As a manufacturer of gas discharge tubes (GDT) and gas-filled spark gaps (GSG), CITEL has a wide range of products that can be used to meet most configuration requirements and specifications on the market specifications on the market:
- Gas arrestors in 2- and 3-electrode configuration
- Flashover voltages from 75 to 3500 V
- Discharge capacities from 5 to 150 kA (8/20 μs)
- Optional external short-circuit element
- Can be mounted on bases, PCBs or surface-mounted components.
Technology & Functionality
GDT and GSG consist of two or three electrodes in a housing filled with a (non-radioactive) inert gas. A defined pressure prevails in this housing. The housing is a ceramic tube that is closed at the ends by metal caps that also serve as electrodes. They are mainly used to protect telecommunication lines, but are also suitable for other applications.
Structure of 2-pole and 3-pole gas arresters (GDT)
This is characterised by a practically unlimited high insulation resistance.
At the response voltage, the conductivity increases abruptly. If the current derived from the GDT is less than about 0.5 A (this is an approximate value depending on the component), the glow voltage at the terminals is in the range of 80-100 V.
As the current increases, the arc voltage (approx. 20 V) is established across the gas arrester instead of the glow voltage. In this operating state, the GDT is most effective because the discharged current can reach several thousand amperes without the arc voltage applied to its terminals increasing.
At a bias voltage approximately equal to the glow voltage, the GDT resumes its original insulating properties.
A GDT is characterised by the following main electrical properties:
- DC flashover voltage (V)
- Impulse flashover voltage (V)
- Leakage current strength (kA)
- Insulation resistance (GΩ)
- Capacitance (pF)
DC spark voltage:
This is the most important parameter describing the operating behaviour of a GDT. It is the voltage at which a voltage flashover occurs between the electrodes when a slowly increasing voltage (dV/dt = 100 V/s) is applied to the component. It depends on the electrode spacing, the pressure and the properties of the gas mixture and the emitting substance.
Available DC overvoltages:
- Minimum voltage: 75 V
- Medium voltage: 230 V
- High voltage: 500 V
- Very high voltage: 1000 to 3000 V
The tolerance range of the sparkover voltage is generally ± 20%.
This quantity depends on the gas properties, the volume and the electrode material as well as its treatment. The leakage current is an important parameter of a GDT and distinguishes it from other protective components such as varistors or Z-diodes. For standard components, it is between 5 and 20 kA for an 8/20-μs pulse. The component can withstand this current repeatedly (typically for ten pulses) without being destroyed or having its basic specifications changed.
Pulse sparkhover voltage:
Flashover voltage in the presence of a steep rising edge (dV/dt = 1 kV/μs): The pulse flashover voltage increases with increasing dV/dt value.
Isolation resistance and capacitance:
These parameters make the GDT practically "invisible" in a power line that is in a steady state:
The insulation resistance is very high (>10 GΩ), the capacitance very low (<1 pF).
3-electrode configurationIf a two-wire line (for example a telephone line) is protected with two GDT in a 2-electrode configuration, each connected to one wire and earth, the following problem may occur:
. A common-mode overvoltage occurs on the line. Due to the dispersion of the sparkhover voltages (± 20%), the sparkover of one of the two GDT occurs very shortly (a few microseconds) before that of the other. Thus, the wire that caused the flashover is earthed (neglecting the arc voltages), turning the common mode overvoltage into a differential overvoltage. This is very dangerous for the end device to be protected. This danger only ceases to exist when the sparkhover also occurs at the second GDT (a few microseconds later). The 3-electrode configuration eliminates this disadvantage. The sparkhover of one pole triggers a "general" sparkhover of the component almost instantaneously (within a few nanoseconds), since all electrodes are in the same gas-filled housing.
End of lifeGDT are designed to withstand multiple pulses without destruction or loss of their initial characteristics (typical pulse tests use ten 5 kA pulses of each polarity). On the other hand, a sustained high current (e.g. with an rms value of 10 A for 15 seconds, which corresponds to the case of a high-voltage overhead line falling on a telecommunication line) will defi nitively destroy the component. If fail-safe behaviour at the end of the operating life is desired (whereby a short-circuit signals a line malfunction to the user when it is detected), GDT with this fail-safe function (external short-circuit) should be selected.
StandardsCITEL GDT meet the specifications of the main telecommunications operating companies (Deutsche Telekom, France Telecom, British Telecom, etc.) as well as the international recommendation ITU-T K12 and the IEC 61643-31x set of standards.
Mechanical characteristicsCITEL GDT are available in various mechanical configurations to adapt to the desired setting: - Bare version for mounting adapted support - S" wire outlet version (diameter 0.8 or 1 mm) for mounting on printed circuit board - SMD" version for surface mounting, with optional" SQ " (anti-roll rectangular electrode version).
Most of the GDT in the CITEL range are available for surface mount (SMD), with optional "anti-rolling" version with square electrode (SQ). The welding profile with reflow must follow the recommended curve (opposite).
The 3-pole BMSQ CMS FL gas line, with its anti-roll "electrode and its exclusive external short-circuit system, which is adapted to this type of mounting, is particularly suitable for surface mounting technology.
Most CITEL GDT are available with wire outlet (diameter 0.8 or 1 mm) for mounting on a PCB. Different output types possible depending on the range:
Axial, radial, straight, folded output.
Wave soldering must be done according to the recommended profile (opposite).
The CITEL GDT with wire outlet are supplied in a radial belt in a pack of 500 components according to the ranges (plan opposite) and in accordance with the IEC 286-1 specification.
Tape and Reel
The CITEL GDT with SMT assembly are supplied in tape and Reel Pack, reel with 500,800 or 1000 components (facing plan) and according to IEC 286-1 specification.
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