Practical Guidelines For Electrical Area Classification In

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Practical Guidelines for Electrical Area Classification in
Combustion Turbine-Generator Power Plants
Ram K. Saini, P.E.
Principal Engineer,
Chuck Emma, P.E.
Principal Engineer,
Burns and Roe Enterprises, Inc.
1.0 INTRODUCTION
Combustion Turbine-Generator (CTG) power plants
utilize flammable or combustible liquids and gases
thatpose fire or explosion hazards. It is essential to
know the characteristics of all of the hazardous
materials used in the power plant, define their
locations and potential sources of leaks, and
determine the extent of the Hazardous Area
associated with each leak source. Each Hazardous
Area should be classifiedaccording to applicable
industry codes and standards. Such Area
Classifications are used in the selection and
installation of suitable electrical equipment, wiring
devices, and wiring methods that prevent the ignition
of flammable or explosion mixtures. This paper
describes the approach being used by Burns and Roe
Enterprises, Inc. inthe classification of Hazardous
Areas for a typical CTG power plant. Others may
find this paper useful in developing their own
guidelines for classifying Hazardous Areas in power
plants. Burns and Roe Enterprises, Inc. takes no
responsibility for classifications prepared by others
using these guidelines.
2.0 DESIGN BASICS
The NationalElectrical Code (NEC), NFPA 70,
defines Hazardous (Classified) Locations as those
where fire or explosion hazards may exist due to the
presence of flammable gases or vapors, flammable
liquids, combustible dust, or ignitable fibers or
flyings.
Electrical Area Classification is the process of
determining the existence and extent of Hazardous(Classified) Locations in a facility handling any of
the substances stated above. The result of such
determination is usually called the Electrical Area
Classification of the facility. The terms Area
Classification, Hazardous Locations, Hazardous
(Classified) Locations, and Classified Areas, when
used in the context of electricalequipment, are all
synonymous with the term “Electrical Area
Classification,” which is being used in these
guidelines to avoid confusion with other non-
electrical classifications.
For a fire or explosion to occur:
• Flammable or combustible material must be present,
• The material must be mixed with air in the proportions required toproduce an ignitable mixture, and
• There must be sufficient release of energy to cause ignition of the mixture.
The above conditions constitute the well-known “fire
triangle.” Within the context of Electrical Area
Classification, the source of energy is understood to
be within the electrical system of the facility.
Areas are classified in accordance with materialtype,
material properties, and the likelihood that an
ignitable mixture will be present. The NEC permits
categorization of hazardous locations in two ways:
• Class I, II and III, Division 1 and 2 concept
(NEC Article 500)
• Class I, Zone 0, 1, and 2 locations concept (NEC
Article 505) 2
The first method has traditionally been used in theUnited States, while the second method is in
accordance with International Electrotechnical
Commission standards and has been used in Europe.
Since 2002, both methods have been accepted by the
NEC.
This paper is based on the Class, Division concept
(NEC Article 500) since it is most commonly used in
U.S. power plants.
Hazardous Locations are then...