A security scheme of the power systems should protect the integrity of the electric networks and carry out fast operations on the entire power system to prevent a possible blackout. Blackouts started as local failures led to electrical disturbances, and the complete collapse of the power systems. This work proposes a security scheme based in two stages. In the first stage, the controlled separation of the power system allows to isolate the failure area. In the second stage, under frequency load shedding control actions should be applied to maintain a balance between consumption and generation power. This work presents some schemes in the IEEE-39 test system.