Gas discharge tube
A gas arrester is a gas discharge tube that serves as a surge arrester to protect against surge pulses such as those used in B. due to lightning strikes in the vicinity of networks (telephone network, electricity network) can occur in these. Often the English term gas discharge tube or its abbreviation GDT is used as an alternative for gas discharge tubes.
The overvoltage is reduced in the gas discharge tube by the automatic ignition of a gas discharge which, depending on the current and voltage , develops as a glow discharge , spark or arc discharge .
function
Below the ignition voltage, the component connected in parallel to the line to be protected behaves like an insulator and does not affect it. From a component-specific ignition voltage, a gas discharge ignites in the gas discharge tube and the terminal voltage on it is reduced to approx. 10… 20 volts within a few microseconds with currents of around 1 ampere or more due to an arc discharge. It is more or less a glow lamp or encapsulated spark gap without a series resistor. In contrast to other surge arresters ( suppressor diodes , varistors ), the terminal voltage may drop well below the nominal voltage, which is equivalent to a short circuit in network applications .
Gas arresters react more slowly than varistors or suppressor diodes, but can dissipate high pulse energies.
Gas arresters, like spark gaps, are therefore referred to as coarse protection elements or coarse protection. Gas arresters differ from spark gaps in that the response voltage is more closely specified and tolerated. This is achieved through the hermetic encapsulation and a defined gas filling.
There are versions from 90 to approx. 4500 volts ignition voltage. The actual ignition voltage is far above the nominal value for steep pulses; this effect is particularly pronounced for spark gaps with a lower nominal voltage due to a stronger field inhomogeneity.
While the current flows after the response, a burning voltage of <25 volts (arc discharge, from approx. 1 A) or approx. 50… 200 volts (glow discharge, with current ≪ 1 A) is established. At higher operating voltages, the leakage current after the end of the overvoltage pulse must be limited via an upstream fuse or protective resistors. The active operating time may only be short; continuous or cyclical operation is not possible.
construction
In a glass or ceramic body with two solid metal connections (often these are also the sealing caps) there is noble gas. Traces of radioactive substances can be mixed with the noble gas in order to achieve pre-ionization. There are also versions made of two elements in a row with three connections. They serve z. B. the protection of symmetrical lines, the middle connection is then at ground potential.
Small gas discharge tubes sometimes resemble small wired glow lamps in terms of shape and size, such as those found in u. a. can be used in illuminated buttons. Such small arresters are z. B. sometimes used for overvoltage protection of the electrodes of picture tubes.
Application / advantages and disadvantages
- advantages
- very low self-capacitance compared to varistors and suppressor diodes
- high leakage energies and peak currents (usual: 2 to 20 kA [kiloampere])
- inexpensive
- Active overvoltage limitation through (low-resistance) short circuit in the event of tripping
- Gas arresters are manufactured for nominal operating voltages (effective value) from 70 V up to a few tens of kilovolts
- very long service life with correct dimensioning
- high internal resistance (gigaohm) at rest
- Destruction by a one-off high energy voltage pulse is unlikely with gas arresters, but it often occurs with varistors
- disadvantage
- Wear, not suitable for periodic response (the response voltage of the gas discharge tube must be designed to be significantly higher than the effective nominal AC voltage to be protected)
- Quasi-short circuit after responding, resulting in a so-called "line follow current" with initially high amplitude, which decays sinusoidally, therefore back-up fuse or a current-limiting load resistor (or line resistance) required for line voltage applications
- higher response time than varistors and unidirectional suppressor diodes
- After tripping, the arrester needs a few milliseconds to become high-resistance again, the so-called deionization time , during which the system follow current is still flowing
- Gas arresters cannot be used to protect DC voltages, as they only go out when the current crosses zero, so if DC voltage is used, the gas arrester would not become high-resistance again and current would continuously flow through it (permanent short circuit)
- At high voltage rise rates ( ), i.e. steep voltage peaks, the arrester only trips at a voltage even higher than its nominal ignition voltage, as it needs a few microseconds (usually 1.5–2 µs) to ionize
- Use in cold conditions increases, use under high room temperatures reduces the ignition voltage
The gas discharge tube is often supplemented by other protective elements, such as VDR ( varistor ) and suppressor diode (also called TVS, Transzorb or Transil diode).
- Applications
- Lightning protection at mains connections, also in surge protection adapter plugs or socket strips. When responding, they trigger the upstream fuse.
- Lightning and surge protection for telephones, modems and network cards.
- Lightning protection in antenna cables (particularly suitable here due to the low capacity).
See also
Web links
credentials
- ↑ http://www.epcos.co.jp/products/ceramics/pdf/arrester_3.pdf Gas discharge tube behavior (company publication from epcos)
- ^ H. Singer, JL ter Haseborg, F. Weitze, H. Garbe: “Response of Arresters and Spark Gaps at Different Impulse Steepnesses”, 5th International Symposium on High Voltage Engineering, Braunschweig, August 1987
- ↑ K. Borgeest: "Optimization and simulation of the transient response and transmission behavior of non-linear protective circuits for HF systems", 1998