Testing insulation resistance is essential to keeping wires and motors working properly. Megohmmeters provide a quick and easy way to determine the condition of the insulation on wire, generators, and motor windings. A megohmmeter is an electric meter that measures very high resistance values by sending a high voltage signal into the object being tested. Typically, however, this is the only function a megohmmeter performs.
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While megohmmeters are often informally referred to as insulation testers, strictly speaking this is not accurate. Why? What&#;s the difference between a megohmmeter and an insulation tester? An insulation tester performs the basic measuring function a megohmmeter does &#; measuring very high resistance values by sending a high voltage signal into the object being tested &#; and it often does much more; it usually performs more functions, including more complex testing and recording of measurements.
A full-featured insulation tester can perform high-voltage insulation resistance tests and much more
For example, unlike megohmmeters, insulation testers can also measure voltage and current. The Fluke FC insulation multimeter, for example, can perform insulation tests up to volts, and it is a full-function digital multimeter. The Fluke C FC can generate up to volts for insulation tests.
Insulation testers can also perform more complex tests, such as compensating for ambient environmental conditions like humidity and temperature during a test, which provides information about how motors perform in changing conditions. Because environmental conditions and/or chemical contamination accelerates the rate at which insulation degrades, it&#;s critical to compare insulation resistance test results that are corrected for different test conditions.
Insulation testers like the Fluke FC and the Fluke C FC offer another advantage over megohmmeters. Memory storage through Fluke Connect® saves measurements to your or the cloud so you don&#;t have write down results. This saves time, reduces errors, and saves data for tracking over time.
Choosing between an insulation tester and a megohmmeter depends on your business needs. A meg test may be all you need. But if you want increased power, convenience, prevention, and safety, an insulation tester may be your best choice.
Basically, there are three different tests that can be done using a megohmmeter.
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1) Insulation resistance (IR). 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-, 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.
2) Dielectric absorption ratio. This test recognizes the fact that "good" insulation will show a gradually increasing IR after the test voltage is applied. After the connections are made, the test voltage is applied, and the IR is read at two different times: Usually either 30 and 60 sec, or 60 sec and 10 min. The latter reading is divided by the earlier reading, the result being the dielectric absorption ratio. The 10 min./60 sec. ratio is called the polarization index (PI).
For example, let's assume we apply the megohmmeter as described earlier with the appropriate test voltage impressed. The one min. IR reading is 50 megohms, and the 10 min. IR reading is 125 megohms. Thus, the PI is 125 megohms divided by 50 megohms, or 2.5.
Various sources have tables of acceptable values of dielectric absorption ratios (see Table 2).Various sources have tables of acceptable values of dielectric absorption ratios (see).
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