ASTM_D_3284_-_05_2011.pdf

Designation:D328405(Reapproved 2011)Standard Practice forCombustible Gases in the Gas Space of ElectricalApparatus Using Portable Meters1This standard is issued under the fixed designation D3284;the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.Asuperscript epsilon()indicates an editorial change since the last revision or reapproval.1.Scope1.1 This field practice covers the detection and estimation ofcombustible gases in the gas blanket above the oil or in gasdetector relays in transformers using portable instruments.It isapplicable only with transformers using mineral oil as thedielectric fluid.Gases dissolved in the oil and noncombustiblegases are not determined.A method of calibrating the instru-ments with a known gas mixture is included.1.2 This practice affords a semi-quantitative estimate of thetotal combustible gases present in a gas mixture.If a moreaccurate determination of the total amount of combustiblegases or a quantitative determination of the individual compo-nents is desired,use a laboratory analytical method,such asTest Method D3612.1.3This standard does not purport to address all of thesafety concerns,if any,associated with its use.It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.Specific precau-tionary statements are given in Section 7.2.Referenced Documents2.1 ASTM Standards:2D3612 Test Method for Analysis of Gases Dissolved inElectrical Insulating Oil by Gas Chromatography2.2 IEEE Standard:3C57.104 Guide for the Interpretation of Gases Generated inOil-Immersed Transformers3.Summary of Practice3.1 A sample of gas is diluted to a fixed ratio with air andintroduced into the meter at a pressure of approximately oneatmosphere.Any combustible gases present are catalyticallyoxidized on the surface of a sensor which is an element of aWheatstone bridge.When combustible gases oxidize on thesurface,they increase the temperature of the element,whichchanges its resistance and upsets the balance of the bridge.3.2 The change in the resistance of the indicating elementsin the bridge circuit is indicated on a meter,which is usuallycalibrated to read in percent total combustible gas.4.Significance and Use4.1 Arcing,partial discharge,and localized overheating inthe insulation system of transformers result in chemical de-composition of the insulating oil and other insulating materials.This may generate various gases,some of which are combus-tible.Typically,gases are generated in the oil and thenpartitioned into the gas space according to their individualsolubilities.Gases which are highly oil-soluble,such as acety-lene,may not be in significant quantities in the gas space untilan incipient fault has progressed to a very serious condition orfailure of the transformer.Gases such as carbon monoxide andhydrogen which have low solubilities in oil can make up alarge fraction of the combustible gases in the gas space.Detection of these gases is frequently the first availableindication of a malfunction.Portable combustible gas metersare a convenient means of detecting the presence of generatedgases.4.2 Normal operation of a transformer may result in theformation of some combustible gases.The detection of anincipient fault by this method involves an evaluation of theamount of combustible gases present,the rate of generation ofthese gases,and their rate of escape from the transformer.Referto IEEE C57.104 for detailed information on interpretation ofgassing in transformers.5.Interferences5.1 In this practice it is essential that sufficient oxygen bepresent in the gas mixture to oxidize the combustible gases.Since the gas blanket in a transformer is usually an inert gas,itis necessary to dilute the sample gas with a known amount ofair.This is usually accomplished by either introducing air andthe sample gas into the instrument in known ratios throughfixed orifices,or by mixing known quantities of air and test1This practice is under the jurisdiction of ASTM Committee D27 on ElectricalInsulating Liquids and Gasesand is the direct responsibility of SubcommitteeD27.03 on Analytical Tests.Current edition approved May 1,2011.Published June 2011.Originallyapproved in 1974.Last previous edition approved in 2005 as D3284 05.DOI:10.1520/D3284-05R11.2For referenced ASTM standards,visit the ASTM website,www.astm.org,orcontact ASTM Customer Service at serviceastm.org.For Annual Book of ASTMStandards volume information,refer to the standards Document Summary page onthe ASTM website.3Available from Institute of Electrical and Electronics Engineers,Inc.(IEEE),445 Hoes Ln.,P.O.Box 1331,Piscataway,NJ 08854-1331,http:/www.ieee.org.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 specimen externally b