The New ITU-T Telecommunication Equipment Resistibility Recommendations
Designers and test houses are presented with increased resistibility
levels and testing complexity from these new equipment recommendations.
the new millennium, a revised set of telecommunication recommendations
for equipment resistibility against overvoltages and overcurrents came
into force from the International Telecommunication UnionTelecommunication
Standardization Sector (ITU-T). Beginning in February 2000, four recommendations,
totaling 139 pages, replaced the previous 32 pages of the 1996 recommendations.14
The changes are substantial and could result in previously compliant
equipment becoming nonconforming and presenting a more challenging protection
Applying the previous recommendations revealed unfulfilled needs, sometimes
manifested as field-failure problems. These unfulfilled needs have been
addressed in the new recommendations, by expanding or adding sections
that cover all equipment ports, the access class of equipment, equipment
with embedded primary protection, primary-secondary coordination verification,
and additional (enhanced) levels of resistibility.
These recommendations now cover all equipment connected to twisted-pair
or coax cable anywhere between the telecommunications center and the customer
premise. Only repeaters are covered elsewhere (recommendation K.17). In
the new recommendations, testing covers ports connected to external symmetric-pair
cables, coaxial cables, and dedicated power feeds, as well as mains power
ports. This article provides an overview of testing ports connected to
The document is divided into four recommendations: K.44 is the test-circuit
standard. K.20, K.21, and K.45 give the performance levels required
for different telecommunications equipment locations. Recommendation
K.20 covers equipment located at a telecommunications center (central
office), and K.21 covers equipment located at customer premises (subscribers).
Recommendation K.45 addresses access equipment, which includes any equipment
located between the telecommunications center and the customer premises.
Although no single application guide is currently available, several
support documents are given in the normative references of these standards.
The ITU-T recommendations do not use the dictionary definition of resistibility
(to remain undamaged by the action). Rather, resistibility is defined
as "to withstand with or without damage." Hence, K.44 presents two test-outcome
criteria: criterion A (withstand without damage) and criterion B (the
equipment, although damaged, shall not cause a fire hazard).3
Equipment resistibility testing must comprise all overstress levels
up to the maximum specified. Two testing approaches are available to
choose from: either test at a large number of incremental levels, or,
based on the equipment, test at specific levels that maximize the stress
on particular components, e.g., just before the inherent equipment protection
starts to limit.
The new recommendations specify six test generators: three voltage
surge (10/700, 1.2/50, 2/10), one current surge (8/20), one combination
(1.2/508/20), and one ac. In addition, a supplementary appendix
provides 2/10 and 10/350 current-surge-generator diagrams. The International
Electrotechnical Commission's electrostatic discharge (ESD) standard is
referenced as the ESD test method.5
The testing of symmetric-pair cable ports uses the 10/700 voltage-surge
generator, the 8/20 current-surge generator (for equipment containing
high-current-carrying components), and the ac generator. Ground wire voltage-drop
testing is under study, and the possible use of the Telcordia GR-1089-CORE
2/10 ground-coupling test is mentioned.
Test Circuits and Testing Approach
In the new recommendations, test configurations are now consistent
(see Figures 1 and 2). Single-port testing is done both transversely
(differential mode, metallically) and longitudinally (common mode) on
equipment that does not contain high-current-carrying components. Multiple-port
testing is done only longitudinally. Equipment that contains high-current-carrying
components (which eliminate the need for primary protection) is tested
only longitudinally with the 8/20 current-surge generator.
Figure 1. Transverse testing, single port.
(Decoupling, biasing, and terminations are not shown.)
|Figure 2. Longitudinal testing, single port.
(Decoupling, biasing, and terminations are not shown.)
K.20 ac testing has been unified with K.21 by including three transverse
ac-test configurations. The recommendations include new tests for both
K.20 and K.21: a multiple-port longitudinal impulse test with the special
agreed primary protector fitted and tests for equipment that contains
primary-protection components. Primary-equipment protection coordination
now must be verified during longitudinal- and transverse-impulse testing.
Two resistibility levels are specified for equipment. The test and
acceptance levels provided in the 1996 recommendations are incorporated
mostly into the basic resistibility requirement in the new recommendations.
For more severe conditions, the new recommendations provide a higher-level
enhanced resistibility requirement. It is important to note that this
enhanced requirement concerns manufacturers. With two levels available,
customers may tend to opt for the enhanced-level performance, while
expecting to pay no more than for equipment that meets the basic resistibility
level. Four protector definitions are provided:
Primary: This is a surge-protective device placed at a
location to prevent most of the overstress from reaching the equipment.
The device must be maintainable and connected to equipotential bonding.
Agreed primary: This is the manufacturer and network operator's
agreed-upon protector for use with the equipment. In cases in which
the equipment needs no primary protector, the agreed-upon primary
protection can be no protection at all.
- Special agreed primary or special test protector: This device
is an end-of-life, agreed-upon primary protector with maximum voltage
and current let-through values used for primary-equipment protection
High-current-carrying components: Such
components are used in primary surge-protective devices or sometimes
as the built-in inherent protection of the equipment.
