Insulation testers, usually called megohmmeters or high resistance meters, are widely used to measure the insulation of generators, motors, power transformers, wiring, electrical appliances and other electrical devices (such as cables for control, signal, communication and power) resistance. They are often used in routine maintenance procedures to indicate changes in the insulation resistance of motors over months or years. A major change in the insulation tester may indicate a potential failure. Therefore, the megohmmeter needs to be calibrated regularly to ensure that the meter itself does not change over time.

The megohmmeter uses a voltage to excite the device or network under test, and then measures the current generated by the excitation, and uses Ohm's law to measure resistance. A good megohmmeter calibrator includes a variety of optional resistors, which is not much different from the resistors provided by modern calibrators using the synthetic resistance function. The difference between the insulation tester megohmmeter calibrator and the DC/low frequency calibrator lies in the required resistor range and the ability to withstand voltage. For example, compared to the ohmmeter function equipped on a digital multimeter (DMM), these electrical testers apply a much higher voltage when making resistance measurements. The voltage range used by the megohmmeter is usually from 50V to as high as 5 kV; and the voltage of a typical digital multimeter is generally less than 10 V. For insulation testing, the resistance value range that needs to be measured is very large, the upper limit can reach 10TΩ, and the required voltage is higher.