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In most cases the infinite sequential stressing method can be applied in the analysis of multi-factor aging of insulation systems. Although the simultaneous multi-factor aging characteristics cannot be reproduced strictly by sequential stress tests, the characteristic equation for multi-factor aging can be derived by combining all single-factor aging characteristics in sequential stress aging. Generally, such characteristic equations can be represented only in numerical form, but in the case of mica-epoxy insulation systems in which the value of n in the inverse n-th power law remains unchanged during thermal aging, and also the value of thermal aging rate remains unchanged during voltage aging, the characteristic equations for multi-factor aging can be derived in an analytical form. The mechanical aging characteristics determined by repeated bending tests seem to be of two types, uniform degradation, similar to thermal aging, and local degradation, similar to voltage aging. The ratio of these two types depends on the mica (mica-paper or mica-splitting). Characteristic equations for simultaneous thermal, voltage, and mechanical aging also can be derived by the infinite sequential stressing method. This method assumes an infinite repetition of the four kinds of aging: thermal, voltage, uniform mechanical, and local mechanical.