IEC 61643-312:2013

IEC 61643-312 ®
Edition 1.0 2013-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Components for low-voltage surge protective devices –
Part 312: Selection and application principles for gas discharge tubes

Composants pour parafoudres basse tension –
Partie 312: Principes de choix et d’application pour les tubes à décharge de gaz

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IEC 61643-312 ®
Edition 1.0 2013-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Components for low-voltage surge protective devices –

Part 312: Selection and application principles for gas discharge tubes

Composants pour parafoudres basse tension –

Partie 312: Principes de choix et d’application pour les tubes à décharge de gaz

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX S
ICS 31.100; 33.040.99 ISBN 978-2-83220-740-6

– 2 – 61643-312 © IEC:2013
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references . 6
3 Terms, definitions and symbols . 6
3.1 Terms and definitions . 6
3.2 Symbols . 10
4 Service conditions . 10
4.1 General . 10
4.2 Low temperature . 10
4.3 Air pressure and altitude . 10
4.4 Ambient temperature . 10
4.5 Relative humidity . 11
5 Mechanical requirements and materials . 11
5.1 General . 11
5.2 Robustness of terminations . 11
5.3 Solderability . 11
5.4 Radiation . 11
5.5 Marking . 11
6 General . 11
7 Construction . 12
7.1 Design . 12
7.2 Description . 12
7.3 Fail-short (failsafe) . 13
8 Function . 14
8.1 Protection principle . 14
8.2 Operating mode . 14
8.3 Response behaviour . 14
8.3.1 Static response behavior . 14
8.3.2 Dynamic response behavior . 14
8.4 Fail-short (failsafe) . 15
9 Applications . 16
9.1 Protective circuits . 16
9.1.1 General . 16
9.1.2 2-point (signal line) protection . 16
9.1.3 3-point protection . 17
9.1.4 5-point protection . 18
9.2 Telephone/fax/modem protection . 19
9.3 Cable TV/coaxial cable protection . 19
9.4 AC line protection . 20
Bibliography . 21

Figure 1 – Voltage and current characteristics of a GDT . 8
Figure 2 – Symbol for a two-electrode GDT . 10
Figure 3 – Symbol for a three-electrode GDT . 10
Figure 4 – Example of a two-electrode GDT . 12

61643-312 © IEC:2013 – 3 –
Figure 5 – Example of a three-electrode GDT . 12
Figure 6 – Failsafe construction of a three-electrode GDT using a solder pill as
sensitive spacer . 13
Figure 7 – Failsafe construction of a three-electrode GDT, using a plastic foil as
sensitive spacer . 13
Figure 8 – Typical response behaviour of a 230 V GDT . 15
Figure 9 – Spark-over voltages versus response time . 15
Figure 10 – Current through the GDT versus response time of fail-short (failsafe) . 16
Figure 11 – 2-point (Signal line) protection . 17
Figure 12 – 3-point protection using two-electrode GDTs . 17
Figure 13 – 3-point protection using three-electrode GDTs . 17
Figure 14 – 3-point protection using two-electrode GDTs with fail-short . 18
Figure 15 – 3-point protection using three-electrode GDTs with fail-short . 18
Figure 16 – 5-point protection using two-electrode GDTs . 18
Figure 17 – 5-point protection using three-electrode GDTs . 18
Figure 18 – 5-point protection using two-electrode GDTs with fail-short . 19
Figure 19 – 5-point protection using three-electrode GDTs with fail-short . 19
Figure 20 – Telephone/fax/modem protection using two-electrode GDTs . 19
Figure 21 – Telephone/fax/modem protection using three-electrode GDTs . 19
Figure 22 – Cable TV/ coaxial cable protection . 20
Figure 23 – AC line protection . 20

– 4 – 61643-312 © IEC:2013
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
COMPONENTS FOR LOW-VOLTAGE SURGE PROTECTIVE DEVICES –

Part 312: Selection and application principles for gas discharge tubes

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
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agreement between the two organizations.
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5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
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expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61643-312 has been prepared by subcommittee 37B: Specific
components for surge arresters and surge protective devices, of IEC technical committee 37:
Surge arresters.
The text of this standard is based on the following documents:
FDIS Report on voting
37B/114/FDIS 37B/120/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above Table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of IEC 61643 series, under the general title Components for low-voltage
surge protective devices can be found on the IEC website.

