Insulation Resistance Test

  1. What is an Insulation Resistance test?
  2. What is the difference between a Megohmmeter and an Insulation Resistance tester?
  3. What is the difference between a Hipot test and an Insulation Resistance test?
  4. What is considered to be an acceptable Insulation Resistance reading?
  5. What is the Polarization Index?
  6. What is trending?
Q. What is an Insulation Resistance test?
A. An Insulation Resistance test measures the resistance offered by an insulating material to the flow of current resulting from an applied DC voltage. The DC potential is normally applied between current carrying conductors and ground. This test is usually done on a product after it is manufactured, installed or repaired. It is also commonly performed as a routine maintenance test on products such as motors or generators and can help to predict if a product is due to fail.
Q. What is the difference between a Megohmmeter and an Insulation Resistance tester?
A. They are essentially the same. A Megohmmeter refers to an instrument whose meter is usually scaled to read in MΩ (millions of Ω’s). An Insulation Resistance tester refers to an instrument that measures the resistance of a product’s insulation system.
Q. What is the difference between a Hipot test and an Insulation Resistance test?
A. The Insulation Resistance test is a qualitative test that gives an indication of the relative quality of a product’s insulation system. This is an ideal test for studying, measuring and recording long term stability of insulating materials over time. The Hipot test is commonly specified as a production line test for electrical products. The Hipot applies an excessive voltage between the current carrying and non-current carrying parts of the DUT to stress the insulation system and verify its integrity. The Hipot test is a more stringent test and is most often a go – no go tester, where an IR Test is done at a lower potential.
Q. What is considered to be an acceptable Insulation Resistance reading?
A. Acceptable Insulation resistance readings can vary dependent upon the standard. A common rule of thumb for large rotating machinery is a "1 megohm for every 1000 volts of operating voltage plus 1 megohm." So if a product operates below 1000 volts the minimum acceptable IR reading would be 2 megohms.
Q. What is the Polarization Index?
A. Polarization is the ability for a material’s dipoles (equal and opposite charges separated by some distance) to line up in the presence of an electric field. As the dipoles align themselves absorption current is created that decreases over time. Initially the absorption current is high, but as fewer and fewer dipoles are left to align themselves with the applied field the current drops. Thus an IR measurement taken after 1 second would yield a lower resistance than an IR measurement taken at say, 60 seconds. In fact, the most common test times are a 10 minute test divided by a 1 minute test. The resulting ratio of the test measurements is known as the Polarization Index.
Q. What is trending?
A. Trending is a term used to describe the process of taking repeated test measurements over time. In the case of Insulation Resistance testing, trending the test results can provide useful information about a product’s integrity and can yield clues as to when its insulation should be repaired or replaced.