I. Introduction
Maintaining the life of machine insulation requires a non-destructive diagnosis which can detect and identify problems. Online partial discharge measurement has been popular for the detection of local defects especially in ground-wall insulation, but gaining more information on overall health (or global problem) is also necessary. Insulation Resistance measurement has been applied for decades in the detection of surface contaminants especially water and carbon dust. Several methods were introduced to determine absorption current from the total charging current during the insulation resistance measurement, in order to identify the aging type, but none is simple.