Identification of fault type is a critical component to dissolved gas analysis and assessing a transformer's condition. Fault conditions occur primarily from the thermal and electrical deterioration of oil and electrical insulation. Each combustible gas level will vary depending upon the fault process.

Arcing
Arcing is the most severe of all fault processes. Large amounts of hydrogen and acetylene are produced, with minor quantities of methane and ethylene. Arcing occurs through high current and high temperature conditions. Carbon dioxide and carbon monoxide may also form if the fault involved cellulose. In some instances, the oil may become carbonized.

Corona  
Corona is a low-energy electrical fault. Low-energy electrical discharges produce hydrogen and methane, with small quantities of ethane and ethylene. Comparable amounts of carbon monoxide and carbon dioxide may result from discharge in cellulose.

Sparking
Sparking occurs as an intermittent high voltage flashover without high current. Increased levels of methane and ethane are detected without concurrent increases in acetylene, ethylene or hydrogen.

Overheating
Decomposition products include ethylene and methane, together with smaller quantities of hydrogen and ethane. Traces of acetylene may be formed if the fault is severe or involves electrical contacts.

Overheated Cellulose
Large quantities of carbon dioxide and carbon monoxide are evolved from overheated cellulose. Hydrocarbon gases, such as methane and ethylene, will be formed if the fault involved an oil-impregnated structure. A furanic compound and/or degree of polymerization analysis may be performed to further assess the condition of the insulating paper.

Each type of fault produces different types of gases. In general, the kind of fault gases which are generated depend upon the type of insulation which is being degraded and the temperature of the fault site, vis :

• Faults involving overheating cellulose insulation generate mainly carbon monoxide and carbon dioxide.
  
  At low temperatures carbon dioxide predominates with increasing amounts of carbon monoxide as the temperature rises.
  
  Under normal operating conditions, there is a continued production of carbon dioxide and carbon monoxide in the ratio of about 3:1, and relatively large amounts of these gases can be found in a normally operating transformer. Very high levels of both gases with carbon monoxide approaching or exceeding the carbon dioxide level are required before suspecting a localized fault involving cellulose.
  
  

• Faults which result in breakdown of the oil.
  
  At the low temperature and energy dissipation of partial discharges or corona, virtually the only gas produced is hydrogen H₂.  
  
  Low temperature and generalized over-heating produces methane CH₄ and ethane C₂H₆, and some hydrogen H₂.
  
  As the temperature increases, ethylene becomes the predominant gas. At the very high temperature of an Arc, acetylene and hydrogen predominate.

The causes of fault gases can be divided into  3 main categories, vis :

• Corona or partial discharge
• Thermal heating or pyrolysis
• Arcing

The most severe intensity of energy dissipation occurs with Arcing, less with thermal heating and least with corona.

A list of the more significant gases can be classified into 3 groups :

• British Standards 5800:1979 (I.E.C. 599:1978) which compares 3 ratios of gases i.e. C₂H₂/C₂H₄, CH₄/H₂, and C₂H₄/C₂H₆, to generate a 3 digit integer code called "Rogers ratio".
  
  This method was developed for use by the Central Electric Generating Board (C.E.G.B.) of U.K. and it is to be noted that no significance has been given to the magnitude of the code units and that fluctuations in values cause very large changes in the ratios.
  
  Hence, this method is hardly practicable for use in industry.
  
  

• IEEE C57.104 : 1991 "Guide for the interpretation of Gases generated in oil-immersed transformers" provides a comprehensive list of the limits of each gas (in ppm) dissolved in the oil sample. This method is more straightforward and practical for most industrial references.
  
  

• Various other bodies and some larger private testing facilities have also adopted the IEEE Standard with modifications, in accordance with their own experiences.
  
  Namely, these are Columbia State University Syracuse U.S.A. and the Northern Technology and Testing, U.S.A.