Breakdown of high-voltage cable near the root of plug: usually by the two ends of high-voltage cable long-term erosion by transformer oil or Valin, high-voltage insulation rubber corrosion, so that insulation resistance is reduced and cause breakdown in addition to the cable ends with the ball often up and down, left and right movement, or torsion bending radius is too small, resulting in small cracks When there is an air gap in the root of the cable plug, the air part receives a higher potential gradient, which is easy to dissociate first, so that the insulation strength is damaged, and a longer time will inevitably lead to breakdown.