Class I: Flammable Gases, Vapors, Liquids | Area Classifications
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2018-4-13
Class I: Flammable Gases, Vapors, Liquids | Area Classifications
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CLASS I
CEC/NEC (North American) Area Classifications
Flammable gases, vapors or liquids
CLASS I (Gas and Vapors) Hazardous Substances CHART (PDF)
Class I locations are those in which flammable gases or vapors are or may be present
in the air in quantities sufficient to produce explosive or ignitable mixtures. (“Gases”
refers to materials that are in a gaseous state under normal atmospheric conditions.
Examples of this would be hydrogen or methane. “Vapors” refers to the gases over a
material that is a liquid under normal atmospheric conditions but emits gases within the
flammable range under these atmospheric conditions.)
Division 1
locations are those where the volatile flammable gases or vapors exists under normal
conditions, or where volatile flammable gases or vapors may exist frequently because of
repair or maintenance operations or leakage, or where breakdown or faulty operation of
electrical equipment or processes might release ignitable concentrations of flammable
gases or vapors, and might also cause simultaneous failure of electrical equipment in
such a way as to directly cause the electrical equipment to become the source of ignition.
An example of this might be an area where a flammable liquid is stored under cryogenic
conditions, and a leak directly into the electrical equipment could cause a failure of the
electrical equipment at the same time the vapors of the evaporating liquid are within the
flammable range.)
Division 2
locations are those locations where ignitable concentrations of flammable gases or
vapors are not likely to exist under normal conditions, or in locations that are adjacent to
Class I, Division 1 locations where there are no barriers or partition to separate the
Division 1 space from a non-hazardous location or where ventilation failure extend the
area where flammables exist under normal conditions.
Groups
The gases and vapors of Class I locations are broken into four groups (NEC 500-503), by
the codes A, B, C & D. These divisions are because different materials have different
explosion and ignition characteristics. The grouping permits equipment to be tested
based on the type of flammable material in which it is intended to be used. It also permits
area classification to be based on the type of material anticipated in that location. The
grouping is based on two major factors: the explosion pressure generated during an
explosion; and the maximum gap between ground flat mating metal surfaces that will
prevent propagation of an explosion through the gap to a flammable atmosphere of the
same flammable material and concentration. This chart shows common gases in Class I
and how they differ by ignition temperature and explosive pressure:
NFPA
C Ignition
F Ignition
Hydrostatic
Zone
Class I
Group
Temperature
Temperature
Pressure *
Gas Groups
Acetylene
A
305
581
6,000 PSI
IIC
Hydrogen
B
520
968
6,000 PSI
IIC
Ethylene
C
450
842
1,200 PSI
IIB
Gasoline
D
280
536
600 PSI
IIA
* Typical tested hydrostatic pressure required for electrical fittings such as unions,
elbows, seals, etc.
Ambient temperature is the surrounding temperature of the environment in which a piece
of equipment is installed, whether it is indoors or outdoors. A heat producing product is
considered acceptable for the location, provided the minimum ignition temperature of the
hazardous material present and the ambient temperature of the location do not exceed
the limits set by the manufacturer. If the ambient temperature is higher than the
maximum stated on the nameplate, it might still be acceptable to use the product under
certain conditions, providing the minimum ignition temperature of the hazardous material
has not been exceeded. In all cases, consult the factory for assistance.
Operating temperature
The rated operating temperature for hazardous (classified) products is determined by
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conducting laboratory tests in an ambient temperature of 40 ° C. Products certified by the
various agencies consider products certified to their standards to be suitable for different
temperature ranges. The range for CSA ( Canada) is -50 ° C to +40 C to +40 ° C; the
range for UL ( United States) is -25 ° C to +40 ° C; the range for IEC and CENELEC
(International) is -20 ° C to +40 ° C.
Temperature Codes (T-Codes)
The ignition temperature or auto-ignition temperature (AIT) is the minimum temperature
required to initiate or cause self-sustained combustion in a substance without any
apparent source of ignition. The lowest published ignition temperature should be the one
used to determine the acceptability of equipment. This is of particular concern when
selecting heat producing equipment such as lighting fixtures or motors which could
generate sufficient heat to ignite the surrounding atmosphere.
Class I and Class II areas use T-Codes or are subject to maximum temperature
limitations as shown in the following chart. North America and the IEC are consistent in
their temperature or T-Codes. However, unlike the IEC, North America includes
incremental values as shown. Equipment tested must have nameplates and marked
showing class, group and operating temperature based on operation in a 40 ° C ambient
(see NEC 500-3 for exceptions). Non-heat producing equipment does not have this
requirement.
North America Temp.
