Working principle of MCB; Miniature arc circuit breaker

MCBs or Miniature Circuit Breakers are electromechanical devices which protect an electrical circuit from an over-current. 

Where Miniature circuit breaker means minimum arc circuit breakers.

The over-current, in an electrical circuit, may result from short circuit, overload or faulty design. A MCB is a better alternative other than Fuse since it does not require replacement once an overload is detected. Unlike fuse, an MCB can be easily reset and thus offers improved operational safety and greater convenience without incurring large operating cost.

MCB is most widely used for industrial and domestic applications i.e. for lighting loads and small motor loads.

MCB or Miniature Circuit Breakers

The working principal of MCB is simple. An MCB functions by interrupting the continuity of electrical flow through the circuit once a fault is detected. In simple terms MCB is a switch which automatically turns off when the current flowing through it passes the maximum allowable limit. Generally MCB are designed to protect against over current and over temperature faults.

There are two contacts one is fixed and the other moveable. When the current exceeds the predefined limit a solenoid forces the moveable contact to open (i.e., disconnect from the fixed contact) and the MCB turns off thereby stopping the current to flow in the circuit. In order to restart the flow of current the MCB is manually turned on. This mechanism is used to protect from the faults arising due to over current or over load.

To protect against fault arising due to over heating or increase in temperature a bi-metallic strip is used. MCBs are generally designed to trip within 2.5 millisecond when an over current fault arises. In case of temperature rise or over heating it may take 2 seconds to 2 minutes for the MCB to trip.

This article covers the insight of a single pole MCB commonly used in the house hold. The following image shows the different internal parts of an MCB with top casing removed. The subsequent sections will examine each part and its function.

Internal Parts of MCB

Internal parts of MCB is as shown below:-

Above pic is self explanatory and very helping in understanding working principle of MCB.