Megger Testing with a Megohmmeter

A megohmmeter, or megger, is a device used to test electrical and insulation resistance. This is usually done by sending a high-voltage signal into the object being tested, typically a wire or motor. Using a megger is important for avoiding electric shock and equipment damage. This article will walk through how and when to use a megohmmeter, and discuss how it compares to other tools.

To understand how megohmmeters work, it’s important to understand the measurements they use. The measurements produced by a megohmmeter, in their smallest part, can be reduced down to ohms. But what are ohms? Ohm is a measure of electrical resistance. The amount a material reduces the electric current that runs through it is the amount of electrical resistance.

Megohmmeters get their names by measuring large numbers of ohms. Meggers read values out in megohms, where 1 megohm is equal to 1,000,000 ohms.

But then why are megohmmeters sometimes called insulation testers? This is because, in order to test the electrical resistance of wires, for example, megohmmeters measure insulation resistance. Taking this measurement assesses the integrity of the insulation, which is important in avoiding electric shock and equipment damage. However, even though meggers are sometimes referred to as insulation testers because they can perform this function, they’re also usually limited to this function. Other tools referred to as insulation testers may have more capabilities, such as readings for voltage and current output. 

To test insulation, meggers use a high-voltage, low-current DC charge that measures the resistance within wires and motor windings to identify current leakage and faulty or damaged insulation. This is known as a meggering test. Meggering tests help verify the integrity of the wires or motors you are working with.

Meggers generate voltage to determine the high resistance value of insulation. Typically, the smallest value that a megohmmeter can supply is 1,000 volts, while some hand-crank meggers can supply as much as 10,000 volts or more through a small generator inside the meter. To run a megger test, follow these steps and refer to the megohmmeter's manual for complete safety guidelines.

Step 1. Cut power

Make sure you eliminate any voltage running through the wires you want to test. 

Step 2. Uninstall wires 

Disconnect the wires you want to test from both ends of the circuit and all feeding wires in motors.

Step 3. Connect the ground lead

Connect one of the megger’s leads to a ground connection such as the wire insulation, the electrical frame, or the ground.

Step 4. Connect to conductor

Connect the other megger lead to the conductor such as the exposed copper of the wire or motor terminal.

Step 5. Build voltage

Crank the generator handle to build voltage. This could take anywhere from two to five seconds.

Step 6. Read meter

Determining a safe reading depends on what you’re testing. Generally, a reading should equal one megohm for each 1,000 volts of operating voltage. For a motor that has a 1,500 operating voltage, a perfect reading would be 1.5 megohms. A minimum reading should never be less than one megohm.

Step 7. Complete testing

Complete testing of remaining wires or terminals.

It’s important to note that multimeters and megohmmeters are not the same thing. Multimeters measure electrical resistance and megohmmeters measure insulation resistance. While electrical resistance is a component in using a megger, the purpose of a megger test is to measure insulation resistance. This is done to check for faulty insulation that could cause electrical problems or damage. Multimeters do not have the ability to measure insulation resistance.

Megohmmeters are an essential device to ensure safety during electrical jobs. Understanding how to use them and read them can help prevent damage to expensive equipment, unplanned shutdowns, and dangers to personnel.  

The product statements contained in this guide are intended for general informational purposes only. Such product statements do not constitute a product recommendation or representation as to the appropriateness, accuracy, completeness, correctness or currentness of the information provided. Information provided in this guide does not replace the use by you of any manufacturer instructions, technical product manual, or other professional resource or adviser available to you. Always read, understand and follow all manufacturer instructions.