Abstract Steady-state security assessment primarily addresses the question of the adequacy of generating and transmission capacity to meet load demand. In security assessment, there are two aspects of system failure that should be considered, namely, load curtailment and system collapse. The study of steady-state security has become essential, specially in modern large interconnected power systems. In this paper, a comprehensive conceptual framework and technique for on-line steady-state security assessment is suggested. As security of a power system is characterized in terms of power injections, namely, load demand and generation, the technique mainly depends on the idea of the partial solution of the electric power system. It assumes a subsystem built from three concentric tiers of buses around the faulted component while the rest of the power system outside the third tier is represented by the active and reactive power injections whose values are those before the contingency occurrence. The basic idea of the conceptual framework and technique for the unified approach to steady-state security assessment assumes that the changes are concentrated only inside the subsystem and those changes outside the third tier are small and can be neglected. The accuracy of the technique is compared with the exact solution, the full AC and DC solutions, of the selected IEEE 30-bus power system model with the same contingency cases and is found satisfactory.