What are the Insulation Resistance Test Standards?

Why do you need IR testing?

No matter how new your insulation is, it can deteriorate fast, especially if it is in a harsh environment. In such a scenario, an insulation resistance test is required to make sure the integrity of the circuit is not compromised. A megohmmeter, also called teraohmmeter is used to measure the ohmic value of insulation. To measure a high-value resistance, low-value current measuring techniques are used. A steady voltage is passed through the circuit and the resultant current is read on a highly sensitive ammeter circuit which displays the resistance value.

Insulation test

The objective of the test is to measure the ohmic value of the insulation under a direct voltage of great stability, which is usually in the range of 50, 100, 250, 500, or 1000 VDC. To conform to specific standards, the insulation resistance test is performed under voltages up to 1500VDC. It is important to maintain a stable voltage during the test. A non-regulated voltage will fall sharply in case of bad insulation, which will lead to faulty measurement.
You can perform three different tests using a megohmmeter. They are-

Insulation resistance (IR)

IR is the simplest test of the lot. In this, the necessary connections are made and the test voltage is applied for one minute. During this period, the resistance will drop or remain somewhat steady. After one min, read the resistance value and record it for reference.
IR is temperature-sensitive. So if the temperature is high, IR will be low and vice versa. So, to get an accurate reading, you will need a base temperature – 20°C or 40°C is the standard. No test result should be considered in isolation. It should be compared with previous readings for effective analysis

Dielectric absorption ratio

This is a test based on the fact that good insulation will show an increasing IP once the test voltage is applied. IR is read at two different times once the voltage is applied – at 30 or 60 seconds and 10 minutes. The latter reading is then divided by the earlier reading to get the dielectric absorption ration. This ratio is called the polarization index (PI).

Step voltage test

This is a test used for evaluating old or damaged insulation. A dual voltage test is preferred here. The IR test is performed at a lower voltage and the test specimen is discharged. The test is repeated at a higher voltage this time. If there is more than a 25% difference in the results, the diagnosis is in favor of damaged or deteriorated insulation.
Whatever tests you perform, you must have a reliable IR tester to give you the correct results.