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Electric-power machines such as generators, large rotary machines, and transformers play an important role in electrical energy delivery. Since the beginning of electrical-energy distribution to consumers, offline diagnostics of these important machines has been performed. In time-based maintenance that is based on offline diagnostics, diagnosis is carried out regular intervals, followed by maintenance or repair as required. Personnel have to be transported to the site to perform the measurements. Currently, online diagnosis is being used important electric equipment around the globe; measurements are performed continuously, which helps minimize the number of faults and therefore network outages. The data collected the diagnostic system, e.g., voltage, current, or temperature, are sent via a computer network to the supervisory service. In this paper, the authors describe a system that uses data from online diagnostic systems to calculate the remaining lifetime of an electrical insulation system. The paper also presents experimental accelerated aging data and use them as input data for remaining-lifetime calculations.