Abstract: An online system for diagnosing operability of a rotating electrical apparatus includes sensors producing electrical variables corresponding to operating conditions of the apparatus, data converters for converting the electrical variables to digital values, a comparator for comparing the values to corresponding predetermined baseline values of the apparatus and producing a corresponding comparison value, and a signalling mechanism for outputting signals related to a period of predicted operability of the apparatus whenever the comparison value exceeds a corresponding predetermined deadband value. The operating conditions may be non-electrical operating conditions, such as a condition of a lubrication system or a bearing of the apparatus. Alternatively, the sensors may sense electrical insulation non-thermal parameters during operation of the apparatus, in order to produce signals related to the operability of an insulator of the apparatus.