"Explosion proof" is both a casually-used generic designation and a precise technical term. In its more generally used form it means that the equipment being discussed is safe for use in a "Hazardous Location". Its strict technical use refers to a specific methodology for making equipment safe for use in hazardous locations.
It is common and costly to misclassify an area, forcing the use of an expensive Hazardous Classified Fixture when a lower level Classification or unclassifeid fixture would surfice. The opposit may also be true; a fixture is specified that is not adequate to the areas hazardous environment. Always confirm that an area has been properly classified before proceeding. The following general guidelines for the Classification of Hazardous Areas explain how and why certain areas are considered hazardous and how to recognize them. The information is greatly simplified and readers must refer to the appropriate codes for precise guidelines about their particular circumstances.
In the United States and many other countries the standard classification system used is that of the National Electric Code (NEC). In Europe and other areas, hazardous locations are classified on the basis of the CENELEC system (See our Knowledge Based article on >> "IP" Classification system for more detail on EN60529. In all areas, specific codes such as the NFPA 496 and 497, Underwriters Laboratories specification 1206, and applicable rules of ANSI, ASHRAE, ASTM, FM Approvals and CENELEC specify what steps must be taken to make equipment safe for use in the areas as they are classified. The following simplifications should help the user to recognize potentially risky situations which require appropriate precautions.
The NEC classification system categorizes hazardous areas in three steps: First by Classes, then further by Groups, and finally by Divisions or Zones. The three Classes are generally distinguished as follows: Class I: Areas which are, or may be, contaminated by hazardous gases or vapors in sufficient quantities to produce explosive or ignitable mixtures. Class II: Areas which are, or may be, contaminated by combustible or explosive dusts. Class III: Areas which are or may be contaminated by combustible or explosive fibers or flyings, but in which the fibers or flyings are not likely to be suspended in the air in sufficient quantities to produce ignitable mixtures.
The Groups differentiate among the broad range of contaminants in each class and divide them into groupings largely on the basis of their relative hazardousness. A list of common items found in each group will be found in the PDF article at the end of this document.
There are two Divisions and three Zones under the NEC system. A simplified explanation:
- Division and Zone "2" applies to areas in which the hazardous contaminants are found in the atmosphere in the minimum quantities to be dangerous only when something goes wrong;
- Division and Zone "1" covers areas where are such contaminants are found more frequently than in Division/Zone 2 areas and in the normal course of business (Note that Class II Group E areas are always classified Division 1.)
- "Zone 0" is a classification normally used under the CENELEC system and refers to areas where contaminants are found commonly or even constantly. Under the normal NEC system these areas would be classified as Division 1, but the NEC is being modified to include a Zone 0 in order to help standardize classifications worldwide.
A practical, although not strictly accurate, way to look at the difference between the two divisions is that Division"2" refers to those locations where the hazardous materials are stored or handled. Division "1" refers to locations where they are handled, manufactured or used: A simple--although by no means precise--rule of thumb for distinguishing between Division 1 and Division 2 areas: If the hazardous material is present in the atmosphere in dangerous quantities more than once or twice a year or if it is ever present for fifteen (15) minutes or more, then the area should be classified as Division 1. If the hazardous material is present less often it should be classified Division 2. Only if hazardous material is virtually never present in ignitable quantities and then only when ventilation is excellent should the location be regarded as non-classified or "ordinary"
This rule of thumb stems in part from the requirements for making equipment safe for use in the different areas. Division 2 equipment is generally made with protections which are not impenetrable by the explosive gases or particles but which do delay the permeation for a certain minimum amount of time.
A good example is that certain totally-enclosed lighting fixtures are permitted in Division 2 areas because the housings are somewhat isolated from the outside environment and as a result it would generally take longer for the hazardous material to permeate into the lighting fixture than it would take to either correct the problem or remove power from the lighting fixture. However, the seals are not hermetic, and therefore if dangerous gases are present long enough they may gradually penetrate into the light fixture where sparks may occur.
Consequently, Division 2 protection is designed and considered to be temporary, i.e. only adequate to allow the user time enough to eliminate the dangerous material from the atmosphere or to remove power from the equipment. Division 1 protection assumes that the contaminant will always be present, and therefore, all protection must be permanent and either impermeable or able to contain any explosion which occurs so that it can not escape from the equipment and spread into the surrounding atmosphere.
A classification of an area as Division 2 relies on the improbability of a hazardous material being present in ignitable quantity at precisely the same time the equipment has a problem which would allow the material rapid access to sparks or other potential sources of ignition. The concept is that even if one condition did exist (a spill of a can of gasoline, for instance), the area is classified as Division 2 if it is extremely unlikely that simultaneously a piece of electrical equipment would have a problem which would allow the vapors from the spill to penetrate to a hot or sparking part of the equipment during the time it takes to clean up the spill or remove power from the equipment.
The ideas of Zones under the NEC system are almost the same as those of Divisions with the same number. Zones were recently added to the NEC code (Section 501 of the code) to begin to bring the classifications and terminology of the NEC and CENELEC systems closer together. There is a Zone 0 in both the modified NEC and the CENELEC systems and although they mean somewhat different things in each system, in the most general terms Zone 0 refers to the idea that the hazardous materials are generally present. Under the normal NEC system Zone 0 locations would be classified Division 1.
UL 844 is the standard used for Luminaires for Use in Hazardous (Classified) Locations (see excerpt below): Scope: 1.1 These requirements cover fixed and portable luminaires for installation and use in hazardous (classified) locations, Class I, Divisions 1 and 2, Groups A, B, C, and D; Class II, Division 1, Groups E, F, and G; Class II, Division 2, Groups F and G; and Class III, Divisions 1 and 2, in accordance with the National Electrical Code, NFPA 70. 1.2 These requirements also cover explosion-proof luminaires for installation and use in Class I, Zone 1, Groups IIA, IIB, IIB plus Hydrogen, and IIC hazardous (classified) locations. 1.3 These requirements cover luminaires for use only under the following atmospheric conditions: a) A minimum ambient temperature of minus 25°C (minus 13°F), b) An oxygen concentration not greater than 21 percent by volume, and c) A nominal barometric pressure of one atmosphere.
Should you or your customerhave any further questions related to UL or NEC Hazardous Location classifications, or any Crownlite product, Please >> Contact Crownlite.
Please see the COMPLETE ARTICLE below for a more complete listing of UL/NEC/IP Ratings and their meaning, as they relate to lighting products.
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