Why Your Multimeter is Lying: A Mentor's Guide to Insulation Resistance Testing

If you’re an electrician, a motor technician, or a serious DIYer, you’ve been here: a motor keeps tripping its breaker. You unhook it, pull out your trusty multimeter, set it to Ohms, and test the windings to ground.

The meter reads “O.L.” (Open Line).

“Well, the insulation’s good,” you mutter. You replace the $1,000 drive, and the system still trips. You’re frustrated, the client is angry, and you’re baffled.

As your mentor in this field, I’m here to tell you the hard truth: your multimeter lied to you.

It didn’t mean to. It just wasn’t built for the job. You brought a ruler to a weight-lifting competition. To understand why, let’s use the simplest analogy in our trade: the water pipe.

The Water Pipe Analogy (But With a Critical Update)

The [Original Article] was right to use the water pipe analogy, but it missed the most important part.

• The Copper Wire = The Pipe: It’s the path for the “stuff” to flow.
• The Electricity = The Water: It’s the “stuff” flowing through the pipe.
• The Insulation = The Pipe Wall: Its only job is to keep the water inside the pipe.

But here is the key concept that changes everything: * The Voltage = The WATER PRESSURE.

Now, let’s look at your tools through this new lens.

What Your Multimeter Actually Tests

When you set your multimeter to the Ohms (Ω) or Continuity setting, you are performing a low-pressure continuity test.

Your multimeter is powered by a tiny 9-volt battery. When you test for insulation, you are essentially sending a 9-volt trickle of water through a massive industrial fire hose that is rated for 1,000 PSI.

What can this 9V trickle tell you? It can only tell you one thing: if the pipe is catastrophically severed in two. If the “pipe wall” (insulation) is so completely destroyed that the copper is touching the metal frame, your meter will beep.

But what if the pipe wall is 99% worn through? What if it’s covered in microscopic cracks, brittle from heat, and damp with moisture? Your 9V trickle will flow right past it without a single leak. The meter will read “O.L.,” and you’ll declare the pipe “good.”

Then, you hook it back up to the “city water main”—a 480-volt, three-phase system—and that 480V of “pressure” blasts water right through those invisible cracks, creating a dead short.

Your multimeter didn’t test the strength of the insulation. It only tested if it existed.

What a “Megger” Actually Tests

This is why professional electricians use an Insulation Resistance Tester, often just called a “Megger” (after the brand that popularized it).

A megohmmeter is not a continuity tester. It is a high-pressure tester.

A tool like the Megger MIT2500 is a portable, high-voltage DC generator. It doesn’t use a 9V battery. It uses its battery to generate 500V, 1000V, or even 2500V of test voltage.

Now, you are doing a real test. You are connecting this tool to your “fire hose” (the motor winding) and filling it with 1000V of “water pressure” to see if the “pipe wall” (the insulation) can hold it.

The tool then measures the leakage. It’s not looking for “O.L.”; it’s looking for an extremely high resistance value, measured in megohms (millions of ohms) or even gigaohms (billions of ohms). * A High Reading (e.g., 2,000 MΩ): The pipe wall is strong. It’s holding the pressure. The insulation is good. * A Low Reading (e.g., 2 MΩ): The pipe wall is leaking. The 1000V of pressure is forcing electricity through all those microscopic cracks and pores. This motor is a failure waiting to happen.

You’ve just found the “invisible” problem your multimeter completely missed.

Why Would You Need 2500 Volts?

This brings us to the tool itself. Why does a handheld device like the Megger MIT2500 cost over $2,000 and, as one user pointed out, generate a massive 2500V DC?

This isn’t for testing your refrigerator (which is why the [Original Article]’s example was so mismatched). This is for professionals working on high-voltage systems. * The Rule of Thumb: You test insulation at twice the operating voltage. * Standard Motors (480V): You’d use a 1000V (1kV) test. * Medium-Voltage Systems (above 1000V): For applications like industrial switchgear, large-scale motors, or utility-grade cables, you need to test at or above the operating voltage.

The MIT2500’s 2500V capability is what makes it a professional, certified tool for industrial and utility applications, all in a handheld, battery-powered device. That is the engineering marvel you are paying for.

Conclusion: The Right Tool for the Right Job

Let’s go back to our failed motor.

The technician with only a multimeter is stumped. He’ll probably replace the motor ($$$$) or the drive ($$$$), and the problem will still be there, because the “leak” was actually in the cable feeding the motor.

The technician with an insulation tester does this:
1. Test Motor: Disconnects and tests motor windings to ground at 1000V. (Reading: 1,500 MΩ). Result: Motor is healthy.
2. Test Cable: Disconnects and tests the cable (Phase A) to ground at 1000V. (Reading: 1.2 MΩ). Result: Cable is failing.

In five minutes, you have diagnosed the problem with 100% certainty. You’ve saved the client thousands of dollars in unnecessary parts and saved yourself from a costly, embarrassing callback.

A multimeter is for checking continuity (is it dead or alive?). A megohmmeter is for checking integrity (is it healthy or sick?). Stop asking your multimeter to do a job it was never designed for.