Circuit breakers
Operating mechanism

Operating mechanism

The operating mechanism creates and stores energy to operate the circuit breaker.  It must always be able to trip the circuit breaker. Depending on the force required to operate the breaker, the circuit breaker may be equipped with one operating mechanism per phase or one mechanism for all three phases. The operating mechanism includes the energy storage medium, actuating circuit and interlocking systems. 


  • Spring; a spring-operated mechanism is one driven by the mechanical energy stored in springs. Typically, the “closing spring” is mechanically charged by a motor and is held in its compressed position by a closing latch. When a close signal releases this latch, this spring pushes against a mechanical linkage to force the breaker contacts closed and, at once, charges the trip spring. The closing spring is then immediately recharged by the motor. Another latch will hold the tripping spring in the compressed position until an open signal releases this latch.
  • Hydraulic; a hydraulic-operated mechanism uses pressurised gas to direct the flow of oil, thus actuating the linkage(s) connected to the interrupter(s).
  • Pneumatic; a pneumatic-operated mechanism uses compressed air as the energy source for closing and tripping.
  • Magnetic; uses a solenoid, or electromagnet, whose pulling force increases with the current. Certain designs utilize electromagnetic forces in addition to those of the solenoid. The circuit breaker contacts are held closed by a latch. As the current in the solenoid increases beyond the rating of the circuit breaker, the solenoid's pull releases the latch, which lets the contacts open by spring action.


According to a 2005 CIGRE CB survey, out of total faults identified in circuit breaker components, 70 percent were associated with the operating mechanism and over 50 percent of major circuit breaker failures were identified as originating with the operating mechanism. (See recommended circuit breaker analysers for testing designed to assess the condition of the operating mechanism.)