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Insight - How MCB works

Written By: 

Arpit Jain


Arc quenching in MCB (Miniature Circuit Breakers)
Another important point to be considered in the design of circuit breakers is Arc quenching. To understand how MCB carries out the arc quenching, it is important to know about the electric arc and how is it produced. When an overcurrent is interrupted by the circuit breaker by opening its contacts, current tries to bridge the gap. In an attempt to maintain the circuit, the air heats up and becomes a conductor. As a result an arc forms. In general, when air and gases are heated, they become electric conductors. The hotter they get, the better they conduct. The following image shows how an electric arc looks like.
Arc quenching
The heat from an uncontrolled arc in a circuit breaker can cause a rapid and violent expansion of the nearby air and could severely damage the circuit breaker. Therefore, besides separating the electrical contacts, a circuit breaker also has to quickly extinguish the arc. A number of factors can be employed for quenching the arc, such as, speed, distance, dielectric strength, cooling etc.
a)      Speed : When the contacts separate rapidly, there is less time for the arc to form and maintain itself.
b)      Distance : When the distance between opened contacts is more, the arc has to stretch more to maintain the current flow which requires more voltage.
c)      Cooling : When the arc is forced against a cold material, it absorbs and dissipates the heat.
d)     Dielectric Strength : When the arc is submerged in a medium with higher dielectric strength than air (sulfur hexafluoride, SF6), the insulating nature of the medium helps in quenching the arc.
In an MCB, arc chutes or arc dividers are used for arc quenching. When the contacts of an MCB separate, generating an electrical arc between them through air, the arc is moved into the arch chute where it is divided into small segments. The overall energy level of the arc gets split up which is not sufficient to sustain the arc and therefore it gets dissipated.

Arc chutes in 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.




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




Thanks for the breif discussion of MCB the information about the MCB is brilliant..
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


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