Abstract: At a measuring point serving at least one adjacent line segment instantaneous values of current and voltage are measured in sequential pairs, either continuously or at intervals by delaying these values by discrete amounts of delay corresponding to twice the wave propagation time on the line from the measuring point to a number of selected reference (appraisal) points along the line segment, and then combining the delayed value pairs corresponding to each appraisal point with a real time value pair. A voltage value at each appraisal point at the mid-point of the delay period is determined by use of delayed measured values that take account of line propagation characteristics such as damping.