This document describes the recommended procedure for measuring insulation resistance of armature and field winding in rotating machines rated 1hp, 750 W or greater. It applies to synchronous machines, induction machines, dc machines, and synchronoous condensers. Contained within this document is the general theory of insulation resistance (IR) and polarization index (P.I.) as well as factors affecting the results, test procedures, methods of interpretation, test limitations, and recommended minimum values.