'Division' or 'zone' by any other name is still hazardous

Not all parts of processing plants or manufacturing facilities are created equal. In fact, some areas of petrochem plants, underground mines, even grain silos pose the potential of accidental explosion, due to ignitable gases, vapors or dust that may be present. Various classifications have been set up to identify these potentially dangerous industrial environments so that electrical equipment ...

05/01/2003


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Not all parts of processing plants or manufacturing facilities are created equal. In fact, some areas of petrochem plants, underground mines, even grain silos pose the potential of accidental explosion, due to ignitable gases, vapors or dust that may be present.

Various classifications have been set up to identify these potentially dangerous industrial environments so that electrical equipment operating in those areas can be regulated to prevent ignition and explosion, thereby ensuring the safety of workers and facilities.

Two main classification systems have emerged: A North American system based on the U.S. National Electrical Code (NEC), and a European approach derived from CENELEC and International Electrotechnical Commission (IEC) methods. The latter system is gaining international acceptance; and although incorporated into NEC in 1996, it has received substantially less support in the U.S.

Division or zone; new directive

In North American usage, class, division, and group labels define safety levels required for equipment installed in hazardous areas. Three general classes define flammable materials in the atmosphere. Two divisions refer to the hazard probability of these materials—(1) for danger present during normal conditions, and (2) for danger present only under abnormal conditions, as in the case of a tank rupture or system failure. Seven groups classify the specific offending flammable materials. (See North American classification table.)

With the IEC approach used in Europe (and outside North America), three zones , rather than two divisions, define the probability of flammable materials being present. Protection type, such as flameproof, purging, and intrinsic safety, denotes the level of safety provided for the device. Groups carry the same general designation as in the U.S. method, but are differently characterized. For either approach, temperate class specifies limits on equipment surface temperature that could act as a source of ignition for gases or vapors.

Plant regions designated Zone 0 or Zone 1 in the IEC system are basically equivalent to Division 1 in the North American classification system, while Zone 2 matches Division 2—see Zone Hazards table. Progressing from Zone 2 through Zone 0 indicates an increasing level of risk. One notable difference between methods is that Zone 0 sets apart regions of highest hazards from Zone 1, while Division 1 lumps both regions under one probability.

A related development comes from the growth of "CE marking" directives issued by the European Union (EU). The so-called ATEX Directive (French for "ATmospheres EXplosible") becomes mandatory on July 1, 2003, after operating on a voluntary basis since March 1996. The ATEX Directive will regulate applicability of equipment for use in the three zones. It adds specific time durations to the presence of hazards and a prefix "2" to indicate dust hazards. The directive applies to all EU member states and several other European countries. New certification will be needed for products shipped to the affected countries.

Frank J. Bartos, executive editor fbartos@reedbusiness.com

North American Hazardous Area Classifications

Source: Control Engineering

Class I: Flammable gases and vapors

Group A: Acetylene

Group B: Hydrogen, butadiene ethylene oxide, propylene oxide

Group C: Ethylene, coke oven gas, diethyl ether, dimethyl ether

Group D: (Partial list) Propane, acetone, alcohols, ammonia, benzene, butane, gasoline, methane, pentanes, toluene

Class II: Combustible dust

Group E: Metal dust

Group F: Coal, coke dust

Group G: Grain, plastic dust

Class III: Combustible Flyings and fibers

No group designators defined: Wood flyings, paper fibers, cotton fibers


Zone Hazard Categories, Comparisons

Continuous hazard >1,000 hr/yr

Intermittent hazard 10-1,000 hr/yr

Hazard under abnormal conditions, &10 hr/yr

Source: Control Engineering with data from Bently Nevada and R. Stahl Inc.

Europe:

Zone 0

Zone 1

Zone 2

IEC/CENELEC/NEC505

(Zone 20, dust)

(Zone 21, dust)

(Zone 22, dust)

North America: NEC500-503

Div.1 (gases, dust)

Division 1

Division 2

North America: NEC505

Zone 0

Zone 1

Zone 2

Safety categories-vapors

G1

G2

G3

Safety categories-dust

D1

D2

D3