Achieving Compliance with ITU-T K.20 and K.21

Phillip Havens

A simple solution is offered for addressing the newest technical requirements for coordination between the primary protectors and the secondary protectors.

The International Telecommunication Union (ITU) is a specialized agency of the United Nations devoted to international harmonization. The ITU Telecommunication Standardization Sector (ITU-T) produces the K series of recommendations covering protection against interference. Recommendations are nonbinding standards, which allows modifications to the recommendation to cover local conditions, such as testing with the local ac supply of 120 V rather than the recommendation value of 230 V. Most European countries recognize the ITU-T standards.

The most recent revisions of ITU-T K.20 (February 2000) and ITU-T K.21 (October 2000) added technical requirements for coordination between the primary protectors and the secondary protectors. A previous article provided a thorough overview of the standards.1 This article examines the surge and power-fault requirements as called out in the updated ITU-T standards. A schematic solution for achieving compliance with these two standards is discussed in this article.

Both ITU K.20 and ITU K.21 address the protection of telecommunications equipment. Such telecommunications equipment includes T1/E1 lines, x-type digital subscriber
line (xDSL) technologies, integrated services digital networks (ISDN), Ethernet links, or just plain old telephone service (POTS). The physical location of the equipment determines whether K.20 or K.21 is observed.

ITU-T K.20 and K.21 Surge and Power-Cross Requirements

Specifically, ITU-T K.20 addresses the need for telecommunications equipment installed in a telecommunication center to resist overvoltages and overcurrents. This equipment is more often thought of as central office (CO) equipment, but it can also include remote terminals. ITU-T K.21 addresses the need for telecommunications equipment installed in customer premises to resist overvoltages and overcurrents. Both of these standards refer to ITU-T K.44, "Resistibility Tests for Telecommunication Equipment Exposed to Overvoltages and Overcurrents--Basic Recommendation." The telecommunications equipment installed in a CO must also comply with the earthing and bonding requirements specified in ITU K.27.

The sources of these overvoltage and overcurrent events include lightning, short-term induction of ac from nearby power lines, and ground potential rise due to power faults. Overvoltage and overcurrent events also result from direct contact between telecommunications lines and power lines. Both ITU-T K.20 and ITU-T K.21 have a protection-level requirement termed basic and a more severe protection-level requirement termed enhanced.

Basic resistibility is intended for equipment having a low exposure to interference due to the nature of its environment. Enhanced resistibility is intended for equipment having a high exposure to interference. The selection of basic or enhanced resistibility is determined by either the company's administration or by the network operators. Local laws and covenants may also influence the selection of the appropriate protection level. The resistibility requirements for both basic and enhanced protection levels have two acceptance criteria:

• Criterion A. The tested equipment shall operate properly after the tests.

• Criterion B. The tested equipment shall not present a fire hazard as a result of the tests.

The basic requirements of ITU-T K.20 have surge and power-fault test conditions that are applied in metallic mode (also known as differential mode or transverse mode) and longitudinal mode (also known as common mode). Some of the surge and power-fault tests are applied only when primary protectors are not used and some test conditions are not applied when primary protectors are always used. A summary is provided in Table I.

The enhanced ITU-T K.20 requirements increase the surge- current rating resistibility and add an "A" acceptance criterion to certain levels of the mains power-contact test. A summary of these requirements is provided in Table II.

The ITU-T K.21 requirements for customer premises equipment are slightly more demanding than the ITU-T K.20 requirements. The basic level of ITU-T K.21 is similar to the enhanced level of ITU-T K.20. Because of the low pair count of cables entering a residence, a higher resistibility to surges is needed at customer premises. By contrast, a CO site has many cables bundled together at the point of entry. These parallel wires divide the induced event across all cables, thereby reducing the level on any single port. The customer premises application does not have the advantage of this type of load sharing. A summary of the requirements is provided in Table III.

Circuit Protection for Surge and Power-Cross Requirements

By using appropriately rated SIDACtor-type devices (crowbarring solid-state protectors), equipment can be protected from the lightning events. A SIDACtor device is simply a thyristor without a gate lead. It is a crowbarring device that turns on with voltage and turns off with current. The unique structure and characteristics of the thyristor are used to create an overvoltage protection device with precise and repeatable turn-on characteristics that provide low voltage overshoot and high surge-current capabilities.

The SIDACtor device operates very much like a switch. In the off state, the device exhibits leakage current (IDRM) of less than 5 µA, which makes the leakage current invisible to the circuit that the crowbarring device is protecting. As a transient voltage exceeds its VDRM, the device enters its protective mode with characteristics similar to an avalanche diode. When sufficient current (IS) is supplied, the device switches to an on state, shunting the surge event away from the circuit the device is protecting. The VI curve in Figure 1 demonstrates the device's first- and third-quadrant characteristics (bidirectional capability).

Figure 1. VI characteristics of a SIDACtor (crowbarring) device.

