General Guidelines for the Classification of Hazardous Areas
Defining "Explosion Proof" and Explaining the Classification of Hazardous Locations
"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.
Aero Conditioner Company has prepared the following general information to 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.
What are hazardous locations?
The determination that areas can be classified as hazardous locations is based on the following:
- the possible presence of an explosive atmosphere such as flammable gases, vapors, or liquids (Class I), combustible dusts (Class II) or ignitable fibers andflyings (Class III);
- the likelihood that the explosive atmosphere is present when equipment is operating; and
- the ignition-related properties of the explosive atmosphere that is present.
An area may also be considered "hazardous" for other reasons. These may include the use of electrical equipment in the vicinity of water, the risk of personal injury from moving or falling parts, or even the presence of biological hazards. While hazards are associated with all of these conditions, areas are only considered hazardous (classified) locations under conditions defined by the United States (National Electrical Code), Canada (Canadian Electrical Code), Europe (CENELEC EN60079-10) and throughout the world (IEC 60079-10), as applicable.
In all classified (hazardous) areas, specific codes such as the NFPA 496 and 497, Underwriters Laboratories UL 1604 (Division 2 / Zone 2) or UL 1203 (Division 1 /Zone 1) guide construction of the units. Other organizations with related rules and standards include ANSI, ASHRAE, ASTM, FM Approvals, ATEX, and CENELEC specify what steps must be taken to make equipment safe for use in the areas as they are classified. More details are available from the UL website and the other website links.
I. 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 Hazards by Group page on this website.
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 2 / Zone 2 areas and in the normal course of business (Notethat 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
II. 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 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 motors are permitted in Division 2 areas because the windings are somewhat isolated from the outside environment and as a result it would generally take longer for the hazardous material to permeate into the windings than it would take to either correct the problem or remove power from the motor. However, the seals are not hermetic (for instance continuously welded), and therefore if dangerous gases are present long enough they may gradually penetrate into the windings 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.
III. 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.