The insulation resistance test is one of the methods used to determine insulation integrity. Especially when combined with other types of tests, it can help you determine whether an installed cable should be replaced. When used in conjunction with baseline test data, insulation resistance testing can identify which cables are now deteriorating at an accelerated rate. It has a similar use for motor winding insulation.
Insulation deteriorates because of many factors, including excess heat, power spikes, excess vibration, mechanical abrasion, moisture, oil, or corrosive vapors. Time in service is also a factor, even under the best of operating conditions.
The insulation resistance tester works by injecting a voltage into the conductor and measuring the current that leaks through the insulation and back to the test unit. The test unit divides the applied voltage by the total current to give you a reading in ohms (yet another great use of Ohm’s Law).
Why “total current” rather than just “current?” Because you’ve got to account for three modes of current flow:
- Capacitive current. When you first trigger the meter to apply the selected voltage, you’re basically charging the capacitor that is formed with the insulation as the dielectric. A dielectric has to be charged before it stops conducting. If you had an analog meter, you’d initially see the needle swing way to the right, and then quickly drop down to the left.
- Absorption current. That analog meter would react in the same way as for capacitive current, except the swing back is fairly gradual. Rather than charging a capacitor, you’re looking at how long it takes the insulation to store potential energy (thus “absorption”) not only along the insulation but also in it. Insulation that has deteriorated will absorb less of this current, thus leaving a greater potential after X time than good insulation will.
Good insulation shows a gradual increase in resistance over a set time. One way we make use of this fact is by comparing readings at two set intervals (60 seconds and 10 minutes are commonly used).
- Leakage current. This is the small current that steadily flows over and through the insulation. That analog meter needle wouldn’t swing when measuring this; you would just get an indication. An insulation resistance tester typically would show an increased leakage current as a decreased resistance.
As you may have surmised, this is the current that, instead of being supplied to the load, is leaking out and flowing along and in the insulation. The energy loss may be significant, but even more significant is the fact that lost energy must flow to somewhere. It will take all paths before it, in inverse proportion to the impedance of those paths.
This summary of the three currents gives you a little insight if you’re wondering what they mean. There is much more to these concepts and their application; taking the time to get full understanding is definitely worthwhile.