tripping mechanism both of which are independent of each other. MCBs are reusable devices and can also act as ON/OFF switches in case of maintenance.  Their selection depends on multiple factors, applications being the primary one.

Next time you need to save your equipment and circuits from damage, install an MCB to save your day!


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