| 摘要 |
PURPOSE:To exactly decide insulation deterioration, by compensating a voltage dropped portion due to a leakage current from a terminal part of a cable, and always maintaining normal residual charge. CONSTITUTION:Between a electric conductor 2 of a cable 1 to be measured and a shielding layer 3, DC voltage 4 of high voltage is applied through a high voltage switch 5 for a specified time, by which the cable 1 is charged. When charging voltage has reached a specified value, the switch 5 is opened, and after that, self-discharge is executed by residual charge of the cable. When the measurement is being executed, if a part of residual charge drops residual voltage in the cable by a leakage current flowing through the outside surface of a bushing 7 constituting a cable terminal part 6, by its portion, the measurement becomes incorrect. Therefore, a dropped portion of the residual voltage is compensated by feeding back an output of an amplifier 9 connected between the electric conductor 2 and a guard electrode 8 provided on the terminal part 6 by separating from its tip part by necessary length, to the DC voltage 4. In this way, the insulating deterioration can be decided exactly. |
