The steady care for electric systems security improvement will more and more incite the grid designer to take the major failure risks into account. Electricite de France has therefore adopted a methodology based upon the study of possible degradation mechanisms, then upon the verification of their plausibility by simulating the system dynamic behaviour. Only this approach can fit with the complexity of great meshed systems. It enabled the performances of a new protection device against losses of synchronism to be defined; this ''defence plan'' will be operational in 1995 on a part of the FRENCH system.