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%. For ac electric machines voltages between 3.3kV to 30kV are considered to be high voltage machines.