61643-312 © IEC:2013 – 5 –
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
The contents of the corrigendum of July 2013 have been included in this copy.

– 6 – 61643-312 © IEC:2013
COMPONENTS FOR LOW-VOLTAGE SURGE PROTECTIVE DEVICES –

Part 312: Selection and application principles for gas discharge tubes

1 Scope
This part of IEC 61643 is applicable to gas discharge tubes (GDT) used for overvoltage
protection in telecommunications, signalling and low-voltage power distribution networks with
nominal system voltages up to 1 000 V (r.m.s.) a.c. and 1 500 V d.c. They are defined as a
gap, or several gaps with two or three metal electrodes hermetically sealed so that gas
mixture and pressure are under control. They are designed to protect apparatus or personnel,
or both, from high transient voltages. This standard provides information about the
characteristics and circuit applications of GDTs having two or three electrodes. This standard
does not specify requirements applicable to complete surge protective devices, nor does it
specify total requirements for GDTs employed within electronic devices, where precise
coordination between GDT performance and surge protective device withstand capability is
highly critical.
This part of IEC 61643
– does not deal with mountings and their effect on GDT characteristics. Characteristics
given apply solely to GDTs mounted in the ways described for the tests;
– does not deal with mechanical dimensions;
– does not deal with quality assurance requirements;
– may not be sufficient for GDTs used on high-frequency (>30 MHz);
– does not deal with electrostatic voltages;
– does not deal with hybrid overvoltage protection components or composite GDT devices.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60068-2-1, Environmental testing – Part 2-1: Tests – Test A: Cold
IEC 60068-2-20, Environmental testing – Part 2-20: Tests – Test T: Test methods for
solderability and resistance to soldering heat of devices with leads
IEC 60068-2-21, Environmental testing – Part 2-21: Tests – Test U: Robustness of
terminations and integral mounting devices
IEC 61643-311, Components for low-voltage surge protective devices – Part 311:
Specification for gas discharge tubes (GDT)
3 Terms, definitions and symbols
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply:

61643-312 © IEC:2013 – 7 –
3.1.1
arc current
current that flows after sparkover when the circuit impedance allows a current to flow that
exceeds the glow-to-arc transition current
3.1.2
arc voltage
arc mode voltage
voltage drop across the GDT during arc current flow
Note 1 to entry: See Figure 1a, region A.
3.1.3
arc-to-glow transition current
current required for the GDT to pass from the arc mode into the glow mode
3.1.4
current turn-off time
time required for the GDT to restore itself to a non-conducting state following a period of
conduction.
Note 1 to entry: This applies only to a condition where the GDT is exposed to a continuous d.c. potential (see d.c.
holdover).
3.1.5
d.c. sparkover voltage
d.c. breakdown voltage
voltage at which the GDT transitions from a high-impedance off to a conduction state when a
slowly rising d.c. voltage up to 2 kV/s is applied
Note 1 to entry: The rate of rise for d.c. sparkover voltage measurements is usually equal or less 2 000 V/s.
3.1.6
d.c. holdover
state in which a GDT continues to conduct after it is subjected to an impulse sufficient to
cause breakdown
Note 1 to entry: In applications where a d.c. voltage exists on a line. Factors that affect the time required to
recover from the conducting state (current turn-off time) include the d.c. voltage and the d.c. current
3.1.7
d.c. holdover voltage
maximum d.c. voltage across the terminals of a gas discharge tube under which it may be
expected to clear and to return to the high-impedance state after the passage of a surge,
under specified circuit conditions
3.1.8
discharge current
current that flows through a GDT after sparkover occurs
Note 1 to entry: In the event that the current passing through the GDT is alternating current, it will be r.m.s. value.
In instances where the current passing through the GDT is an impulse current, the value will be the peak value.
3.1.9
discharge voltage
residual voltage of an arrester
peak value of voltage that appears across the terminals of a GDT during the passage of GDT
discharge current
– 8 – 61643-312 © IEC:2013
3.1.10
discharge voltage current characteristic
V/I characteristic
variation of peak values of discharge voltage with respect to GDT discharge current
Figure 1c Figure 1a
v
v
V
s
G
V
g
V
e
A
V
a
i
t
A
G
Figure 1b
i
t
IEC  527/13
Legend
V spark-over voltage V arc voltage G glow mode range
s a
V glow voltage V extinction voltage A arc mode range
gl e
Figure 1a – Voltage at a GDT as a function of time when limiting a sinusoidal voltage
Figure 1b – Current at a GDT as a function of time when limiting a sinusoidal voltage
Figure 1c – V/I characteristic of a GDT obtained by combining the graphs of voltage and current
Figure 1 – Voltage and current characteristics of a GDT
3.1.11
extinction voltage
voltage at which discharge (current flow) ceases
3.1.12
fail-short
failsafe
thermally-activated external shorting mechanism

