Technical Paper
Hazardous Locations
The Explosion Triangle
This triangle illustrates the three basic
ingredients necessary for combustion to
occur. In hazardous locations, the fuel source
can be in the form of flammable vapors,
liquids, gases, combustible dust or fibers with
the oxidizer being oxygen in the surrounding
air. With these two ingredients present in
their most easily ignitable concentration and
the introduction of sufficient electrical or
thermal energy, ignition will result. Either confining the explosion or
preventing the combustion from occurring can provide a means of protection
in this hazardous environment.
What are hazard locations?
Hazardous locations, defined in the National Electrical Code (NEC), are
areas with the possible presence of an explosive atmosphere containing
fuels such as flammable gases, vapors, liquids, combustible dust or
ignitable fibers & flyers. The presence of an explosive atmosphere can exist
at all times, only during abnormal operations or only when system
components or processes fail. There are two systems for hazard area
classification. The first is the National Electrical Code (NEC) Division
Requirements. This system includes six Articles regarding hazardous
locations. Article 500 defines the general class and division requirements,
while the following Articles 501 to 505 define more specific requirements.
The second system for hazard area classification is the IEC Zone
Classification Code. In both cases the classification specifies the properties
of the flammable material that may be present and the probability that it will
be present in explosive concentrations.
Rittal Corporation
One Rittal Place
Springfield, Ohio 45504
Product information:
1-800-477-4000
Technical Hotline:
1-800-637-4425
Email: Rittal@rittal-corp.com
Website: www.rittal-corp.com
Hazardous Location – Classes
The NEC standard identifies three classes of hazardous locations. These
classes define the generic type of hazard materials that may be present.
• Class I:
Flammable gases and vapors
• Class II:
Combustible dusts
• Class III:
Ignitable fibers (e.g. flyings)
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Technical Paper
Class I – Gases and Vapors
Class I locations are those areas which are deemed hazardous due to the
presence of gases or vapors that are present in the air in sufficient quantity
to produce explosive or ignitable mixtures. Class I is further subdivided into:
• Divisions (pre-1998 version of the NEC)
• Zones (IEC Classification – 1998 version)
The Division system is to be used for existing buildings. For new
constructions, the IEC classification must be used.
Class II – Combustible Dusts
Class II locations are those areas which are deemed hazardous due to the
presence of combustible dusts that are present in the air in sufficient
quantity to produce explosive or ignitable mixtures. Class II is also
subdivided into divisions, but the IEC Zone standard has not been adapted
to this class yet.
Class III – Ignitable Fibers or Flyings
In accordance to the NEC Article 503, Class III areas are locations that are
hazardous due to the presence of ignitable fibers or flyings, but not in
quantities sufficient to produce ignitable mixtures. As with Class I and II,
Class III is subdivided into Divisions, but the IEC Zone standard has not
been adapted to this class.
The Divisions and Zones
Divisions and Zones indicate the probability that a combustible or flammable
concentration of material may be present. The pre-1998 version of the NEC
standard works with Divisions while the actual 1998-version adopted the
IEC Zone standards and is now working with Zones. The Division system
will still be used for existing buildings. For new constructions, the IEC
classification must be used.
Rittal Corporation
One Rittal Place
Springfield, Ohio 45504
Product information:
1-800-477-4000
Technical Hotline:
1-800-637-4425
Email: Rittal@rittal-corp.com
Website: www.rittal-corp.com
In the pre-1998 NEC standard two Divisions were defined:
• Division 1 - Hazard commonly exists under normal operating
conditions.
• Division 2 - Hazard exists only during faulty operating conditions.
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Technical Paper
The actual NEC and IEC standard defines three different Zones:
• Zone 0 – Locations in which explosive gas atmospheres are
continuously present or are present for long periods.
• Zone 1 – Locations in which:
1. explosive gas atmospheres are likely to occur in normal
operating conditions.
