MCB Working Principle and MCB Rating Avoid MCB 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.

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:

Single Pole:

Single Pole MCB
Single Pole MCB


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)

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 three major types according to their instantaneous tripping currents. 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.

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)

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

How to Select MCB:

  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

      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…..

Also see: 25 Number of Why Questions

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