Miniature Circuit Breaker

Miniature Circuit Breakers, commonly known as MCB’s are widely used in both household and industrial power distribution and safety applications. They are very robust switches that can act both as a regular switch and a safety device. This article aims to discuss in-depth about MCBs, their construction, operating principle and applications.

What is a Miniature Circuit Breaker?

Miniature Circuit Breakers are found almost everywhere where electrical power needs to be controlled. MCBs have their applications from typical households to industrial machines and premises. If you are someone who’s new to MCBs, you might have seen one of these in distribution boards in your house:

 

MCBs are typically placed after the RCCB (Residual Current Circuit Breakers) in a distribution board to switch power to a particular circuit or an appliance. In a household application, this can be the electrical supply to a room or a part of it like the lights or wall outlets.

 

Miniature Circuit Breakers are basically on/off switches that have an additional safety feature called ‘over-current/short-circuit protection’. MCBs are rated for a particular maximum current. If the current through the MCb is higher than that value, it can automatically shut off and protect the rest of the circuit.

 

This is very advantageous over traditional fuses because fuses need to be replaced after such an over-current incident. MCBs do not need to be replaced as we can easily turn them back on after fixing the fault in the system. The actuators in them are spring loaded and the off position is at the bottom. This is done on purpose to provide the quick shutoff action if a fault in the circuit occurs.

 

Miniature Circuit Breaker Working Principle

Miniature Circuit Breakers are relatively simple devices that act as mechanical switches with overcurrent shut-off. They are very easy to install and operate and often do not cost a fortune to install. Let’s have a look at the internal working of an MCB and how it works.

 

The functional principle of an MCB is based around a simple yet interesting phenomenon in physics. It is called ‘thermal expansion’ of a bi-metal strip. This is the same principle used in irons to automatically on/off the heating element to maintain the set temperature.

 

Normally, when electric current travels through a conductor, it tends to heat up. This causes the metallic conductor to ‘expand’ lengthwise. If the metal is uniform and made of the same material, this increase is linear and can be observed by measuring the length difference before and after heating up. Different materials expand differently under the same temperature.

 

Bi-metal strips, as the name suggests, are made of two different metals connected together. When heated up, instead of expanding in length in a linear way, bi-metal strips tend to ‘bend’ due to different expansion coefficients of the two metals. This bi-metal strip is a part of the conductive path. When current travels through this conductor when the switch is in ‘on’ position, it gets slightly warmed up.

 

This strip is specially designed to allow a certain amount of current (i.e. 20A) without causing too much of a deflection. And this is the rating of the MCB. When the current exceeds this value, the bi-metal strip bends and releases an electro-mechanical latch to immediately disconnect the conduction path. This takes only a fraction of a second therefore protects the devices connected through the MCB.

 

The switch inside the MCB is mechanically latched under a spring-loaded tension. Because of this, even though the bi-metal strip cools down after the switch shuts off, it will not turn back on automatically. We need to manually switch it on again. This allows us to troubleshoot and fix the issue before turning the MCB back on.

 

Miniature Circuit Breaker Types

MCBs are available in a multitude of sizes, ratings and types you can choose for your particular application. While the basic functionality is the same for all types and models, you need to select the best possible type for the application to make sure it works perfectly.

 

There are six types of miniature circuit breakers. They are type A, B, C, D K and Z. Type B, C and D are the main types. K and Z are less common and are highly specialized for certain specific applications. These ratings are based on the parameter called ‘trip-curve’. The trip curve is the MCB’s response to the current flow.

 

Type A MCBs

MCBs that belong to category A are extremely sensitive to the current flow. They are so sensitive that they are very rarely used. Type A MCBs can trip at about 2x to 3x the rated current of the device. These are used to protect highly sensitive devices like semiconductors.

Type B MCBs

Type B MCBs trip at around 3x to 5x the rated current. These can act as fast as 40 milliseconds or as slow as 13 seconds depending on the model. These are generally used to control domestic and industrial lighting circuits.

