Insulation Resistance Testing, Part 4

In our previous issue, we described time-resistance and dielectric absorption ratio tests. Two other useful insulation resistance tests are the short time and step voltage tests.

• Short time. This is also called a “spot reading” test. It's a quick check that reveals gross defects in the insulation. Apply the test voltage for a designated interval, and then record the results. That interval is usually 60 sec. Though you can use any interval you wish, make your choice only once. Use the same interval consistently, so spot readings make sense when compared to "typical" readings.
• Step voltage. Another name for this test is “tip-up.” It's essentially a spot reading done at one voltage and a second SR done at a higher voltage. Good insulation exhibits the same resistance reading at either voltage. If the readings differ, then there’s a problem with the insulation (e.g., pinholes, contamination).

Always perform one of these tests before incurring the time and expense of installing a motor.