Updated: Jun 11, 2021
The ratio of 10 minute IR value to 1 minute IR value is called polarization index. The ratio of 60 seconds IR value to 30 seconds IR value is called dielectric absorption ratio(DAR or DA) When current is applied to insulating material, the current may starts flowing through the surface if it has dust, dirt and moisture on it. The insulation resistance of the specimen under test may low if taken normal insulation resistance. The polarization index test is carried out to measure the actual health of the insulation. It shows more accurate insulation health condition. The ratio of 10 minute IR value to 1 minute IR value is called polarization index.
How it is performed?
The voltage is applied to equipment for its insulation measurement and the value of the insulation resistance is recorded after one minute. The second reading of the insulation resistance is taken after 10 minutes. As the time passes after 01 minute the insulation resistance value increase if surface current starts decreasing and capacitive current value also gets stabilize. Thus, the polarization index is the ratio of insulation resistance value after 10 minutes and 01 minute.
Polarization Index or PI = R10/R1
R10 - Resistance Value after 10 minutes
R1 - Resistance Value after 01 Minute
Suppose insulation resistance value after 01 Minute and 10 Minutes are 500 Mega Ohm and 2000 mega Ohms, then the polarization index is;
PI = R10/R1 = 2000/500 = 4
The polarization value above 1.5 shows insulation quality is good.
Dielectric Absorption Ratio (DA or DAR ) Test
The ratio of insulation resistance value after 30 seconds and 60 seconds is recorded in DA test.
The ratio of IR resistance after 60 seconds and 30 seconds is called the dielectric absorption ( DA) index. The test is conducted to see the stabilization of absorption current in the insulating material.
Dielectric Absorption Ratio, DAR or DA = R60/R30
R60 = Insulation resistance after 60 Seconds
R30 = Insulation resistance after 30 seconds
If the IR value after 30 seconds and 60 seconds are 50 mega Ohms and 75 Mega Ohms, the DA ratio is;
DAR = R60/ R30 = 75/50 = 1.5
Why these tests important?
These both tests shows healthiness of the insulating material. The IR value is affected with change in temperature,moisture and other environment condition. However, in PI and DA test the ratio is used so there is no need to have temperature correction. Both the readings in the ratio test is based on the same temperature conditions.
For more Details Please Read: Polarization current Mechanism