If only the two conditions; ON & OFF; is considered. Than electrical system seems very easy & simple. But safety of human being is the top most priority which can not be neglected at any cost.
It is the safety & security of human beings & machines which further makes the electrical system complex & bulky. In houses, Industries & complexes; We have seen MCB which are being connected at the back ends of connected loads. These MCB are the part of electrical safety system.
Whenever fault condition arises such as short circuit, It not only damages the faulty circuit but it also create Burdon on the back-end distribution system. Which further interrupt the working of rest of the healthy system. That’s why it is important to eliminate the faulty circuit so that rest of the healthy system works without any interruption.
A GIF below shows the working of single phase distribution system without MCB. It is clearly seen in the GIF, At normal condition total load connected across the power supply. But when fault arises in one of the connected load 4, Than heavy current flows towards the load4. At such condition, Main distribution system starts burning due to excessive current. At last, Due to heavy current, Distribution line get burnt down. Rest of the health circuit also get eliminated.
Gif shows; Single phase distribution Circuit without MCB
Another GIF is shared next, In which same distribution circuit is designed through MCB. Under fault condition, Heavy current flows across the circuit of load 4 in a series through the MCB4. At last, MCB4 get trip but rest of the healthy circuits remains in working condition.
Gif shows; Single phase distribution Circuit with MCB
MCB is the component which make us safe, It eliminates the faulty circuit so that rest of the healthy circuit remains active. In this post, We will discuss about the working mechanism of MCB. If you want to know about the MCB specification or MCB selection criteria, You can go throughMCB Selection.
MCB
MCB stands for “miniature circuit breaker“.(MINIATURE: Something much smaller than its usual size)
MCB is an electromechanical protection device which provides protection in overload & short circuit conditions. MCB also work as a safety switch to isolate the load from power supply.
” Overload– It is the Phenomenon in Electrical system in which current flows more than the rated capacity of the component. It is generally a slow process phenomenon.”
” Short circuit– It is the rapid phenomenon in which current rises exponentially, Rapidly. short circuit situation has much larger current As compare to overload which can be multiple times the rated current of the system.”
Understanding both the situations overload & short circuit thorough an example below:
Let suppose we have 2pole C32Amps MCB &Total 3 single phase Loads are connected across it.
Which are taking current as 10Amps,10Amps & 5Amps in total current 25Amps. System will work smoothly.
Now another load get connected across it which is taking a current; 15Amps. Which results total current across MCB will get 40Amps which is more than the rated capacity of MCB. After sometime under total load of 40Amps, MCB get trips. This situation is known as ‘Overload‘.
Now let suppose, potential wire get shorted with earth or neutral directly. At this situation; heavy current flows rapidly which is multiple times the rated capacity of the MCB. This situation is known as ‘Short circuit.
2 POLE MCB C32 BY SIEMENS | 1. Terminal Blocks 2. MCB details like Make, Type, Tripping Curve, Current Rating & Voltage Rating 3. Switching Capacity 4. Switching Symbol 5. Operator for manual actuation
A Collage images of MCB is shown below. Which reflects the on & off situation of MCB. At normal condition when MCB handle in ON(Uplift) position, Both the terminals, Up side & down side are in connected state. At tripping or at OFF(down) position, Both up & down terminals get disconnected.
4 POLE MCB C40 BY SIEMENS
MCB WORKING PRINCIPLE
As we have discussed, MCB provides protections in Short circuit & overload conditions. Both the phenomenon’s have two different working mechanism which works independently for a one common goal of tripping. There are two tripping mechanism in MCB :
Thermal Tripping
Magnetic Tripping
At overload condition– Thermal(delay) tripping works.
At Short circuit condition– Magnetic(instantaneous) tripping works.
Whatever the situation of tripping; “overload“ or “Short circuit“, Arc get generated between the contacts. To extinguish that arc, Extinguishing Area/Chamber is been presented inside the MCB.
In Total MCB has following components:
Terminals block
Thermal Bimetallic Strip
Electromagnetic coil
Tripping mechanism
Contacts
Operating lever
Fire extinguishing chamber
All the components are clearly explained in an image below :
DETAILED INNER VIEW OF MCB |
Before Explanation, We should have to learn about the path of current flow; first. Lets start from the top of the MCB As shown in an image below.
Input supply is connected at terminal block at the top of MCB. After that current passes through the Thermal bimetallic strip & than through main contacts. After the main contacts, Current passes through electromagnetic coil & last exit through the exit contact. All the components are connected in series, Same current flows through thermal bimetallic strip & electromagnetic coil.
Inner view MCB | Current path inside the MCB |
Thermal bimetallic strips works in overload condition which is basically a slow process in which thermal bimetallic strip take time to get heated up under overload condition than a bending movement from thermal bimetallic strip produces a movement in trigger mechanism which results MCB get trip. (Explained in an image below)
Electromagnetic coil works in Short circuit condition which is a rapid process. In short circuit condition, Heavy current flows rapidly through electromagnetic coil which results a sharp magnetism get created. A metal rode freely available inside the coil which moves inside the coil due to magnetism. That metal rode triggers the trip mechanism. (Explained in an image below)
As soon as tripping mechanism works, Manual actuation handle get off parallelly with trip mechanism.
Inner view of MCB; describes its working | Gap between tripping mechanism & thermal bimetallic strip (Down Right) Gap between tripping mechanism & Electromagnetic coil rode (Down Left)
Inner View of MCB | Image shows Electromagnetic coil triggers the trip circuit inside MCB
One more thing is very special in MCB is; Trip free mechanism.
Trip free mechanism
In trip free mechanism, MCB under trip condition can not be reset through manual actuation handle until thermal bimetallic get cool down. Even if we are trying to lift the actuation handle multiple times in trip condition, MCB can not get reset until thermal get cool down.
Arc Generation & Extinguishing
As soon as contact get opened, Arc get generated between the contacts & current starts flowing through the air. Generated arc get shifted to Arc chamber, A very high temperature get generated across the chamber that’s why arc chamber is designed with special thermal insulation material.
As the arc moves further Arc length goes on increasing. Arc chamber consists of arc chutes which are having of equal gaps & equal sizes. Arc get divided broken into smaller sizes on arc chutes. At one time resistance of the path get higher enough such that arc voltage does not able to maintain the arc current & get diminished.
All these phenomenon been clearly shown in an images below :
Arc generation inside MCB
Arc generation inside MCB
SUM UP
MCB is an electromechanical protection device which provides protection in short circuit & overload conditions.
Inside the MCB for both the phenomenon’s Short circuit & Overload, There are two different working mechanism which are working independently for a common goal of tripping.
At overload condition, Thermal(delay) tripping works. Which is a slow process.
At short circuit condition- Magnetic(instantaneous) tripping works. Which is a rapid process.
As the tripping initiated, Arc get generated between the contacts. To extinguish that arc, extinguishing chamber is present inside MCB; That chamber is specially covered with heat insulation material inside the chamber.
Hope, you like the post. If you have any doubt, Suggestion or query please do comments !
For MCB specification details or MCB selection criteria you can go through MCB SELECTION
