Practical guidelines for determining electrical area classification

Quantifying electrical safety risks and reducing them to an acceptable level is a team effort. The designer must follow applicable codes and standards when engineering the plant, and operators must run and maintain it within those guidelines. Does your staff know where the hazardous areas are in your plant? Are hazardous area classifications made years ago still appropriate under current standards?

Cataloging risks

Hazardous area classification is a rigorous method of determining where an explosive environment may be present. The codes and standards used in this process provide guidance for selecting, building, and installing electrical equipment in that area. In general, hazardous areas (HAs) are defined by:

• The type of hazard or hazardous material.
• The likelihood of its presence in flammable concentration.
• The auto-ignition temperature of the hazardous material.

An explosive atmosphere can be created by a gas or vapor, dust, or even fibers. Methane-based hazards dominate the gas-fired generation sector, whereas spontaneous combustion of coal dust plagues plants fired by that fuel. The different zones defined by codes for classifying particular hazardous areas are listed and discussed in the online supplement to this article.

The probability that any hazard exists in combustible concentration is determined by the specifics of the plant or system under consideration. For example, a natural gas vent line is much more likely to contain such a hazard than a lube oil line—unless, of course, the oil line's flanged joint is leaky.

Finally, plant design also must protect against auto-ignition of combustible substances. A good example of this type of hazard is a flammable gas coming into contact with a hot surface. Codes define various temperature classes to guide designers as they specify equipment.

In the field

By design, natural gas-fired power plants are fueled by an extremely combustible substance. But fuel lines and the turbine deck are hardly the only HAs in the plant. The prerequisites for developing an area classification for a gas-fired plant are understanding the characteristics of all of hazardous materials present in it and defining where those materials exist (or may spread due to a leak). This allows mapping out the plant's HAs.

Each HA should be classified according to applicable industry codes and standards. For example, electrical area classifications explain how to select and install electrical equipment and wiring—right down to the wiring method—to minimize the likelihood of ignition of a flammable or explosive mixture.

—Ram K. Saini, PE, is senior principal engineer for Burns and Roe Enterprises Inc. He can be reached at 201-986-4339 or rsaini@roe.com. Charles Emma, PE, is principal engineer and can be reached at 856-638-6920 or cemma@roe.com.

The National Electrical Code (NEC), NFPA 70, defines hazardous (classified) areas as those where fire or explosion hazards may exist due to the presence of flammable gases, vapors, or liquids, combustible dust, or ignitable fibers. Electrical area classification (EAC) is the process of determining the existence and extent of hazardous locations in a facility containing any of those substances. The result of the process is usually called the EAC of the facility. In the context of electrical equipment, the following terms—area classification, hazardous locations, hazardous (classified) locations, and classified areas—are all synonymous with EAC. In the following guidelines, the term EAC is used to avoid confusion with other nonelectrical classifications.

The approach described in this article focuses on a combustion turbine-generator (CTG) plant, although the same methods are applicable to any type of power plant and its systems.