MIT515, MIT525, MIT1025, and MIT1525 insulation resistance testers

The Polarisation Index is the ratio of insulation resistance at 1 minute to 10 minutes. It shows how the insulation is charging up and can determine whether it is clean and dry. For trending purposes, the PI value negates the effects of temperature compared to past results.

As the insulation value increases, the test current decreases and becomes harder to measure with the same level of accuracy.

The answer to this question depends on the test set you are using. It is certainly challenging for instrument manufacturers to produce test sets that deliver good performance when the guard terminal is in use, not least because the guard terminal diverts a lot of current away from the measuring circuits. It is by no means unknown, for example, to have a surface leakage resistance of the order of 0.5 MΩ in a test sample with an insulation resistance of 100 MΩ. In other words, the guard terminal current is around 200 times greater than the current in the measuring circuit. This high level of guarded current can cause many problems in a poorly designed instrument, including greatly impaired accuracy. If you’ve got such an instrument, there’s not much you can do about it. However, if you’re buying a new instrument, the answer is simple. Insist that the manufacturer gives you meaningful data about measurement accuracy when the guard terminal is in use. The latest Megger units, for example, have a maximum error of 2 % when guarding 0.5 MΩ leakage with a 100 MΩ load.

The current rating is important, as an underpowered instrument will take a very long time to charge high capacitive test objects, such as long cables; it may also be unable to maintain the required test voltage when high levels of surface leakage are present. It is, however, necessary to be careful when comparing different instrument current ratings. An instrument with a 3 mA short-circuit capability that incorporates power regulation technology to ensure maximum power transfer into all load types will, for example, almost always be faster and more convenient to use than a 5 mA rated instrument that doesn’t use this technology.

The answer, at least in part, is in the question! An insulation resistance tester is designed to be used only on dead circuits, but that’s no guarantee that it won’t ever be accidentally connected to a live circuit. And if it is, an appropriate CAT rating is essential, especially as the environments in which HV insulation testers are most frequently used often have high supply transients. We recommend a CAT IV 600 V rating, and it’s imperative to be sure that this rating applies to all of the instrument’s terminals, including the guard terminal.

If all you ever want to do is one-off type go/no-go testing, you’re correct to say that an instrument that tops out at a few GΩ is fine. But most people who carry out HV insulation testing are looking for more. Specifically, they want to be able to trend and compare results over time, as this provides a valuable warning of impending problems. Consider, for example, a piece of equipment that, over several years, has consistently had an insulation resistance of, say, 100 GΩ. However, the most recent test shows that this has fallen to 20 GΩ. Clearly, something has changed, and an investigation is in order. However, if you had carried out the tests with an insulation tester that reads “infinity” for all values above 10 GΩ, you would have noted no change, and no warning bells would have sounded!