Testing connections. A megohmmeter usually is equipped with three terminals. The “LINE” (or “L”) terminal is the so-called “hot” terminal and is connected to the conductor whose insulation resistance you are measuring. Remember: These tests are performed with the circuit de-energized. The “EARTH” (or “E”) terminal is connected to the other side of the insulation, the ground conductor. The “GUARD” (or “G”) terminal provides a return circuit that bypasses the meter. For example, if you are measuring a circuit having a current that you do not want to include, you connect that part of the circuit to the “GUARD” terminal.
Insulation starts to age as soon as it’s made. As it ages, its insulating performance deteriorates. Any harsh installation environments, especially those with temperature extremes and/or chemical contamination, accelerate this process. This deterioration can result in dangerous conditions in power reliability and personnel safety. As such, it’s important to identify this deterioration quickly so that corrective steps can be taken. One of the simplest tests and its required test instrument is not universally understood. To help eliminate this lack of understanding, let’s discuss in detail Insulation Resistance (IR) testing and the megohmmeter.
This is the simplest of the tests. After the required connections are made, you apply the test voltage for a period of one min. (The one-min interval is an industry practice that allows everyone to take the reading at the same time. In this way, comparison of readings will be of value because, although taken by different people, the test methods are consistent.) During this interval, the resistance should drop or remain relatively steady. Larger insulation systems will show a steady decrease, while smaller systems will remain steady because the capacitive and absorption currents drop to zero faster on smaller insulation systems. After one min, read and record the resistance value. Note that IR is temperature sensitive. When the temperature goes up, IR goes down, and vice versa. Therefore, to compare new readings with previous readings, you need to correct the readings to some base temperature. Usually, 20°C or 40°C are used as comparison temperatures; tables are available for any correction. However, a common rule of thumb is that IR changes by a factor of two for each 10°C change.
For example, suppose we obtained an IR reading of 100 megohms with an insulation temperature of 30°C. The corrected IR (at 20°C) would be 100 megohms times 2, or 200 megohms. Also, note that acceptable values of IR will depend upon the equipment. Historically, field personnel have used the questionable standard of one megohm per kV plus one. The international Electrical Testing Assoc. (NETA) specification NETA MTS-1993, Maintenance Testing Specifications for Electrical Power Distribution Equipment and Systems, provides much more realistic and useful values. Test results should be compared with previous readings and with readings taken for similar equipment. Any values below the NETA standard minimums or sudden departures from previous values should be investigated.