A short circuit fault of 35 kV dry-type air-core series reactor is described in this paper. It is judged preliminarily by its structural characteristics and site inspection that insulation breakdown between the coil cladding layers is occurred.It is found by way of disassembling the faulty phase that there are cracks on the surface of the cladding layer and the partial layout of the wire is loose after removing the insulation layer.Compared with the disassembly and test results of normal phase equipment

it is inferred that the outer insulation layer at the upper end of the inner side of the encapsulation is cracked due to the winding process of glass fiber cloth and the thermal expansion and cold contraction of the encapsulation

and moisture invades the encapsulation in case of multiple-day rainy weather

which leads to the insulation performance degradation of the cladding layer

and the breakdown and short-circuit burning occurs during AVC switching. On this basis

such preventive measures as inter-turn over-voltage test

provision of rain proof cover

provision of ventilation duct cleaning items and strengthening of measurement of the temperature rise of the encapsulation after rain are proposed so to prevent of similar failures from recurring.