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 symmetric-pair cables.

 

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 (1.2/50–8/20), 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.

 

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:

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.

Click to enlarge

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 currents.

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 voltage–time 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 half cycles.

Figure 6. Polymer PTC partial switching in AC test 2.2.2.

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.

Comments

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.

References

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 Overcurrents—Basic Recommendation," ITU, Geneva, February 2000.

4. ITU-T Recommendation K.45, "Resistibility of Access Network Equipment to Overvoltages and Overcurrents," ITU, Geneva, February 2000.