A trip accident happened due to lightning strike in the Southern Grid. Then the line in struck area was tested with ±500 k V DC composite insulator strings according to the standard DL/T 810—2002. In the steep-wave test

the end-fitting sheaths at high-voltage ends of two composite insulator strings were broken down. To understand the cause of the breakdown

this study adopts the X-ray DR detection technology to detect the broken-down sheaths area with multiple parameters and angles to determine the optimal shooting parameters. The sheath thickness is measured on the X-ray images with the similarity principle. As a result

the breakdown is confirmed to be due to the obvious gap between the sheath and the end-fitting and due to the nonuniform thickness of the insulation layer on the end-fitting tail. Moreover

an internal triangle airway defect is found in the sheath between the end-fitting and the first umbrella skirt. All the optimal parameters and testing procedure in this study may provide a basis for X-ray detection of internal structure of composite insulators

and the X-ray DR testing technology can be taken as an innovative method for highly efficient detection of composite insulators without disassembly.