Abstract: A charging current flows through an insulation for insulating two conductors of an electric apparatus by applying an alternate current voltage thereto. A direct current component of the charging current is detected, and is amplified by biasing a direct current voltage to the alternate current voltage. An insulation deterioration of the electric apparatus is determined by an absolute value of the amplified direct current component.

Abstract: A power cable such as a rubber or plastic insulated power cable is diagnosed to be detected an insulation deterioration resulted from water tree generated in an insulation material thereof. At first, charging current flowing from the power cable through a grounding conductor connected to a metal shielding layer thereof to the ground is measured under a state that a rated AC voltage is applied to the power cable. In accordance with the charging current, stray current flowing from the ground to the power cable is calculated. Then, a direct current component of the charging current is detected, and an intrinsic direct current component is calculated by subtracting the stray current from the direct current component. The insulation deterioration is diagnosed in accordance with the level of the intrinsic direct current component.