Type C MCBs

These trip at currents about 5x to 10x times the rated current. The tripping time can be between 40 milliseconds upto 5 seconds. Type C MCBs are a good choice for circuits that use motors such as fans, electric motors, transformers and microwave ovens.

Type D MCBs

Type D MCBs are the least sensitive type of MCBs. These can withstand upto 10x to 20x times rated current to protect heavy machines such as UPS, large motors, welding machines and x-ray machines. These draw large inrush currents therefore the type D MCBs can safely withstand the current while also providing safety if something shorts.

 

Type K MCBs

These are similar to type D MCBs, but are more responsive than them. These offer faster trip times therefore can sometimes be more suitable than type D MCBs.

 

Type Z MCBs

These are a more responsive version of type A MCBs. Type K MCBs operate at a current flow of 2 to 3 times the rated value within 0.1s (100 milliseconds).

 

Miniature Circuit Breaker Diagram

Let’s have a look at the internal diagram of an MCB. MCB stands for Miniature Circuit Breaker, a switch that can automatically cut-off electric power supply to a circuit in an overcurrent event. There are many electromechanical components inside an MCB’s housing.

 

The primary parts that are visible to the outside are the terminals themselves. There are two terminals, one for input and one for output. When wiring, care must be taken to not to swap these.

 

There can be a single or multiple levers tied together in a MCB (depending on the number of poles) for manually switching on and off the device. These are connected to metallic contacts (one being a bi-metal strip and the other being a fixed metal contact) that form the electrical connection between the input and output terminal(s).

 

To trigger the protection, there is an electromagnetic solenoid that releases the spring mounted contacts to separate them instantly. Newer MCB models contain an additional unit called ‘arc chamber’ that helps prevent the electric arc formation when disconnecting. This is very useful in high current inductive load applications such as large motors to protect the MCB when opening its contacts while the load is powered. 

 

The figure below shows a simplified circuit diagram of an MCB which showcases the trip bar (lever), latch and the metal contacts along with the bimetal filament.

Now, let’s have a look at the typical domestic wiring that uses miniature circuit breakers:

MCB Wiring Diagram

 

The following diagram shows a single-phase electrical wiring of a distribution board for a domestic application.

 

The electricity from the national grid enters the premises through the kilowatt-hour meter and a two-pole MCB to completely isolate the premises if and when needed. This also acts as a secondary protection since this is nothing else but a MCB.

 

Then, the supply is passed through an RCCB (Residual Current Circuit Breaker) device. This is extremely sensitive to current leaks and trips if there is a current imbalance in live and neutral wires. This is what protects users from electric shock.

 

After the RCCB, the neutral wire connects to a bus-bar and is distributed to the subcircuits. The live is connected to one or more MCBs’ inputs to form subcircuits. The outputs of each MCB are connected to a different subcircuit. In a typical household application these subcircuits can be living rooms, bedrooms, porches and kitchens. These logical subcircuits allow more control over the electrical safety of the premises.

 

Miniature Circuit Breaker Price

Miniature circuit breakers are available in many different types and configurations. There is no fixed price per category and can vary between manufacturers as well. Also, it is worth noting that there are two major types of MCBs, AC and DC breakers. For direct current (DC) applications, AC MCBs should never be used.

 

Starting from domestic and general purpose, low current applications, MCBs can cost you from $3-$4 up to hundreds of dollars based on their features and reaction times.

 

Miniature Circuit Breaker Use

MCBs find their applications in many different scenarios.

