MCCB Working Principle

 

26 Nov, 2021.

In this post, we will understand the working principle of MCCB. (Click on the images for a more zoomed view)

In electrical engineering, circuit breakers play a very important role in controlling the flow of current in a circuit. As the name implies, a circuit breaker is an electrical device which is used to protect the electrical circuit from fault or damage by interrupting the flow of current and thus, breaking it. The three major types of faults that can be prevented by a circuit breaker are overcurrent, short circuit current and overload.

Before we proceed ahead, let us discuss a simple switch operation. Refer the above image. A switch can be open or closed. It consists of two contacts – fixed and moving. In the image, the black line is the moving contact and the grey line is the fixed contact. The moving contact is controlled and when this contact touches fixed contact, the circuit is closed and current will flow through it. When the moving contact moves apart from fixed contact, the circuit is open and the current will not flow through it.

Molded Case Circuit Breaker:

Now, there are many types of circuit breakers; but we will consider MCCB in this post. MCCB stands for molded case circuit breaker. It is used for current ratings of up-to 2500 A. Due to a large amount of protection provided, it has a wide amount of use in large scale systems for better isolation. You should know first before proceeding ahead that every circuit breaker has five main types of components – frame, operating means, contacts, arc extinguisher and tripping unit.

1. Frame:

Let us first start from it’s physical properties. It is a sealed-case circuit breaker which houses all the components inside it. This assembly type is made of glass-fiber reinforced thermoset polymer and is the base cover. This provides high mechanical strength and also a high dielectric strength. Due to the structure of MCCB, it’s seal cannot be opened or closed normally or frequently; except for cleaning, inspection or maintenance purposes.   

    

2. Operating Means:      

 As discussed earlier, as a circuit breaker is basically a switch operation, the switch inside has three positions – open, close and trip. Refer the below image. In MCCB, when the circuit is in open condition, the outer switch will be in down position. When the circuit is in closed condition, the outer switch will be in up position. When the circuit is in trip condition, the outer switch will be in middle position. The circuit inside has a trip latch. When the circuit is opened or closed in normal operation, the trip latch does not move. The latch is interconnected in such a way that it assists the opening or closing operation as seen in figure. But, when a trip condition is occurred and sensed by the internal circuitry, the mechanical assembly activates the trip latch. This opens and breaks the circuit to prevent any malfunction. It can then be only brought to a normal condition by pressing reset button outside the assembly. 

As discussed above, refer the below image for it’s outer switch position and how it operates.  Usually, many ask that why the circuit breaker switch is in down position for open condition as compared to our normal domestic switches that we use. The reason is that if any heavy object falls on the switch or any accident touches the switch, then the mechanical spring will bring it down in open condition. This is safe than bringing the switch in closed condition by moving it up; if it was designed as a normal switch that we use.  

3. Contacts: 

Due to a large amount of current handling involved, the switching contacts used inside the assembly that moves the outer switch must be of a very high quality. Normally, an electrolyzed brass material is used inside. They have to handle a high amount of arc generated during tripping and so; the material used must have high arc resistance, low resistance with good corrosion resistance. It is such a hard alloy that can it handle large arc currents and load currents. 

4. Arc Extinguisher:

When the tripping current or fault current occurs, the contacts inside break the circuit as discussed earlier. So, as this current does not get any path to flow, a huge arc is generated between the contacts. This discharge of electricity between the gaps is extremely dangerous and hot to handle. This arc must be extinguished before it causes any damage. This is done by an arc chute or extinguisher.

It is a structure that contains arc dividers. As there are a large number of dividers, the arc is drawn between these contacts. Due to rise in temperature, it is stretched by the arc dividers which reduces it’s temperature ultimately and so, it is diminished. 

5. Tripping Unit:

This is one of the main components of MCCB. It is this part which triggers the trip circuit and breaks the current path. If you see the second image above, we had seen the bimetallic contact which bends the normal contact in trip condition and breaks the path. This bimetallic contact consists of two metals that expand at different rates when it is exposed to a high temperature. If you see, in normal condition, the contact is straight and allows the current to pass. But, in trip condition, it’s temperature rises and starts to bend due to thermal rate of heat expansion. This bend will push the trip bar and unlatch it as shown. This is the basic protection theory for overload.

For short circuit, earth fault or instantaneous fault, a set of current transformer is used. It’s output is connected to a solenoid coil which generates an electromagnetic field when current passes through the circuit. The field is very small in normal condition, but very large in trip condition. This large field attracts the trip bar and unlatches it.  

I have covered the general working principle of MCCB. I have also not attempted to cover every theory of these design deeply; you can learn it easily once you get familiar with them. I have just given you an insight of these types of controls. A great electrical understanding is required by the engineer who is dealing with such system. Learn the basics and explore a new type of study in this type of automation.

Thank you guys; I hope you enjoyed reading the practices normally used for this type of study in industrial automation.