It is possible to ignite combustible gases or vapors either directly from an arc or spark at the electrical equipment or from the heat generated by the electrical equipment, so extraordinary care must be taken when installing electrical equipment in these areas. Special precautions are necessary where combustible gases or vapors and electrical equipment are located in the same general area. Extra care must also be taken to ensure the electrical equipment is well maintained.
One of the most important precautions to be taken in a combustible gas or vapor atmosphere is to ensure the electrical equipment to be used is able to contain an explosion without igniting the gases or vapors in the area surrounding it. Some gases or vapors have very high explosion pressures generated as a result of the ignition of the material.
Very high flame speeds often accompany their ignition. The combination of high flame speeds and high explosion pressures can subject an electrical enclosure to an enormous amount of internal pressure.
Enclosures must be designed to withstand these pressures, which can be exceptionally high. Sudden release of energy from a gas ignition in a confined space can result in explosive pressures as high as eight times for many gas or vapor mixtures or as much as hundreds of times the initial pressure inside the enclosure. Electrical enclosures manufactured for use in a hazardous atmosphere must use special design considerations that will permit hot gases to vent from the enclosure without rupturing the enclosure and igniting the surrounding atmosphere.
Some history on the factors involved in the design of electrical equipment to withstand internal explosion may help in understanding the requirements in Section 500-5(a) for group classifications of chemicals.
There are four Class I group classifications in Section 500-5(a): Groups A, B, C, and D. They were introduced into the National Electrical Code (NEC) in the 1930s. By grouping many combustible liquids or gases into four major groups, electrical enclosures could then be designed to withstand internal explosion pressures without flame propagation if the gases or vapors within were ignited.
Electrical equipment was required to be marked with the group classification of the liquids or gases with which the enclosure could successfully contain the explosion.
These groups were defined based upon the level of hazard associated with the explosion pressures of specific atmospheres and the likelihood that an explosion could be transmitted outside the enclosure.
Group A was defined as an atmosphere containing acetylene, and Group B as an atmosphere containing hydrogen or gases of equivalent hazardous nature. Group C was defined as an atmosphere containing ethyl ether or gases or vapors of equivalent hazardous nature, while Group D contained by far the most chemicals of the four groups with atmospheres of gasoline, alcohol, acetone, natural gas, and similar materials.
In the 1960s, a test apparatus was developed at Underwriters Laboratories that provided a means to determine more precisely how these liquids or gases reacted during actual test conditions. This new apparatus permitted the testing of many combustible liquids, gases, or vapors that had previously been grouped as theoretically the same chemical characteristics. It was called the Westerberg Explosion Test Vessel. The test apparatus provided documented proof that if a specific gap was provided in an enclosure for a specific gas, the enclosure could be vented safely without igniting the outside atmosphere. The gap was called the Maximum Experimental Safe Gap (MESG).
The Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas in Table 2-1 of NFPA 497 (1997 edition) is a complete list of the most combustible liquids and gases. This table provides an asterisk adjacent to each liquid or gas that has been group classified by an actual test. The table also contains many of the chemicals’ MESG information and other information, such as upper and lower flammable limits, autoignition temperature, etc.
The definition of MESG is the maximum clearance between two parallel metal surfaces that has been found, under specified test conditions, to prevent an explosion in a test chamber from being propagated to a secondary chamber containing the same gas or vapor at the same concentration.
FPN No. 2 in Section 500-5(a) further explains that “explosion characteristics of air mixtures of gases or vapors may vary with the specific material involved. For Class I locations, Groups A, B, C, and D, the classification involves determinations of maximum explosion pressure and maximum safe clearance between parts of a clamped joint in an enclosure. It is necessary, therefore, that equipment be approved not only for class but also for the specific group of the gas or vapor that will be present.”
Electrical equipment installed in a hazardous (classified) area must be marked with the appropriate group classification. The integrity of the enclosure must be maintained, and the gaps designed to permit the gas to escape must not be blocked.
Understanding the importance of this marking and the proper use of electrical equipment in these areas will provide a safer installation.
ODE is staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or via e-mail at firstname.lastname@example.org.