Megohm
A megohm (MΩ) is a unit of electrical resistance equal to one million ohms, used to express very high resistance values, particularly insulation resistance. In manufacturing, a megohm reading is obtained with a megohmmeter or megger, which applies a high DC test voltage and measures leakage current to compute resistance. Higher megohm values indicate lower leakage and healthier insulation.
On a CNC shop floor, megohm testing is a routine pre-commissioning and troubleshooting step for motor windings, servo drives, power cables, heaters, and control panels. A technician connects a megohmmeter between a conductor and ground, applies a high DC voltage, and reads the insulation resistance in MΩ or GΩ. Ordinary multimeters do not reach this range, so a megger is required. This matters because coolant washdowns, humidity, and metal dust can create surface leakage paths that lower insulation resistance. By catching moisture ingress, cracked insulation, carbon tracking, or aging dielectric breakdown early, the machinist avoids nuisance trips, ground faults, and unexpected downtime. A consistently high reading confirms the equipment can handle operating voltage safely; a low reading signals the need for cleaning, drying, or component replacement before returning the cell to production.
- Moisture contamination after washdown: Coolant, condensation, or wet dust lowers insulation resistance, producing a poor megohm reading and causing nuisance trips or unsafe operation even though the machine appears fine.
- Testing without isolating electronics: Applying megger voltage to VFDs, PLC input modules, or surge suppressors can damage sensitive components or produce misleading readings because these circuits are not designed for high-voltage insulation testing.
- Dirty terminals and carbonized surfaces: Conductive dust, oil film, or heat-damaged insulation creates surface leakage paths that reduce measured megohms, leading to intermittent faults, ground leakage, or insulation breakdown under load.
Is a megohm a unit of resistance or a test instrument?
A megohm is a unit of electrical resistance equal to one million ohms. The instrument used to measure it is called a megohmmeter or megger, which applies high DC voltage and measures leakage current.
Why are megohm readings used instead of ohms when checking insulation?
Insulation resistance is extremely high, so expressing it in raw ohms is impractical. Megohms (MΩ) and gigohms (GΩ) provide a convenient scale for comparing leakage levels and spotting insulation degradation.
What does a high megohm reading mean on a CNC motor winding?
A high reading means less leakage current and generally healthier insulation. It indicates the winding is dry, clean, and free of carbon tracking or dielectric breakdown, making it safe for commissioning or continued operation.