The level of security is defined taking into account credible contingencies, called “normative incidents”, for which the power system is supposed to remain within the limits defined above after fault conditions have been cleared. A typical normative incident is the loss of a GTG or STG. For contingencies more severe than these normative incidents such as the simultaneous loss of several GTGs, a defense plan is required that will allow the controlled shutdown of the installation under the best possible conditions (defense plans are not covered in this paper).