A Megger insulation tester is a small, portable instrument that gives a direct reading of insulation resistance in ohms or megohms. Essentially, a Megger insulation tester is a high-range resistance meter that consists of a built-in direct-current generator. The Megger insulation tester is specially constructed with both current and voltage coils, which enables the device to read true ohms. Because of this design, ohms are read independent of the actual voltage that is being applied. The benefit of the Megger insulation tester is that its testing method is non-destructive, meaning there is no deterioration of the insulation from testing.


Primarily, a Megger insulation tester is used to determine if insulation is “good” or “bad.” Before we get into detail about what the two mean, it is important to discuss how “good” and “bad” insulation is measured. One rule of thumb to follow when using a Megger insulation tester is Ohm’s law. This equation is what is used to measure insulation:

Ohm’s Law Equation: E = I x R

E -voltage in volts
I – current in amperes
R- resistance in ohms

This equation is essential when using a Megger insulation tester to measure insulation.


To be able to truly understand the importance of a Megger insulation tester, it is crucial to understand the difference between “good” and “bad” insulation.

“Good” insulation does not mean that the insulation is perfect. As a matter of fact, no insulation is ever perfect. “Good” insulation essential means that the insulation itself has a relatively high resistance to the current. It also means that the insulation has the ability to keep a high resistance, meaning chances of fast deterioration are minimal.

“Bad” insulation is considered any insulation which low resistance. Obviously when a piece of new equipment is purchased, electrical insulation will likely be in tip-top shape, but after continual use and stresses to the insulation, it begins to lose its resistance rate over time. For example, time leads to pinholes or cracks, development of moisture and in some causes penetration of foreign matter. These stresses cause a leakage in the current, which in turn provides low resistance.