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Power grids have traditionally been planned and operated to withstand without resulting in cascading events transmission outage contingencies of the N-1 or N-2 kind. However, in actual operation, power grids are potentially vulnerable to cascading outages. Reliable operations therefore require a risk-based approach to monitoring and managing the probability and impact of potential cascading outages. Extending into the transmission planning area, the probabilistic reliability assessment (PRA) method should also study the cost-effectiveness of transmission investment in mitigating potential cascading outages. This paper defines and quantifies a measure that relates to the vulnerability of the power grid to cascading outages. The approach is to simulate and identify potential cascading modes (PCM) and to compute the probability and impact of each stage of the cascade and display the risk of each potential cascade stage on the two-dimensional display of likelihood and consequences. In this display, the traditional N-1 or N-2 contingencies will also be plotted for comparison. This approach enables grid operators and transmission planners to be fully aware of not only the contingencies for which they are required to assess and mitigate, but also the potential cascading modes. This places both types of potential adverse events on the same basis for comparison. This approach to study the PCMs will make potential solutions to the mitigation of cascading outages more apparent, such as transmission additions, automatic islanding, generation and/or load shedding. An example of applying this method is illustrated in this paper.