Hydraulic-magnetic breakers secure circuits after a time delay by cutting power.
As a tripping device, they are not dependent on temperature, so they sometimes see use where there are high ambient temperatures. The insensitivity of these breakers to temperature often allows them as candidates to protect low-current circuits, as it is with sensitive coils, which even in the case of a short one would not produce much heat. For similar reasons, when a fault has cleared, they do not need to cool down before they can restart.
Traditional hydraulic-magnetic breakers consist mainly of a solenoid and an armature which connects to a collection of contacts through a linkage assembly. The charging current is connected so that it passes through the solenoid coil. The spring-charged plunger (or core) of the solenoid travels back and forth in a hermetically sealed tube which is filled with silicone damping fluid.
Currents below the circuit breaker rating do not generate sufficient magnetic flux to overcome the spring’s force to the core. But overloading currents make the core travel to and ultimately reach the pole piece. Once these two parts are in contact, the magnetic circuit’s reluctance drops significantly. This draws the armature with enough force to break the mechanism of the latch and trip the breaker to the pole face. The action isolates the contacts and disrupts the current flow through the solenoid coil, thus allowing the core spring back to its position of rest.
Silicone fluid controls the speed at which the core travels. The damping produces a regulated pause in time prior to breaker trips. The waiting time is inversely proportional to current magnitude. The time delay is useful as part of their normal procedure for having loads draw short-duration overcurrents. A typical example is the one in a motor which is starting up.
In a situation where there is a short circuit, the process is quite different. The magnetic flux created by the coil here is sufficient to draw the armature to the face of the pole and to trip the breaker even as the core has not moved. This action takes place within the immediate trip area of the circuit breaker.
Such breakers can be used up to 10 kA at 125 and 240 Vac in a broad range of sizes including current ratings up to 100 A and short-circuit interference capacities. Conversely, for systems controlling sensitive equipment, trip points can be as small as 20 mA.
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