61643-312 © IEC:2013 – 9 –
3.1.13
follow (on) current
current that the GDT conducts from a connected power source after sparkover
Note 1 to entry: The GDT is expected to extinguish after sparkover to avoid overheating
3.1.14
gas discharge tube
GDT
gap, or several gaps with two or three metal electrodes hermetically sealed so that gas
mixture and pressure are under control, designed to protect apparatus or personnel, or both,
from high transient voltages
3.1.15
glow current
glow mode current
current that flows after breakdown when the circuit impedance limits the follow current to a
value less than the glow-to-arc transition current
Note 1 to entry: See Figure 1a region G.
3.1.16
glow-to-arc transition current
current required for the GDT to pass from the glow mode into the arc mode
Note 1 to entry: See Figure 1a region G.
3.1.17
glow voltage
glow mode voltage
peak value of voltage drop across the GDT when a glow current is flowing
Note 1 to entry: See Figure 1a, region G.
3.1.18
impulse sparkover voltage
highest value of voltage attained by an impulse of a designated voltage rate-of-rise and
polarity applied across the terminals of a GDT prior to the flow of the discharge current
3.1.19
nominal d.c. sparkover voltage
voltage specified by the manufacturer to indicate the target value of sparkover voltages of a
particular type of GDT products
Note 1 to entry: The nominal value is generally a rounded number such as: 75 V, 90 V, 150 V, 200 V, 230 V,
250 V, 300 V, 350 V, 420 V, 500 V, 600 V, 800 V, 1 000 V, 1 200 V, 1 400 V, 1 800 V, 2 100 V, 2 700 V, 3 000 V,
3 600 V, 4 000 V et 4 500 V
Note 2 to entry: Values in between should be agreed jointly between the manufacturer and the user.
3.1.20
sparkover
breakdown
abrupt transition of the gap resistance from practically infinite value to a relatively low value

– 10 – 61643-312 © IEC:2013
3.2 Symbols
A
A
C
C B
IEC  528/13 IEC  529/13
Figure 2 – Symbol for a two-electrode GDT Figure 3 – Symbol for
a three-electrode GDT
Figures 2 and 3 show the symbols for two- and three-electrode GDTs.
4 Service conditions
4.1 General
The basic GDT is relatively insensitive to temperature, air pressure and humidity. GDTs fitted
with a fail-short mechanism have a lower high temperature rating due to the thermal nature of
the fail-short. Manufacturer’s guidelines shall be followed when soldering fail-short
mechanism GDTs to avoid premature operation of the shorting mechanism. For reference,
standardised values and ranges of temperature, air pressure and humidity are given in
Subclauses 4.2 to 4.5.
4.2 Low temperature
GDT shall be capable of withstanding IEC 60068-2-1, test Aa –40 °C, duration 2 h, without
damage. While at –40 °C, the GDT shall meet the d.c. and impulse sparkover requirements of
Table 1.
4.3 Air pressure and altitude
Air pressure is 80 kPa to 106 kPa.
These values represent an altitude of +2 000 m to –500 m respectively.
4.4 Ambient temperature
For the purposes of Subclause 4.4, the ambient temperature is the temperature of the air or
other media, in the immediate vicinity of the component.
operating range (GDTs without failsafe): –40 °C to +90 °C
operating range (GDTs with failsafe): –40 °C to +70 °C
NOTE This corresponds to class 3K7 in IEC 60721-3-3.
storage range (GDTs without failsafe): –40 °C to +90 °C
storage range (GDTs with failsafe): –40 °C to +40 °C

