Abstract:A 220 kV transformer tripping accident caused by heavy gas misoperation of oil-flow relay in on-load tap-changer (OLTC) is analyzed. Through the test and examination of the insulation resistance of reed switch in oil-flow relay and combined with the fault recording wave, the accident action logic is repeatedly deduced and analyzed. The root causes of the accident and a design defect of reed switch in oil-flow relay are found out, and some reasonable suggestions are put forward.