A breakdown failure of prefabricated cable joint occurred on the 220 kV cable line

and the spare joint with the same batch and specification cracked under normal pre-expansion during the process of urgent repair.The inspection

testing and cause analysis are conducted in this paper.According to the surface examination and discharge tracking analysis of the cracked EPDM rubber insulation on the broken-down joint

the main reason of this failure is the sudden crack of the insulation during operation process of the joint.Moreover

the cracked EPDM rubber insulation of the spare joint after normal expansion was checked by insulation aging test

and the result shows its breaking elongation ratio is low

which directly reduces the mechanical strength of the middle joint and leads to entire crack of the spare joint insulation.