The polarisation index test of electrical insulation is the key criteria for testing the healthiness of electrical system.
The failure of the insulation cause the device to fail. The insulation resistance value goes down as the temperature increases. The insulation resistance gets halved with  every increase in temperaure by 10 degree centigrade.The moisture also cause the insulation failures. The checking of insulation resistance is one of the method for ensuring healthiness. However, it is not the foolproof criteria for ensuring the reliablity. One very important parameter known as the polarisation index (PI) needs to be checked.
The polarisation index gives the indication about the moisture content absorbed in the imsulating material.
The ratio of the ten minutes to one minute insulation resistance value is known as the PI. The higher the value of the PI, the better is the insulation.
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