MCB
MCB stands for ‘MINIATURE CIRCUIT BREAKER’
MCB is an electromechanical protection device which provides protection from overload & short circuit conditions. MCB also works as a switch to isolate the load from power supply.
In short, we can say MCB a safety switch !
An image of 32Amps 2pole MCB is shown below. Image explains the different part & specifications marked on MCB.
Different parts of the MCB are marked in a colored rectangle with numbering.
- Terminal block
- Make, Type, Tripping Curve, Current Rating & Voltage Rating
- Switching Capacity
- Switching symbol
- Operator for manual actuation
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Multiple details been marked on MCB; in an image above. Which are very important to consider before installation of the MCB.
Factors for MCB selection
- Type of power supply(AC/DC)
- Type of load (Curve Type)
- Rated load current
- Rated Voltage
- Number of poles
- The breaking capacity
- MCB Accessories
In this post, We will briefly discuss the selection of MCB according to our requirements. If you want to know about the working mechanism of MCB, You can get through another post MCB Working Principle
MCB SELECTION CRITERION
There are multiple types of MCBs. The selection of MCB entirely depend upon the requirement. Next, All the important specification is discussed.
An collage image of C63Amps 4 Pole MCB by ABB is shown below :
- Lower left image shows MCB details such as curve rating C, current rating 63,ABB mcb series SH 204 m, designed voltage rating 400, Breaking capacity amps 10000.
- While lower right image shows the symbol of switching contact.
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1. SUPPLY TYPE
Supply type is the first step for MCB selection which means the type of supply AC or DC which further categorized on operational voltage.
As shown in an image, Operational voltage is 400Volts ,
Sinusoidal sign in the image indicates that supply voltage is AC .
2. RATED VOLTAGE
MCB are specially designed for tripping condition. In tripping scenario arc get created Which directly links with the operational voltage. That’s why MCB are designed for particular Operational voltage rating known as rated voltage.
3. RATED CURRENT
Rated current is the current up to which MCB works precisely without getting trip.
An image1, Rated Current is 32Amps.
An image2, Rated Current is 63Amps.
As discussed above, MCB provides protection in overload & Short circuit conditions !
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, Uncontrollably in undefined direction. Short circuit situation has much larger current As compare to overload which can be multiple times the rated current of the system.
EXAMPLE
” Let suppose we have 2pole C32Amps MCB
@Total 3 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 current 15Amps which results total current across MCB will be 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 Short circuit.
// Now let suppose, potential wire get shorted with earth or neutral directly. At this situation heavy current flows rapidly which is multiple times more than the rated capacity of the MCB. This situation is known as short circuit. MCB will trip instantly. “
MCB current rating should be selected on two factors; load current & type of load. Generally, We do choose MCB rating at least 1.2 times of the load current. So that even in surge current position at the initial time of switching, MCB do not trips.
For example: 1.5 KW 3phase Induction motor with Rated current of 3Amps. We do choose a MCB of 6Amps.
4. POLES
Poles defines number of lines or number of current paths.
In single phase, 1phase & 1neutral is required that means there are two lines ‘phase’ & ‘Neutral‘. For single phase system, 2 pole MCB is required.(shown in an image1)
In 3phase+1neutral supply, 4 Pole MCB is required.(shown in an image below)
In multipole MCB more than 1, all the MCB poles are connected externally not internally. All the Poles are connected through the cap of operating handle in such a way that in tripping of one single pole, all the connected poles trip simultaneously.
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5. CURVE TYPE
As discussed, MCB do not trip at their rated current. They are designed to operate at the current higher than the rated current; after a particular time interval.
“MCB actually follows the inverse time characteristics which means higher the fault current lower will be the trip time.”
There are different types of loads such as electronic circuitry, Light loads, heating elements, motors working in different places likes houses, labs, workshops & industries.
Electronic circuitry (not including power electronics) has lower surge current while motors such as Induction motors have high starting/surge current. Power surge in houses is lower than the power surge in industries. That’s why level of MCB tripping should be different according to the place & type of load. Lets understand this through an example.
Example
MCB having a Rated current of 25 Amps.
@ Home loads which includes loads like lights, fans, TVs, geysers etc. consuming current up to 11Amps can precisely works on 25Amps normal MCB.
@ 3 phase 5.5kw Monoblock pump with rated current approx. 11Amps is connected across 3pole 25amps MCB. At the time of starting pump may take current up to 55Amps, if MCB is normal which is not suitable for high inrush current than MCB will trip.
@ Similarly If MCB is being installed for sensitive semi-conductor equipment, Than demand of MCB tripping; for the safety of equipment; will be rapid even at small fluctuation.
That’s why for different types of load situations, MCB tripping should be different.
By understanding all the requirements, MCB tripping is categorized in 5 ranges according to ABB that are Z-curve, B-curve, C-curve, k-curve & D-curve
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- Z-curve– Z curve MCB are designed for semi-conductors & sensitive equipment’s, Having a tripping range from 2*In to 3*In. (In=Rated current)
- B-curve– B curve MCB are designed for control circuit & lighting In houses & living apartments, Having a tripping range from 3*In to 5*In.
- C-curve– C curve MCB are designed for medium magnetic start-up current protection such as low rating motors in industries, Having a range from 5*In to 10*In.
Both image-1 & Image-2 MCBs are C-curve designed.
- K-curve– K curve MCB are designed for High magnetic startup current such as Induction motors & transformers, Having a range from 10*In to 14*In.
- D-curve– D curve MCB are designed for very high magnetic startup current, Having a range from to 10*In to 20*In.
Tripping ranges of all the curves tells that MCB will trip under the range with inversely proportional to the time.
Example
3Pole 10Amps Z-curve MCB
According to the curve, Tripping range will be= 2*In to 3*In= (2*10 to 3*10)
20amps to 30amps [8Sec. to 2Sec.]
At 20Amps MCB will take 8Seconds Approx. to get trip while at 30Amps MCB will take 2Seconds.
This doesn’t mean MCB will not trip below 20Amps. Below 20Amps & above 10Amps it takes time to get trip until bimetallic thermal produces bending movement.
6. BREAKING CAPACITY
It is the maximum current that MCB can interrupt successfully without being getting damaged. These are the very high current with very less probability & These breaking capacity current is much higher than the rated capacity.
An image1, Breaking capacity is 7500Amps.
An image2, Breaking capacity is 10000Amps.
7. AUXILIARY ACCESSORY
All the MCBs are not compatible for auxiliary accessories. Types of MCBs are there; through which an auxiliary accessories can be connected. Auxiliaries such as signal contacts, shunt trips & undervoltage-overvoltage releases. MCB must be chosen accordingly whatever the auxiliary required in along with MCB.
Signal contacts are the contact element either NO or NC or NO+NC. Signal contacts get mechanically connected with the MCB. According to the position of MCB ON or OFF, signal contact changes its contact position.
Shunt trips also get connected with MCB mechanically. Shunt trip contains a electromagnetic coil, On getting a trip signal electromagnet get energizes & produces a tripping command to the MCB.
Undervoltage – Overvoltage releases are the type of safety also get connected with MCB. Whenever supply voltage goes out of the selected range, It produces tripping command to the MCB.
SUM-UP
• MCB is an electromechanical protection device which provides protection in overload & short circuit conditions. MCB also works as a switch to isolate the load from power supply.
In short, we can say MCB a safety switch.
• MCB is a type of safety & selection of safety should be done carefully. For MCB selection there are total 7 criteria’s.
Hope you like the post if you have any doubt, suggestion or query please do comments. If you want to know about the working mechanism of MCB than you can go through MCB Working Principle