Insulation resistance testers are usually called megohmmeters or high-resistance meters, which 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 large change in insulation resistance may indicate a potential failure. Therefore, the megohmmeter needs to be calibrated regularly to ensure that the meter itself does not change over time.

Working principle of insulation resistance tester:

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 a megohmmeter calibrator and a 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 50 V to as high as 5 kV; while 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 10 TΩ, and the required voltage is higher.

Almost all insulation testers use DC voltage as excitation, so there are few AC requirements for megohmmeter calibrators. Many megohmmeters are two-terminal devices that provide a voltage and measure the current determined by the device under test. Megohmmeters with ranges up to 1 TΩ and higher usually have a third terminal called Guard, which is very useful for eliminating the leakage path and parallel components of the unknown resistance Rx under test. The purpose of the protection terminal is to eliminate the leakage current that may be generated to selectively reduce the influence of the output parasitic resistive element to zero.

One of the main problems when calibrating these instruments is to find a suitable resistor, of course, first of all, it must be enough; it also needs to be large enough to withstand high DC voltages. In addition, the manufacturer of the megohmmeter does not have a unified standard for what resistance value should be used for calibration, so a variety of resistance values are required. By understanding the various insulation testers, you can know that they require different performance check points. For example, one tester needs to test 50 kΩ, another tester needs to test 60 kΩ, another one needs to test 100 kΩ, and so on.

"Universal" multifunctional electrical/electronic calibrators cannot be used to calibrate insulation resistance testers, because their resistors are usually only able to handle a limited voltage, which is often no more than 20 V. The challenge for the insulation resistance calibrator is to integrate these special needs into an economical, compact and portable solution.