TESTech tell you how to choose an insulation resistance test machine
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Insulation resistance testing puts a qualitative value on the condition of conductor insulation and the internal insulation of different pieces of electrical equipment. As an insulation resistance test begins, apply a direct current (dc) voltage to the conductor or equipment under test. Some current flows out of the test equipment into the conductor and begins to charge the insulation. This current is called capacitive charging current and can be observed on the meter face.

As the charging current first begins to build, the resistance reading on the meter face will indicate a low value. Think of this as electrons beginning to flow into and become stored in the insulation itself. The more current that flows out of the test set, the lower the megohm reading. The insulation becomes charged quickly and the meter indication will begin to settle out at a higher megohm value—provided the quality of the insulation is good!

The second current that flows is the absorption or polarization current. The amount of absorption current is dependent upon the contamination of the insulation. For example, if moisture is present in the insulation, absorption current will be high, indicating a lower resistance value. However, it is important to realize this absorption current takes longer to build than the capacitive charging current. Therefore, an insulation tester operated for too short a period will observe only the capacitive charging current and not begin to show the presence of contaminants in the insulation.

Finally, the current that leaks through deteriorated insulation and into the non-current carrying metal components is the leakage current. This current is most often considered in an insulation resistance test. However, for more accurate troubleshooting and maintenance, the absorption or polarization current must also be considered. Some insulation resistance testers can be programmed to perform the tests needed to consider all currents.

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