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National Electrical Manufacturer’sAssociation (NEMA) Standards250-1991, Enclosures for Electrical

Equipment (1000 Volts Maximum)

NEMA Enclosure Rating

Enclosures may be tested to determine their ability to prevent the ingressof liquids and dusts. In the United States,equipment is tested toNEMA 250. Some of the more common

enclosure ratings defined in NEMA 250 are as follows.General Locations

_ Type 3 (Dust-tight, Rain-tight,

or Ice-resistance, Outdoor enclosure):

Intended for outdoor use primarilyto provide a degree of protection against rain, sleet, windblown,dust, and damage from external ice formation.

_ Type 3R (Rain-proof, Ice-resistance,

Outdoor enclosure): Intended for outdoor use primarily to

provide a degree of protection against rain, sleet, and damage from external ice formation.

_ Type 3S (Dust-tight, Raintight, Ice-proof, Outdoor enclosure):Intended for outdoor use primarily to provide a degree of protection against rain, sleet, windblown dust, and to provide for operation of external mechanisms when ice ladened.

_ Type 4 (Water-tight, Dusttight,Ice-resistant, Indoor or outdoor enclosure): Intended for indoor or outdoor use primarily to provide a degree of protection against windblown

dust and rain, splashing water,hose-directed water, and damage from external ice formation.

_ Type 4X (Water-tight, Dusttight,Corrosion resistant, Indoor or outdoor enclosure): Intended for indoor or outdoor use primarily to provide a degree of protection against

corrosion, windblown dust and rain, splashing water, and hose-directed water, and damage from external ice formation.

Hazardous (Classified) Locations Two of the four enclosure ratings for hazardous (classified) locations are described as follows in NEMA 250:

_ Type 7 (Class I, Division 1,Group A, B, C or D, Indoor hazardous location, Enclosure): For indooruse in locations classified as Class I, Division 1, Groups A, B, C or D as defined in the NEC and shall be marked to show class, division, and group. Type 7 enclosures shall be capable of withstanding the pressures resulting from an internal explosion of specified gases, and contain such an explosion sufficient that an explosive gas-air mixture existing in the atmosphere surrounding the enclosure will

_ Type 9 (Class II, Division 1, Groups E, F or G, Indoor hazardous location, Enclosure): Intended for use in indoor locations classified as Class II, Division 1, Groups E, F and G as defined in the NEC and shall be marked to show class, division, and group. Type 9 enclosures shall be capable

of preventing the entrance of dust.

The above two NEMA ratings are now often misunderstood. For example, the above definition of Type 7 is essentially

the same as that for explosion

–proof. Therefore, when an approval

agency approves equipment as

explosion–proof and suitable for Class

I, Division 1, the equipment automatically

satisfies the Type 7 requirement;

however, the agency does not require

that the equipment be labeled Type 7.

Instead it is labeled as suitable for

Class I, Division 1. Similarly, Type 9

enclosures would be labeled as suitable

for Class II, Division 1.

Hazardous Location

Classification

Hazardous areas in North America are classified by class, division, and group.

Note

The method of classifying locations as zones instead of divisions was introduced into the 1996 edition of the NEC as an alternate method, but it is not yet in use. The zone method is common in Europe and most other countries.

Class: The Class defines the general nature of the hazardous material in the surrounding atmosphere.

_ Class I—Locations in which flammable gases or vapors are, or may be, present in the air in quantities sufficient to produce explosive or ignitable mixtures.

_ Class II—Locations that are hazardous because of the presence of combustible dusts.

_ Class III—Locations in which easily ignitable fibers or flyings may be present but not likely to be in suspension

in sufficient quantities to product ignitable mixtures.

Division: The Division defines the probability of hazardous material being present in an ignitable concentration in the surrounding atmosphere.

_ Division 1: Locations in which the probability of the atmosphere being hazardous is high due to flammable material being present continuously, intermittently, or periodically.

_ Division 2: Locations that are presumed to be hazardous only in an abnormal situation.

Group: The Group defines the hazardous material in the surrounding atmosphere.The specific hazardous materials within each group and their automatic ignition temperatures can be found in Article 500 of the NEC and in NFPA 497M. Groups A, B, C and D apply to Class I, and Groups E, F and G apply to Class II locations.

The following definitions are from the

NEC.

_ Group A: Atmospheres containing

acetylene.

_ Group B: Atmospheres containing hydrogen, fuel and combustible process gases containing more than 30 percent hydrogen by volume, or gases or vapors of equivalent hazard such as butadiene, ethylene oxide,propylene oxide, and acrolein.

_ Group C: Atmospheres such as ethyl ether, ethylene, or gases or vapors of equivalent hazard.

_ Group D: Atmospheres such as acetone, ammonia, benzene, butane, cyclopropane, ethanol, gasoline, hexane,methanol, methane, natural gas,naphtha, propane, or gases or vapors of equivalent hazard.

_ Group E: Atmospheres containing combustible metal dusts, including aluminum, magnesium, and their commercial alloy, or other combustible dusts whose particle size, abrasiveness, and conductivity present similar hazards in the use of electrical equipment.

_ Group F: Atmospheres containing combustible carbonaceous dusts,including carbon black, charcoal, coal, or dusts that have been sensitized by other materials so that they present an explosion hazard.

_ Group G: Atmospheres containing combustible dusts not included in Group E or F, including flour, grain,

wood, plastic, and chemicals.

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