PURPOSE: To facilitate assembly and dismantle and performance of testing such as dielectric strength by using a testing core whose cross section is reduced as compared to the proper core at the time of assembly in the factory.

CONSTITUTION: A core, which has a cross section necessary for a dielectric strength test is used as the testing core instead of a proper core to facilitate the assembly of the core and the winding for the dielectric strength test of the winding and improve the performance efficiency of the dielectric strength test of the winding. Among dielectric strength tests for an oil-filled transformer winding, an induction dielectric strength test requires a large core cross section. Usually frequency of a source for test is 3W4 times of commercial frequency. Therefore, in the case of the induction dielectric test whose magnification of excitation is about two times, the test can be carried out using the testing core whose cross section is reduced to 2/3W1/2 of the proper value.