scope:

This Recommendation gives a procedure in order to protect telecommunication lines using metallic symmetric conductors (i.e., twisted pair cables) against overvoltages and overcurrents imposed on the lines due to nearby lightning flashes.

Its calculation procedure allows the assessment of the expected annual number of damages (N_Z) and the risk of damage due to indirect lightning flashes (R'_Z). The risk of damage due to indirect 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 (R'_Z) shall be compared with the tolerable risk due to indirect flashes (R_Ti), 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.

Depending on the user's desire, the resistibility of the external port of network terminal equipment (e.g., exchange line card, modem, POTS telephone set, etc.) may or may not be included in the calculation. This decision implies the selection of the appropriate impulse withstand voltage (UW) of the relevant line node and the result shall be limited to the damage caused to the external port of the equipment. However, the protection of other equipment ports (e.g., power port) and of equipment that does not have a direct interface with the line is outside the scope of this Recommendation and should be evaluated using the risk assessment applied to the exchange or to the customer's structure.

The protection of persons using telecommunication equipment inside the exchange or the customer's structure from dangerous situations caused by touch voltages is outside the scope of this Recommendation and should be evaluated using the risk assessment applied to the exchange or to the customer's structure.