ITU-T K.47 PDF

This Recommendation gives a procedure to protect telecommunication lines using metallic components against direct lightning flashes to the line itself or to the structures that the line enters. Lines made by the following types of cables are covered by this Recommendation.

– Symmetric cable: Cable with a core made of one or many metallic symmetric pairs, which may have a metallic sheath, and/or a plastic covering, and/or a supporting wire.

– Coaxial cable: Cable with metallic inner and outer conductors separated by a dielectric, which may have a plastic covering and/or a supporting wire.

– Optical fibre cable: Cable with optical fibres, which may also have metallic components such as an inner strength member and/or an outer metallic sheath.

Its calculation procedure allows the assessment of the expected annual number of damages (Nd) and the risk of damage due to direct lightning flashes (Rd). The risk of damage due to direct lightning flashes can be used in the risk assessment of a larger system to which the telecommunication line is a part, according to [IEC 62305-2]. The calculated risk can also be used by the line owner (e.g., telecommunication operator) in order to evaluate the need of implementing additional protection measures on the line. In this case, the calculated risk (Rd) shall be compared with the tolerable risk due to direct flashes (RTd), the latter calculated based on [ITU-T K.72]. If the calculated risk is lower than the tolerable limit, the line is adequately protected. Otherwise, it is necessary to implement additional protection measures until the risk of damage is lower than or equal to the tolerable limit.

The protection against direct flashes for equipment connected to the line is not considered by this Recommendation and it should be evaluated using the risk assessment applied to the structure where the equipment is located (i.e., exchange, customer's building or remote site).

The protection of persons using telecommunication equipment inside a structure from dangerous situations caused by direct flashes (e.g., touch voltages) is outside the scope of this Recommendation and should be evaluated using the risk assessment applied to the relevant structure.