MCB | MCB Working Principle | MCB Rating Avoid failure

MCB Operating Principle and MCB Rating Avoid MCB failure:

Control MCB:

This is required to control/protect the control circuit. During fault condition it will trip or isolate the control circuit from the power source to avoid further damages. I will come varies model 1 P (single pole), 2 P (two pole), 3 P (three pole), 4 (Four Pole) and according to your application. While selecting MCBs, consider which type of power source you are going to use. Different model MCB will come for different power source. For AC Source AC MCB and DC source DC MCB will come.

It protects from the short circuits and does not protect from the earth fault.

Get More Electrical Concept And Interview Questions By Using This Link

Cost wise:    DC MCB > AC MCB

Cost according to Make: Siemens > ABB > Hager > L & T > Telemechanic (these all are mostly I used)

According to the Quality of Operation: ABB > Telemechanic > Siemens > L&T > Hager.

According to life time: Siemens > ABB > Hager > L&T > Telemechanic

Standard Rating: 100A > 63 A > 40 A > 32 Amps > 16 Amps > 10 Amps > 6 Amps > 5 Amps> 4 Amps > 2 Amps> 1 Amps

Note: Do not use MCBs where your demand is more than 40 A. Use 63 A & 100 A MCCB instead of MCB 63 A & 100 A. Increase in load current through the MCB which will reduce the life of MCB

Type of MCB As per Number of Poles:

Single Pole:

Single Pole MCB
Single Pole MCB

Get More Electrical Concept And Interview Questions By Using This Link


Only one Pole is presented. You can use them in

  1. Single phase lighting
  2. NO or NC Contact bypass
  3. Limit switch contact bypass
  4. Panel accessories control (like exhausted fan, illumination light etc.)

Two Pole:

Two Ploe MCB
Two Ploe MCB

Two Single pole MCB are riveted together. It will operate at the same time for all the operations.

  1. Lighting arrangement.
  2. Two phase supply power distribution (P-P) like welding machine, Control transformer etc.
  3. Single phase power distribution (P-N) like heaters, external single phase power distribution etc.
  4. All Power source control (AC power and DC power)
Also Know About:   Reason For Transformer Core Saturation

Three Pole:

Three Ploe MCB
Three Ploe MCB

Three single pole MCBs are riveted together. All will operate at the same time for all the operation.

  1. Small size motor input (P-P-P)
  2. PT (Potential transformer) input/output (3phase)
  3. Welding transformer
  4. All three phase distribution

Four Pole:

Four Pole MCB
Four Pole MCB

Four single pole MCBs are riveted together and operates all at a time.

  1. Power supply distribution, Where the neutral is required (P-P-P-N)
  2. CT output (current transformer)
  3. PT output (mostly are used to monitor HT line voltage)
  4. Lighting DB input power supply

Types of MCB according to its Tripping Current:

MCBs are classified into five major types according to their instantaneous tripping characteristics. They are

Type B MCB: Trip the circuit when the current flow reaches 3 to 5 times of its full load current. Mostly they are used in resistive and inductive loads. They are recommended in Household application such as lighting, small ceiling fan application etc.

Type C MCB:Trip the circuit when the current flow reaches 5 to 10 times of its full load current. Mostly they are used high inductive loads where switching surges such as small inductive motor, fluorescent lighting. They are preferred to handle high short circuit currents. They are recommended for Industrial application.

Type D MCB:Trip the circuit when the current flow reaches 10 to 25 times of its full load current. Mostly they are used for very high inductive loads where frequent switching surges such as small electrical elevators, X Ray machine, welding Machine etc.

Type K MCB: The MCB trips The circuit If the current flow reaches to 8 to 12 times. It protects the electrical equipments such as transformers, Induction Motors, and other auxillary loads from short circuits and over loads.

Type Z MCB: The MCB operates when the fault current reaches 2 to 3 times of its full load current. Since they are suitable for commercial application and switching on the small loads.

Get More Electrical Concept And Interview Questions By Using This Link

Also Know About:   What is Balance Load & Unbalance Load EEE Student

What is MCB tripping Curve:

MCB tripping curve is consisting of thermal overload protection and magnetic short circuit protection.

Thermal overload protection:

When the current flow increases the bi-metallic strip, operate the MCB against Overload condition. The operating time will be few seconds.

Magnetic Protection:

It works same as current transformer. There is an magnetic coil is placed inside of the MCB, when the current flow increases in short time, the magnetic coil produce sufficient magnetic flux to pull the tripping plunger. These type of magnetic protection operates only between the current exceeds 8 to 10 times (it is depending on the type of characteristics curve).

types of MCB tripping curve

Video Explanation of MCB in English:

How to avoid failure of MCB:

  1. Do not Load MCB more than 70 % of its full load rating (Load means current)
  2. Make sure terminal Screws are in tight condition.
  3. Use lugging to connect cables and MCB’s terminals Click this to know about lugging.
  4. Do not operate it too fast (continuous on/off), Give at least 5 secs for each operation.
  5. Do not switch it on during fault condition frequently (max two or three times are allowed). 80 % of the MCBs failure due to this reason only. Frequent high current breaking leads to damages in inner bimetallic strip.
  6. In correct closing (while closing mcb, you have to lift the closing nob down to up correctly)
Get More Electrical Concept And Interview Questions By Using This Link

How does it operate:

It is just like same thermal overload relays. But it works according to the principle of IDMT. when the fault current rises, the bimetallic strip’s temperature will rise. Due to increase in temperature of bimetallic strip will trip the circuit fastly.

It does not reset before the temperature of bimetallic strip coming to normal value.

Where we should not use this:

  1. If the load current is more than 40 Amps, I do not recommend to use MCB. Instead of MCB you can use MCCB, which gives best performance in higher load.
  2. Do not connect multiple number of single stand cable without lugging all together.
  3. Do not use high starting current applications like Coal crusher, Coal grinder, Crane, long trolley, cross trolley etc…
  4. Do not use it as a main incomer of lighting circuits without RCCB (Residual case circuit breaker) or ELCB (Earth leakage Circuit breaker), because it does not protect the circuit from earth faults
Also Know About:   What is Motor In Tamil மோட்டார் என்றால் என்ன?

How to Select MCB for Industrial Use as per Installation:

  1. Calculate load current, put 150 % X load current = MCB rating (to avoid loading factor and future load expansion)
  2. Use 2 Amps/1 Amps MCB for control circuit (it will withstand up to 200-400 watts coil)
  3. Use 6 Amps MCB for Panel accessories like Exhaust fan, illumination lamp, Space heater.
  4. Select Box type power terminal connection (you can see it on the bottom or top side of MCB power terminal side), it will give more tightness(torque) to the terminal for single stand cable connection. By this you can avoid heating factor at the power terminal of MCB

How to Select MCB As per application:

Type B, C and D can be used for surge protection and over load protection. According to IEC/EN 60898-1 these mcb can be used for cable protection.

Type K MCB: motors and transformers and simultaneous overcurrent protection of cables with overload tripping based on IEC/EN 60947-2

Type Z MCB: control circuits with high impedances, voltage converter circuits and semicable protection

      5. Disadvantage of Screw type MCBs: Do not Select it.

  1. In my carrier I had found almost 10 % of MCBs (around 80 numbers) failure per year due to this model. it does not give high compression at the terminal connection.
  2. You cannot connect multiple number of single stand cable and single stand cable with multi stand cable. if you do so, finally you will get lose contact.
  3. Loose contact leads to failure of MCBs

            Thanking You!!!! Have A Nice Day…..

LEAVE A REPLY

Please enter your comment!
Please enter your name here