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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