Spark-gap

A spark-gap is an electric device used to limit the Surtension S (function of Parasurtenseur) and to carry out Court-circuit high-speed eurs.

Spark-gaps with two electrodes

The spark-gaps are composed of two electrode S face to face in a medium which can be the ambient air (spark-gap with Air), air but in a closed medium (with encapsulated air) or gas (spark-gaps with gas).

Beyond a certain value of tension between the terminals, therefore of some Electric field between the electrodes, a starting occurs and a running is established between the two electrodes by forming a Electric arc. In general one of the electrodes is connected to a phase, and the other with the ground. After starting, the phase is thus practically with the potential of the ground: it is said that the Surtension was deviated with the ground.

The spark-gaps are very robust and make it possible to deviate of the important currents, however the electric arc which appears during operation is maintained by the current one that outputs the system, it is necessary to envisage the cut of this Court-circuit phase/ground.

It is the principle of the horns of starting which one sees on the equipment of distribution of electricity but they are also used in low tension in the form of modular switchgear and for the installations of Télécommunication with a limited effectiveness. Indeed slowness with starting (approximately 1µs) is a defect of this type of component for protection: the Overpressure S rapids as those caused by the lightning have a boarding time which can be lower than the time of starting of the spark-gap, it lets pass the peak of overpressure and deviates most of energy but does not protect the equipment effectively significant: the spark-gap lightning protectors with gas on the lines telecom are of a relative effectiveness to protect the equipment from telecommunication and are sometimes associated with secondary components of standard protection Varistance or Diode of chopping. Nevertheless the spark-gaps with gas get a very high current of discharge (flow of the transitory waves of current) and offer a very low stray capacity thus allowing a minimal degradation of the transmitted signals.

Generally, the Parafoudre S containing Varistance S offer a more effective protection against overpressures than the spark-gaps.

In the laboratories high voltage one uses spark-gaps with Sphère. In this case the 2 electrodes are spheres; one can regulate their spacing and thus precisely define the tension of starting between the spheres.

Tripolar spark-gaps

Spark-gaps with three electrodes particularly adapted to the protection of the appairées lines (standard line of telecommunication), they offer a protection in common mode (wire 1 compared to the ground, and wire 2 compared to the ground), as well as a protection in differential mode (between wire 1 and wire 2).

Spark-gaps with trigger

The spark-gap is equipped with an auxiliary electrode makes it possible to force the establishment of the arc by an external device. Functionally, a spark-gap with trigger is similar to a Triac.

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