With the advantages of small volume, light weight and high accuracy, the epoxy pouring current transformer is widely used in power systems of 35kV and below. The safety of its own equipment directly affects th1e safe and stable operation of power systems. This paper details an insulation breakdown fault of the epoxy pouring current transformer. The cause of the fault is clear and definite. In the manufacturing process of the current transformer, in order to make the primary copper winding located in the middle of the secondary winding, an insulating support block is installed between the primary winding and secondary winding in a fixed position. Then, in the pouring process, the insulating support block has not fully fused into an organic whole with the surrounding insulating medium, leaving a gap around the insulating support block. The insulating performance of the current transformer is reduced, which leads to the insulation breakdown after running for a long time. Based on this fault, the research on manufacturing technology of pouring current transformer is carried out, and the fault prevention measures are put forward.