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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

 

 

 

Comments

So Simply explained..was feeling the urge to know about the MCB mechanism .

does the frequency matters for a MCB? 

 

VERY NICE

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