An insulation resistance test involves applying a specific voltage across a dielectric and using the measured current to calculate the resistance. Insulation resistance testing is important, as the performance of electrical insulation degrades as it ages.
An insulation resistance test is performed by connecting the positive and negative leads of a tester across an insulation barrier. Where the leads go will depend on the type of electrical equipment being tested. Next the test voltage is applied for 1 minute - this is the industry standard testing time and will allow comparisons with other readings. During the interval, the reading will likely drop or remain mostly steady, however, only the reading after 1 minute is recorded.
When performing multiple insulation resistance tests it is important to be as consistent as possible. Each test should be performed in the same way, but ideally also in the same environmental conditions. Insulation resistance is affected by temperature, and tests at different temperatures can have drastically different results. In general, resistance is doubled for every 10 degrees Celsius lost and halved for every 10 degrees gained.
The International Electrical Testing Association provides standard measurements for most types of electrical equipment, and it is often sufficient to compare readings with those from other pieces of equipment under similar conditions.