That the specter of a major system shutdown is prominent in the thoughts of many operators is evidenced by the 1939 symposium, covering methods of modernization of station-switching facilities, and the 1940 symposium, discussing means of restoring service to systems after a major shutdown. In this paper, system and apparatus performances are studied, during the period between the time allotted for the functioning of the high-speed relay protection, and the point where the system becomes so unstable that a major shutdown occurs. With a view to the further improvement of system operation, the paper studies: 1. Phenomena preceding and during instability; 2. Method of lengthening the period between the fault and instability; 3. A local high-speed back-up relay scheme; 4. Means of automatically and quickly stabilizing an unstable system; and 5. In case all of the above fail, progressive isolation of the system by zones selected by degree of low voltage.