Recommendation ITU-T K.20 specifies resistibility requirements and test procedures for telecommunication equipment that is attached to or installed within a telecommunication centre. Overvoltages and overcurrents covered by this Recommendation include surges due to lightning on or near the line plant, short term induction from adjacent a.c. power lines or railway systems, earth potential rise due to power faults, direct contact between telecommunication lines and power lines, and electrostatic discharges (ESDs). The sources for overvoltages in internal lines, between equipment or racks, are mainly inductive coupling caused by lightning currents being conducted in nearby lightning strikes or lightning currents being conducted in nearby conductors. Changes compared with Recommendation ITU-T K.20 (2017) include: - external Ethernet power contact test; - test exemption for internal short cables; - renaming of some test titles for clarity; - Intermediate Ethernet test level added.