This crowbarring device would be placed in either a combination metallic/longitudinal mode or possibly in metallic mode only. The appropriate choice is made by consideration of both the design parameters and the applicable standards. For example, the standoff voltage level is determined by the steady-state voltage levels of the intended application. A T1/E1 system may have voltages as high as 135 V when it is remotely powered, even though its data signal is typically less than 3 V. Therefore, the standoff voltage of a crowbarring device would have to be greater than 135 V. However, if compliance with TIA-968-A is also required, then the minimum switching voltage would be 170 V for any component placed in a longitudinal mode. Therefore, the appropriate choice of the crowbarring device for a T1/E1 application depends on:

• Steady-state conditions, which provide guidance in choosing VDRM parameter (remote dc powering levels).

• Applicable regulatory requirements (TIA-968-A), which may require minimum levels for the VDRM parameter.

• Necessary surge robustness, which requires reviewing regulatory requirements ITU-T K.20/21 to determine whether to use an A-, B-, or C-rated SIDACtor device.
   

The power-fault events require careful selection of overcurrent protection. This overcurrent protection must be a surge-tolerant device. The acceptance criterion level "A" does not allow a permanent open (nonoperational) condition after the surge events. A device such as an F1250T TeleLink fuse (Teccor/Littelfuse) will not open during these surge events.

The enhanced power-cross requirement of ITU-T K.21 (1500-V, 7.5-A test condition) also requires this "A" level of compliance for the equipment. Coordination is a requirement of ITU-T K.20/21 when a primary protector is used. Some examples of coordination include:

• Use of a high-wattage-rated line-feed resistor placed between the primary and the secondary protector.

• Proper selection of switching-voltage parameters of the secondary protector as compared with the primary protector (higher switching-voltage [VS] parameter for the secondary protector).

• Guaranteed minimum length of wire, which results in sufficient resistance or transmission delay between the primary and the secondary protectors to ensure that the primary protector will turn on.

The diagram in Figure 2 shows a sample schematic that indicates both the overvoltage and the overcurrent protection necessary for a T1/E1 application. The two fuse elements (F1250T Telelink) provide overcurrent protection by opening (interrupting) the current flow during power-cross events that exceed the I2t rating. Two devices are required because two separate paths to ground are possible. A path from tip to ground and a path from ring to ground exist, and both paths must be protected.

Figure 2. This sample schematic shows both the overvoltage and overcurrent protection for a T1/E1 application.

The A2106 SIDACtor device provides the overvoltage protection. During a metallic (differential) surge event, the A2106 crowbars at a 50-80-V level and protects the equipment downstream. During a longitudinal (common mode) event, the A2106 crowbars at a 170-250-V level and protects the equipment downstream. It is important to note that the SIDACtor device remains in the off-state mode during normal operating conditions (with the remote power feed active and with data being transmitted). The secondary protector device (P0080SA MC) located on the integrated circuit (IC) side of the transformer provides a lower threshold. A lower threshold can be used behind the transformer because it is isolated from the dc power supply.

Conclusion

The ITU-T K.20 and K.21 standards outline requirements for resistibility to overvoltages and overcurrents in telecommunications equipment. Equipment installed either in a central office environment or at customer premises locations may require compliance to ITU-T K.20 or to ITU-T K.21, respectively.

With the addition of technical requirements for coordination between the primary protectors and the secondary protectors in the most recent revisions of K.20 and K.21, it is important to understand the steps necessary for achieving compliance. This article has provided a succinct solution for addressing both overvoltage and overcurrent conditions. Both conditions can be managed by inserting two overcurrent devices such as the F1250T (Telelink) fuse and by inserting two overvoltage protective devices such as the A2106UC (SIDACtor) and the P0080SA MC.

Together, these three devices help provide protection for the sensitive ICs so that the telecommunications equipment remains functional (for criterion A) and safe (for criterion B) during and after surge and power-cross events. The same concepts discussed here can be used for xDSL, 10 BaseT, or any other telecom equipment.

Designers must consider several parameters when selecting the appropriate protection levels. First, they must take into account the exposure to metallic (differential) surge and power-cross events as well as longitudinal (common mode) surge and power-cross events. Second, the standoff levels of the overvoltage protection are determined by two factors: the steady-state levels of the system being protected and any specific regulatory requirements. Finally, the required standard and the intended compliance level (basic or enhanced) determine the surge rating of the overvoltage device.

Reference

1. Mick J Maytum, "The New ITU-T Telecommunication Equipment Resistibility Recommendations," Compliance Engineering 19, no. 1 (2002): 30-37. Also available on the Internet at http://www.ce-mag.com/archive/02/01/Maytum.html.
 
Phillip Havens is a technical product management engineer for Teccor Electronics/Littelfuse (Irving, TX). He can be reached via e-mail at phavens@littelfuse.com or at 972-518-9427.