| 摘要 |
PURPOSE:To eliminate an influence of a power apparatus which has been con nected in parallel to a cable, by applying a DC voltage between a cable shielding layer and a ground, and switching and connecting two kinds of insulation resis tance measuring resistances whose resistance values are different from each other, between a bus and a ground. CONSTITUTION:A voltage V between a cable shielding layer 16 and a ground is measured by applying a DC power source 52 between the cable shielding layer 16 and a ground. On the other hand, between a bus 24 to which a cable, etc. have been connected, and a ground, two kinds of insulation resistance measuring resistances 75, 76 whose resistance values are different from each other are switched and connected, and voltages v1, v2 across the resistance are measured successively. In this case, in addition to a cable 10 to be measured, the cable which has been connected to the bus 24 and an insulator resistance 50 of a power apparatus 40 such as a motor, a transformer, etc. also enter in parallel to insulation resistance measuring resistances 74, 76, therefore, an insulation resistance RX is derived by an operational expression RX={(V-v2)/ v2-(V-v1)/v1}R2 for eliminating an influence of the insulator resistance 50.
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