Breaking in a large electric power system causes torsional vibrations on the turbogenerator shafts. Damping of such vibrations is very low compared with the damping of the electrical system. Therefore, during a strong electric transient, the shafts connecting the turbine and the generator rotors may undergo heavy loading and considerable deformations, both elastic and plastic, for prolonged periods of time. As a result, the fatigue life of the shafts may be substantially reduced. The paper introduces an improved mechanical dynamic model of the system for torsional response, which can account for the plastic material model, to yield the cumulative damage during the transients due to high and low cycle fatigue. Moreover, a procedure for the calculation of fatigue life expenditure is developed based on the ‘equivalent strain range’ derived from a known twist angle history of the shaft. A detailed discrete model for both the mechanical and the electrical system was used in conjunction with modelling of the fast reclosing mechanism. In this manner, fatigue life reduction due to a certain fast reclosing strategy was evaluated. The system thus developed can lead to a better understanding of the mechanism of fatigue life reduction due to the electrical transients.