PURPOSE: To reduce floor area or height required for an electrical test and reduce assembly or disassembly works exclusively required for the electrical test by connecting a bushing case for tests having a bushing for tests with an adapter at the hole of a wall bushing.

CONSTITUTION: A transformer 2 which has an iron core wound with a winding is housed in a tank 1 with insulation medium, e.g., insulation oil and a wall bushing 3 through the insulation oil and a low voltage bushing 4 are provided. A bushing case 19 for tests provided with a bushing 18 for tests is connected by an adapter 21 to a hole 10 for the wall bushing 3 and a lower terminal 18a of the bushing 18 for tests from a manhole which is not shown in the drawing and is provided at the side of a connection box 6 for the transformer and a connecting conductor 11 are connected by a connecting conductor 22 for an electrical test of the transformer. Insulation oil is filled in the tank after the connections are finished and, for example, voltage is applied to a terminal 18b of the bushing 18 for tests to carry out an electrical test such as withstand voltage.