Area Classification
The type of protective technique selected and the level of protection it
must provide depend upon the potential hazard caused by using electrical
apparatus in a location where a combustible, flammable, or ignitible substance may be present. Area
classification schemes and systems of material classification have been
developed to provide a succinct description of the hazard so that appropriate
safeguards may be selected. All useful area classification systems specify the
kind of flammable material that may be present and the probability that it will
be present in ignitible concentrations.
4.1 North American methods
4.1.1 In the United States, area classification principles are stated in
Article 500 of the National Electrical Code, ANSI/NFPA 70. Similar requirements
in Canada are given in the Canadian Electrical Code, Part 1, Section 18, (CSA C22.1).
Various organizations have developed numerous guides and standards that
have substantial acceptance by industry and governmental bodies for area
classification.
Area classification descriptions used in the United States and Canada
include the following:
Locations are classified
(1) by CLASS—the generic form of the
flammable materials in the atmosphere (gas or vapor,
dusts, or easily ignitible fibers or flyings);
(2) by DIVISION—an indication of the
probability of the presence of the flammable material in ignitible concentration;
(3) by GROUP—the exact nature of the
flammable material.
4.1.1.1 Classes
Class I locations are those in which flammable gases or vaporsare, or may be, present in the air in quantities
sufficient to produce explosive or ignitible mixtures.
Class II locations are those that are hazardous because of the presence of
combustible dusts.
Class III locations are those in which easily ignitible fibers or flyings may
be present but not likely to be in suspension in sufficient quantities to
produce ignitible mixtures.
4.1.1.2 Divisions
Class 1, Division 1 locations are those in which:
a) hazardous concentrations exist
continuously, intermittently, or periodically under normal operating conditions;
b) hazardous conditions may exist
frequently because of repair or maintenance operations or because of leakage; or
c) breakage or faulty operation of process
or other nonelectrical equipment or processes might release flammable
concentrations or gases or vapors and might
also cause simultaneous failure of electrical equipment that causes a source of
ignition.
Class I, Division 2 locations are those in which:
a) hazardous volatile liquids, vapors, or gases are normally confined within closed
containers or closed systems from which they
can escape only in case of accidental
rupture or breakdown of such containers or
systems, or in case of abnormal operation
of equipment;
b) flammable concentrations are
normally prevented by positive mechanical ventilation but might become
hazardous through failure or abnormal operation of the ventilating system; or
c) areas adjacent to Division 1
locations to which hazardous concentrations of gases or
vapors might occasionally be communicated.
Class II, Division 1 locations are those in which:
a) combustible dust is, or may be, in
suspension in the air continuously, intermittently, or periodically under
normal operating conditions, in quantities sufficient to produce explosive
or ignitiblemixtures;
b) breakage or faulty operation of a
process or machinery may produce combustible
concentrations of dusts and might also cause
simultaneous failure of electrical equipment, which, in turn, may act as a
source of ignition; or c) electrically conductive combustible dusts may be
present.
Class II, Division 2 locations are those in which:
a) combustible concentrations of
suspended dust are not likely, but where deposits or
accumulations of dust may interfere with the
safe dissipation of heat from electrical
equipment or apparatus; or
b) combustible concentrations of
suspended dust are not likely, but where deposits or
accumulations of dust on, in, or in the
vicinity of electrical equipment might be ignited
by arcs, sparks, or burning material from
such equipment.
Class III, Division 1 locations are those in which easily ignitiblefibers or materials producing
combustible flyings are handled,
manufactured, or used.
Class III, Division 2 locations are those in which easily ignitiblefibers may be stored or handled (except in
the process of manufacture).
NOTE: Some plant areas in the manufacture, handling, and storage of
explosives or
ammunition and nitrocellulose products (such
as celluloid photographic films), etc., involve conditions that are not covered
by NEC Classifications. This is particularly true where black powders,
smokeless powder, dust from TNT, and other explosives are present. See NFPA
495, Reference Code for Explosive Materials, for guidance.
4.1.1.3 Groups
The United States and Canadian standards recognize seven groups: Groups A,
B, C, D, E, F, and G. Groups A, B, C, and D apply to Class I locations; Groups
E, F, and G apply to Class II Locations. These groups include the following:
Group A—Atmospheres containing acetylene.
Group B—Atmospheres such as butadiene*, ethylene oxide*, propylene
oxide*, acrolein*, or hydrogen (or gases
or vaporsequivalent in hazard to hydrogen, such
as manufactured gas).
Group C—Atmospheres such as cyclopropane, ethyl ether, ethylene,
hydrogen sulfide, or gases or vapors of equivalent hazard.
Group D—Atmospheres such as acetone, alcohol, ammonia*, benzene, benzol, butane,
gasoline, hexane, lacquer solvent vapors, methane, naphtha, natural gas, propane, or gases
or vapors of equivalent hazard.
In the United States:
Group E—Atmospheres containing combustible metal dusts regardless of
resistivity or other combustible dusts of similarly hazardous characteristics
having resistivity of less than 102 ohmcentimeter (magnesium, aluminum, bronze powder, etc.)
Group F—Atmospheres containing carbon black, charcoal, coal, or coke dusts
that have more than 8 percent total volatile material (coal and coke dusts per
ASTM 3175-82) or atmospheres containing these dusts sensitized by other
materials so that they present an explosion hazard and having resistivity
greater than 102 ohm-centimeter but equal to or
less than 108 ohmcentimeter.
Group G—Atmospheres
containing combustible dusts (flour, starch, pulverized sugar and cocoa, dairy
powders, dried hay, etc.) having resistivity of 108 ohm-centimeter or
greater.