Fault gases are caused by corona (partial discharge), thermal heating (pyrolysis) and arcing.

Partial discharge is a fault of low level energy which usually occurs in gas-filled voids surrounded by oil impregnated material. The main cause of decomposition in partial discharges is ionic bombardment of the oil nolecules.

The major gas produced is hydrogen. The minor gas produced is methane.

Thermal faults
A small amount of decomposition occurs at normal operating temperatures. As the fault temperature rises, the formation of the degradation gases change from methane (CH4) to ethane (C2H6) to ethylene (C2H4).

A thermal fault at low temperature (less than 300 degrees Celsius) produces mainly methane, ethane and some ethylene.

A thermal fault at higher temperature (more than 300 degrees Celsius) produces ethylene. The higher the temperature becomes, the greater the production of ethylene.

Arcing is a fault caused by high energy discharge.

The major gas produced ruing arcing is acetylene. Power arcing can cause temperature of over 3000 degrees Celsius to be developed.

If the cellulose material (insulating paper etc.) is involved, carbon monoxide and carbon dioxide are generated.

A normally aging conservator type transformer having a CO2/CO ratio above 11 or below 3 should be regarded as perhaps indicating a fault involving cellulose, provided the other gas analysis results also indicate excessive oil degradation.