Security assessment is decomposed into successive steps.
1. General system robustness is assessed by the pre-contingency LPM, which we suggest to compute using one of the ``system theory'' approaches proposed in the literature.
2. If the latter margin is sufficiently large, we propose to analyze each contingency in terms of its impact on the LPM, using the above approximate models. The regression trees provide also sensitivity type of information about the most influencing system parameters.
3. If all post-contingency margins are large enough, we should search for combinations of contingencies most likely to lead to voltage collapses.
The last problem may be reformulated as the search for the shortest path to insecurity, where the length of the path is determined by the probability of the corresponding combination of contingencies leading from the present state to a situation where the LPM is reduced below an acceptable threshold. This latter analysis aims thus at completing ``classical'' security assessment, which often leads to rather uninformative answers, by a more informative identification of the system weak points. This is the adaptation to voltage security assessment of the means-ends analysis proposed by Talukdar and Christie [14].