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An analysis of the most important time dissemination methods in current use is presented and the difficulties encountered or foreseen in applying such methods in electrical substations is described. The latter part of the paper is devoted to methods developed and applied at Hydro-Qu??bec. There are many requirements for a common time scale in electric power systems. For example, interest has arisen in the chronology of protection operations, where the accuracy must be in the order of milliseconds. A more recent application is for measurement of the voltage phase-angle (or power angle) between remote points on a power system, a technique which calls for an accuracy in the order of 10 ??s. A distinction is established here between time dissemination and frequency dissemination, which are often confused.