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This study examines how byproducts from thermal decomposition of sulfur fluoride (SF6) can be measured and applied to diagnose an overheating fault occurring in SF6 insulated equipment. Using a series of experiments performed on a SF6 thermal fault simulation system, the thermal decomposition characteristics of SF6 were obtained for temperatures below 400°C. Based on these results, the formation rate was extracted to indicate the progress of the thermal fault's development, and the characteristic component ratio was used to identify the type and severity of the thermal fault. The relationship of the fault temperature and the formation rate along with the correlation between the characteristic ratio and the fault temperature were studied. The results show that: the formation rate of the components effectively reflects the severity and development trend of the thermal fault; by integrating the IEC standard 60480‐2004, the obtained thermal fault maximum safe time (TMS) can accurately indicate the largest safe time when thermal fault happens; component characteristic ratios C(SO2)/C(SO2F2) and C(SO2F2+SOF4)/C(SOF2) can be used to evaluate whether the fault type is a thermal fault; and the thermal correlation ratio (TR) can be employed to recognize the temperature value from 300°C to 400°C. [ABSTRACT FROM AUTHOR]