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Explosion protection standards 4-3-00

Article-Explosion protection standards 4-3-00

Explosion protection standards 4-3-00

Explosion protection standards get in sync

The move toward global standardization gives engineers more options for specifying electrical equipment for use in hazardous areas

By John Day, Contributing Editor

Enclosure manufacturers and OEM manufacturers who design electrical equipment for use in hazardous (classified) locations are applauding recent changes in the National Electrical Code (NEC) and the Canadian Electrical Code (CEC). The changes give engineers in North America new options for classifying hazardous areas and specifying the types of electrical equipment that can be used in those areas. For manufacturers and buyers alike, the changes represent a step toward a global market.

For the past several decades, the NEC and CEC defined hazardous areas as either Division 1 (hazardous under normal operations) or Division 2 (hazardous under abnormal operations). Throughout most of the rest of the world, hazardous areas are classified according to International Electrotechnical Commission (IEC) standards as Zone 0, Zone 1, or Zone 2. Zone 0 and Zone 1 together roughly correspond to Division 1, with some overlap of Zone 1 into Division 2, and Zone 2 is the same as Division 2 (see chart).

Areas containing Class I hazards (flammable gases or vapors) can now be reclassified as Zone 0, Zone 1 or Zone 2. Other hazards fall into Class II (combustible dusts) or Class III ("flyings," or ignitable fibers such as cotton). Only Class I hazards are covered under the Zone system.

The need for specialized electrical equipment for hazardous locations first became apparent in the early years of the twentieth century. The age of electricity coincided with the age of the automobile, and electric "arcs and sparks" don't play nicely with volatile vapors.

Global differences. Paul Babiarz, international marketing manager for Crouse-Hinds, Syracuse, NY, a manufacturer of products for hazardous locations, says that less than 10% of hazardous locations in North America are classified as Division 1. In Europe, more than 60% of hazardous areas are classified as Zone 1.

As a result, most hazardous location electrical equipment in North America is developed for Division 2 areas, and most electrical equipment in the rest of the world is developed for Zone 1. Today, according to Babiarz, there are relatively few products designed for Zone 2 locations, but that will change as the trend toward Zone 2 and Division 2 reclassification grows.

One of the barriers to globalization of hazardous-location electrical equipment is the requirement that products must be labeled as meeting IEC, CENELEC (European Committee for Electrotechnical Standardization), and North American standards.

Babiarz suggests that the labels can be difficult for non-experts to decipher and, on a practical basis, are too large to fit smaller products. "However, as individuals on both sides of the ocean become more comfortable with globalization, these types of non-tariff barriers will become easier to understand," he notes.

The hazardous location protection methods most commonly used in North America, according to Babiarz, are "explosionproof," intrinsic safety, and purging/pressurization. IEC alternatives include flameproof, increased safety, intrinsic safety, pressurization, powder filling, and oil immersion.

The intrinsic safety concept, used in North America and in Europe, limits the energy available (even under specified fault conditions) for igniting flammable gases, usually by inserting energy-limiting devices within control and instrumentation circuits. Increased safety eliminates the source of ignition by arcing, sparking and hot spots under normal conditions by the use of increased electrical spacings and highly reliable terminations.

Babiarz explained that increased safety ratings allow other than explosionproof enclosures to be used in Zone 1 areas. Wire terminals rated as increased safety are not considered to be sources of ignition. In contrast, only explosionproof enclosures can be used in Division 1 areas and, under the Division system, wire terminations are considered to be sources of ignition.

"Under the Division system, explosionproof enclosures are designed to contain explosions. Under the Zone system, increased safety products are designed to ensure that explosions never happen," says Joe Smargie, product sales manager at Rose+Bopla, an enclosure maker based in Frederick, MD. Smargie says increased safety enclosures, which have been used successfully in Europe since the end of World War II, are significantly lighter, less costly, and less prone to installation error than the explosionproof enclosures favored by large U.S. manufacturers.

Understandably, European firms are encouraging acceptance of Zone-classified enclosures. Michael Herzog, product line manager for Human Machine Interface devices at Rittal Corp., Springfield, OH, says his firm, headquartered in Germany, was among the first standard enclosure manufacturers to enter the Ex market.

Both protection methods work, however, notes David Leveille, manager of technical services at Hoffman Enclosures (Anoka, MN), and U.S. engineers are comfortable with the Division system. He thinks it's unlikely there will be a mass migration to Zone classification for existing U.S. facilities. "It's been argued that the Zone system is not as expensive, but there's no empirical evidence to support that."

Crouse Hinds' Babiarz said the decision of which system is safer, less costly, or easier to install depends on user preference, how areas are classified, and the wiring systems used in the facility.

Paul T. Kelly, Associate Managing Engineer for Hazardous Locations at Underwriters Laboratories, Inc. (Northbrook IL), explains that Division 1 locations involve conduit as the predominant wiring system, while Zone 1 locations allow for the possibility of more flexible options centered around cabling. "This potential use of more cabling instead of conduit is possible in Zone 1 locations because Zone 1 locations are less hazardous than Division 1 locations; also, in the increased safety protection method, no normally arcing parts are to be contained by the wiring compartment and additional measures-such as larger spacings among wiring connections-are taken to minimize the possibility of high temperatures or sparks."

While the greater use of cabling over conduit would seem to make Zones the classification system of choice, there's a "catch" in the U.S., according to Kelly. "As opposed to IEC and European installation codes, the NEC hasn't yet broadened the use of cabling for Zone areas beyond that allowed for Division areas, so petrochemical plants in the U.S. that use the Zone classification system still use conduit as their primary wiring system. With the next few (NEC) Code cycles, a greater flexibility should begin to appear regarding wiring systems suitable for U.S. Zone-classified locations."

Nevertheless, U.S. manufacturers and users alike are embracing Zone classification. Bill Lawrence, senior engineer in the electrical section of Factory Mutual Research Corp. (Norwood MA), says his organization, a third-party testing laboratory, has recently approved a fluorescent lighting system for use in Zone 1 locations.

The system was used on a large oil project in Alaska, and Lawrence said the alternative to the Zone-classified system would have been a Division 1 fixture that cost more than $1,000 and weighed several hundred pounds.

Lawrence said the manufacturer used both increased safety and flameproof methods to provide explosion protection and to reduce weight and cost. Instead of housing the light in an explosionproof case, it protected individually protected electrical components. Additional spacing was provided between the increased safety wiring terminals to reduce the likelihood of arcing, for example, and the lighting system's ballast was housed in a small, flameproof case. The system itself is housed in a lightweight, shatterproof case for environmental protection.

How the hazardous area classifications for electronic enclosures compare
National Electrical Code (NEC)
Division 1:
Where ignitable concentrations of flammable gases, vapors, or liquids can exist all of the time or some of the time under normal operating conditions
International Electrotechnic Commission (IEC)
Zone 0:
Where ignitable concentrations of flammable gases, vapors, or liquids can exist all of the time or some of the time under normal operating conditions

Division 2:
Where ignitable concentrations of flammable gases, vapors or liquids are not likely to exist under normal operating conditions

Zone 1:
Where ignitable concentrations of flammable gases, vapors, or liquids can exist some of the time under normal operating conditions

Zone 2:
Where ignitable concentrations of flammable gases, vapors, or liquids are not likely to exist under normal operating conditions.
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