Abstract: A circuit breaker fault occurring to the overhead lines causes trip protection, the reclosing breaker failed to eliminate the abnormality,and the B-phase insulation sleeve pile head is found to have obvious ablation and blackening phenomenon. Through carrying out the insulation resistance test and bushing terminal levelness test to the fault circuit breaker of the lower pole, the fault part is located. Then,the paper views the feeder terminal action report, disassembles and checks the components in the box, and carries out X-ray fluoroscopy and electric field simulation analysis on the insulation sleeve. It finds out that B-phase charged main circuit penetrates the damaged epoxy resin layer, which produces disruptive discharge to the shell and leads to the single-phase grounding fault after the surface flashover. The mechanical and electric performance tests have been taken to the same batch of sleeves to eliminate their own defects, and the reason for sleeve cracking is the external force produced by instantaneous collision or artificial lifting causes the epoxy resin layer to bear internal stress to generate cracks invisible to the naked eye. Therefore, the paper puts forward preventive measures such as strengthening the production process of epoxy resin pouring and carrying out power-frequency voltage-withstand test in time.

    Key words: circuit breaker; insulation sleeve; breakdown; epoxy resin; fault analysis