Electrical Testing

Tip of the Week: Does That IR Spec Still Mean Anything?

To detect deterioration, insulation resistance testing should be conducted over time.

Insulation resistance (IR) testing is widely used to assess the condition of insulation in such things as service cables, feeder cables and the wiring in major branch circuits. It’s also used to assess the condition of motor winding insulation.

The problem is that an IR test taken in isolation seldom tells you anything useful unless the value is really low (as in almost no resistance). When a motor supplier states an IR testing value or range for maintenance purposes, that’s really just a guide for you to use in your baseline testing, prior to placing the motor in service. Unless the winding insulation is severely damaged, whatever value you get for that motor is the value that goes into the test data history of that motor.

Notice, “test data history” not “PM procedure.” Why is this? Because to correctly assess the condition of the winding insulation, you are looking for deterioration. And that happens over time. Thus, it’s not that the test tech takes a reading and can tell that the winding insulation is deteriorating.

It’s that someone “in the back office” must review the data in the computerized maintenance management system (CMMS) or motor maintenance software and look at the trend. A small deviation downward over time is normal, but if the curve suddenly “breaks downward,” you need to schedule that motor for service or replacement.

If the motor is critical, you should conduct IR testing more frequently (if possible) or consider an automated system that performs this test.

Winding deterioration also means the motor may draw more current to do the same work (depending upon how bad that deterioration is), so you can also monitor the current on each phase. This can be useful as a condition alert. For example, if one phase starts pulling more current relative to the other two, you may have a phase-to-frame fault developing.

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