The K.20 multiple-port longitudinal-impulse test voltage has been increased
from 1.0 to 1.5 kV. The multiple-port longitudinal-impulse test in now
repeated at 4.0 kV with the special agreed primary protector and coordination
verified. Some enhanced tests increase the voltage level to 6 kV (from
4 kV previously).
K.20, K.21, and K.45 provide the impulse tests for symmetric-pair cables
Table I ). The table includes the number of ports tested,
recommendation test number, test type, stress levels, number of
tests, primary protection used, acceptance criterion, and general
notes. When the stress levels of adjacent columns are the same,
the table cells have been merged. This table provides a way to
compare quickly the similarities and differences between K.20,
K.21, and K.45.
Inherent Protection, 10/700 Voltage Surge. This testing is straightforward,
but the extra-high-voltage 6-kV enhanced longitudinal test in K.21 should
be noted. Field returns of modems have shown electromechanical relay
arc failures between their contacts and coils. These relays arced at
4.5 kV, so the 6-kV test level is a realistic value. Fitting overvoltage
protective devices to ground will certainly reduce the isolation voltage
stress level. Reducing the isolation voltage stress level may have been
the thinking behind reducing the test value to the general 1.5-kV level
for this type of equipment.
The environmental stress, however, does not change because the equipment
has inherent overvoltage protection to ground. Prudent designers should
check the performance of any series elements that arc before the overvoltage
protection at the 6-kV level. Many components, such as positive-temperature-coefficient
(PTC) thermistors, typically do not specify their maximum impulse withstand.
With overvoltage protection devices to ground fitted, most of the 6-kV
10/700 generator output voltage appears across any series overcurrent
element (e.g., a PTC thermistor). To survive, the PTC thermistor must
be rated for this 6-kV 10/700 condition.
Coordination, 10/700 Voltage Surge. This is a new requirement.
This testing usually requires testing at least three generator levels:
maximum protected component stress, maximum inherent protection stress,
and coordination verification at the highest stress level (4 kV or 6
kV). The testing must maximize the stress in three circuit areas: the
protected equipment components, the equipment's protection circuit,
and the primary protector. The equipment components must be tested with
an equipment voltage just below the voltage threshold of the equipment's
protection circuit. The protection circuit must be tested just below
the voltage threshold of the primary protector, and the primary protector
must be tested at the maximum generator voltage specified.
Figure 3 is based on the test circuits shown in K.44's Figures I.1-1
and I.1-10. Unfortunately, the K.44 editorial process allocated some
wrong waveforms to these circuits. This article shows some actual measured
waveforms that occur with the circuit.
Figure 3. Coordination and ac evaluation circuit.
Figure 3 shows the ac and 10/700 generators that connect to the equipment.
As in K.44, the nominal 230-V sparkover gas discharge tube (GDT) is
replaced with a selected 350-V GDT (2027-35). This is the special test
protector. The 28-W coordinating resistance
is formed by a polymer PTC thermistor (MF-SM013/250-2) and a line-feed
resistor (4B08B-BM1-000). Inherent overvoltage protection is provided
by a unidirectional buffered-gate silicon-controlled-rectifier (SCR)
thyristor, whose switching voltage tracks the subscriber-line interface
circuit (SLIC) supply voltage (50 V in this example). The use
of a buffered-gate SCR prevents an overvoltage in the SLIC due to excessive
gate current.6 The protected SLIC
is represented by a 300-W resistor.
Setting the 10/700 generator to 53 V results in 50 V across
the SCR, which is just above its switching voltage (52 V).
This is the maximum long-term voltage stress condition for the
SLIC (dark green line in Figure
4). In the positive polarity, the SCR diode always limits
the SLIC voltage to a few volts. The positive-polarity SLIC stress,
therefore, is only minimally dependent on the generator voltage.
|Figure 5. GDT just sparking over and maximum
dynamic sparkover voltage.
Increasing the generator voltage to ±1000 V sets the voltage to
just below the GDT sparkover voltage for this surge waveform (blue line),
and this condition represents the maximum stress on the inherent protection.
The voltage across the coordination resistance will be the difference
of the GDT and SCR voltages, amounting to a ±370 V 10/700 impulse.
These components should be rated for this condition, particularly for
designs that use only a PTC thermistor.
Increasing the generator voltage to ±1050 V causes the GDT to
spark over at ±390 V after a 35-microsecond delay. Coordination
first occurs at this point. At the 4-kV generator setting, coordination
is shown, and the GDT sparkover voltage rises to 640 V. The coordinating
components must not break down under this short-term condition. For
the coordinating component, specifying a 2/10 impulse voltage rating
in the region of 1000 V should be considered.
The lowest value of coordinating resistance comes from the longitudinal
test. Here, one of the primary protectors could spark over first, placing
the 25-W feed resistance in shunt with the generator output and
reducing the available voltage to the other primary. For a 390-V sparkover
and a 4-kV test, the lowest coordination resistance is 6.4 W.
The enhanced level (6 kV) in K.21 would need a 4-W resistance.
However, this resistance would result in equipment that coordinated
at the enhanced level (6 kV), but failed at the basic (4 kV) level.
