The insulation fault occurred between different slices of the iron cores will seriously affect the operation of the generators since the iron core is one of the most important components. With the experiments carried out on the iron cores, the faults can be detected more early to avoid the deterioration of the issues. The fundamental and specific operation methods of ELCID (Electro-magnetic Core Imperfection Detector) are introduced in the paper, and the detailed precautions analyzed with several examples are also proposed. The iron-loss tests are proved to be feasible for the judgment of the condition of the iron cores.