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Hidden failures in protection systems have been identified as key contributors in the cascading of power system wide-area disturbances. This paper describes a methodology to evaluate the hidden failure's effects based on regions of vulnerability and areas of consequence in protection systems. The mechanisms of the hidden failures as well as the development of the regions of vulnerability from the hidden failures are shown. A number of the regions of vulnerability in a sample 179 test system are represented as physical areas. Two scenarios evaluate the consequences of the unwanted disconnections of the transmission lines caused by hidden failures. The development of the index of severity, which combines the magnitude of the regions of vulnerability and the consequences of unwanted disconnections caused by hidden failures, is proposed. This index would identify the critical protection systems, whose unwanted operations would result in a significant loss of the power system integrity and/or a large cascading event.