The diagnosis of FRSL methods is based on the flow of leakage flux through the windings, and the FRSL method has the advantage of being sensitive to variations in eddy current losses in the windings due to variation in measurement frequency. The method is capable of detecting the degradation of the insulation in localized area, which causes several parallel conductors to be short circuited in one area of the winding or shorted lamination, with the result that the conductor is made larger and consequently the eddy current losses are increased.