A Megohmmeter, colloquially known as a megger, is an instrument designed to measure high resistance values. While a conventional voltmeter uses a low voltage to measure resistance. A typical megger applies high voltage to the circuit with voltages ranging from 250 up to 10,000 volts.
The megger, also known as an insulation resistance tester, is often used to measure the resistance (health) of motor windings, coaxial cables (with caution), and even the health of power cables used in electric vehicles. A hand-crank generator was used in the early meggers. Modern units such as the Fluke 1507 pictured in Figure 1, feature specialized high voltage probes and the prominent TEST button to activate the high voltage.
Note that the term megger may be used as a noun to identify a specialized piece of test equipment, or as a verb to describe the procedure. The use is context dependent. For example, it would be acceptable to say. “Go megger the old motor using the new meggers 1,000 V setting.”
Figure 1: Image of the Fluke 1507 with a large TEST button to active the high-voltage.
** Tech Tip**: Close inspection of Figure 1 reveals a specially designed probe used for insulation testing. The thicker probe has a built-in pushbutton used to remotely activate the high-voltage test. The probe also has a unique connection to the meter to communicate a button press.
How is a megger used?
A common megger application is to measure the resistance (health) of an induction motor’s stator windings. To perform the test:
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Warm up the motor for an extended time to drive out moisture. If commissioned, run the motor for a few hours. Alternatively, activate the motor’s anti-condensation heaters (if so equipped). Another potential option is to apply a controlled low-voltage, high-current source to heat the windings in accordance with the manufacturer’s instructions.
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Lock Out and Tag Out (LOTO) the system.
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Disconnect the motor.
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Connect the megger’s ground lead to the motor’s frame and the high voltage lead to a motor winding. Avoid contact with auxiliary devices such as the heater or temperature sensor.
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Perform the high voltage test and record the results. Be sure to perform the test at the manufacturer’s recommended voltage.
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Periodically perform the test to capture long-term motor health trends.
Tech Tip: As implied in the previous steps, moisture degrades the megger’s insulation tests skewing the megger results. Heat is often used to drive out the moisture.
Does DigiKey offer a megger?
This last year we received over 100 questions asking if DigiKey offers a megger. Yes, DigiKey offers several solutions including:
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Fluke 1507 as pictured in Figure 1
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Klein ET600 as pictured in Figure 2
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At least 50 other units are available by searching for the keywords “insulation tester”
Figure 2: Image of the Klein ET600 insulation tester.
Tech Tip: Be certain you know what is connected to your megger before you push the test button. Never megger yourself or another person as the high voltage can be dangerous. A megger may destroy sensors or sensitive electronic assemblies if used carelessly. For example, a megger applied to a coax cable will destroy a Low Noise Amplifier (LNA) if it is connect during the high-voltage test.
Best wishes,
APDahlen
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About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.