PURPOSE: To remove an electrical trouble by testing after the insulation oil filled in an elephant case is circulated or while being circulated when a direct current charging test is carried out on the cable head in the elephant case of an elephant bushing type transformer.

CONSTITUTION: A bushing 3 is installed through the wall 1 of the container of a transformer filled with insulation oil 2 and an elephant case 4 housing the tip of the bushing 3 and having a peep window 11 is fixed on the wall 1 and filled with insulation oil 5. A cable head 6 is projected through the bottom of the case 4 with a cable 8 and is connected to the bushing 5 with a lead wire 7 which will be removed while testing. In this construction, an oil supplying inlet 9 and an oil exhaust port 10 are installed on and beneath the case 4 and between these, a filter 13 and an oil pump 14 are connected with valves 12 and 12'. A charging test is carried out circulating or after circulated the insulation oil 5, which is cleaned to increase the withstand voltage of the cable head 6.