Superseded by 43-2000. Insulation resistance characteristics of rotating-machine windings and the manner in which these characteristics may serve to indicate the condition of the winding are described. The recommended procedure for the measurement of insulation resistance of windings of rotating machines rated 1 hp, 1 kW or greater is presented. The recommended minimum value of insulation resistance of alternating-current and direct-current rotating-machine windings is given. Synchronous machines, induction machines, direct-current machines, and synchronous converters are covered, as well as armature windings and field windings. Fractional-horsepower machines are not included.

The purpose of this publication is to:1) Describe and define insulation resistance as applied to the winding of a rotating machine; 2) Review the factors which affect or change insulation resistance characteristics and recommend uniform test conditions; 3) Outline and recommend uniform methods for measuring insulation resistance together with precautions for avoiding erroneous results; 4) Provide a basis for interpreting insulation resistance test results to estimate the suitability of the winding for service or for overpotential test.; 5) Present equations, based on machine ratings, for the calculation of recommended minimum insulation resistance values for various types of rotating machines.