This Recommendation deals with protection principles, e.g. risk management, safety and reliability, Surge Protective Devices and Surge Protective Components. It gives guidance of the protection of telecommunication equipment, installations and cable plants exposed to the results of external sources of interference such as overvoltages and overcurrents due to lightning or effects related to power lines and electric traction systems. It gives general information about: " the origin of overvoltages and overcurrents (lightning, power induction, power contacts, earth potential rises); " types of protective devices (voltage and current limiting devices) and their residual effects; " risk assessment; " protection of telecommunication lines; " protection of exchange and transmission equipment; " protection in access networks. Reference is made, in the bibliography, to some K-series recommendations and IEC standards related to: " power supply effects and low frequency interference; " lightning effects; " surge protective devices and components; " resistibility of telecommunications equipment.