A regular program of testing insulation resistance is strongly recommended to prevent electrical shocks, assure safety of personnel and to reduce or eliminate down time. It helps to detect deterioration of insulation in order to schedule repair work such as: vacuum cleaning, steam cleaning, drying and rewinding. It is also helpful when evaluating the quality of the repairs before the equipment is put back into operation.
This method is fairly independent of temperature and often can give you conclusive information without records of past tests. It is based on the absorption effect of good insulation compared to that of moist or contaminated insulation. Simply take successive readings at specific times and note the differences in readings (see curves, Figure 2). Tests by this method are sometimes referred to as absorption tests.
The insulation resistance test of transformer complements other transformer tests. The solid winding insulation tests are not conclusive in themselves but provide valuable information on winding conditions, such as moisture content, and carbonization. These tests are considered non-destructive even though at times they may cause a winding failure. It should be pointed out that a winding failure results from an incipient failure that the test was supposed to detect. If it had gone undetected, it might have occurred at an unplanned time.