Basically, there are three different tests that can be done using a megohmmeter.
- 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. 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. 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. 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.
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.
- Dielectric Absorption Ratio (DAR) and Polarization Index (PI)
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 later 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).
|INSULATION CONDITION||60/30-SECOND RATIO||10/1-MINUTE RATIO (POLARIZATION INDEX)|
|Dangerous||—||Less than 1|
|Questionable||1.0 to 1.25||1.0 to 2***|
|Good||1.4 to 1.6||2 to 4|
|Excellent||Above 1.6**||Above 4**|
- Step voltage test
This test is particularly useful in evaluating aged or damaged insulation not necessarily having moisture or contamination. A dual voltage test instrument is required here. After the connections are made, the IR test is done at a low voltage, say 500V. The test specimen then is discharged and the test is done again, this time at a higher voltage, say 2500V. If more than a 25% difference exists between the two IR readings, age deterioration or damaged insulation should be suspected.