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Maintaining dynamic security of a power system subjected to large disturbances is of utmost importance. Fast and accurate online detection of instability is essential in initiating certain emergency control measures. The techniques reported in the literature involve mainly the application of global phasor measurements and heuristic algorithms. In this paper, an accurate technique for the online detection of loss of synchronism based on voltage and current measurements in a line is presented. The technique makes use of the concept of potential energy in a line. The conditions for the system instability are derived from energy function analysis. However, no assumptions are made regarding the power-angle relationship in a line, nor are any data on the system equivalents necessary in implementing the detection scheme.