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This paper reports on a comparative study between mineral and ester oils basing on DGA (Dissolved Gas Analysis). Electrical (breakdown of low energy) and thermal (stray gassing) faults are realized at laboratory scale and gas formation as well as diagnosis methods are compared. Two extraction methods described in IEC 60567 were used: total vacuum degassing and stripping method. Three different diagnosis methods were used: IEC 60599 gas ratios, Duval's triangle and key gases. It appears that for electrical faults and especially for low energy discharges, the same gases (Hydrogen and Acetylene), in relatively same ratios, are created. It is shown that classic Duval's triangle is well applicable for this type of fault confirming thus the results reported in literature. For thermal faults and especially for stray gassing issue (low thermal faults), Ethane (associated with Hydrogen) constitutes the key gas of natural esters and the new Duval's triangle can be applied for mineral oil whereas some arrangements need to be introduced for ester oils.