The Number That Saved the Motor
Insulation Resistance Testing with a Megohmmeter
Eric W. Rogers
April 2026 · 8 min read
The motor had been running for eleven years without a single unplanned outage. It drove a large cooling fan in a chemical processing facility — not glamorous work, but critical. When that fan stopped, the whole line stopped.
The maintenance tech, Johnny, had been assigned to do a routine preventive maintenance check on the motor during a scheduled weekend shutdown. Most of his crew would have just greased the bearings, checked the coupling, and called it done. Johnny pulled out his megohmmeter.
The reading came back at 0.8 megohms.
Why that number mattered
The general rule of thumb for a 480V motor is a minimum of 1 megohm. At 0.8 megohms, Johnny's motor was already below that threshold. It was not failed yet — but it was trending toward failure. He flagged it for replacement before the plant came back online.
Three weeks later, the motor was swapped out during a planned maintenance window. The old motor was sent to a rewind shop. The shop confirmed what the megohmmeter had already told Johnny: the winding insulation had been compromised by moisture and heat cycling. A catastrophic failure was weeks away.
What a megohmmeter actually does
A megohmmeter — commonly called a megger — is a high-voltage resistance tester. It applies a DC test voltage (typically 500V, 1,000V, or higher) to the insulation of a motor winding, cable, or other electrical component and measures how much current leaks through that insulation.
The result is expressed in megohms (MΩ) — millions of ohms. High resistance means the insulation is doing its job. Low resistance means current is leaking where it should not be — through degraded insulation, moisture contamination, or physical damage.
Why not just use a multimeter?
A standard multimeter uses only a small test voltage — typically 9V or less — for resistance measurements. That is not enough to stress the insulation and reveal degradation. A megohmmeter applies hundreds or thousands of volts to simulate the stress the insulation experiences in service. That is what reveals the true condition of the insulation.
How to perform the test safely
Insulation resistance testing is a de-energized test. The equipment must be completely shut down, isolated, and locked out before you connect the megohmmeter. You are applying a high-voltage test signal — if the equipment is still connected to a live source, you risk equipment damage, instrument damage, or serious injury.
Pre-test checklist
- De-energize and isolate the equipment completely. Apply lockout/tagout.
- Disconnect the motor leads from the drive or starter. You are testing the motor winding in isolation — not the connected circuit.
- Select the correct test voltage. A common starting point: 500V for 240V-rated equipment, 1,000V for 480V equipment. Follow the equipment manufacturer's recommendation and IEEE 43 guidance.
- Connect the megohmmeter — one lead to the winding terminal, the other to the motor frame (ground).
- Apply the test voltage for the specified duration (typically 60 seconds for a spot reading).
- Record the result with the date, temperature, and test conditions.
- Discharge the winding after testing. The capacitive charge stored in the insulation must be safely dissipated before you disconnect the leads.
Interpreting the results
A single test result tells you where you are. A series of tests over time tells you where you are headed. That distinction is the difference between reactive maintenance and predictive maintenance.
Above 100 MΩ
Excellent
Insulation in good condition. Continue routine monitoring.
2–100 MΩ
Acceptable
Insulation acceptable but trending should be established.
Below 1 MΩ
Investigate
Below minimum threshold. Investigate before returning to service.
Temperature affects insulation resistance significantly — resistance drops as temperature rises. For valid comparisons over time, readings should be corrected to a standard reference temperature (typically 40°C). Most megohmmeter manufacturers provide correction factor tables.
Trending is the real tool
Johnny's motor read 0.8 MΩ. That number alone told him the motor was below the minimum threshold. But what if the motor had read 50 MΩ? That sounds fine — until you check the records and see it read 800 MΩ two years ago, 400 MΩ last year, and 50 MΩ today.
That trend is a warning. The insulation is degrading. The motor may still pass the minimum threshold today — but it is heading toward failure. Trending over time is what transforms insulation resistance testing from a pass/fail check into a genuine predictive maintenance tool.
What to record every time
- •Date and time of test
- •Equipment ID and location
- •Test voltage applied
- •Ambient temperature and humidity
- •Resistance reading (MΩ)
- •Temperature-corrected reading (if applicable)
- •Technician name
Mini Quiz — Test Your Knowledge
Select your answer for each question, then click Reveal Answers.
1. What does an insulation resistance tester (megohmmeter) measure?
2. Why can't a standard multimeter be used to test insulation resistance?
3. Per NFPA 70E and general industry practice, what must be done BEFORE connecting a megohmmeter to a motor or cable?
4. What is the general minimum acceptable insulation resistance for a 480V motor winding?
5. True or false: A megohmmeter test result of 500 megohms always means the insulation is in perfect condition.
The Bottom Line
Insulation resistance testing is not a one-time event. It is a discipline. One test tells you where you are. A series of tests over time tells you where you are headed — and that is the information that prevents failures before they happen.
What you must know cold
De-energize and Isolate First
Equipment must be fully de-energized, isolated, and locked out before connecting the megohmmeter
Select the Right Test Voltage
Match test voltage to the equipment rating — 500V for 240V systems, 1,000V for 480V systems is a common starting point
1 MΩ Minimum — But Trend Matters More
1 MΩ is the floor for 480V motors per IEEE 43. Declining trends are the real warning sign
Discharge After Testing
Always discharge the capacitive charge stored in the insulation after the test — before disconnecting leads
Record and Compare
Log every test result with date, temperature, and conditions. Trending over time is the diagnostic tool
Temperature Correction
Insulation resistance drops as temperature rises. Correct readings to a standard temperature (typically 40°C) for valid comparisons
A megohmmeter is not just a tester. It is a crystal ball — if you know how to read it.
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