Arcing, partial discharge, and localized overheating in the insulation system of transformers result in chemical decomposition of the insulating oil and other insulating materials. This may generate various gases, some of which are combustible. Typically, gases are generated in the oil and then partitioned into the gas space according to their individual solubilities. Gases which are highly oil-soluble, such as acetylene, may not be in significant quantities in the gas space until an incipient fault has progressed to a very serious condition or failure of the transformer. Gases such as carbon monoxide and hydrogen which have low solubilities in oil can make up a large fraction of the combustible gases in the gas space. Detection of these gases is frequently the first available indication of a malfunction. Portable combustible gas meters are a convenient means of detecting the presence of generated gases.

Normal operation of a transformer may result in the formation of some combustible gases. The detection of an incipient fault by this method involves an evaluation of the amount of combustible gases present, the rate of generation of these gases, and their rate of escape from the transformer. Refer to IEEE C57.104 for detailed information on interpretation of gassing in transformers.

1.1 This field practice covers the detection and estimation of combustible gases in the gas blanket above the oil or in gas detector relays in transformers using portable instruments. It is applicable only with transformers using mineral oil as the dielectric fluid. Gases dissolved in the oil and noncombustible gases are not determined. A method of calibrating the instruments with a known gas mixture is included.

1.2 This practice affords a semi-quantitative estimate of the total combustible gases present in a gas mixture. If a more accurate determination of the total amount of combustible gases or a quantitative determination of the individual components is desired, use a laboratory analytical method, such as Test Method D 3612.

1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Specific precautionary statements are given in Section 7.