The water tree diagnoses of 6. 6kV class XLPE cables by DC component method are described. This method measures the DC current component by energizing the cable with AC high voltage

therefore it would be suitable for on-line diagnoses. The DC component is considered to appear due to the rectifying effect of water trees inside the insulation.In the laboratory test

the DC component showed a good correlation with the degradation. However

almost no correlation was seen in the on-site measurement. The cause of this bad correlation found by the authors is galvanic effect inside the jacket. The jacket resistivity being low

this generates the noise current through the grounding circuit. Based on this finding

the equivalent circuit of on site measurement wes proposed.Considering the equivalent circuit

two methods were proposed to discriminate the true DC component out of noise current. One is to by-pass the noise current from the measurement apparatus

and the other is to statistically estimate the true DC component by using the transition in the jacket resistivity.The on-site test were carried out to confirm the applicability of these methods.