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!