Power security planning becomes an important issue in the actual growth of power system due to the dependence of modern society to a reliable, secure and flexible network. Major blackouts are started by cascade failures which successively weaken the system and make further failures more likely. With the advance of utilization of distributed generation (DG), this is a feasible and cost-effective solution to mitigate cascading failures impacts. This paper presents a methodology based on the Manchester Model to evaluate the effectiveness of DG in the mitigation of impacts due to cascade failures. The expected value of energy not supplied (EENS), with and without DG, is computed as security index for comparisons. Then, a practical identification of strategic nodes for the installation of DG using distribution factors is presented.