Meggers

Megger is a type of electrical instrument used to measure the insulation resistance of electrical equipment by applying a high direct current voltage and measuring the resulting leakage current. The term megger originally derives from the Megger brand, which has become a generic name for insulation resistance testers. The plural form Meggers can refer to multiple such devices or, informally, to practitioners who perform insulation tests with them.

Meggers are used in the maintenance and testing of a wide range of equipment, including cables, wiring, motors, transformers, and switchgear. They are commonly employed during commissioning, periodic maintenance, and fault investigation to assess insulation integrity. Readings are typically reported in megaohms (MΩ); higher values indicate better insulation, while low readings suggest insulation degradation, moisture, contamination, or damage. Test voltages usually vary by application, ranging from tens to thousands of volts, and may be accompanied by longer tests to obtain more stable results.

Operation and interpretation: A megger supplies a DC voltage to the insulation under test and measures the leakage current to calculate resistance. Some tests use additional indices, such as polarization index (PI) and dielectric absorption ratio (DAR), to provide a fuller picture of insulation condition. Proper discharge of capacitive insulation after testing is essential to safety.

Safety and standards: Testing involves high voltage and can pose shock or arc risks. Tests should be performed only on de-energized equipment by trained personnel, with appropriate personal protective equipment and adherence to local electrical safety practices.

See also: insulation resistance testing, polarization index, dielectric absorption ratio.