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A new algorithm for protection against voltage collapse is proposed. The algorithm makes use of the magnitudes and angles of the local phasors, i.e. bus voltages and load currents. The change in an apparent-power line flow during a time interval is exploited for computing the voltage-collapse criterion. The criterion is based on the fact that the line losses in the vicinity of the voltage collapse increase faster than the delivery of apparent power and, at the voltage-collapse point, the losses consume all the increased power. The selected criterion equals 0 when a voltage collapse occurs. The proposed algorithm could be easily implemented in a numerical relay. The information obtained by the relay can be used at two levels - for the coordinated system-wide control action or for automatic action on the local level. The algorithm is simple and computationally very fast. It was tested on the IEEE 118-bus test system.