The analysis of severe blackouts has become an essential part of transmission grid planning and operation. This may include evaluation of rare-event probabilities, which can be difficult to estimate. While simulation offers flexibility to model large complex systems, efficiency remains a big concern when estimating very small probabilities. This paper presents an effective simulation technique to evaluate rare-event probabilities associated with cascading blackouts in an electric grid. We test our technique on an IEEE 118-bus electric network and show that it can dramatically improve simulation efficiency. We also demonstrate that the proposed technique can effectively locate vulnerable links. These are links whose failures lead to the highest probabilities of a blackout event.

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