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Difference between Insulation Resistance (IR) and Polarizing Index (PI)

Insulation Resistance (IR)

  • IR of an electrical equipment refers to the resistance measured between conducting part and earth  or between two circuits separated by insulation
  • It is expressed in MW
  • It is of the order of several tens/ thousand MW or infinity for healthy insulation
  • It reveals quality of insulation of the equipment and the degree of dryness/moisture
  • The subsequent high voltage test can be avoided if  ‘IR’ value found is low

Polarizing  Index (PI)

  • The subsequent high voltage test can be avoided if  ‘IR’ value found is low
                IR (10 minutes )
   i.e.PI=  ---------------------
                IR (1 minute ) 
  • It being a ratio, it is unit less 
  • Its value depends on class of insulation of equipment. For class A insulation, PI should be ≥1.5 and for B class, PI should be ≥ 2.0
  • It gives quantitative information about the insulation with respect to moisture, dirt and contamination.
  • PI < 1 indicates need for immediate reconditioning.
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