61643-312 © IEC:2013 – 11 –
4.5 Relative humidity
In this clause the relative humidity is expressed as a percentage, being the ratio of actual
partial vapour pressure to the saturation vapour pressure at any given temperature, 4.4, and
pressure, 4.3.
normal range: 5 % to 95 %
NOTE This corresponds to code AB4 in IEC 60364-5-51.
5 Mechanical requirements and materials
5.1 General
Clause 5 lists standardised requirements for terminations, solderability, radiation and marking.
The radiation requirement is a key item to check as GDTs containing radio active elements
are still manufactured.
5.2 Robustness of terminations
If applicable, the user shall specify a suitable test from IEC 60068-2-21.
5.3 Solderability
Solder terminations shall meet the requirements of IEC 60068-2-20, test Ta, method 1.
5.4 Radiation
Gas discharge tubes shall not contain radioactive material.
5.5 Marking
Legible and permanent marking shall be applied to the GDT as necessary to ensure that the
user can determine the following information by inspection:
Each GDT shall be marked with the following information
– nominal d.c. sparkover voltage
– date of manufacture or batch number
– manufacturer name or trademark
– part number
– safety approval markings
NOTE 1 The necessary information can also be coded.
When the space is not sufficient for printing this data, it should be provided in the technical
documentation after agreement between the manufacturer and the purchaser.
6 General
Due to the high complexity of the gas discharge physics on which the functioning of the GDTs
is based, the performance of the GDTs depends very much on the technical expertise of the
manufacturer. Thus the electrical properties and characteristics (tolerances, ignition values,
etc.) are varying.
– 12 – 61643-312 © IEC:2013
7 Construction
7.1 Design
The GDTs consist of two or more metallic electrodes that are separated by gap(s) in a
hermetically sealed envelope containing an inert gas or mixture of gases, usually at less than
atmospheric pressure. Some of the gases used are argon, helium, hydrogen, and nitrogen.
Electrode spacing is maintained by means of ceramic, glass, or other insulating materials,
that may form a part of the sealed envelope. The electrodes are fitted with a variety of
terminations suitable for mounting on circuit boards, clip terminals, sockets, or for
incorporation in a protector.
7.2 Description
The electrical properties of an open gas-discharge path depend greatly on environmental
parameters such as gas type, gas pressure, humidity and pollution. Stable conditions can only
be ensured if the discharge path is shielded against these environmental influences. The
design principle of GDTs is based on this requirement. A proven technique of connecting
insulator and electrode ensures hermetic sealing of the discharge space.
The type and pressure of the gas in the discharge space can thus be selected on the basis of
optimum criteria. The rare gases argon and neon are predominantly used in gas arresters
since they ensure optimum electrical characteristics throughout the entire useful life of the
component.
An activating compound is applied to the effective electrode surfaces to enhance the emission
of electrons. The electrodes are typically separated by less than 1 mm. The combination of
the activation compound and the electrode separation distance lower the electrode work
function and increase the ignition voltage stability over repetitive current surges.
To achieve optimal response characteristic at fast rise times, ignition aids are attached to the
cylindrical internal surface of the insulator. These ignition aids distort the electric field, which
enhances the ionization speed of the gas. The electrical characteristics of the GDT, such as
d.c. spark-over voltage, pulsed and a.c. discharge current handling capability as well as its
service life, can be optimized to the specific requirements of various systems. This is
achieved by varying the gas type and pressure as well as the spacing of the electrodes and
the emission-promoting coating of the electrodes.
Figure 4 and Figure 5 show construction examples of two- and three-electrode GDTs.

