A new power system controlled separation scheme is proposed to prevent the propagation of cascading failures across a transmission network should it undergoes a major disturbance, thereby reducing the possibility of a large-scale blackout. This scheme is developed based on a set of conjectures, which state the following: (i) the locations of out-of-step operations are independent of the severity and the location of the initial faults; (ii) these out-of-step operations occur sequentially over a sufficiently long duration so that relay blocking and transfer tripping can take place to minimize the load-generation imbalance in the formed islands. To verify these conjectures, extensive dynamic stability simulations are executed on a 30-bus and a 517-bus system, which exhibit characteristics suitable for this study. Furthermore, we verify that these out-of-step operations do depend on the prevailing system topology and the operating conditions.