Tech Basics: Testing Insulation

Remember when it was your job to fill the swimming pool for younger siblings? Oh, the pain — the baseball field and all your friends were waiting. After all, filling a pool eight feet in diameter and two feet deep takes some time. So, you decide to drag out the garden hose, secure it so it won’t fall out, turn on the water, and then sneak out to the front yard for a couple of innings. Unfortunately for you, little Bobby has decided to choose this day to retaliate for all the brotherly torture you bestowed upon him this summer. Bobby gets an ice pick out of Mom’s kitchen drawer. With the deftness and speed of a 20-year lawn care guy, he aerates the garden hose — with two hundred holes. How much water do you suppose is making it to the pool now?

This example illustrates what can happen to wire when its insulation breaks down. Electrical current leaks out. The wire is called a conductor, and its job is to transport electric current. Insulation protects the wire and its job is to “resist,” or keep the current from leaking out. All insulation will leak some current, even if it is a minute amount. Some factors that cause insulation to break down include moisture, dirt, acid, humidity, mechanical damage, and vibration.

To complete our comparison to the garden hose, picture the hose as insulation. Resistance is a byproduct of the hose’s construction. The water is current, and the water pressure is voltage. Open up the faucet and you get more pressure. With increased pressure comes more water. Increased voltage equals less resistance and more current.

Insulation And The HVACR Field

The conductors and insulation we are most concerned with in the HVACR field are in electrical motors, the most common of which are in air conditioning and refrigeration compressors. The arch rivals of a good, clean, operating compressor are liquid refrigerant, moisture, and acid. Because we cannot see exactly what is happening within the compressor, we need to create a window.

The Window

Our window to the inner workings of a compressor’s conductors and insulation is a megohmmeter. The megohmmeter is a piece of electrical test equipment that reads high-range ohms called megohms. In order to accurately read megohms, the megohmmeter generates its own electrical current, which travels over and through the insulation being tested. When testing insulation, the power source should always be disconnected from the compressor. For convenience and safety purposes, some megohmmeters include a built-in voltage test circuit making it easy to ensure the compressor is not energized. The megohmmeter’s true value is realized over time and through multiple looks inside the window.

Your Best Friend

The megohmmeter is the maintenance technician’s best friend. Ideally, when a piece of equipment is installed, a baseline reading should be taken on the compressor’s electrical insulation. This value gives the technician a basis of comparison for future measurements. During each maintenance visit, a reading should be documented. A collection of these readings is called a trend. If the trend shows megohm readings are continuously decreasing over time, it can indicate a potential problem. At this point, the technician can make recommendations to the customer. Depending on the situation, the best course of action might be to recover refrigerant, replace driers, and perform a good evacuation. Or it might be to replace the compressor prior to complete failure and save the customer considerable aggravation.

Professional Discipline

The practice of obtaining and documenting information like megohm readings during each maintenance visit is the sign of a true professional technician. This information helps to determine the efficient mechanical operation of the equipment. It also helps to separate the professional technician from the “fly-by-nighters” in the customer’s eye.

Rothacker is a member of the National Comfort Institute’s Advisory Board and a National Comfort Team Founding Member. For questions or comments on Tech Basics, contact Rothacker at

Publication date: 09/22/2003

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