Okay, let's break down what's happening and address your concerns. You're right to be skeptical and thorough. Here's a comprehensive look at your situation, testing procedures, and how to move forward

Your Concerns are Valid:
The Fluke 757 vs. a "Real" Megger

You're correct to differentiate. While the 757 is an insulation resistance tester and can provide valuable information, it has limitations compared to a high-end megohmmeter, especially when it comes to higher voltage testing, polarization index (PI), and dielectric absorption ratio (DAR) measurements. These advanced features are crucial for a thorough motor insulation assessment.

Lack of Baseline & Confidence

The biggest problem is the lack of a baseline and the uncertainty in interpreting the readings. You need a system to determine what constitutes a "good" vs. "bad" motor.

The Constant 2 MΩ Reading

This is a major red flag. It indicates something is wrong with your test setup, the instrument itself, or your understanding of how to use it.

Analyzing Your Current Procedure



Voltage Setting (1000V)

This is likely too high for 480V motors. While the 757 can go that high, you generally want to use a voltage that's roughly twice the motor's rated voltage + 1000V (per IEEE standards) unless the motor manufacturer specifies something different. So, for a 480V motor,

a 500V test voltage is generally recommended. Exceeding the recommended test voltage doesn’t necessarily mean you’ll damage the insulation, but it can accelerate degradation. Lead Connections

This sounds correct based on your description, but double-check your Fluke 757 manual to be absolutely certain. Pay close attention to where the "test" lead (with the button) should be connected. Incorrect lead placement will absolutely skew the readings.

Testing Between Phases and to Casing

This is the correct procedure for insulation resistance testing. You need to test all phase-to-phase combinations (A-B, B-C, C-A) and each phase to ground (A-Ground, B-Ground, C-Ground).

The 2 MΩ Reading with No Leads Connected

This is the most concerning point. This should not happen. It suggests:

A Faulty Meter

The 757 itself might be malfunctioning. This is the first thing to investigate.

Incorrect Settings

Double-check that you have the meter set to Insulation Resistance (Megohm) mode, not some other resistance or continuity test.

Lead Issues

Although less likely, test the leads for continuity and isolation from each other. A shorted lead could cause this behavior.

Steps to Rectify and Improve Your Testing


Verify Meter Functionality



Use a Known Resistor

Ideally, get a high-value, precision resistor (e.g., 1 MΩ or 10 MΩ, rated for the test voltage). Connect it to the 757 and see if you get an accurate reading. This will help determine if the meter is actually functioning correctly.

Check the Manual Again (Carefully)

Read the Fluke 757 manual very carefully, paying close attention to the test setup instructions, voltage ranges, and troubleshooting sections. Pay attention to the setup for insulation resistance testing.

Contact Fluke Support

Fluke has technical support. Contact them and explain the issue of getting a 2 MΩ reading with no leads connected. They can offer specific troubleshooting advice for your meter model.2.

Establish a Correct Testing Procedure



Voltage Selection

Use 500V for your 480V motors unless the manufacturer specifies otherwise.

Complete Testing

Test all phases to each other (A-B, B-C, C-A) and all phases to ground (A-Ground, B-Ground, C-Ground). Record each reading.

Test Time

Apply the test voltage for a minimum of 60 seconds per test. Insulation resistance readings tend to increase over time as the insulation becomes polarized.3.

Understanding and Interpreting Results (Establish a Baseline)



IEEE Standards

IEEE Standard 43-2000 (Recommended Practice for Testing Insulation Resistance of Rotating Machinery) is the gold standard for motor insulation testing. You should get a copy or access to this standard. It provides guidance on acceptable insulation resistance values based on motor voltage and other factors.

Temperature Correction

Insulation resistance is very temperature-dependent. Colder motors will have higher resistance readings. IEEE 43 provides correction factors to adjust readings to a standard temperature (usually 40°C). You must record the motor temperature at the time of testing.

Minimum Acceptable Values

IEEE 43 provides formulas and tables to determine minimum acceptable insulation resistance values. A simplified rule of thumb is:

1 MΩ + (kV x motor HP) = minimum acceptable resistance to ground. This is a very general guideline. Trending Data

The most valuable information comes from trending insulation resistance readings over time. Test your motors periodically (e.g., every 6 months or annually) and record the results. A sudden drop in insulation resistance is a sign of insulation degradation.4.

Consider Polarization Index (PI) and Dielectric Absorption Ratio (DAR)

These are more advanced tests that require taking readings at multiple time intervals (e.g., 30 seconds and 10 minutes for PI). PI and DAR are more sensitive indicators of insulation health than a simple spot reading.

Limitation with the Fluke 757

The Fluke 757 may not have the ability to easily perform PI and DAR tests. You may need to manually record the readings at different times. If PI and DAR are important, consider a meter that can automate these tests.5.

Educate Maintenance



Share Your Findings

Once you have a better understanding of the testing process and how to interpret the results, share your findings with the maintenance team. Explain the importance of proper wiring and the difference between a wiring fault and a true insulation failure.

Establish Clear Criteria

Work with maintenance to establish clear criteria for when a motor should be pulled from service. Base these criteria on insulation resistance readings, trending data, and visual inspection.

Training Resources



IEEE

IEEE offers training courses and webinars on motor insulation testing.

Megger (Ironically)

Even though you didn't buy their megger, Megger has excellent application notes and webinars on insulation testing principles.

YouTube

Search for "motor insulation testing," "megohmmeter," and "IEEE 43." Look for videos from reputable sources like Fluke, Megger, or electrical training companies. But be critical of the information – make sure it aligns with industry best practices.

In Summary: Address the 2 MΩ reading issue immediately. It indicates a problem with the meter or your setup. Refine your testing procedure, paying attention to voltage selection, test duration, and temperature correction. Focus on establishing a baseline and trending data over time. Consider the limitations of the Fluke 757 and whether you need a more advanced megohmmeter for PI and DAR testing. Educate maintenance on proper wiring practices and the criteria for identifying a "bad" motor.By following these steps, you'll be able to confidently assess the condition of your motors and make informed decisions about their maintenance. Good luck! Flag for review