Deals with protection against lightning of outdoor telecommunication lines using metallic conductors (for example, access networks, lines between buildings). These lines concern: - telecommunication lines connecting a switch with a network termination (NT1) - telecommunication or signal lines connecting equipment located in different buildings, e.g. ISDN lines or signal lines between computers. The object of this standard is to protect telecommunication lines and connected equipment against the direct and indirect influence of lightning by limiting the risk of damage due to overvoltages and overcurrents, liable to occur in these lines, to values which are lower than or equal to tolerable risk of damage. The type of building can also have an effect on the risk assessment of lightning damage to telecommunication lines, as well as the physical layout of the equipment installation. However, these and other similar aspects are covered by appropriate, specific standards and are beyond the scope of this standard. Fibre optical cable with metallic pairs in the cable core must be protected, following the requirements of this standard, together with those requirements defined in IEC 61663-1.