The hazardous locations covered by Chapter 5 of the National Electrical Code (NEC) are classified in accordance with the properties of flammable liquids, gases, vapors, combustible dust, or ignitable fibers or flyings that may be present in the area where electrical equipment may be installed. A study of a few of the important aspects of this subject should provide a better understanding of the classification of hazardous locations.
If an area is considered to contain materials that are likely to form ignitable concentrations, the NEC treats it as a special occupancy with special requirements for any electrical equipment installed there. If possible, electrical equipment should be installed and connected at a location outside the hazardous (classified) location. If electrical equipment must be installed in the hazardous area, Section 500-4 provides a selection of the acceptable protection techniques that should be used for a particular hazard.
Explosion-proof apparatus, dust ignition-proof equipment, and purged and pressurized equipment are examples of protection techniques that can be used in certain hazardous (classified) locations.
One of the first considerations in determining the area classification of a potentially hazardous location is to analyze the possible hazard for each room or area where flammable or combustible material may be encountered. By analyzing each area within a facility individually, it will be less likely that the overall facility would be over-classified.
The ambient temperature of the air surrounding the ignitable material, and the temperatures of the ignitable material and of the electrical equipment in the general area of the material must be considered. Temperature is a major factor in area classification, because too much heat added to an ignitable material can cause the material to autoignite without any kind of external energy.
This is called the autoignition temperature of a material. The actual definition of autoignition temperature is “the minimum temperature required to initiate or cause self-sustained combustion of a solid, liquid, or gas independently of a heated element.” As more heat is added to the material, less energy is required from an outside source to provide ignition of the material.
Electrical equipment installed in the area where combustible material (chemical) may be present must not operate at a temperature that is higher than the autoignition temperature of the combustible material. Consult the Material Safety Data Sheet (MSDS) that is available from the manufacturer of the chemical, which should contain the chemical data. If the MSDS is not available, consult National Fire Protection Association (NFPA) 497, the “Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors, and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas” for the autoignition temperature of most combustible materials.
If the chemical is a dust, consult NFPA 499, the “Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas.” Make sure that the electrical equipment operates at or below the autoignition temperature of the material.
Most liquid combustible materials have a flash point. Flash point is the minimum temperature at which a flammable or combustible liquid gives off vapor in sufficient concentration to form an ignitable mixture with air near the surface of the liquid. This is usually a value verified by an actual test.
The gas or vapor may stay close to the surface of the liquid if the density of the gas or vapor in the area of the liquid is about the same density as that of the air. The gas or vapor may migrate away from the liquid, either up or down, depending upon whether it is lighter or heavier than air.
The ignition characteristics of the combustible material and any ventilation or air movement in the area are also important factors in determining hazardous area classifications. The quantity of material in the area and the pressure at which the material is being stored will also affect the extent of the classification. The larger the quantity or the higher the storage pressure, the larger the area that may be affected by an inadvertent release of the combustible material.
It is very important to determine any special conditions that might affect the burning or ignition characteristics of the hazardous material. For example, oxygen-enriched atmospheres where flammable or combustible materials are present or handled add a dimension that is outside the scope of normal hazardous classification techniques. This is not covered by the NEC. Exceeding the normal 21 percent (approximate) by volume of oxygen that is found in air often changes ignition and burning characteristics of materials.
Also, if a material could be ignited just by introduction into air, such as occurs with many pyrophoric materials, the electrical equipment would not be considered as a source of ignition. Therefore, the area would not require special electrical protection techniques. Where oxygen-enriched atmospheres are encountered, the user should refer to the specialized documents that deal with these types of areas and comply with those documents for electrical equipment installation.
These are just a few of the factors that must be taken into consideration for area classification. Verification of the materials (chemicals) available in an area and an understanding of the concepts of area classification can help ensure safe electrical installations.
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 email@example.com.