Many hand-held multimeters can't measure more than 100 megabytes. Usually, it's 600 megabytes. Insulation resistance meters can measure in the range of megom or hundreds of megom. Some models reach megohm!

The reason a multimeter cannot measure such a high resistance is because the measurement is done at low voltage (usually with a 9V battery inside the multimeter). The multimeter circuit provides a constant current and will measure the voltage at both ends of the probe to calculate the resistance. If a 9V battery is used to drive the current source, very small voltage differences need to be measured to have high resistance measurement capability, and the resolution of the multimeter ADC is limited. In addition, for wiring and systems using operating voltages higher than 9V, it is important to test at these higher voltages, as the insulation resistance changes at higher voltages. A regular multimeter will not "see" it, but report it open.

Therefore, the solution is to use a higher voltage, which is the role of the insulation resistance tester. Internally, it will raise the voltage to hundreds or thousands of volts. This does mean that insulation resistance meters should not be used on electrical circuits. High pressure may cause component failure. Insulation resistance meters are suitable for use in passive equivalent circuits. This also explains why it is called an insulated resistance meter instead of an ohmmeter. Ohmmeters, such as those found in ordinary hand-held multimeters, are unlikely to break the electroless electronic circuits they are used in. Due to high voltage, such equipment should only be used in the case of disconnected electrical connections, non-electric motors or transformers.