An insulation tester, also known as a megohmmeter, is a portable instrument that directly measures insulation resistance in ohms or megohms. It is used to test the insulation in electrical devices and equipment to prevent hazards from electric shock or short-circuits caused by degraded insulation over long periods of use. Major brands that produce insulation testers include Fluke, EXTECH, MEGGER, UNI-T, and GEZELLE.