Insulation resistance testing is one of the most useful predictive maintenance tests available. This test gives insight into the integrity of insulation. The higher the resistance across the insulation, the less current is escaping through it. Conversely, a low reading means that insulation is leaking.
An obviously low resistance (e.g., 500 ohms instead of 150 megohms) indicates a bad cable or motor winding. Performing insulation resistance testing during or immediately following installation will enable you to identify insulation failures so they can be corrected (replace the cable or motor) prior to energization. That’s an obvious benefit of this testing. But if you’re installing new cables and new motors, why bother?
One reason is that cables sometimes do get damaged during installation. You want to catch this as early as possible. A more important reason is that testing at this juncture is the only way to get the baseline data you need for maintenance trending.
The insulation resistance test really proves its worth when used over time rather than as a single go no-go condition test. If you have baseline data and perform subsequent insulation resistance tests at set intervals using the same test voltage, you can plot the results.
Many tools exist for tracking and plotting insulation resistance testing results. You can even make your own by using a spreadsheet. Now imagine you’re looking at a chart of a series of insulation resistance test measurements for a particular cable, and each measurement is a plot point on that chart. The vertical axis is the resistance value and the horizontal axis is time. You’ll notice a line drawn from each point gradually trends downward over time.
This downward slope is very slight. What you’re looking for is a “break” in that line, where the slope suddenly is steeper than normal. When you see this, you know the deterioration of that insulation has accelerated beyond the normal rate.
Depending upon the rate of acceleration, you may decide to do any of several things, such as:
• Add the cable to the list of cable to be replaced during the next shutdown.
• Increase the test frequency to monitor the problem more closely.
• Replace the cable (or motor) as soon as the work can be scheduled.
Note that the value obtained to create the break in the trendline could easily be within an acceptable range. That is, the insulation resistance isn’t low on an absolute scale. It’s low relative to the previous set of readings.