MCB: MINIATURE CIRCUIT BREAKER OPERATION BASIC

What is MCB?

MCB is a device designed to protect an electrical circuit's wiring from the serious damage which would be caused if it has to carry a current which is too high for the diameter of its wires.

The MCB or Miniature Circuit Breaker is actually works for both electrical circuit make or break switch and protection device fuse.

Likely normal fan/light   switches, MCB also can operate manually as circuit ON or OFF; specialty is, it can break the circuit automatically  to protect from over current.

Fuse is a very first protection device for electrical circuit. Fuse actually nothing but a thin wire that enclosed in a protective casing and plugged into the circuit.

Fuse carries the same circuit current that flows in any other point, but it designed such a way that when current reach too high unexpectedly, the fuse burns up and break and safe the wiring from damages.

What Is Differences Between Fuses And MCB?

The limitation of fuses is that it works only once, every time burning the fuses, it needs to replace by new fuse-wire. To overcome this limitation and avoid fuse-wire replacing hassle, miniature circuit breaker or MCB are commonly using rather than conventional fuse in low voltage  electrical network system.

MCB also works for over current and opens the circuit as soon as current reach unsafe levels. The main difference of MCB from fuse is that it can use over and over time.

How MCB works?

The basic MCB or miniature circuit breaker consists a simple switch to operate automatically at danger level of over current. The main part as known trip unit of MCB actually do this mechanism.The working principle of miniature circuit breaker or  MCB is as below-
  • A simple bimetallic strip design for MCB in such a way that when current limit cross a safe level then unsafe high current bends the thin strip to break the current flow;
  • An electromagnet uses in MCB that magnetizes by electricity; magnetize electromagnet experiences magnetic forces which pull or push to break the circuit when current flow reach an unsafe level;
  •  Semiconductor devices are using for breaker operating in advanced circuit breaker. This is very much précised to monitor the circuit current level, when reach the current at  preset value then circuit breaker can operate. This is  expensive that makes it limited use in more sophisticated devices.
Due to nature of fault current amount and time- a bimetallic protection against over load current and an electromagnetic protection against short-circuit current is suitable.    
Basic Components of MCB
Basic components of MCB are shown in MCB construction figure as below:
      1. Internal components of MCB
        Tunnel Terminal;
      2. Jaw Terminal;
      3. Arc Runner;
      4. Arc Chamber;
      5. Thermo-metal Element;
      6. Calibration Mechanism;
      7. DIN Rail Mounting;
      8. Moving Contact;
      9. Fixed Contact;
      10. Latch Mechanism;
      11. Operating Handle;
      12. Coil Assembly;
      13. Trip Bar.
 
MCB Operating Mechanism
The operating mechanism of MCB is limited on manual and auto combination. It can break the circuit during excessive over current, but can not make the circuit again, need manual operation.
Latch has three operation positions- ON, OFF & TRIPPED. In close condition of MCB the latch positioned at ON and manually switch off the MCB, the latch positioned at OFF. When MCB automatically break the circuit to safe from over current then latch positioned at TRIPPED.
MCB Operating Time & Current
Type of MCB Tripping Current Operating Current
Type  B 3 to 5 times of full load current 0.04 to 13 seconds
Type  C 5 to 10 times of full load current 0.04 to 5 seconds
Type  D 10 to 20 times of full load current 0.04 to 3 seconds

For example, let us consider a MCB with rating is 6A; 
What will be the short circuit current for different classes like B, C & D.

For class B, short circuit current will be 3 to 5 times of it's rated current. So short circuit current will be 6x3 to 6x5 or 18A to 30A;

For class C, short circuit current will be 5 to 10 times of it's rated current. So short circuit current will be 6x5 to 6x10 or 30A to 60A;

For class D, short circuit current will be 10 to 20 times of it's rated current. So short circuit current will be 6x10 to 6x20 or 60A to 120A;

So, class should be select considering the interruption of short circuit current label.

What Is difference Between MCB, MCCB, RCCB & ELCB

MCB: Miniature Circuit Breaker
MCB: SP/DP/TP
Physiognomies


  1. Rated current up to 100 A;

  2. Tripping point not non-adjustable;
  3. Thermal-magnetic operation.
  4. Three types according to pole:
MCB- SP (Single Pole)
MCB- DP (Double Pole)
MCB- TP (Triple Pole)
MCCB: Molded Case Circuit Breaker
Physiognomies
  1. Rated Current up to 100A ;
  2. Tripping point adjustable/non-adjustable;
  3. Thermal and thermal-magnetic operation.
RCCB: Residual Current Circuit Breaker
Physiognomies
  1. Residual current rating 10mA, 30mA, 100mA, 300mA, 500mA, 1000mA & adjustable 2000mA;
  2. Not operate for over current;
  3. Detect difference between phase and neutral current; if difference found breaks the circuit immediately;
  4. Mainly used for shock protection;
  5. Nuisance tripping may happen due to aged or less performed appliances.
ELCB: Earth Leakage Circuit Breaker
Also known as Ground Fault Circuit Interrupter, in short GFCI.
Physiognomies
  1. ElCB works on earth leakage current;
  2. ELCB or GFCI was designed only to detect a current flowing in the safety ground or earth wire.
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