A circuit breaker is an automatic device which is designed to stop the flow of current in an electrical circuit and save the electrical circuit from damage caused by overload or short circuit.

Its main function is to detect a fault condition and interrupt current flow.

A circuit breaker can be reset either manually or automatically to resume usual operation.


  1. Thermal magnetic circuit breaker.
  2. Instantaneous trip circuit breaker.
  3. Current limiting circuit breaker.
  4. Instantaneous magnetic trip circuit breaker.

Circuit breaker must safely make into and interrupt prospective short circuit fault current expected at that point into circuit. Main contact must open rapidly when resulting arc is transferred to special arcing contacts above main contacts.

Arc chutes with arc splitters must stretch quickly to cool down the arc until it snubs. The circuit breaker remain open during quenching of arc.

Typical circuit breaker consists of fixed contacts arranged so that arcing contact make before and open after main contacts.

Working Principle of Circuit Breaker

The circuit breaker normally consists of fixed contacts and moving contacts. In the “ON” condition of the circuit breaker, the fixed and moving contacts are normally connected to each other due to applied mechanical pressure on the moving contacts. There is an arrangement to stored potential energy in the operating mechanism of circuit breaker which is released if the switching signal is given to the breaker. The P.E.(Potential Energy) can be stored in the circuit breaker by different ways like by deforming metal spring, by compressed air, or by hydraulic pressure. But whatever the source of potential energy, it should be released during operation. The release of potential energy makes the sliding of the moving contact in a speedy manner.

All circuit breaker have operating coils, whenever these coils are energized by switching pulse, and the plunger inside them displaced. The working coil plunger is normally attached to the operating mechanism of circuit breaker, as a result the mechanically stored P.E.(Potential Energy) in the breaker mechanism is released in forms of K.E.(Kinetic Energy), which tends to move the moving contact to move as which is mechanically attached through a gear lever arrangement with the operating mechanism. After a number of time of operation of circuit breaker the total stored energy is released and hence the potential energy again stored in the operating mechanism of the circuit breaker with the help of spring charging motor or air compressor or by any other means.

Survey of Circuit breakers

  • Checking of main, arcing and auxiliary contacts for wear and damage and misalignment.
  • Checking of fixed and moving isolator contact for signs of wear, damage and misalignment.
  • Arcing chutes should be clean, no arcing debris prevent and should be correctly alignment.
  • Checks all wings for damage and proper connection at terminals. Loose connections results in sparking.
  • All mechanical linkage should be checked for signs of wear and damage.
  • The test of circuit breakers requires close and trip operations while it is in isolated position.
  • The racking mechanism which moves the circuit breaker from fixed to isolated place should be checked for free movement.
  • Check that fixed main contact terminals are shattered when circuit breaker is removed.
  • Under voltage release and overcurrent trip settings for level and time delay.
  • The time delay dash pot filled with oil up to correct level and of correct oil is used as per manufacturer.