Power System Protection: 4 Basic MCB Concepts Every Beginner in Power Industry Should Know

MCB or Minature circuit breaker is commonly used in our homes, industries, and substation for protection purposes. A Miniature Circuit breaker is most commonly used a piece of protection. Today you’ll learn the basic working and construction of MCB.

Arc Phenomenon

A circuit breaker contains two contacts, the moving and the static contact. During its working, a relay signals the moving contact to move apart. A fault current on breaker results in the motion of contacts which generates a lot of heat ionizes the air around contacts thus producing an arc.

The arc itself acts as a conductor and continues to carry the current. It is desired to interrupt this arc as quickly as possible.

A circuit breaker uses Relays to detect a fault

A CB on its own can’t detect the fault, a relay is incorporated inside the MCB assembly. In case of fault the relay system in energized and it signals the circuit breaker to interrupt the circuit.

Circuit breaker vs Fuse (Pros vs Cons)

A fuse is a standalone device (components) which detects a fault on its own and it has fast operating time. However one needs to replace fuse after every operation. A CB on the other hands just trips and one can simply switch it after removing fault.

Learn the complete difference between the fuse and circuit breaker here.

Types of MCB

On the basis of the time required for tripping, the MCBs are classified into three types:

  1. Type A: They trip at an overload of 3 to 5times of rated current.
  2. Type B: They trip at an overload of 5 to 10 times.
  3. Type C: They trip at an overload of 10 to 20 times.

Working of MCB

The video below illustrates the working and construction of MCB.

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