Centre electrode “c”
Activating compound
Activating compound
Electrode “b”
Electrode
Electrode Electrode “a”
Discharge space
Insulator Ignition aid
Ignition aid
Ignition aid
Insulator
IEC  717/13 IEC  718/13
Figure 4 – Example of a Figure 5 – Example of a
two-electrode GDT three-electrode GDT

61643-312 © IEC:2013 – 13 –
7.3 Fail-short (failsafe)
GDTs are usually designed for pulse-shaped loads. If permanent overloads are possible (e.g.
mains contact), GDTs with integrated failsafe should be used. This external short-circuit
mechanism prevents the generating of excessive thermal energy of the operating GDT by
bridging it.
The failsafe mechanism usually consists of a mechanical short-circuit spring and a
temperature sensitive spacer, which prevents the bridging of the GDT until a defined
temperature is reached.
The fail-short mechanism performance is dependent on its thermal environment. The
soldering profile used for the GDT could be critical. Recommendations made by the
manufacturer for mounting and processing should be followed. The fail-short spacer, used to
keep the switch open, has typical melting temperatures of >200 °C for solder spacer types.
For plastic foil spacer types, typical melting temperatures are 140 °C or 260 °C depending on
their composition. If an inappropriate soldering profile and mounting arrangement used the
spacer will melt and the GDT will be shorted after soldering. When a permanent current
overload occurs the GDT temperature rise operates the fail-short switch. Caution should be
used in the coordination between the soldering temperature of the GDT to the board and the
operating temperature of the fail-short mechanism to avoid desoldering of the GDT. Under
current overload conditions the GDT thermal radiation to adjacent components is another
factor to be considered.
Failsafe constructions are available for two- and three-electrode GDTs. For three-electrode
GDTs two examples are shown in Figures 6 and 7.
Short-circuit spring Solder pill
Short-circuit spring
Solder pill
Not activated Activated
IEC  719/13 IEC  720/13
Figure 6 – Failsafe construction of a three-electrode GDT
using a solder pill as sensitive spacer
Foil Foil
Short-circuit spring Short-circuit spring

Not activated Activated
IEC  721/13 IEC  722/13
Figure 7 – Failsafe construction of a three-electrode GDT,
using a plastic foil as sensitive spacer

