The SF

6

decomposition products are intrinsically related to the type and severity of fault discharge of SF

6

switchgear

and the fault diagnosis can be realized through the analysis of SF

6

decomposition products. H

2

O has a significant effect on the SF

6

decomposition products under arc discharge

but its influence law and related mechanism are still unclear

which s

eriously restricts the engineering application of SF

6

decomposition product analysis method. In order to solve this problem

a research platform for SF

6

decomposition products of switchgear under arc discharge was built. The arc discharge experiments were carried out by using H

2

18

O as tracers. The change law of

18

O isotopic substances was analyzed by gas chromatography-mass spectrometer. The formation pathway of SF

6

decomposition products and the influence of H

2

O on SF

6

decomposition products were clarified

and the reaction mechanism was clarified. It has found that H

2

O will participate in the formation reactions of SOF

2

SO

2

SO

2

F

2

and CO

2

. The different reaction mechanisms lead to significant differences in the formation and evolution laws of each substance. SOF

2

and SO

2

F

2

are dominated by the decomposition reaction and composite reaction during the discharge. SO

2

is dominated by the intermolecular reaction after discharge. CO

2

comes from the combined action of the two.