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A new philosophy for the analysis and design of large electric power interconnections has evolved following the Northeast power failure of November 9, 1965. Most contingencies that can cause wide-spread outages occur too infrequently to be included in the criteria for system design. Nevertheless, systems should be tested for combinations of events that cause system instability and separation so that the consequences of these unlikely occurrences can be evaluated and system designers can provide disaster control procedures to limit the extent and duration of system outages. The design and disaster control procedures should provide successive lines of defense against increasingly severe and unlikely events. Disaster control procedures can be coordinated so that governor action, load shedding, and system separation can be integrated according to time and frequency in such a way that maximum reliability and security will be provided.