Electrical ageing of polyethylene at impulse and AC test voltages

The main purpose of the tests was to establish a relationship between growing tree length and voltage excitation on low density polyethylene samples with point-to-plane electrode arrangement. Test cell made of Perspex enabled microscopic examination of the specimen under test. Standard impulse voltage of either polarity up to 70 kV, and intermittent AC 50 Hz voltages up to 20 kV were applied in these tests. The specimens in the form of small blocks (20×5×3 mm) were tested in silicone oil bath at room temperature. The distance `d´ between needle electrode with tip radius r=0.01 mm, and plane electrode 5 mm in diameter, was fixed at 1.4 mm apart. A linear distance from needle tip to the end of propagating tree (tree length `1´) was measured against a corresponding number of consecutive impulses applied, or against a total time of exposure of the specimen to AC voltage, up to the breakdown. To prevent damage on needle tip the experiment had been stopped just before the tree reached the plane electrode, so the same needle electrode could be used again for several trials, thus partly reducing scatter of results