A big bang in enclosure designA big bang in enclosure design

Until recently, U.S. and Canadian rules governing the design of electronic enclosures used in hazardous locations differed significantly from international regulations. That made it difficult or impossible to exchange equipment and posed major problems for engineers designing products for a global market.

But that is changing, thanks to an addition to the 1999 edition of the National Electrical Code (NEC) that will bring U.S. and International Electrotechnical Commission (IEC) standards more closely into alignment with each other. The most significant impact: design engineers can now use lighter equipment in some areas that previously required heavy, explosion-proof boxes.

Up until now, domestic and international regulations differed in the way in which hazardous areas are classified and in their specific strategies for explosion management. The North American classification system is based on divisions, while the international system is based on zones (see chart).

After taking a first step three years ago by allowing the international zone system, NEC now addresses the explosion protection technology that can be associated with each zone, says Donald Zipse. He serves on a panel of the National Fire Protection Association that oversees the application of electrical equipment.

And this is precisely where the different philosophies regarding explosion management come into play. Three elements are necessary for an explosion, namely oxygen, a fuel source, and an ignition source, explains Dave Leveille, manager of technical services at Hoffman, which offers a range of enclosures designed for both zone and division hazardous environments. Take away any one of these elements, he says, and you eliminate the potential for an explosion.

The division system concentrates on cutting off the fuel source by sealing the enclosure, resulting in a so-called "explosion-proof box." The zone system, on the other hand, cuts out the ignition source, creating an "explosion-protected'' box.

What this means in practice is that in all of Division 1 any electrical connections must be inside an explosion-proof box, generally made of heavy cast metal (usually aluminum), gasketed and secured with many heavy bolts. In Zone 1, on the other hand, explosion-protected boxes can be made of lighter-gauge steel, aluminum, or fiberglass composite. Enclosures made of these materials are typically less expensive, easier to mount, require fewer lid screws, and feature integral gaskets. They are nearly maintenance-free.

The electrical technology that goes inside these boxes is of another order entirely. Intrinsic safety has been around for years as a concept in the division system, and it has always meant that voltage and amperage are kept so low that even a short would ignite nothing. Intrinsic safety devices tend to be low-voltage sensors. Now, attention is also focusing on increased safety, which has always been an important goal in the zone system.

One of its most important principles, says Zipse, is the separation of any arcing device from the terminal strips in the enclosure. "Any arcing device such as a starter, push button or circuit breaker, is encapsulated in a small compartment,'' explains Zipse. "The wires communicate through a channel to the enclosure where they connect to a terminal block."

Joe Smargie, Rose+Bopla's product sales manager for the company's new line of explosion-protected enclosures, describes the inside of the box. "The terminals are clamped down to either spring clamps or screws and are positioned far enough from the box walls to eliminate arcing. All the metal is recessed to eliminate sparking from one terminal to another,'' he says. Similarly, the cross-section of the electrical wiring is increased so that it is balanced with the voltage and no hot spots are possible. Furthermore, because the connections are regulated, the neat and easily identified terminals make for low maintenance.

And there are other Zone 1 spark-, arc- and hot-spot-eliminating strategies. Connections can be capsulated; immersed in insulating, flame-proof oil; buried in quartz powder that melts and absorbs the energy from any spark, or the entire box can be pressurized and/or purged with nonflammable atmosphere.

Equipment must be identified as meeting either American or European standards, and currently, apparatus labeled for the U.S. is not acceptable in Europe and vice versa. However, individual products can carry both markings.

Hazardous area classifications for electronic enclosures

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.

Division 2:

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

International Electrotechnical 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.

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.