This document describes very low frequency (VLF) testing of ac electric machines. It covers acceptance testing of new machines in the factory or on-site after erection. Also covered is the routine maintenance testing of machines that have been in service. In order to facilitate communication and comparison among investigators, this document recommends that the VLF used be 0.1 Hz ± 10%. The purposes of this recommended practice are as follows: a) To provide a uniform procedure for testing the stator (armature) insulation of ac electric machines with VLF voltage, in order to obtain consistent results b) To recommend constants for relating VLF tests to power-frequency and direct-voltage tests to obtain equally effective test levels c) To describe VLF test equipment and wave shape d) To define terms that have specific meaning in VLF testing Revision Standard - Inactive-Reserved. This recommended practice describes very low frequency withstand and diagnostic tests that are performed on the stator windings of ac electric machines.