The paper gives details of development work on a protective scheme based on the travelling waves present on major transmission lines after the incidence of faults. The basic principle of the scheme is outlined. Analytical treatment, based on an ideal single-phase line, is included to indicate how the initial travelling wave propagated from a fault and a later wave resulting from a reflection at the fault position may be identified. The time interval, between the arrivals of these two waves at the transducers which would energise the protective scheme, is proportional to the distance to the fault. Its position may therefore be determined with acceptable accuracy. Indication is given of the differences resulting from faults both on and external to a protected line, and discriminatory checks which could be incorporated are outlined. The application of the scheme to 3-phase lines is considered and a further discriminatory check is described. To demonstrate the potential of the proposed scheme, its responses to a number of fault conditions are provided.