2. explosive gas atmospheres may exist frequently because of
repair or maintenance operations or because of leakage.
3. the location is adjacent to a Zone 0 location.
• Zone 2 – Locations in which:
1. explosive gas atmosphere are not likely to occur in normal
operation and if they do occur they will exist for a short time
only.
2. flammable volatile liquids, flammable gases or vapors are
handled, processed or used, but in which liquids, gases or
vapors are normally confined within closed containers or
closed systems from which they can escape only as result of
abnormal conditions.
3. explosive gas atmospheres are normally prevented by
adequate ventilation but they may occur as a result of failure
or abnormal operation of the ventilation system.
4. the location is adjusted to Zone 1 and no adequate positive
pressure ventilation is provided. If a ventilation system is
provided and there no safeguards against ventilation failure
exist, this is also a Zone 2 area.
Area Classification – Division vs. Zones
Continuous Hazard
Intermittent Hazard
Zone 0
Zone 1
Division 1
Hazard under abnormal conditions
Zone 2
Division 2
Groups in the NEC standard
Rittal Corporation
One Rittal Place
Springfield, Ohio 45504
Further the NEC standard defines Groups. These Groups are split into two
major categories, one for gases, and the other for dusts. Inside these two
major categories, there are Groups that represent a specific kind of gas or
dust.
Product information:
1-800-477-4000
Technical Hotline:
1-800-637-4425
Email: Rittal@rittal-corp.com
Website: www.rittal-corp.com
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Technical Paper
The definition of these groups is as follow:
• Group A, B, C, D – Gases
o Gr. A: acetylene
o Gr. B: hydrogen
o Gr. C: ethyl ether
o Gr. D: acetone, gasoline
• Group E, F, G – Dusts
o Gr. E: Metallic dusts (aluminum)
o Gr. F: Carbonaceous dusts (coal)
o Gr. G: Agricultural Chemical and Plastic dusts (flour, grain)
Types of Hazard Protection
The protection methods for hazardous locations are differentiated by Class
and further divided into the Divisions and Zones. The protection methods
are shown in the following tables.
Class I Protection Methods
Area
Division Protection Method
Division 1
Explosion proof
Intrinsically safe (2 fault)
Purged or pressurized (Type X or Y)
Division 2
Rittal Corporation
One Rittal Place
Springfield, Ohio 45504
Product information:
1-800-477-4000
Technical Hotline:
1-800-637-4425
Email: Rittal@rittal-corp.com
Website: www.rittal-corp.com
Hermetically sealed
Nonincendive
Non-sparking
Oil immersion
Sealed device
Purged/pressurized (Type Z)
Any Class I, Division 1 method
Any Class I, Zone 1 or 2 method
Class II Protection Methods
Area
Division Protection Method
Division 1
Dust-Ignition-Proof
Intrinsically safe
Pressurized
Division 2
Dusttight
Nonincendive
Non-Sparking
Pressurized
Any Class II, Division 1 method
Area
Zone 0
Zone 1
Zone 2
Zone Protection Method
Intrinsically safe, ‘ia’ (2 fault)
Class I, Division 1 Intrinsically safe
Encapsulated, ‘m’
Flameproof, ‘d’
Increased safe, ‘e’
Intrinsically safe, ‘ib’ (1 fault)
Oil immersion, ‘o’
Powder filling, ‘q’
Purged/pressurized, ‘p’
Any Class I, Zone 0 method
Any Class I, Division 1 method
Energy limited, ‘nC’
Hermetically sealed, ‘nC’
Nonincendive, ‘nC’
Non-sparking, ‘nA’
Restricted breathing, ‘nR’
Sealed device, ‘nC’
Any Class I, Zone 0 or 1 method
Any Class I, Division 1 or 2 method
Class III Protection Methods
Area
Zone Protection Method
Division 1
Dusttight
Intrinsically safe
Division 2
Dusttight
Intrinsically safe
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