Codes
(NEC-500) & CSA
Maximum
Temperature
IE / CENELEC / US
(NEC 505) T-CODES
°C
°F
T1
T1
450
842
T2
T2
300
572
T2A
280
536
T2B
260
500
T2C
230
446
T2D
215
419
200
392
T3A
180
356
T3B
165
329
T3C
160
320
135
275
120
248
T3
T4
T3
T4
T4A
T5
T5
100
212
T6
T6
85
185
NEC ARTICLE 505 TO INCLUDE ZONES
The rewritten Article 505 of the 1999 National Electrical Code (NEC) provides for the use
of the Zone Classification System from the Division System. Prior to insertion of the Zone
Classification, the IEC/CENELEC (for the European Union) utilized this classification
system whereas the Division System was utilized only by the NEC (for the United
States). Article 505 was rewritten because the Division Classification and the Zone
Classification were difficult to merge.
Both the Zone System and the Division group classify the different gases in ascending
order of “more easily ignitable”; Group “A” in the NEC (Division System) is the most
easily ignitable – Acetylene, whereas Group “A” in the IEC (Zone System) is the least
easily ignitable. Further, the IEC uses three groups for the representative gases and the
NEC uses four. AD to this confusion, the IEC and NEC do not use the same terminology
and neither have the same number of categories to identify the likelihood of the hazard
being present.
Both Division and Zone Classification Systems start with a definition if what the hazard is
and the probability the hazard will be present. The NEC Division Classification Systems
uses Classes and Groups to identify hazards:
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• Class I represents gases & vapors
• Class II represents explosive dusts
• Class III represents hazardous (flammable) fibers
Groups further define the hazard in Class I and Class II areas. In Class I, there are Group
A (Acetylene and similar gases), Group B (Hydrogen and similar gases), Group C
(Ethylene and similar gases) and Group D (Propane and similar gases). Hazardous dusts
and fibers (Class II & III respectively) are not treated in Article 505.
The IEC Zone Classification System identifies the hazard by 2 Main Groups
• Group I for mining (underground locations and Group II for surface (not
underground) locations. [Further Section 90-2(b)(2) states the NEC does not cover
underground mining installations.)
• Group II is divided into 3 sub-groups in order of hazard to threat of ignition –
Group A – the least likely to ignite (such as Propane); Group B (gases such as
Ethylene) and Group C – the easiest to ignite (such as Acetylene and Hydrogen).
DEFINITION OF DIVISIONS
A Class I, Division 1 location is one where an explosive atmosphere is presumed to be
present in normal operation either all or part of the time.
A Class I, Division 2 location is one where volatile flammable liquids or gases are
handled, processed or used, but which are normally enclosed in containers from which
they can only escape in the case of accidental rupturing or abnormal operation of
equipment.
DEFINITION OF ZONES
Class I locations can further be divided into Zones based upon the frequency of
occurrence and duration of and explosive gas or vapor atmosphere as follows:
Zone 0 areas are locations in which explosive gas or vapor atmospheres are present
continuously or for long periods of time. Zone 0 areas are those where there is a
flammable mixture typically more than 1,000 hours per year.
Zone 1 areas are those in which (a) explosive gas atmospheres are likely to occur in
normal operation, or (b) explosive gas atmospheres may exist frequently because of
repair or maintenance operations or because of leakage, or (c) the location is adjacent to
a Class I, Zone 0 location from which explosive gas atmospheres could be transmitted.
Zone 1 areas are those where there is a flammable mixture more than 10 hours per year
and less than 1,000 hours per year.
Zone 2 areas are those in which (a) explosive gas or vapor atmospheres are not likely to
occur and if they do occur, will exist for a short period of time, or (b) flammable liquids,
gases or vapors are handled, processed or used, but in which liquids, vapors or gases
are normally confined within closed containers or closed systems from which they can
escape only as a result of accidental rupture or breakdown of the containers or systems
or the abnormal operation of the equipment by which the liquids, gases or vapors are
handled, processed or used, or (c) explosive gas atmospheres are normally prevented by
adequate ventilation but which may occur as a result of failure of the ventilation system,
or (d) the location is adjacent to a Class I, Zone 1 location from which explosive gas
atmospheres could be transmitted unless this is prevented by adequate positive-pressure
ventilation from a source of clean air and effective safeguards against failure of the
ventilation system. Zone 2 areas are those where explosive gas atmospheres will exist
for less than 10 hours per year.
EXAMPLES OF AREA ZONE CLASSIFICATION
ZONE 0 (Continuous Grade – 1,000 hours per year)
Areas within process equipment developing flammable gas or vapors
Areas within enclosed pressure vessels or storage areas
Areas around vent pipes which discharge continually or for long periods
Areas over or near the surface of flammable materials
ZONE 1 (Primary Grade – 100 hours per year)
Areas above roofs outside storage tanks
Areas above floating storage tanks
Areas within specified radii around the outlet pipes and safety valves
Rooms without ventilation openings from a Zone 1 area
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Areas around flexible pipelines and hoses
Areas around sample taking points
Areas around seals of pumps, compressors and similar primary sources
ZONE 2 (Secondary Grade – 10 hours per year)
Areas around flanges and connecting valves
Areas outside Zone 1 around outlet pipes and safety valves
Areas around vent openings from Zone 2.
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http://www.ldpi-inc.com/resources/codes/Class1
2018-4-13