Gas discharge tubes (GDTs) have evolved to a level of providing very reliable and effective surge protection in telecommunications systems and equipment, safeguarding against lightning and power-fault conditions.
Due to their robust nature and superior electrical characteristics, GDTs have already become the preferred replacement for carbon blocks in traditional telephone-service applications. Because of their ultra-low capacitance plus low insertion and return loss performance, they’re also replacing solid-state semiconductor solutions in broadband-communication designs.
General GDT Operation Finding extensive employment, GDTs protect against overvoltages caused by lightning, power switching, and fault conditions. When a voltage disturbance reaches a GDT’s spark-over value, it will switch into a virtual short, known as the arc mode. The GDT virtually shorts the line, diverting the surge current through the GDT to ground, and removes the voltage surge.
At normal operating voltages below the GDT’s rated dc breakdown (DCBD) voltage, measured at a rise rate between 100 to 2000 V/s, the GDT remains in a high-impedance OFF state. With an increase in voltage across its conductors, it will enter into the glow-voltage region, which is where the gas in the tube starts to ionize due to the charge developing across it.
In the glow region, the increase of current flow will create an avalanche effect in the gas ionization process that will transition the GDT into a virtual short-circuit mode. The current, which depends on the impedance of the voltage source, will pass between the two conductors. We refer to the voltage across the GDT during the short-circuit mode as the arc voltage.
The transition time between the glow and arc region depends on the available current of the impulse, distance and shape of the electrodes, gas composition, gas pressure, and the proprietary emission coatings. The active emission coating allows the tubes to transition into arc mode at currents lower than 500 mA with arc voltage specified at less than 10 V at 1 A.
The GDT will switch back or reset into a high-impedance state once there is not enough voltage and/or current to keep it in the arc condition. This is known as the extinguishing voltage, holdover voltage, or impulse reset voltage.
Due to the typical arc voltage of less than 10 V, a GDT is ideal for protecting against high-energy impulses and acpower cross conditions. Under ac, the power dissipated in the device needs careful monitoring. A switch-grade, fail-short mechanism can protect against thermal overload under these ac conditions.
At elevated GDT temperatures, a spring-loaded clip operates like a switch to short the TIP/RING conductors to ground. Importantly, it is not a good practice to hold a GDT in its glow region as this will significantly reduce its life expectancy. In this condition, significant heat can develop on the electrodes that can damage the emission coatings and cause premature failure of the tube.
Also avoid using a variable ac source such as a curve tracer or equivalent to vary the voltage and power across the GDT’s DCBD voltage. It is highly unlikely, however, that a condition could exist in the field that would maintain a GDT in the glow mode (Fig. 1).
Dynamic Performance With Telecom Surges Characterization of a GDT’s impulse spark-over specifications is via an impulse voltage waveform such as 100 V/µs or 1 kV/µs. This ramp voltage is not an accurate representation of a real-world scenario, i.e., disturbances entering a twisted-pair cable from a lightning strike.
Based on rise and decay times, a lightning strike is viewable as a charge dissipating through the impedance of the line. There is a relationship between the DCBD, ramp-impulse, and surge-impulse voltages, which translates into similar GDT performance between the 100-V/µs impulse breakdown voltage and the 100-A, 10/1000-µs surge-impulse voltage (Fig. 2).
Field studies show that lightning-strike energy is similar to a 10/250-µs waveform for a positive stroke. Primary-protection telecom standards such as Telcordia GR-974-CORE additionally specify end-of-life mode tests using a series of surge generators delivering 10/1000-µs, 10/250-µs, and 8/20-µs type wave shapes. The dynamic performance of the GDT with these wave shapes is less known.
As secondary protection is often required to protect voltagesensitive equipment from the primary protector let-through, it is important to know how the GDT performs under these types of impulses. Standards such as Telcordia GR-1089-CORE for secondary protection still rely on worst-case, carbon-block primaryprotection technology to cover legacy equipment while the GR- 1089-CORE, issue 4 release allows secondary protection to depend more on the primary-protector technology used in the field.
Variable Dynamic Parameters Designers often question the inconsistent electrical measurements of a GDT. These inconsistencies are the result of contaminants introduced into the gas from normal operation. Contaminants in the gas change the electrical characteristics of the GDT, causing increases in DCBD voltage values.
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