This Recommendation gives the methodology for evaluating the need for protection measures against lightning of telecommunication lines using metallic conductors. This method is based on the risk assessment: protection measures are necessary when the risk is greater than the tolerable risk. Maximum value of the tolerable risks is suggested. The Recommendation considers the risk of loss of service of the telecommunication network. The risk is evaluated through the risk components caused by lightning, which is the source of damage on telecommunication line as function of the striking point respect to the telecommunication line (lightning flashes direct to the line, near the line or direct to the structures connected at the ends of the line). This Recommendation shall be used together with Recommendation K.47, which provides the risk assessment and the protection measures against lightning flashes direct to telecommunication line and to structures connected at the ends of telecommunication line, and Recommendation K.46, which provides the risk assessment and the protection measures against lightning flashes near the telecommunication line.