What Is a Motor Insulation Tester? Megohmmeter Basics and Pass Limits
What Is a Motor Insulation Tester? Megohmmeter Basics and Pass Limits
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A motor insulation tester is a meter that measures how well the insulation between a motor’s winding and its frame, or ground, resists current. In most shops it is a megohmmeter, which applies a high test voltage and reads resistance in megohms. A low reading suggests the insulation is damaged, wet, or contaminated, and a high reading suggests it is sound. The tool is used before a motor is energized after service and as part of routine trend checks.
Quick answer
- A motor insulation tester is usually a megohmmeter. Fluke notes that megohmmeters are often called insulation testers, though it says this is not strictly accurate.
- It sends a high test voltage into the winding and reads very high resistance values, in megohms.
- Fluke’s failed-motor guidance gives a minimum of 2 megohms to ground for AC devices and 1 megohm to ground for DC devices.
- Always isolate the motor and lock out and tag out the disconnect before testing. Fluke’s guidance says to do both.
The person typing “what is a motor insulation tester” wants
The person typing this phrase wants to know what the tool is, what it measures, and what a reading means for a motor. They are often trying to decide whether to buy one, or whether a motor reading they have seen is a problem. This guide covers the tool, the basic test, and the pass limits given by Fluke. It does not give a test voltage for every motor, because the sources do not set one.
What the tool measures
Insulation resistance is the resistance between conductors and ground, or between windings that should be isolated. A motor winding is wrapped in insulation to keep current in the copper. Over time, heat, moisture, dust, and oil can lower that resistance. An insulation tester applies a voltage and measures the small leakage current, then shows the result as resistance.
Fluke describes the megohmmeter in similar terms. Its page on insulation testers says a megohmmeter “sends a high voltage signal into the object being tested” to measure very high resistance values. It adds that megohmmeters are often informally called insulation testers, though it says “strictly speaking this is not accurate.”
Insulation tester or megohmmeter?
The two names overlap, so the difference matters when you buy. Fluke’s comparison page says an insulation tester “performs the basic measuring function a megohmmeter does,” and that it “usually performs more functions, including more complex testing and recording of measurements.” It also says that “unlike megohmmeters, insulation testers can also measure voltage and current.”
For a basic motor check, a megohmmeter does the core job. For a logging or multi-function job, an insulation tester with recording and voltage functions may fit better. The Fluke comparison page gives the Fluke 1550C FC as an example of a unit that “can generate up to 5000 volts for insulation tests.” That figure is for that model only, and you should check the test voltage of your own unit before choosing a setting.
How a basic motor test is done
Fluke’s failed-motor guide lists the steps that come before the reading. Read the full page before you test, but the key points are these.
- Isolate the motor. Fluke says to isolate electronic controls and devices from the circuit, and to “lock out and tag the disconnect to the motor starter.”
- Set the tester to the right test voltage. The guide says, “Set the tester to the appropriate test voltage.” It does not give a voltage for every motor, so use the motor nameplate, the tester manual, and your site procedure.
- Take the reading. Record the value and the time, and compare it with the pass limits in the next section.
- Compare line and load circuits. Fluke says that “To pass these tests, the line and load circuits need to show high resistance.”
Fluke also says the winding test “strictly provides a point-in-time measurement.” That is why the same guide recommends tracking readings over time, and the HVAC compressor page calls trending “the key to predictive maintenance.”
Pass limits in the sources
The pages we opened give two sets of limits. They are not the same, so it helps to know which applies to your motor.
| Source | Limit stated | Context |
|---|---|---|
| Fluke failed-motor guide | Minimum 2 megohms to ground for AC devices; 1 megohm to ground for DC devices | Pass limit in a troubleshooting context |
| Fluke HVAC compressor guide, citing IEEE 43 | Minimum of 1 megohm plus 1 megohm per kilovolt of motor operating voltage | For a 460 V motor, the guide works out to about 1.46 megohms |
| Fluke HVAC compressor guide | A motor “may be unacceptable for use if IRT values are below 100 megohms” | Stated for modern insulations, regardless of surface contamination |
The IEEE 43 formula gives a floor. The 100-megohm figure in the HVAC guide is a stricter concern for modern insulation. A reading between the floor and 100 megohms is not an automatic failure, but it calls for a closer look and a trend check.
