1. Nitrogen and around 5% oxygen: Transformer operates normally.

2. Nitrogen and over 5% oxygen: Check tightness of transformer.

3. Nitrogen, carbon monoxide and carbon dioxide: Transformer overload or cellulose is damaged by heat during operation. Operation condition should be checked.

4. Nitrogen and hydrogen: Corona discharge, water electrolyte.

5. Nitrogen, hydrogen, carbon monoxide and carbon dioxide: Corona discharge or transformer heavily overload.

6. Methane, small amounts of ethane and ethylene: Sparking or other external fault to damage oil.

7. Methane, carbon monoxide/carbon dioxide or small amounts of hydrocarbons (usually without acetylene): Under presence of cellulose, sparking or other external fault to damage oil.

8. Hydrocarbons in acetylene fluid: energetic arc causes deterioration of oil.

9. Ethylene, methane and acetylene: There is high temperature arc of oil but confined in a limited area, usually bad connection or short-circuit in oil.