Gas discharge tube (GDT) A gas discharge tube (GDT) is a sealed glass-enclosed device containing a special gas mixture trapped between two electrodes, which conducts electric current after becoming ionized by a high voltage spike.GDTs can conduct more current for their size than other components. Like MOVs, GDTs have a finite life expectancy, and can handle a few very large transients or a greater number of smaller transients. The typical failure mode occurs when the triggering voltage rises so high that the device becomes ineffective, although lightning surges can occasionally cause a dead short.
GDTs take a relatively long time to trigger, permitting a higher voltage spike to pass through before the GDT conducts significant current. It is not uncommon for a GDT to let through pulses of 500 V or more of 100 ns in duration. In some cases, additional protective components are necessary to prevent damage to a protected load, caused by high-speed let-through voltage which occurs before the GDT begins to operate.
GDTs create an effective short circuit when triggered, so that if any electrical energy (spike, signal, or power) is present, the GDT will short this. Once triggered, a GDT will continue conducting (called follow-on current) until all electric current sufficiently diminishes, and the gas discharge quenches. Unlike other shunt protector devices, a GDT once triggered will continue to conduct at a voltage less than the high voltage that initially ionized the gas; this behavior is called negative resistance. Additional auxiliary circuitry may be needed in DC (and some AC) applications to suppress follow-on current, to prevent it from destroying the GDT after the initiating spike has dissipated. Some GDTs are designed to deliberately short out to a grounded terminal when overheated, thereby triggering an external fuse or circuit breaker.
Many GDTs are light-sensitive, in that exposure to light lowers their triggering voltage. Therefore, GDTs should be shielded from light exposure, or opaque versions that are insensitive to light should be used.
The CG2 SN series of surge arrestors formerly produced by C P Clare, are advertised as being non-radioactive, and the datasheet for that series states that some members of the CG/CG2 series (75-470V) are radioactive.
Due to their exceptionally low capacitance, GDTs are commonly used on high frequency lines, such as those used in telecommunications equipment. Because of their high current handling capability, GDTs can also be used to protect power lines, but the follow-on current problem must be controlled.
General Operation of a GDT
When a voltage disturbance reaches the GDT sparkover value, the GDT will switch into a virtual short,
known as the arc mode. In the arc mode, the GDT practically shorts the line, diverting the surge
current through the GDT to ground and removes the voltage surge from the plant and equipment. At
normal operating voltages below the GDT rated DC sparkover voltage (measured at a rate of rise between
100 to 2000 V/s), the GDT remains in a high impedance off-state condition. All of our GDTs are rated at
a minimum insulation resistance of 109
Ω at 200 V, 100 V bias or 50 V bias for the different breakdown
values. With an increase in voltage across its conductors, the GDT will enter into its glow voltage region.
The glow region is where the gas in the tube starts to ionize due to the charge developed across it. During
the glow region the increase of current flow will create an avalanche effect in gas ionization that will
transition the GDT into a virtually short circuit mode
and current (dependent on the impedance of the
voltage source) will pass between the two conductors.
The voltage developed across the GDT with a short
circuit condition is called the “Arc Voltage” VARC.
The transition time between the glow and arc region
is dependent on the available current of the impulse,
the distance and shape of the electrodes, the gas
composition, gas pressure and the proprietary emission
coatings. Bourns active emission coatings allow the
tubes to transition into the arc mode at currents much
lower than 500 mA.
The arc voltage is the voltage rating that is specified
as <10 V @ 1 A for Bourns® GDTs. The GDT will
switch back or reset into a high impedance state once
there is not enough energy to keep the device in
the arc condition. This is sometimes also called the
“Extinguishing Voltage, Holdover Voltage or Impulse
Reset Voltage”. Figure 1 shows the basic switching
characteristic of a GDT.
Due to the typical arc voltage of less than 10 V, a GDT is ideal for protecting against high-energy impulses
and AC power cross conditions. Under AC, the power dissipated (VARC x IGEN) in the device needs to
be carefully monitored. The switch grade fail-short mechanism is available for the 2026/2036 series and
protects against thermal overload under AC conditions. At elevated GDT temperatures, a spring-loaded
clip will operate like a switch to short the TIP/RING conductors to ground. The 2026-xx-xx-F series is
recommended where high AC or long duration AC conditions exist or to meet Telcodia GR-974-CORE
requirements. The switch grade fail-short mechanism cannot be used to meet Telcordia GR-1089-CORE
requirements as there are first level AC tests where the equipment must work correctly after the test. It is
also not a good practice to hold a GDT in its glow region as this will significantly reduce the life expectancy
of the device. In this condition, significant heat can be developed on the electrodes that can damage the
special emission coatings and cause premature failure of the tube. Using a variable AC source such as a
curve tracer or equivalent to vary the voltage and power across at its DCBD voltage should be avoided. It
is highly unlikely that a condition could exist in the field which would maintain a GDT in the glow mode.
Bourns uses an active proprietary coating that provides a very low glow to arc transition current, helping to
reduce or eliminate these issues.
