The function of the charging motor (M) is to compress the main closing spring which is the mechanical stored energy mechanism. The energy required to trip or open the circuit breaker is provided by the tripping spring, while the energy required to close the circuit breaker is supplied by the closing 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.