Sealed Winding Conformance Testing and Recent Revisions to NEMA MG-1

The sealed winding conformance test is often performed during the final acceptance test sequence for industrial motor stator insulation. The stator winding is first completely soaked with water, then receives a 10-minute insulation resistance (IR-io) measurement, followed by a 60-second high potential (hipot) test at 1.15 times rated line-to-line voltage (Vll). A 1-minute insulation resistance (IR-i) test is performed immediately after the hi-pot test. To define the test requirement, API 541 and 546 standards reference NEMA MG-1 20.18, which specifies an acceptance criterion for both IR measurements of (MOmega) ges V_LL + 1, where [V_LL + 1] is in kV. NEMA MG-1 20.18 refers to IEEE Std 43 for the insulation resistance test method and minimum insulation resistance. Until its most recent revision, IEEE Std 43 recommended the minimum IRi (MOmega) ges V_LL + 1 (kV). However, the current version of IEEE Std 43 (2000) recommends IR₁ ges 100 MOmega for all stator windings built after about 1970. Modern epoxy-mica insulation systems should be able to meet this value when fully clean and dry. The wet conditions encountered during sealed winding tests may produce a different response in the insulation system. The sealed winding test is outside the scope of IEEE 43. The next revision of NEMA MG-1 20.18 will clarify the sealed winding test acceptance level of IR (MOmega) ges V_LL + 1 (kV). This paper investigates the effect of test solution conductivity on the IR measurement, provides the NEMA-approved clarification of the sealed winding test specification and suggests further clarifications to the sealed winding test procedure.