Multiple-Port Testing, 10/700 Voltage Surge. Because the generator
output is resistively divided, the individual port stress is generally
lower for multiple-port testing than for
single-port testing unless the inherent overvoltage protector is shared
across several lines. This test will result in the highest ground-return
Single- and Multiple-Port Testing, 8/20 Longitudinal Current Surge.
The 8/20 current generator has a single output. Each tested wire is
fed via a current-limiting resistor and coupling network. These tests
specify a current-limiting resistance of zero, which effectively parallels
all the tested wires. If the inherent protection does not share the
current, then the first wire protector to operate hogs all the current.
The coupling network may also contribute to dissimilar current flows.
Testing should start at the single-wire current value to determine where
that current flows. If the current is found to flow in just one wire,
higher test currents cannot be used.
Ac Test Levels
K.20, K.21, and K.45 present the ac test levels. Table
II shows the ac tests for symmetric-pair cables. The table includes
the number of ports tested, recommended test number, test type, stress
levels, number of tests, primary protection used, acceptance criteria,
and general notes.
Inherent Protection, Power Induction. Inherent protection is
a 600-V rms, 0.2-second, 600-W test applied to the different configurations.
Some equipment designs use a 250-V PTC for series overcurrent protection.
To prevent the PTC from switching into the 600-V source voltage, the
PTC trip time should not be less than 0.2 second.
Coordination, Power Induction. Basic coordination testing is
a 600-V rms, 1-second, 600-W test applied to the different configurations
with the special agreed primary protector. For this test, the 250-V
PTC thermistor will have its switched voltage limited by the special
test protector. Although called a coordination test, a coordination
verification is not specified. Enhanced testing is a series of tests
done with a 200-W generator source with various voltagetime
values set between 1500 V rms for 0.18 second and 450 V rms for 2 seconds.
Figure 6 uses the waveforms from Figure 3 for the 366-V rms test specified
in K.44. After about 0.2 second, the heating from the 2-A peak current
causes the PTC thermistor to switch. The GDT then limits the voltage
and sparks over. In conduction, the GDT limits the power to the PTC
thermistor, which then cools, lowering its resistance. After the next
zero-voltage crossing, the PTC thermistor takes sufficient current (0.2
A) to rise and spark over the GDT. This process repeats on successive
|Figure 6. Polymer PTC partial switching in AC
Power Contact. For power-contact testing, the recommendation
specifies eight source-resistance values ranging from 10 to 1000 W.
A subset of these can be used if the highest stress conditions for the
equipment are known. Enhanced testing also requires that the equipment
meet criterion A for the resistance range of 160 to 600 W.
If the inherent protection conducts during this test, then the easiest
way to meet criterion A is to use a PTC thermistor overcurrent protector.
The new ITU-T recommendations are more demanding than before. One conformance-testing
facility estimated a doubling of test time for the new recommendations.
For example, power-cross testing now involves eight resistor values
instead of three. Lightning testing with the special agreed protector
must now measure the circuit conditions during the coordination test
as well as the equipment afterward. The special agreed protector must
be provided for conformance testing along with the equipment. For equipment
being sold to several network operators, there could be just as many
special agreed protectors. Unless the special agreed protectors could
be rationalized down to a single worst-case protector, the number of
coordination tests required will multiply accordingly. If a new customer
has a special agreed protector worse than the one used for the original
conformance testing, then the coordination tests will need to be repeated.
These changes force the designer to adopt a rigorous design approach
and component specification. Coaxial cables, dedicated power feeds,
and mains power ports have not been covered here because these components
have their own set of tests. As items under study are resolved and feedback
is received on applying the recommendations, revisions of these recommendations
can be expected during the next year or two. Keep a watch on http://itu.int/ITU-T.
1. ITU-T Recommendation K.20, "Resistibility of Telecommunication Equipment
Installed in a Telecommunications Centre to Overvoltages and Overcurrents,"
International Telecommunication Union (ITU), Geneva, February 2000.
2. ITU-T Recommendation K.21, "Resistibility of Telecommunication Equipment
Installed in Customer Premises to Overvoltages and Overcurrents," ITU,
Geneva, October 2000.
3. ITU-T Recommendation K.44. "Resistibility Tests for Telecommunication
Equipment Exposed to Overvoltages and OvercurrentsBasic Recommendation,"
ITU, Geneva, February 2000.
4. ITU-T Recommendation K.45, "Resistibility of Access Network Equipment
to Overvoltages and Overcurrents," ITU, Geneva, February 2000.
5. IEC 61000-4-2, "Electromagnetic Compatibility (EMC)Part 4-2:
Testing and Measurement TechniquesElectrostatic Discharge Immunity
Test," International Electrotechnical Commission, Geneva, April 2001.
6. MJ Maytum et al., "Coordination of Overvoltage Protection and SLIC
Capability" in The 2000 International IC China Conference Proceedings
(Beijing: Global Sources Ltd., March 2000), 8792.
Mick J. Maytum is the applications
director for Power Innovations Ltd., a Bourns company (Bedford, UK).
He can be contacted at email@example.com.