This recommended practice provides guidance for the calculation of power station GPR and LI voltages, as well as guidance for their appropriate reduction from worst-case values for use in metallic telecommunication protection design. Information is also included for the determination of the following: a) The fault current and the earth return current. (The probability, waveform, and duration of these currents and the impedance to remote earthing points used in these GPR and LI calculations as well as the effective X / R ratio are discussed.) b) The zone of inßuence (ZOI) of the power station GPR. c) The calculation of the inducing currents, the mutual impedance between power and metallic telecommunication facilities, and shield factors. d) The channel time requirements for metallic telecommunication facilities where non-interruptible channels are required for protective relaying. To review and update existing standard 367-1987 as required. Revision Standard - Superseded. Guidance for the calculation of power station ground potential rise (GPR) and longitudinal induction (LI) voltages is provided, as well as guidance for their appropriate reduction from worst-case values, for use in metallic telecommunication protection design.