– 14 – 61643-312 © IEC:2013
8 Function
8.1 Protection principle
Generally, a spark-over occurs whenever surge voltages exceed the electric strength of a
system’s insulation. To prevent this system sparkover, a GDT with appropriate voltage limiting
capabilities needs to be installed. A surge event exceeding the GDT spark-over voltage
causes it to conduct, entering first into the glow mode, which in turn begins to limit the surge
voltage magnitude. As the current increases the GDT then transitions from the glow mode to
its arc mode. This further limits and lowers the surge voltage to around 10 to 35 V depending
on the GDT technology. GDTs utilize this natural principle of limiting surge voltages. For the
test circuits used to determine the parameters of a GDT see IEC 61643-311.
8.2 Operating mode
A simplified GDT can be compared with a symmetrical low-capacitance switch whose
resistance may jump from several Gduring normal operation to values 1  after ignition
caused by a surge voltage. The GDT automatically returns to its original high-impedance state
after the surge has subsided.
Figure 1a shows the voltage curve at the GDT and Figure 1b the current as a function of time
when limiting a sinusoidal voltage surge. Virtually no current flows during the time that the
voltage rises to the spark-over voltage V of the GDT. After ignition, the voltage drops to the
s
glow voltage level V (70 to 200 V depending on the type, with a current of several 10 mA up
gl
to about 1,5 A) in the glow-mode range G. As the current increases further, transition to arc
mode A occurs. The extremely low arc voltage V of 10 to 35 V typical for this mode is
a
virtually independent of the current over a wide range. With decreasing over-voltage (i.e. in
the second half of the wave), the current through the GDT decreases accordingly until it drops
below the minimum value necessary to maintain the arc mode.Consequently, the arc
discharge stops suddenly and, after passing through the glow mode, the GDT extinguishes at
a voltage V .
e
The V/I characteristic of the GDT shown in Figure 1c was obtained by combining the graphs
of voltage and current as a function of time.
8.3 Response behaviour
8.3.1 Static response behavior
If a voltage with a low rate of rise (typically 100 V/s) is applied to the GDT, the spark-over
voltage Vs will be determined mainly by the electrode spacing, the gas type and pressure, and
by the degree of pre-ionization of the enclosed noble gas. This ignition value is defined as the
d.c. spark-over voltage.
8.3.2 Dynamic response behavior
At fast rate of rise, the spark-over voltage Vs of the GDT exceeds d.c. spark-over voltage.
This effect is caused by the finite time necessary for the gas to ionize. All these dynamic
sparkover voltages are subject to considerable statistical variation.
However, the average value of the spark-over voltage distribution can be significantly reduced
by attaching the ignition aid to the inside surface of the GDT. This reduces the upper limit of
the tolerance field considerably and also limits the spread of the spark-over voltage. The
ignition voltage in this dynamic range is defined as the impulse spark-over voltage.
In general the two voltage rates of rise of 100 V/s and 1 kV/s are used to evaluate the
dynamic characteristic of surge arresters (Figure 8).

61643-312 © IEC:2013 – 15 –
1 200
V
10 kV/µs
1 000
1 kV/µs
V 100 V/µs
S
100 V/s
Max.
Min.
2 4 6 8 10
10 10 10 10 10
V/s
dv/dt
Static response
Dynamic response
IEC  723/13
Figure 8 – Typical response behaviour of a 230 V GDT
Figure 9 shows an example of the correlation between the response time and the spark-over
voltages.
1 000
1 kV/µs 100 V/µs
V
V
S
Impulse
spark-over
voltage
100 V/s
DC spark-over
voltage
0 2 4 6 8 µs 0,5 1 1,5 2 s 2,5
t
Dynamic response
Static response
IEC  724/13
Figure 9 – Spark-over voltages versus response time
8.4 Fail-short (failsafe)
In the case of influences such as a direct contact between the power and telecommunication
lines, current will flow through the ignited arrester for a long period of time. The GDT then

– 16 – 61643-312 © IEC:2013
heats up. When this happens, the hardware must be protected from thermal overload. The
heating is detected by a fail-short (failsafe) mechanism. A spacer (solder pellet, plastic foil or
mechanical device), which initially keeps the short-circuit spring at a distance from the
electrodes, melts at a temperature determined by the choice of material used. The short-
circuit spring, which is pre-stressed, then drops onto the electrodes and short-circuits them.
Furthermore, careful consideration must be given to long term power fault events that can
cause GDT heating causing loss of the solder connections to the circuit board, before the
operating temperature of the fail-short mechanism is reached.
Figure 10 shows a typical short-circuit characteristic as a function of the current flowing
through the GDT. This characteristic can be affected by the thermal conductivity of the holder.
Therefore the coordination between component and package must be subsequently verified
by a type test.
I  (A)
Time to short-circuit  (s)
IEC  725/13
Figure 10 – Current through the GDT versus response time of fail-short (failsafe)
9 Applications
9.1 Protective circuits
9.1.1 General
The following basic circuits illustrate standard configurations for GDTs used in protection
circuits for the telecommunications sector. 2-point and 3-point protection solutions typically
contain GDTs only, whereas 5-point protection solutions can make additional use of current-
limiting components such as PTC thermistors, heat coils, fuses, or electronic current limiters.
NOTE 1 Designations a and b define the input side. Designations a' and b' define the protected side.
NOTE 2 In some cases series fuses are used to avoid excessive current flow in front of the GDTs (input side).
9.1.2 2-point (signal line) protection
A 2-point protection circuit is connected between A/C wires and operate by limiting the
voltage between A/C and conducting the current from A to C. 2-point (signal) circuits are
often run with no ground conductor. A two-electrode GDT circuit located between the two

61643-312 © IEC:2013 – 17 –
signal lines prevents the formation of large potential differences at the input of the equipment
to be protected before they can cause any damage (Figure 11).

