Megohmmeters

Megohmmeters, commonly known as meggers, are electrical test instruments used to measure the insulation resistance of electrical equipment and installations. They work by applying a controlled high DC voltage to the insulation and measuring the resulting leakage current, from which insulation resistance is calculated and read in megohms. High resistance indicates good insulation, while low resistance suggests degradation or moisture.

Most megohmmeters are portable handheld or benchtop devices that include a regulated high-voltage source, a current meter, a voltmeter, and a range selector. They provide test voltages ranging from tens to thousands of volts depending on the application, with discharge circuits to safely remove stored charges after testing. Operators should isolate the equipment, discharge the insulation after testing, and follow safety procedures because high voltage is involved. Some instruments support advanced tests such as polarization index or dielectric absorption ratio for cables.

Applications for megohmmeters span preventive maintenance and condition assessment of electrical systems, including cables, motors, transformers, switchgear, insulators, and wiring in buildings, industrial plants, and transportation systems. Insulation resistance testing helps detect moisture ingress, contamination, aging, or insulation failures, informing maintenance decisions and safety practices.

The term megger is a generic reference to insulation resistance testers, originating from the Megger brand name. Today, the instrument is widely used in electrical testing and is available in a range of portable and benchtop models to suit different voltage and accuracy requirements.