Abstract: The method of protecting power transmission lines is, accpording to the invention, based on a comparison between the actual quantities of current and/or voltage at the beginning of the protected section of a power transmission line and the same quantities of current and/or voltage calculated by means of current and voltage parameters at the end of the protected section providing there are no faults in this section. In apparatuses for realization of this method the voltage and current at the end of the protected section are multiplied in symmetrical components multipliers by respective coefficients of an equivalent four-terminal network substituting for the protected section and summed up. If there are no faults in this section, this calculated sum is equal to the respective electrical quantity at the beginning of the protected section.