Electrical machines, especially induction machines, are also installed in the open space, for ex ample in cement mills. In the period from November to March, cement production is stopped, engines do not work, they are cold and exposed to weather conditions, which promotes the absorption of moisture by the insulation system. Not all of the motors have heating systems installed, and among those which do, most have heating often turned off by the operators. During the off period for engines, there is time for diagnostic tests off-line. This article presents a methodology of diagnostic tests of main insulation with the use of DC voltage, the criteria for assessing the technical condition of the insulation and also the results of the tests of the insulation of wound-rotor induction motors, that was regularly performed every year in the period (2001-2007) in one of the cement mills. These are the engines installed in the open air. The results of the diagnostic tests are used for engine- repair planning.