This is a report of the initial investigation conducted by Underwriters' Laboratories Inc. to establish a procedure for classifying additional flammable gases or vapors within Groups A, B, C, and D of Class I hazardous locations as defined by the National Electrical Code. This procedure provides a means of increasing the utility of presently recognized explosion-proof electrical equipment without the necessity of additional expensive and laborious testing of such equipment.
Using a test facility designed and constructed by Underwriter's Laboratories Inc. comparative experimental data on flame propagation and explosion effects, with special reference to explosion-proof electrical equipment, have been obtained for fifteen flammable and nonflammable gases or volatile chemical materials net previously classified. Features investigated included the maximum explosion pressure developed by the particular gas- or vapor-air mixture under pertinent test conditions, and the widths and clearances in metal-to-metal joints that will arrest flame propagation. The primary explosion chamber, 12 inch in internal diameter with an internal volume of one cubic foot, is provided with a variable test joint assembly, means for inducing initial turbulence in the test mixture prior to ignition, and an optional 10-foot section of 1-1/2
inch size conduit for studies of explosion pressure piling effects.
To arrive at the proposed classifications, the flame propagation and explosion effects of each gas or vapor,
together with its ignition temperature, were compared with the corresponding characteristics of acetylene
(Group A), hydrogen (Group B), diethyl ether (Group C), and gasoline (Group D) as determined under the
same test conditions. The Group classification of these four reference materials has long been established within Class I hazardous locations, and each of the four is considered representative of the other gases or vapors classed in the respective Group.
On the basis of the results of this investigation, fourteen gases or vapors have been added to the Group classifications within Class I hazardous locations as follows:
Class I, Group B - Butadiene, ethylene oxide, and propylene oxide.
Class I, Group C - Acetaldehyde, isoprene, and unsymmetrical dimethylhydrazine.
Class I, Group D - Acrylonitrile, ammonia (anhydrous), ethylene dichloride, propylene, styrene, vinyl acetate, vinyl chloride, and para-xylene.
Carbon disulfide, the fifteenth material investigated would not be classified in any of the four Groups of Class I hazardous locations. Because of its Iow ignition temperature and the very close clearances between surfaces necessary to arrest its flame propagation, carbon disulfide requires safeguards beyond those required for any of the present Groups.
Part II reports on an investigation conducted by Underwriters Laboratories Inc. to classify additional gases
or vapors within Groups A, B, C, and D of Class I hazardous locations as defined by the National Electrical
Code, NFPA No. 70-1975, ANSI C1-1975.
UL TR 58-1993 history
1993UL TR 58-1993 An Investigation of Flammable Gases or Vapors with Respect to Explosion-Proof Electrical Equipment