A simple, effective approach is presented for measuring the susceptibility of power systems to cascading blackouts. A failure network and an analysis method are proposed, based on line outage distribution factors and network theory. This network is analyzed with metrics that quantify stress. The metrics can be computed rapidly and be used in realtime operations as well as for. The studies presented are remarkable for use of real, large-scale data for the entire Western Interconnection of North

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... merica. The metrics vary reasonably with different seasonal and loading conditions. Metrics for a pre-blackout reconstruction show that the system was highly stressed and correctly identified the most vulnerable and critical areas and branches. Lowering stress is a serious candidate for reducing risk of cascading.