Insulation resistance measurements for machine insulation

In order to gain assurance that a machine winding insulation can safely withstand the service voltage and assess its reliability in operating conditions after a given number of years in service, dc tests have been used for decades. Some specific quantities, such as the 1-minute insulation resistance (IR) or the polarization index (PI), can be used to assess the condition of the insulation system. The minimum IR value recommended by the IEEE Std-43 for most ac windings built after about 1970 is 100 MΩ (corrected to 40°C). Whether this arbitrarily value is too high or too low is still under debate. This paper presents data from windings of various machines and shows how insulation resistance depends not only on the health of the insulation system but also on machine size and on insulation technology. A normalization technique using the winding capacity and considering the insulation technology is suggested in order to compare values between different machines or with individual bars or coils.