How to Use a Megger to Test a Circuit

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Faulty wires causing flickering lights? A Megger is a lifesaver when attempting to find the faults in a wiring's insulation. A Megger insulation tester can offer a direct reading of insulation resistance measure in ohms or megohms. Good insulation has high resistance. Poor insulation has fairly low resistance.

How to Use a Megger to Test a Circuit
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What Is a Megger Tester?

The small, portable instrument is intended to be a high-range resistance meter. A Megger, slang for megohms, has a built-in direct-current generator. It is designed specifically with both current and voltage coils so that a true ohm can be read directly and free of the actual voltage applied. A high direct-current voltage made by either a hand-cranked or line-operated generator causes a small current to ride over and through surfaces of the installation without causing any damage. The current, typically about 500 volts, is measured by the ohmmeter and appears on the indicating scale. Good insulation with a high resistance will read in the megohm range.

How and When to Use

Time and weather can take down the insulation resistance of an electrical system. The measured resistance of the insulation is determined by the applied voltage paired with the resultant current. A few things can affect the current. Temperature and humidity are major causes for the breakdown of an electrical system's insulation resistance. To test, connect one lead of the Megger to the insulation and the other lead to the conductor. If you are testing the insulation for an electrical wire, the outer covering of the wire is the insulator and the copper wire inside the insulation is the conductor. Rotate the crank until you feel the slip clutch beginning to slide. Rotating the crank powers the DC generator that is connected to the meter. When the DC generator is turned on it makes a connection between the test lads, insulation and conductor.

Safety Measures & Tips

Whenever dealing with electrical projects, make sure to have a few things on hand. Safety goggles will be vital for protecting eyes from errant sparks or bits of floating insulation as the work is being completed. Wear gloves when you can, particularly at any point where the bare wires may be exposed. Long sleeves are highly recommended to protect skin from any exposed insulation. Always isolate the component that you intend to test. To test an older Megger model, use a working element, such as a washing machine or dishwasher electrical element, to ensure the Megger is in good working order before you begin.


Kimberley McGee is an award-winning journalist with 20+ years of experience writing for a variety of clients, including The New York Times, Las Vegas Review-Journal Home section and other national publications. As a professional writer she has researched, interviewed sources and written about home improvement, interior design and related business trends. She earned a B.A. in Journalism from the University of Nevada, Las Vegas. Her full bio and clips can be viewed at

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