1.3 PROTECTION DEVICES
Between the more common protection devices of the disturbs of which in 1.1a and other impulsive phenomenons call the gas discharged and the varistor.

GAS DISCHARGED
The devices intervene presenting a short-circuit toward the ground through the the time when the gas is ionized, so from the discharged capsule instant to the same power off instant. The typical start value is chose for the work point, generally is chose even
to the present maxim tension in that circuit point with the addition of a 10% safety limit. If the employed to the current line entrance is 220Vca of the apparatus, the discharged must be selected with a start minimum tension of 380V. After the capsule the tension gets down to the value (depends of the discharged current) of 5---30V routing all the disturb current driven on the line toward the earth. Finished the transitory the current gets down at the values that must extinguish the discharged. Can happen that the gas capsule remains until the passage for the zero of the alternate tension current; this phenomenon had to the fact that the discharged maintain contribute also the wiring current can overheat the device and also destroy it.

VARISTOR
The devices behavior like the resistances with the dependent value from the applicate tension. To a low tension the resistance has the values beyond 10MOhm; as soon as the tension passes the intervention threshold value the resistance gets down to the unimportant values discharging on the earth the impulse current. With the exception of the discharged the intervention threshold like the extinction is not tied o secondary phenomenon but only to the value of the tension to the heads of the varistor. There are available the varistor with the intervention tension value until 1000V and currents until 20KA. The varistor are more faster then the discharged, these last have the vantage to disperse less energy and have a long life.

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