Modern economics demand a reduction in costs and prices, and this usually means reduction in the amount of materials used. The danger is that some of the insulation materials in use have a cellulose base, which means that their ageing may be endangered if the temperature is higher than the 110°C, since, above this temperature, the cellulose materials tend to change quickly their consistency and mechanical strength.It has been generally agreed that the life expectancy of electrical machines should be seven years when continuously under rated load. General equations for change in life expectancy with temperature have been experimentally and partly deductively found and presented by Montsinger and Bussing, and experiments show that the equations are correct for continuous loads. But when the load changes the heating and cooling periods must be taken into consideration. Short-circuits or heavy overloads can be very dangerous.Equations have been developed for load changes and for straight line and exponential temperature changes, and it is shown that the cooling-off period especially can be very dangerous and take a very appreciable part of the life expectancy of the electrical equipment in question. Sample calculations on transformer and intermittent motor loads are included.