Design innovations and rating increases of rotating machines continuously increase the need to develop improved methods of testing insulation for serviceability. Research in the last 15 years evolved a method of 1/10 Hz overpotential testing to supplement traditional power-frequency testing and to complement direct-voltage testing. A background for VLF (very low frequency) testing is available in the technical literature. (See Section 11.) VLF testing permits advantageous application of more easily transportable equipment both in weight and size as compared to non-resonant power-frequency testing. This document provides a uniform method of VLF testing in order to obtain consistent results, accumulate meaningful experience, and lead to establishment of standards of test level. References as listed in Section 10. are necessarily those which have been issued at the time this document was written. When subsequent revisions of the references are issued, they should be used in place of those indicated unless, due to a change in scope, they no longer apply.