In this paper

a CT accident is inspected

and the cause of the accident is analyzed. Air bubble in CT is considered to be the reason of the accident

which cause increase of the partial discharge （PD）

and induce breakdown. In order to avoid this kind of accident

PD test should be carried out. The products

which PD levels are not within the specified value under 1.2 times of the phase voltage

should be out of service. The products

which PD levels are not within the specified value under the line voltage

should be paid more attention to on its operation situation

or be made the further flaw inspection. The products

on which short circuit accident of large current occurred

the PD test should be carried out

and those

which PD level are within the specified value

should be out of service

or be made the further flaw inspection.