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 that is too high for the diameter of its wires.

The MCB or Miniature Circuit Breaker is actually working for both electrical circuits' make or break switch and protection device fuse.

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

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

The fuse carries the same circuit current that flows in any other point, but it is designed so that when the current reaches too high unexpectedly, the fuse burns up and breaks and protects safe the wiring from damage.

What Are the Differences Between Fuses And MCB?

The limitation of fuses is that it works only once, every time burning the fuses, it needs to replace with a new fuse wire. To overcome this limitation and avoid fuse-wire replacement hassle, miniature circuit breakers or MCBs are commonly used rather than conventional fuses in low-voltage electrical network systems.

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

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How MCB works?

The basic MCB or miniature circuit breaker consists of a simple switch to operate automatically at the danger level of over-current. The main part as a known trip unit of MCB actually does this mechanism. The working principle of the miniature circuit breaker or  MCB is as below-

• A simple bimetallic strip design for MCB in such a way that when the current limit crosses 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; magnetized electromagnet experiences magnetic forces which pull or push to break the circuit when the current flow reaches an unsafe level;
•  Semiconductor devices are used for breaker operating in advanced circuit breakers. This is very much précised to monitor the circuit current level, when reaching the current at a preset value then the circuit breaker can operate. This is expensive which makes it limited use in more sophisticated devices.

Due to the nature of fault current amount and time- bimetallic protection against overload current and electromagnetic protection against short-circuit current is suitable.    

Basic Components of MCB

The basic components of MCB are shown in the MCB construction figure below:

1. 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 to manual and auto combination. It can break the circuit during excessive overcurrent, but can not make the circuit again, need manual operation.

Latch has three operation positions- ON, OFF & TRIPPED. In the close condition of MCB, the latch is positioned at ON, and manually switching off the MCB, the latch is positioned at OFF. When MCB automatically breaks the circuit to save from over-current then the latch is 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 an MCB with a rating is 6A; 

What will be the short circuit current for different classes like B, C & D?

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

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

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

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

What Is the Difference Between MCB, MCCB, RCCB & ELCB

MCB: Miniature Circuit Breaker

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)

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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 the 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 safe ground or earth wire.

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