Reading the result
A single reading can mislead. The Fluke failed-motor guide says that “If the readings differ by more than a few percent, the motor is probably unsafe to energize.” That statement refers to readings taken on the same motor under the same conditions, so keep test conditions the same across readings.
The HVAC guide also says that readings change with temperature. It says readings are corrected to a chosen baseline, such as 40 °C. We did not include the correction factor in this guide, because the wording of the factor in the source we opened needs a direct check against the original page before it is quoted.
Use a trend. Log each reading, the date, the test voltage, and the motor temperature if you have it. A steady decline matters more than one value.
Safety notes
An insulation test applies high voltage to the winding. Lock out and tag out the disconnect, confirm the motor is de-energized, and follow the tester’s own safety instructions. Discharge the winding as the tester manual directs before you touch the leads. Do not test a motor that is still connected to a live circuit or a drive.
How we chose
We limited this guide to three Fluke pages that cover the motor insulation test directly: the failed-motor troubleshooting page, the insulation tester versus megohmmeter page, and the HVAC compressor motor page. We did not pick a product. Where a model is named, it is only as an example of the stated function or range.
What we could not verify
- The sources we opened do not give one test voltage for all motors. Use the nameplate, the tester manual, and your site procedure.
- We could not confirm the 60-second spot test length or the temperature correction factor from the pages we opened, so we do not state them here.
- The pages do not state which pass limit a particular motor manufacturer requires. Check the motor manual.
- We did not open the IEEE 43 standard itself. We cite the figure as the Fluke HVAC guide states it.
FAQ
Is a motor insulation tester the same as a megohmmeter?
In most shop use, yes. Fluke notes that the term insulation tester is informal for a megohmmeter, and that insulation testers can also measure voltage and current.
What is a good reading for a motor?
The sources give minimum values, not a target. Fluke’s failed-motor guide says 2 megohms to ground for AC devices and 1 megohm for DC devices. The HVAC guide says readings below 100 megohms may be unacceptable for modern insulation. Use the stricter figure when the motor is important.
Why do I need to lock out the motor?
Fluke says to lock out and tag the disconnect to the motor starter, and to isolate electronic controls before testing. This prevents the motor or its controls from being energized during the test.
Can one reading tell me the motor is bad?
Not on its own. Fluke describes the test as a point-in-time measurement, and recommends tracking readings over time.
What does the megohm per kilovolt rule mean?
The IEEE 43 figure cited by Fluke is 1 megohm plus 1 megohm for each kilovolt of motor operating voltage. For a 460 V motor, that is about 1.46 megohms.
Do I need a special meter?
You need a tester that can apply a test voltage high enough for the winding and show megohm values. The Fluke comparison page describes the functions. Check the test voltage range on any model against your motor.
Bottom line
A motor insulation tester is a megohmmeter that measures winding insulation resistance at a set test voltage. Isolate and lock out the motor, use the pass limits from your motor and standard, and track readings over time rather than relying on one value.
Internal reading
For related insulation testing, read How to Use an M500 Megohmmeter: Basic Steps and Megger Test Electric Motor: 3 Insulation Testers Compared.
Sources
- Fluke, “5 Steps to Test Insulation Resistance in Failed Motors”, https://www.fluke.com/en-us/learn/blog/insulation-testers/troubleshoot-failed-motors-with-insulation-resistance-testing (opened 2026-10-28)
- Fluke, “Insulation Tester vs Megohmmeter”, https://www.fluke.com/en-us/learn/blog/insulation-testers/insulation-tester-or-megohmmeter (opened 2026-10-28)
- Fluke, “HVAC Compressor Motors Insulation Resistance and Testing”, https://www.fluke.com/en-us/learn/blog/hvac/insulation-resistance-and-electrical-testing-for-hvac-compressor-motors (opened 2026-10-28)
