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a device for closing electric circuits by means of an electrical discharge in a gas, a vacuum, or, more rarely, a solid dielectric. It contains two or more electrodes separated by one or more discharge gaps. The conductivity of the gaps changes abruptly when the potential difference between the electrodes becomes equal to a critical value that depends on the given conditions; this critical value is called the breakdown voltage or striking potential. Various types of discharge can occur in a discharger, depending on the state of the discharge gap and on the electric-circuit parameters: spark discharges; glow discharges, including corona discharges; arc discharges; high-frequency discharges; and mixed types.
Dischargers are used in electrical engineering, electronics, automation, and experimental physics. They provide protection for electric circuits and devices from voltage surges. The spark discharger is an example of dischargers used to switch high-frequency and high-voltage electric circuits. Dischargers are also made use of in measuring high voltages. Sometimes they are used to measure pressure in a vacuum system (seeVACUUM MEASUREMENT).
Functionally, there are two basic types of dischargers—protective and control. Protective dischargers, by means of their breakdown, make possible the prevention of excessive voltage rises on power lines or in equipment to which they are connected. The simplest forms of dischargers used to protect electrical networks are rod gaps and horn gaps, which consist of two electrodes—in the shape of rods or curved horns, respectively—separated by an air gap. One of the electrodes is connected with the equipment being protected and the other is grounded. Since the conductivity of a gas-discharge gap increases drastically in a breakdown, the discharge current does not cease after the voltage falls to a normal value. This current is called the follow current and is a short-circuit current to earth of the system or equipment. The follow current causes the protective relaying to function, and a temporary power outage is consequently suffered by the equipment or system.
In the case of alternating currents, the operation of the protective relaying can be prevented by the use of expulsion-type arresters, which extinguish the arc of the follow current. The discharge gap in expulsion arresters is located in a chamber containing a material that produces an insulating gas. Under the action of the heat given off in the arc of the follow current, the material decomposes and evolves a large amount of gas; the pressure in the chamber is thereby increased, and a stream of gas is formed that extinguishes the arc when the follow current passes through zero. Expulsion arresters are generally used to protect AC power lines from lightning surges.
To ensure the efficient operation of protective dischargers, the breakdown voltage must be very stable—that is, it must be independent of atmospheric conditions and the state of the electrodes. In addition, the volt-second characteristic of the discharge gap—that is, the curve of the gap’s breakdown voltage as a function of the rate of increase of the voltage across the gap—should be relatively flat and should lie below the volt-second characteristic of the insulation of the equipment being protected. These requirements are met by valve-type arresters, which provide protection from lightning and switching surges for the insulation of transformers and other electrical equipment.
Control dischargers are used to connect the various elements of pulsed-voltage generators in a specified sequence, to connect a load to high-power pulsed current sources, and to connect the electric-circuit elements of high-voltage test apparatus. The simplest control discharger is the sparking ball, which consists of two spherical electrodes separated by a layer of gas. In some types of control dischargers, to initiate the discharge between the electrodes at the desired moment the electric strength of the discharge gap is decreased by, for example, the injection of hot gas; another method of initiation is the use of a starting pulse, as in trigatrons.
REFERENCESBezrukov, F. V., Iu. P. Galkin, and P. A. Iurikov. Trubchatye razriadniki. Moscow-Leningrad, 1964.
Katsnel’son, B. V., A. M. Kalugin, and A. S. Larionov. Elektrovakuumnye eleklronnye i ionnye pribory, book 1. Moscow, 1970.
Kushmanov, I. V., N. N. Vasil’ev, and A. G. Leont’ev. Eleklronnye pribory. Moscow, 1973.
Kalashnikov, A. M., and Ia. V. Stepuk. Elektrovakuumnye i poluprovodnikovyepribory, 4th ed. Moscow, 1973.
A. M. BRONSHTEIN