Megohmmeter

A megohmmeter, commonly called an insulation resistance tester, is a portable or benchtop instrument used to measure the resistance of electrical insulation. It applies a known DC voltage to the insulation and measures the resulting current; the resistance is displayed in megohms, with higher values indicating better insulation.

Operation: The device charges insulation with a high voltage, typically from 250 V up to several kilovolts, depending on the application. The meter calculates R = V / I and presents a reading in MΩ. Because insulation can polarize, readings may drift; some devices offer polarization index (PI) and dielectric absorption ratio (DAR) tests to assess long-term stability.

Typical equipment ranges: low-voltage instruments for electrical installations use 250–1000 V; high-voltage megohmmeters may apply 2–5 kV or more. Handheld models are common for field work; benchtop units provide higher voltages and features such as data logging and remote interfaces.

Applications: used to assess insulation in electrical installations, motors, transformers, cables, switchgear, insulators, and non-metallic components. Tests are performed with equipment de-energized and capacitors discharged; proper grounding and safety procedures must be observed. Readings should be interpreted considering temperature, humidity, insulation type, and time since last insulation stress.

Limitations: measurements can be affected by surface leakage, moisture, contamination, and polarization; not suitable for live circuits; high voltage requires safety precautions. Calibration and appropriate test duration are important, as short tests may give misleading results. Results guide assessments of insulation integrity and maintenance planning, with acceptance criteria varying by equipment and applicable standards.