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| 28 | ITU-T K-Numbers Explained Below are descriptions of popular recommendations listed in the previous table. K.20: "Resistibility of telecommunication equipment installed in a telecommunication center to overvoltages and overcurrents" The ITU-T K.20 Recommendation specifies resistibility requirements and test procedures for telecommunication equipment that is attached to, or installed within a telecommunication center and Central Office (CO). Overvoltage and overcurrent events that are covered by this recommendation include surges due to lightning on or near the line plant, short-term induction from adjacent AC power lines or railway systems, earth potential rise due to power faults, direct contact between telecommunication lines and power lines, and electrostatic discharges (ESDs). Test requirements given by this recommendation are somewhat less severe than the tests required for the customer premises equipment ITU-T K.21. The reason is that CO installations typically are well planned and the personnel working there are trained, making this environment more controllable. K.21: "Resistibility of telecommunication equipment installed in customer premises to overvoltages and overcurrents" The ITU-T K.21 Recommendation specifies resistibility requirements and test procedures for telecommunication equipment that is attached to or installed within the customer premises. The scope of this recommendation is for the overvoltage and overcurrent events just as in the previously cited ITU-T K.20. K.45: "Resistibility of telecommunication equipment installed in the access and trunk networks to overvoltages and overcurrents" The ITU-T K.45 Recommendation specifies resistibility requirements and test procedures for telecommunication equipment installed between telecommunication centers and between a telecommunication center and the customer premises. It, too, covers overvoltages or overcurrents; only the placement of the equipment is different. The above three recommendations are not complete. There are tests defined, and the setup for performing the tests and an exhaustive explanation of how to perform the tests is in the K.44 Recommendation. K.44: "Resistibility tests for telecommunication equipment exposed to overvoltages and overcurrents – Basic Recommendation" The ITU-T K.45 Recommendation specifies resistibility requirements and test procedures for telecommunication equipment installed between telecommunication centers and between a telecommunication center and the customer premises. It, too, covers overvoltages or overcurrents; only the placement of the equipment is different. The ITU-T K.44 Recommendation seeks to establish fundamental test methods and criteria for the resistibility of telecommunication equipment to overvoltages and overcurrents. Overvoltages or overcurrents covered by this recommendation include surges due to lightning on or near the line plant, short-term induction of alternating voltages from adjacent electric power lines or electrified railway systems, earth potential rise due to power faults and direct contact between telecommunication lines and power lines. There are other recommendations from the K series which complement the approach to the resistibility tests of the telecommunication equipment. The additional K series recommendations supply guidelines on possible scenarios and also offer insights on component selection for protection of telecommunication equipment, such as application guides for surge protective components and surge protective devices.

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