Insulation resistance is obtained by applying a known DC voltage between conductor and earth, or between windings, and measuring the resulting current. The ratio of the two is the resistance, which in healthy insulation ranges from hundreds of megohms to several teraohms depending on the equipment and the test voltage.
It is the most widespread test in electrical maintenance because it is fast, non-destructive and catches the two most common problems: absorbed moisture and surface contamination. Its limitation is that the absolute value depends heavily on temperature and ambient humidity, so it is rarely judged on the figure alone.
The test is therefore completed with two ratios calculated from the same measurement and almost immune to temperature: DAR, between 30 seconds and 1 minute, and the polarization index, between 1 and 10 minutes. The test voltage is chosen from the rated voltage of the equipment, typically 500 V to 10 kV.