IR and PI testing are offline tests performed on rotating machines to assess the condition of the stator winding insulation system before any HV diagnostic testing is applied. Conducted using a controlled DC test voltage (with the machine isolated from the power system), IR testing measures the resistance of the stator windings insulation system to DC leakage current (applied phase to ground), providing an immediate indication of overall insulation integrity.

PI testing extends this assessment by evaluating the change in insulation resistance over time, offering insight into insulation dryness, contamination levels, and general ageing condition. IR & PI testing is performed as per the IEEE 43 standard. As the first test performed prior to the application of higher HV stresses, such as PD and/or TD testing, IR and PI serve as a critical screening pass/fail step to verify that the insulation system is in a suitable and safe condition for further diagnostic evaluation.

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