The convenience of testing the high voltage machine insulation is well known. Test methods used in our country for determining the state of degradation of the insulation are presented with results gathered during three years of experience. The data obtained from measurements on ten machines of different types are presented and analyzed according to international standards and practice. A standardized routine of nondestructive measurements is carried out in twenty machines under the management of our company. Also some tests were made on machines that are already due to refurbishment. Destructive tests were performed on these machines to find out the real state of their insulations and to check the security margin given by testing worldwide practice. Data from measurements before and after a small repair of the stator winding of one machine is presented. The level of partial discharges increased afler the repair. lt is shown that some techniques used for repairing a winding are not always a good practice.