When the current is below the rated value of the circuit breaker, the magnetic flux in the solenoid is insufficient to pull the core towards the pole piece due to the pressure of the spring.

When an overload occurs, meaning the current exceeds the rated value of the circuit breaker, the magnetic flux in the solenoid generates enough pull on the core to start moving it towards the pole piece. During this movement, the hydraulic fluid regulates the speed of the core, creating a controlled time delay that is inversely proportional to the magnitude of the current. The purpose of this time delay is that if the overload is brief, such as during motor startup, the core will return to its resting position once the overload disappears.

If the overload persists, the core will reach the pole piece after a time delay corresponding to the current, at which point the magnetic circuit’s reluctance will significantly decrease, allowing the armature to be attracted to the pole face with sufficient force to collapse the latching mechanism, causing the circuit breaker to trip.

The contacts separate, the current stops flowing, and the core returns to its resting position.