The security analysis of power systems involves the traditional contingency analysis developed since several years by power engineers and researchers. In the last decade, due to the increase in power transfer among areas of interconnected systems and to the lack of reactive power to sustain the voltage profiles, power system planners and operators are beginning to face voltage collapse problems and therefore they need suitable software tools and algorithms to take into account the reactive subproblem. In this paper, second order information are calculated and used to perform in a very fast way an effective ranking of the possible contingencies with respect to the voltage collapse. Following this ranking, a quantification of the risk is carried out by means of an iterative procedure. It provides the highest values of the power transfers in the critical branches that allow the attainment of a feasible point with respect to voltage collapse, even in case of the branch outage.