PURPOSE:To achieve a test automatically, by immersing a mobile electrode into a container holding an insulation oil to read out a breakdown voltage thereof with the electrode for computation. CONSTITUTION:A plurality of containers 3 filled with an insulation oil to be tested are arranged on a turntable 2 and rotated with a driver 12 together with the table 2. Then, when the first container 3 is detected by a container detection means 10, the driver 12 stops and the rotation of the table 2 is halted. Then, a liftable electrode 4 is lowered with a driver 11 and immersed into the insulation oil in the container 3. Here, the electrode 4 is kept for about 10min in standby until bubbles of the insulation oil disappear after the agitation thereof. As the bubbles are gone, a driver 15 of a voltage adjuster 14 starts to raise the voltage of a transformer 5 until a dielectric breakdown occurs in the insulation oil. Then, with the discharging between the oil and electrode 4, an overcurrent is caused to activate a thermal switch 17 stopping the rise in the voltage while a dielectric breakdown voltage is computed 22. The results of the computation are recorded 7 through an interface 25.
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