During the natural aging processes of oil and insulating parts, especially in the case of thermal or electrical failures, cracked gases form and are dissolved in the oil. The rate of decomposition and the type of gases change during defective operation, which could be results of the thermal overloading and / or electrical faults.

Based on the quantity or types of fault gases, the gas increase rates and the proportions between the gases, the types of failure can be deduced.

Partial discharges with lower energy mainly lead to the formation of hydrogen and methane, as well as small quantities of ethane.

Thermal overheating results in the pyrolysis of hydrocarbons. In the temperatures among 300-700ethylene and propylene accompanied with larger quantities of CO and CO2 prevail. Over700mainly ethylene, propylene and hydrogen form, and above1000acetylene are also formed.

Electrical discharges (arcs and spark discharges) cause separation of hydrogen and acetylene, as well as methane and ethylene. By thermal-oxidative cellulose degradation, large quantities of CO and CO2 are formed.