They do not depend on temperature as a tripping mechanism, so they often find use where ambient temperatures are high. Their insensitivity to temperature also makes them candidates for protecting low-current circuits, as with sensitive coils, which would not generate much heat even in the case of a short. For similar reasons, they need not cool down after a fault has cleared before they can reset.

Typical hydraulic-magnetic breakers mainly consist of a solenoid and an armature that connects via a linkage assembly to a set of contacts. The load current is connected so it flows through the solenoid coil. The solenoid's spring-loaded plunger (or core) travels back and forth in a hermetically sealed tube filled with silicone damping fluid.

Currents below the circuitbreaker rating don't generate enough magnetic flux for the core to overcome the force of the spring. But overload currents make the core move toward the pole piece and eventually touch it. Once the two pieces are in contact, the reluctance of the magnetic circuit drops considerably. This attracts the armature to the pole face with enough force to collapse the latch mechanism and trip the breaker. The action separates the contacts and interrupts the flow of current through the solenoid coil, letting the core spring back to its rest position.

Silicone fluid regulates the core's speed of travel. The damping creates a controlled time delay before the breaker trips. The delay is inversely proportional to the magnitude of the current. The time delay is useful for letting loads draw short-duration overcurrents as part of their normal operation. The classic example is that of a motor starting up.

Operation in the case of a short circuit is quite different. Here the magnetic flux produced by the coil is enough to attract the armature to the pole face and trip the breaker though the core has not moved. Such action takes place in the circuit breaker's instantaneous trip region.

These breakers can be found in a wide range of sizes with current ratings to 100 A and short-circuit interruption capacities up to 10 kA at 125 and 240 Vac. Conversely, trip points can be as low as 20 mA for units targeting sensitive equipment.

Circuit Breaker Industries Inc. (cbibreakers.com) provided information for this article.