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The torsional stresses that occur in large steam turbine- generator shafts due to both planned and unplanned system switching operations are discussed. These operations include routine line switching, reclosing line-to-ground faults by either triple-pole or single-pole breaker operation, reclosing line-to-line and three-phase faults, malsynchronization and load rejection. The calculation of cumulative fatigue of individual turbine-generator shafts from computer-plotted torsional swing traces is explained. The benefits of single-pole breaker operation and of dividing the output of large generators between different electrical systems are briefly mentioned.