The nature of the turn-to-turn short circuit in the windings of electric machines with long mean-time-between failures is specified. The choice of the methods for the diagnostics of the presence of short-circuited turns in the windings of electric machines in the process of their repair and current operation is substantiated. The structure of the system of short-circuited turn diagnostics is grounded being based on the proposed methods. The solution to the posed problem is considered taking into account the electric machine magnetic system real condition characterized by the deterioration of its magnetic and electrical properties. The results are verified by the experiments with the use of a computer-aided measuring system.