At the circuit breaker’s rated current or below, the magnetic flux in the solenoid is insufficient to attract the core towards the pole piece, due to the spring force shown in Figure 2. Therefore the circuit breaker remains loaded and the circuit is energised

When an overload occurs, i.e. current is greater than the circuit breaker’s rating, the magnetic flux in the solenoid produces sufficient pull on the core to start its movement towards the pole piece.

During this movement, the hydraulic fluid regulates the core’s speed of travel, creating a controlled time delay inversely proportional to the magnitude of the current. This time delay is useful in that if the overload is of short duration, e.g. start-up of motors etc., the core returns to its rest position when the overload disappears.

If the overload persists, the core reaches the pole piece. As a result the reluctance of the magnetic circuit is reduced, so that the armature is attracted to the pole piece with sufficient force to collapse the latch mechanism (toggle), and trips the breaker (Figure 4). The contacts separate, the current ceases to flow and the core will then return to its rest position

With high values of overloads or short circuit, the magnetic flux produced by the coil is sufficient to attract the armature to the pole face and trip the breaker even though the core has not moved. This is called the instantaneous trip region of the circuit breaker characteristic.
Unlike thermal circuit breakers, the hydraulic-magnetic circuit breaker’s trip point is unaffected by ambient temperature. After tripping, the breaker may be re-closed immediately since there is no cooling-down time necessary. By the nature of the principle of operation, it is possible to obtain any variation of time/current characteristic.