megaohm

A megaohm (MΩ) represents one million ohms of electrical resistance. This unit describes the opposition to electrical current flow across a material or component, and is used in metrology and electrical testing where very high resistances are being measured. Most practical electrical work deals in ohms or kilohms; megaohm-range readings signal either insulation breakdown, leakage paths, or materials designed to block current almost entirely.

Megaohm measurements are routine in insulation resistance testing. A technician uses a megohmmeter (also called an insulation tester or ohmmeter) to verify that cables, motor windings, and installed equipment meet minimum safety standards. For example, a 600-volt motor winding might be required to show at least 1 to 5 megaohms of resistance between windings and ground, depending on the equipment age and standards applicable. Power cables in harsh environments or after water exposure are tested the same way. A reading below specification indicates moisture ingress, contamination, or insulation failure that demands repair or replacement before energizing the circuit.

The megohmmeter works by applying a high test voltage (typically 500 V, 1000 V, or higher) across the item under test and measuring the resulting leakage current. Since current equals voltage divided by resistance, a very small leakage current reflects a very large resistance value in the megaohm range. Digital instruments display the result directly in MΩ; analog instruments use needle deflection across a logarithmic scale that typically spans from fractions of a megaohm to over 100 MΩ.

Common ranges and failure points

Readings above 10 MΩ are usually considered excellent insulation. Readings between 1 and 10 MΩ often indicate acceptable condition but warrant monitoring or investigation depending on the equipment class. Readings below 1 MΩ generally signal a fault. A sudden drop from stable megaohm readings to kilohm or lower values suggests active degradation, wet conditions, or contamination on the insulation surface. Environmental factors matter: temperature, humidity, and salt spray all reduce measured resistance, so baseline testing is always done under standard lab conditions when absolute comparison is needed.

The term megaohm is straightforward SI nomenclature: the prefix mega- means 106, and ohm is the unit of resistance named after Georg Ohm. In the wider electrical trade, megaohm testing sits at the boundary between acceptance testing (new equipment installation), preventive maintenance (periodic re-testing of in-service assets), and fault diagnosis (identifying failure modes before catastrophic breakdown). Regulatory standards for power distribution, industrial motors, transformers, and telecommunications cables all specify minimum megaohm values. A technician who sees megaohm readings is almost always evaluating safety and reliability rather than measuring active circuit behavior.