What Is a Megohmmeter? How It Works and When to Use One

What Is a Megohmmeter? How It Works and When to Use One

ByKen

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What is a megohmmeter? It is an instrument that measures very high electrical resistance, usually the resistance of insulation, and reports the result in megohms (MΩ) or gigaohms (GΩ). It applies a DC test voltage across the material under test and measures the very small current that leaks through it. Less leakage current means higher resistance, and higher resistance means the insulation is in better condition.

This guide explains what the instrument measures, why it uses a high test voltage, what changes the reading over time, and how the terms and test values are used in published guidance. It does not replace the manufacturer’s manual for any specific model.

Quick answer

A megohmmeter is a high-voltage resistance tester built for insulation. A standard multimeter’s ohms function is not designed for the megohm and gigohm range, while a megohmmeter applies a higher voltage, often up to 1,000 V or more, and reads the resulting leakage. Use it when you need to know whether the insulation on a cable, motor winding, or other equipment still keeps current where it belongs.

What the instrument measures

Fluke describes insulation resistance as a qualitative value for the condition of conductor and equipment insulation, reported in megohms. The instrument does not measure a cable’s conductivity. It measures how well the material between conductors, or between a conductor and ground, resists current flow. A reading that falls over time or is lower than expected points to a problem such as moisture, contamination, or deterioration.

Keysight’s guide describes the same basic principle. A megohmmeter sends a high-voltage signal through an insulator and measures how much current passes. The less current that flows, the higher the resistance and the better the insulation. Keysight notes that these readings reach the megohm and gigohm range, which general-purpose meters do not resolve well.

Why it uses a high test voltage

Insulation is designed to block current, so the current that leaks through it is tiny. A low test voltage would produce a current too small to read reliably and would not stress the insulation the way service voltage does. Keysight says megohmmeters operate at higher voltages than regular ohmmeters, up to 1 kV or more. Fluke’s guide notes that the DC test voltage should follow the manufacturer’s recommended value. If the manufacturer does not give one, industry practice data is used instead.

Fluke states that many handheld testers supply up to 1,000 V, while others supply 5,000 V or more. Across the products Fluke compares, test voltages run from 50 V to 10,000 V. The right voltage depends on the equipment, so a higher setting is not automatically better.

The three currents that change the reading

Fluke explains that applying a DC voltage produces three currents at once. These currents change the reading over the course of a test.

  • Capacitive charging current flows first and makes the reading look low at the start. In good insulation, the reading rises as the insulation charges.
  • Absorption, or polarization, current flows more slowly. Moisture or contamination raises this current, which lowers the resistance reading.
  • Leakage current flows through the insulation and across its surface. It is the current that a clean, dry insulation should keep very low.

Because of these three currents, a single reading taken at one instant can be misleading. A reading that climbs and levels off over several minutes usually indicates healthier insulation than one that stays flat or falls.

Megohmmeter and insulation tester: the same instrument?

Fluke’s guides use the terms insulation tester and megohmmeter interchangeably. Some of its product names combine both, and the same family of instruments is described under either name. Keysight also calls a megohmmeter an insulation tester. When you shop for or read about one, the name on the label matters less than the specifications: test voltage, measurement range, and the test modes the unit supports.

Analog and digital models

Keysight describes analog megohmmeters as durable and simple to read, with a needle gauge. Digital models add features such as data logging, but they need batteries or another power source. Digital units can also perform timed calculations for you, which Fluke notes for some newer testers. Choose the format based on whether you need a quick spot reading, a logged trend, or a rugged tool for field use.

Test values and minimum readings

Fluke’s guide lists minimum values from NETA guidance, which applies when manufacturer data is not available. The minimum recommended resistance depends on the nominal voltage of the equipment and the test voltage used:

  • 250 V equipment: 500 V DC test, recommended minimum 25 MΩ.
  • 600 V equipment: 1,000 V DC test, recommended minimum 100 MΩ.
  • 1,000 V equipment: 1,000 V DC test, recommended minimum 100 MΩ.
  • 5,000 V equipment: 2,500 V DC test, recommended minimum 1,000 MΩ.

Fluke states that minimum insulation values should be based on the one-minute spot reading. These figures are reference points from the cited guidance, not a substitute for the equipment manufacturer’s requirements.

Polarization index in brief

Fluke’s guide describes the polarization index (PI) as a way to judge insulation condition over time. Take one reading at one minute and another at ten minutes, then divide the ten-minute reading by the one-minute reading. Fluke notes that ten minutes is the industry standard test duration and that the polarization index should never be less than 1.0. Record both readings and compare new results with earlier ones, since the trend matters as much as a single number.

Common uses

Keysight gives three examples of insulation resistance testing. In cable manufacturing, testing confirms that cable jackets can handle environmental stress without degrading. In solar (PV) installations, routine checks help catch leakage currents that could weaken system integrity. In elevator maintenance, testing motor winding insulation helps reduce the risk of wear-related failures. Fluke’s guide adds that trend testing charts resistance over months or years, because a large change can signal a developing failure.

Safe use: what to check first

Read the manufacturer’s safety instructions for your model before you connect it. Confirm that the equipment is isolated and that the test voltage matches the equipment rating. Check the leads and the probes for damage. Insulation work on mains equipment should be carried out by a qualified person. Keep the test records so that a later reading can be compared with the earlier one.

Fluke’s guide also notes that readings are usually normalized to 20 °C, and that resistance changes with temperature. Record the temperature and humidity with each reading so that results from different days can be compared fairly.

What we could not verify

  • Model-specific discharge times, lockout steps, and any maximum distribution-system voltage for a given megohmmeter. The Keysight quick-start PDF returned no readable text, so its safety figures are not included here.
  • Exact test voltages and measurement ranges for any individual instrument. Check the datasheet or manual for the model you own.
  • Whether the NETA minimum values apply to your equipment type. Confirm the requirement with the equipment manufacturer or the applicable code.

How we chose

This explainer draws on three sources: two Fluke Learn articles on insulation resistance testing and a Keysight Used Learn guide on resistance instruments. Each was opened and read as text. Where a point appeared only in a search summary or an unreadable PDF, it was left out or placed under what could not be verified.

FAQ

Is a megohmmeter the same as an ohmmeter?

No. An ohmmeter measures ordinary resistance at low test voltages. A megohmmeter applies a higher DC voltage to measure insulation resistance, which is typically in the megohm and gigohm range.

What does the reading tell me?

A high, steady reading generally indicates good insulation. A low or falling reading can indicate moisture, contamination, or deterioration. Compare results over time, because a single reading has limited meaning on its own.

Why do the readings change during a test?

Capacitive charging, absorption, and leakage currents all flow at different rates after the voltage is applied, so the reading rises or settles over the test period.

Related guides

For a step-by-step walkthrough, read How to Use a Megohmmeter: 3 Listings Compared. For background on the broader category, see Best Megohmmeter: 6 Insulation Testers Compared.

Sources

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