Electrical Testing

IR Testing, Three Ways

A look at different ways you can test for insulation resistance.

Insulation resistance testing is inarguably valuable, but for the most part it depends upon relative measurements. A single measurement by itself can reveal a gross failure, but it’s not going to tell you that insulation is actually deteriorating. For that, you need to trend the test results and watch for a sharp change.

But there’s more than one type of insulation resistance test. Which one should you use?

Let’s start with finding a particular type of gross failure—miswiring of equipment. A standard test for this is the high potential (hi pot) test. It’s called hi pot because the voltages used are much higher than the voltages used for standard maintenance testing.

To understand how to correctly perform this test (including time durations), consult IEC 60950. Typically, the voltage you are going to apply via your tester is determined based on the equipment’s nameplate rating. Per IEC 60950, to get the basic test voltage for the hi pot test, you double the operating voltage and then add 1,000V.

Motors are typically checked using the short time / spot reading test. This can, if done properly, help you predict motor winding insulation failure and intervene before it actually happens. This test has many other uses, too. But let’s look at how you’d use it for a motor.

To perform this test, connect your tester across the insulation of the motor windings, and leave it there for a set period. Commonly, 60 seconds is used for this purpose. Whatever period you use, it must be the same from test to test or the results are not very useful. Record the resistance reading at that mark. If you took a baseline reading when the motor was new, you just have to record the reading for each subsequent test. If you plot these on a graph, you’ll see a slight slope appear over time. When that slope suddenly isn’t so slight anymore, your insulation has problems.

What if you want a determination without having to wait for several maintenance testing cycles to occur? That’s where the “time resistance method” comes in. You take successive readings at set intervals and plot them. Absorption current starts high and gradually decreases. After several minutes, you should see resistance increase. If you don’t (if the resistance seems to flatten out), your insulation has problems.

The step voltage (aka, “tip-up”) test consists of performing the test using two different voltages, then comparing the readings. If the insulation is good, you should not see any difference. Just make sure both voltages are within what the insulation is rated to handle; this is supposed to be non-destructive testing.

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