An abnormal ablation of 10 kV isolating switch on the low voltage side of the main transformer in a 220 kV substation causes three-side breaker tripping failure.The on-site disassembling inspection of the fault isolating switch is carried out.The inspection results show that the locking mechanism of the isolating switch is not clamped

the contact area and contact pressure between the moving contact arm and the static contact decrease

the contact resistance increases and causes heating

and part of the current flows through the stop bar.The cause of the fault is concluded that the C-phase stop bar of the isolating switch seriously heated under the action of the load current

causing the distortion of the surrounding electric field

which further develops into the three-phase short-circuit fault under the joint action of arc movement

gas expansion and metal melt.Combined with the causes of the isolation switch fai-lure

the preventive measures are proposed to improve safe and reliable equipment operating level.