Recommendation ITU-T K.21 specifies resistibility requirements and test procedures for telecommunication equipment that is installed in or on a customer premises building. Overvoltages or 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 contacts between telecommunication lines and power lines and electrostatic discharges. The sources for overvoltages in internal lines are mainly inductive coupling caused by lightning currents being conducted in nearby lightning strokes or lightning currents being conducted in nearby conductors. Major changes compared with the 2008 version of this Recommendation include: o updating the references; o information on what USB ports should be tested; o information on when to add protection to untested ports; o adding test requirements for external coaxial cable ports; o adding test requirements for multiple conductor internal unshielded cable ports.