A
PD
GDT
C
IEC  726/13
Components
PD protected device
Figure 11 – 2-point (Signal line) protection
9.1.3 3-point protection
3-point protection circuits are connected between the a/b wires and ground and operate by
conducting voltage surges to ground and conducting voltage surges between a and b. Both
two-electrode and three-electrode GDTs are used (Figures 12 and 13).
a a
A
GDT
A
C
c
c
GDT
C
C
B
GDT
A
b b
IEC  727/13 IEC  728/13
Figure 12 – 3-point protection using Figure 13 – 3-point protection using
two-electrode GDTs three-electrode GDTs
Figures 14 and 15 show another alternative using a GDT with fail-short mechanism

– 18 – 61643-312 © IEC:2013
a a
FS FS
A
GDT
ϑ ϑ
A
C
c
c
GDT
C
C
B
GDT
ϑ ϑ
A FS FS
b
b
IEC  729/13 IEC  730/13
Components Components
FS fail short (failsafe) mechanism FS fail short (failsafe) mechanism
Figure 14 – 3-point protection using Figure 15 – 3-point protection using
two-electrode GDTs with fail-short three-electrode GDTs with fail-short
9.1.4 5-point protection
A 5-point protection circuit contains a current-limiting component, usually a PTC thermistor, in
addition to the GDT. The thermistor blocks further current flow through it by assuming a very
high resistance in the event of an overcurrent (see Figures 16 and 17). However, it may not
always be possible to reset an activated thermistor in systems with constant current feed.
a a'
a a'
ϑ
ϑ
A
GDT
A
C
c
c
GDT
C
C
B
GDT
A
b b'
b b'
ϑ
ϑ
IEC  731/13 IEC  732/13
Figure 16 – 5-point protection Figure 17 – 5-point protection
using two-electrode GDTs using three-electrode GDTs
Figures 18 and 19 show another alternative using a GDT with fail-short mechanism

61643-312 © IEC:2013 – 19 –
a a' a a'
ϑ ϑ
FS
FS
A
GDT ϑ
ϑ
A
C
c
c
GDT
C
C
B
GDT
ϑ ϑ
A FS FS
b b' b b'
ϑ ϑ
IEC  733/13 IEC  734/13
Components Components
FS fail short (failsafe) mechanism FS fail short (failsafe) mechanism
Figure 18 – 5-point protection using Figure 19 – 5-point protection using
two-electrode GDTs with fail-short three-electrode GDTs with fail-short
9.2 Telephone/fax/modem protection
Telephones, faxes and modems are increasingly being equipped with sophisticated
electronics. Typical circuits used to protect them with GDTs are shown in Figures 20 and 21.
In the event of an overvoltage, the GDT protects both exchange lines by conducting the surge
current away to ground.
a
a
A
A
PD
PD GDT
GDT
C
C
B
b b
A
GDT
C
IEC  735/13 IEC  736/13
Components Components
a tip a tip
b ring b ring
PD protected device PD protected device
Figure 20 – Telephone/fax/modem Figure 21 – Telephone/fax/modem
protection using two-electrode GDTs protection using three-electrode GDTs
9.3 Cable TV/coaxial cable protection
GDTs are particularly well suited for protecting coaxial cables frequently laid in CATV
networks, as they do not disturb the system even at high frequencies thanks to their low self-
capacitance of typically 0,5 to 1 pF. The GDT is contained in the coaxial protection module

– 20 – 61643-312 © IEC:2013
where it is connected between the central conductor and the shielding. It is recommended to
ground either the shielding or the housing of the prote
...