To investigate an abnormal of potential transformer（PT）in gas insulated switchgear（GIS）detected by livedetection of sulfur hexafluoride（SF

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）decomposition products

the cause of failure is analyzed combining different de-composition products detection techniques

electric tests and equipment disassembling. The obtained results indi-cate that

the observed abnormal phenomenon is a without reported ultrahigh hydrogen content in SF

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. The test re-sults from different electrochemical sensor instruments are quite different

and the data from c

hromatograph is moreaccurate. The main reason of PT defect is introducing some contaminants such as transformer oil from retired siliconsteel sheets. Meanwhile

under the longtime overheating from over-excitation of the defective iron core

evolution ofdissolved gas or decomposition of transformer oil and other materials result in the abnormal high of hydrogen in SF

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.The defect of a PT is treated successfully by treatment suggestions. Some maintenance advices for similar equipmentare proposed. Furthermore

the modified ideas for decomposition products detection techniques and devices and use-ful technical guidance are proposed for security and stability of GIS equipment and power grid.