Specifications

29
●
Discharge current
This depends on the properties of the gas, the volume, and
the material and treatment of the electrodes. It is the major
characteristic of the GDT and the one that distinguishes it
from other protection devices (Varistor, Zener diode, etc.): 5
to 20kA with an 8/20
㎲
impulse for the standard components.
This is the value the device can withstand repeatedly (say for
ten impulses) without destruction or alteration of its basic
specifications.
●
Impulse sparkover voltage
Sparkover voltage in the presence of a steep rise front (dV/dt
= 1kV/
㎲
): the impulse sparkover voltage increases with
increasing dV/dt.
●
Insulation resistance and capacitance
These characteristics make the gas discharge tube
practically
<< invisible >> in a line in a steady-state context:
insulation resistance very high (>1 Gohm), capacitance very
low (<10pF).
3-electrode configuration
Protecting a two-wire line (for example a telephone pair) with
two 2-electrode gas discharge tubes (connected between
the wires and ground) may cause the following problem:
The line is subjected to an overvoltage in common mode;
because of the dispersion of the sparkover voltages (±
20%), one of the gas discharge tubes sparks over a very
short time before the other (a few microseconds); the wire
that has sparked over is therefore grounded (neglecting the
arc voltages), turning the common-mode overvoltage into a
differential-mode overvoltage, very dangerous for the terminal
equipment.
This risk disappears when the second gas discharge tube
arcs over (a few microseconds later).
3-electrode geometry eliminates this drawback: the sparkover
of one pole causes a
<< general >> breakdown of the device
almost instantaneously (a few nanoseconds) because there
is only one gas-filled enclosure.
End of life
Gas discharge tubes are designed to withstand several
impulses without destruction or loss of the initial
characteristics (typical impulse tests: 10 times 5 kA impulses
of each polarity).
On the other hand, a sustained strong current (e.g. 10 A rms
for 15 seconds, simulating the fall of a AC power line onto a
telecommunication line) will put the device out of service
definitively.
If a fail-safe end of life is desired (i.e. a short-circuit that will
report the fault to the user when the line fault is detected),
gas discharge tubes with the fail-safe feature (external short-
circuit) should be chosen.
Standards
CITEL gas discharge tubes comply with the specifications of
main telecom operators (France Telecom, British Telecom,
etc.) and with the ITU-T K12 international recommendation.
The CITEL line
CITEL proposes a full line of gas discharge tubes to meet most
configuration needs and specifications found on the market :
󳀏2- and 3-electrode gas discharge tubes
󳀏Sparkover voltages from 75 to 3000 V
󳀏Discharge capacities from 2.5 to 150 kA
󳀏Optional external short-circuit device
󳀏
Installation on support, on printed circuit, or surface-mounted
devices.
Gas Discharge Tubes