  • Domestic circuits
    • MCBs are used in household applications to prevent overload and any associated fire hazards.
  • Heating and Lighting
    • In both general purpose and industrial applications, heating and high-power lighting systems can put a huge stress onto an electrical system. MCBs help distribute the load between subcircuits to better handle the demand and remove any section when not necessary.
  • Industrial Applications
    • Starting from motor control upto heavy machinery upto 30kA (30,000A) power supplies are controlled using MCBs and their rugged counterparts, MCCBs (Molded Case Circuit Breakers

 

MCB Operation

Like we discussed above, there are a  few key components inside an MCB:

 

  1. Latch
  2. Solenoid
  3. Switch
  4. Plunger
  5. Incoming Terminal
  6. Arc Chutes Holder
  7. Arc Chutes
  8. Dynamic Contact
  9. Fixed Contact
  10. Din Rail Holder
  11. Outgoing Terminal
  12. Bi-metallic Strip Carrier
  13. Bi-metallic Strip

In its normal operation, the dynamic contact and fixed contact are connected together when the switch is in ON position. At this point, the plunger is also tensioned using the built-in spring, ready to release when needed.

The current passes from input terminal to outgoing terminal through the fixed and dynamic contacts and the bimetallic strip.

When current passes the threshold of the MCB, the bi-metal strip starts deflecting towards one side. When it deflects past the maximum threshold (which means the current is too high), it activates the solenoid and instantly releases the plunger. This action breaks the contact between the dynamic and fixed contacts by moving the dynamic contact away from the fixed contact.

The figure below shows the normal working state (left) and the tripped (open) state of an MCB.

 

Once tripped, MCBs cannot automatically turn on. A user has to manually switch on the MCB to restore power to the circuit. This reminds the electrician to check the circuit for any errors and correct them before powering on the circuit again. 

 

How to Select Proper MCB for Different Loads?

While all the MCB types follow the same functional principle, they do not have the same performance in terms of response time and current rating .Therefore, it may not be suitable to replace an MCB with a different type.

 

When purchasing/selecting a miniature circuit breaker for a particular application, the usual practice is to calculate the current requirements of the circuit being switched and select a MCB that has the same or better current rating. This is generally a bad practice as there is a set of information that associates with an MCB that needs attention.

 

Let’s start with the markings that are found on the body of a MCB.

 

There are a number of parameters marked on an MCB housing. Sometimes not all of them will be available on MCBs by a particular manufacturer, but the most important markings will always be printed on the unit.

 

Let’s look at the attributed we need to consider when selecting an MCB:

 

  • MCB Current curve rating
    • Marked as Axx, Bxx, Cxx, Dxx, Kxx or Zxx (where xx is the current in Amperes), this indicates the application the MCB is intended for. For example, B type is more suitable for purely resistive loads such as lighting and heating. C is for inductive loads such as fans and motors. Type C is common as it can easily handle pure resistive loads as well. Type D is for highly inductive loads such as powerful motors (water pumps, submersible pumps ,industrial fans) etc. Although not common, type A, K and Z are also there. Type A is extremely sensitive and K and Z are less sensitive than B, C and D.
  • Operating Voltage
    • This voltage is mentioned related to the phase count of the system. If the MCB is intended to be used in a single phase system, the voltage will be 230V or 240V. For a three-phase system it will be marked as 400V or 415V.
  • MCB Breaking Capacity
    • This is the absolute maximum current that the MCB can safely cut-off in case of an extremely large inrush/short circuit current situation. This is written in numerals. For example, the MCB shown above has ‘6000’ marked. This means that the MCB can safely interrupt currents upto 6000A (6kA).
  • Energy Class
    • Indicated in Joules per seconds, this is the maximum energy that MCB can be continuously allowed to exist in the system. There are three classes, 1 through 3 that indicate this parameter. The MCB shown here is Class 3, which allows 1.5Joules/second 

 

The rest of the information is manufacturer specific and can vary from manufacturer to manufacturer.

 

Miniature Circuit Breaker vs Circuit Breaker

In an electrician’s context, miniature circuit breaker and the circuit breaker are known to be the same device. From an Engineering perspective, the circuit breaker is a more ruggedized version of MCB that allows safe switching of high voltage circuits. MCBs are used in more compact and low-voltage applications.

 

Conclusion

In this article, we discussed in-depth about Miniature Circuit Breakers, their functional principle and applications. MCBs are very popular and effective electrical circuit protection devices that protects appliances and wiring from overcurrent events and can prevent fire and electric shock hazards.

 

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