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Insight - How MCB works

Written By: 

Arpit Jain



MCBs or Miniature Circuit Breakers are electromechanical devices which protect an electrical 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 reset and thus offers improved operational safety and greater convenience without incurring large operating cost.


MCB or Miniature Circuit Breakers

The principal of operation is simple. An MCB functions by interrupting the continuity of electrical flow through the circuit once a fault is detected. In simple terms MCB is a switch which automatically turns off when the current flowing through it passes the maximum allowable limit. Generally MCB are designed to protect against over current and over temperature faults (over heating).
There are two contacts one is fixed and the other moveable. When the current exceeds the predefined limit a solenoid forces the moveable contact to open (i.e., disconnect from the fixed contact) and the MCB turns off thereby stopping the current to flow in the circuit. In order to restart the flow of current the MCB is manually turned on. This mechanism is used to protect from the faults arising due to over current or over load.
To protect against fault arising due to over heating or increase in temperature a bi-metallic strip is used. MCBs are generally designed to trip within 2.5 millisecond when an over current fault arises. In case of temperature rise or over heating it may take 2 seconds to 2 minutes for the MCB to trip.
This article covers the insight of a single pole MCB commonly used in the house hold. The following image shows the different internal parts of an MCB with top casing removed. The subsequent sections will examine each part and its function.

Internal Parts of MCB




I likeD it..I had seen it many times at my home but nver tried to know how it works..thanks to engineersgarage ..evrone shud watch its intresting
explanation is strong. thanks. regards, raja giri


gr8 work.. arc quenching explained nicely. now I know what arc chutes are for.

Arc is like a circle it has on ends ra erri puka

wow, very nice.




wow...its very powerful thing. .& helpfull

mcb is a best of electrical component


nice & informative


Great !!!!!!!!!! nicely Explained!!!!


do it can get failure in its work....

Thank helped me a lot....


Great work..... Really Helpful,,,
Thank you so muchhhh....

Very Nice presentation.

Thank you .




very nice & good presentation.

Thank you .

nice one thanks a lot



Hmmmmm, nice , very helpfulll

in witch case:  solenoid will trip or , Bimetal will trip

WITCH????   I'm sorry but do you have spelling difficulties,it's spelt as WHICH!!!!

Really the diagram of MCB helped me a lot in teaching.

Thanks so much.Nice and simple explanation.Do add some video clips if possible.

it is really understandable to every one easily YES I really like it very much

easy explanation which everyone can understand. awesome ,hats off !!...
Havells 32Amp MCB Can you tell me Input and output Line ?

Many thanks for this piece. Really helped me a lot!!

very good information..

Really the Device is great..functionality!!

Good explain

its very good

Very good explanation. Thanks!

"protect against over current and over temperature faults" what does this really mean... please explain more on this..  if current flow is more than 220V will it trip, or i a use 6Amp tripper and give more load to it then it will trip.. pls help me to understand. 

I want to know if there is loose connection in the joint of two wires at that condition mcb will trip or not


<p>In my system was falut by short circuit but the MCB not it triped. i want to know the reason.</p>

How to calculate what MCB I need for my main switch of home that takes single phase supply line ?  10AMP, 20 AMP, 15 AMP - how to know ?


there is a formula to calculate amp rating of mcb that is. amp rating= total load in watt/ supply voltage * power factor generally we assume power factor = 0.8
Wonderful Really a good article Thnx
Thanks for explanation but I didn't get whether it will work for low currents...
Shall I need stabilizer if I had MCB in my home... Plz anyone give me reply...
Very super in The information thanks




why we don't take short circuit protection by bi-metallic strip?

very useful

how the rated current is fixed for every MCB and how it detects the current rating