One of the most fundamental of electrical installation and maintenance tasks is taking insulation resistance (JR) readings. This is done to verify the integrity of the insulating material, be it wire and cable insulation or motor/generator winding insulation. Any electrical insulation must have the opposite characteristic as the conductor: It should resist the flow of current, keeping it within the conductor.
Using Ohm's Law
To better understand Ohm's Law (E = I x R), let's use an analogy to describe the function of resistance--it's very much like a pipe carrying water. As shown in Fig. 1, water pressure, which is provided by a pump, causes the water to flow through the pipe. There is some resistance to this water flow in the form of friction along the interior pipe wall. If the pipe springs a leak, the water pressure goes down.
[Figure 1 ILLUSTRATION OMITTED]
Looking at the analogy from an "electricity" point of view, voltage is the "electrical pressure" that causes current to flow along the conductor. (See Fig. 2.) There also is a resistance to flow here, but it's much less through the conductor than through the insulation. Obviously, the higher the voltage, the more current we'll have. And, the lower the conductor resistance, the more current we'll have for the same voltage. This basically is what Ohm's Law expresses.
[Figure 2 ILLUSTRATION OMITTED]
We all know that no insulation is perfect (having infinite resistance). As such, there's some electricity flowing along the insulation or through it to ground. This current is called leakage current. It may be only a millionth of an ampere (one micro-amp), but it's current nonetheless. And don't forget that a higher voltage will cause a higher amount of leakage current. Leakage current does not harm good insulation, but it becomes a real problem with deteriorated insulation.
So, how do we determine what's "good" insulation? Based on our discussion here, it would seem that insulation with a relatively high resistance to current would qualify. We also could say "good" insulation has the capability of keeping a high resistance. That said, you would need a way to measure this resistance to make such a determination. This is the basis for IR testing. By taking measurements at regular intervals, you can do a trend analysis on the integrity of any insulation.
To measure IR, you would use an IR tester, which is a portable instrument that's essentially a resistance meter, or ohmmeter, with a built-in, hand-cranked or line-operated DC generator that develops a high DC voltage. This voltage (usually 500V or more) causes a small current to flow through and over the insulation's surfaces. The tester provides a direct reading of IR in ohms or megohms.
So, you use an IR tester and obtain measurements. What do they mean? Based on our prior theoretical discussion, a high resistance value would indicate a "good" insulation while a relatively low resistance value would point to a "poor" insulation. In the real world, however, actual resistance values can be higher or lower due to the effects of factors such as temperature, humidity, the moisture content of the insulation, even the person doing the testing. And, IR readings can be very different for the same motor tested on three different days.
What really matters is the trend of the readings over a period of time. A continued lessening of IR readings through a specific interval should be interpreted as a warning of pending problems. As such, you can get a very good sense of the condition of an insulation through good record keeping and common sense.
One important note to remember: Each of these periodic tests should be made, as much as possible, the same way. In other words, you should use the same test connections at the same applied test voltage for the same length of time. If at all possible, try to do the testing at the same temperature, or correct the measurements to the same temperature. A helpful hint is to record the relative humidity near the tested equipment at the time of each test; this will help you evaluate the readings. IR test set manufacturers have helpful temperature correction and humidity information.
Based on your observations of the test data, you can make some intelligent decisions. The table below provides some useful guidelines.