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Definitions of Classes and Zones |
| Hazardous Areas are locations where the potential
for fire or explosion exists because of gases, dust, or easily ignitable fibers
or flyings in the atmosphere. In
North America,
hazardous areas are separated by classes, divisions, and groups to define the
level of safety required for equipment installed in these locations. Classes
define the general form of the flammable materials in the atmosphere.
Divisions define the probability of the presence of flammable materials.
Groups classify the exact flammable nature of the material.
In
Europe and countries
outside of North America, classification of hazardous areas is
accomplished differently. Zones are used to define the probability of the
presence of flammable materials. Protection Types denote the level of
safety for the device. Groups classify the exact flammable nature of the
material. These groups are separated differently than North American Groups.
Temperature Identifications convey the maximum surface temperature of the
apparatus based on 104° F (40° C) ambient. These temperature codes are selected
carefully not to exceed the ignition temperature of the specific gas or vapor to
be encountered in the application.
Some classifications are not shown here. For
further detailed information, see specific standards published by approval
organizations.
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Common Terms:
Class I, Division 1
A
Class I, Division 1 location is a location where ignitable concentrations
of flammable gases, vapors or liquids:
-
can exist under normal operating
conditions;
-
may exist frequently because of repair or
maintenance operations or because of leakage; or
-
may exist because of equipment breakdown
that simultaneously causes the equipment to become a source of
ignition
Equipment intended for use in a Class I , Division 1 area is usually of
the explosionproof, intrinsically safe, or purged/pressurized type.
Explosionproof apparatus: "Apparatus enclosed in a case that is
capable of withstanding an explosion of a specified gas or vapor that
may occur within it and of preventing the ignition of a specified gas or
vapor surrounding the enclosure by sparks, flashes, or explosion of the
gas or vapor within and that operates at such an external temperature
that a surrounding flammable atmosphere will not be ignited
thereby." (See NFPA 70)
Intrinsically safe apparatus: "Apparatus in which all the
circuits are intrinsically safe." (See UL 913)
Intrinsically safe circuit: "A circuit in which any spark or
thermal effect is incapable of causing ignition of a mixture of
flammable or combustible material in air under prescribed test
conditions." (See UL 913)
Purging: "The process of supplying an enclosure with a protective
gas at a sufficient flow and positive pressure to reduce the
concentration of any flammable gas or vapor initially present to an
acceptable level."
(See NFPA 496)
Pressurization:"The process of supplying an enclosure with a
protective gas with or without continuous flow at sufficient pressure to
prevent the entrance of a flammable gas or vapor, a combustible dust, or
an ignitable fiber." (See NFPA 496)
Class I, Division 2
A Class I, Division 2 location is a location:
-
where volatile flammable liquids or
flammable gases or vapors exist, but are normally confined within
closed containers;
-
where ignitable concentrations of gases,
vapors or liquids are normally prevented by positive mechanical
ventilation; or
-
adjacent to a Class I, Division 1
location, where ignitable concentrations might be occasionally
communicated.
Equipment intended for use in a Class I, Division 2 area is usually of the
nonincendive, non-sparking, purged/pressurized, hermetically sealed, or
sealed device type.
Nonincendive circuit: "A circuit in which any arc or thermal
effect produced under intended operating conditions of the equipment is
not capable, under the test conditions specified, of igniting the
specified flammable gas- or vapor- air mixture."(See UL 1604)
Nonincendive component: "A component having contacts for making
or breaking an incendive circuit and the contacting mechanism shall be
constructed so that the component is incapable of igniting the specified
flammable gas- or vapor-air mixture. The housing of a nonincendive
component is not intended to:
Nonincendive field circuit: "A circuit that enters or leaves the
equipment enclosure and that under intended operating conditions is not
capable, under the test conditions specified, of igniting the specified
flammable gas- or vapor-air mixture or combustible dust." (See UL 1604)
Non-sparking apparatus: "Apparatus that has no normally arcing
parts or thermal effects capable of ignition. Examples of normally
arcing parts are relays, circuit breakers, servo-potentiometers
adjustable resistors, switches, non-latching type connectors and motor
brushes. (See UL 1604)
Purging: See definition under Class I, Division 1
Pressurization: See definition under Class I, Division 1
Hermetically sealed component: "A component that is sealed
against entrance of an external atmosphere and in which the seal is made
by fusion, such as soldering, brazing, welding, or the fusion of glass
to metal." (See UL 1604)
Sealed device: "A device that is constructed so that it cannot be
opened, has no external operating mechanisms, and is sealed to restrict
entry of an external atmosphere without relying on gaskets. The device
may contain arcing parts or internal hot surfaces." (See UL 1604)
Class I, Zone 0
A
Class I, Zone 0 location is a location where ignitable concentrations of
flammable gases, vapors or liquids:
Equipment intended for use in a Class I, Zone 0 area is usually of the
intrinsically safe, "ia," type.
Intrinsically safe: See definitions under Class I, Division 1
Class I, Zone 1
A
Class I, Zone 1 location is a location where ignitable concentrations of
flammable gases, vapors or liquids:
-
are likely to exist under normal operating
conditions;
-
may exist frequently because of repair or
maintenance operations or leakage; or
-
may exist because of equipment breakdown
that simultaneously causes the equipment to become a source of
ignition; or
-
are adjacent to a Class I, Zone 0 location
from which ignitable concentrations could be communicated.
Equipment intended for use in a Class I, Zone 1 area is usually of the
flameproof, purged/pressurized, oil immersed, increased safety,
encapsulated or powder filled type.
Flameproof: "The enclosure of the equipment will withstand an
internal explosion, and prevent passage of flame to the surrounding
atmosphere. Care must be taken to maintain the length and clearance
(gap) of flameproof joints in service." (See UL 2279)
Purged/pressurized: See definitions under Class I, Division I.
Oil immersion: "Arcing contacts are immersed in a protective
liquid." (See UL 2279)
Increased safety: "The equipment contains no normally arcing
parts, and additional measures (such as larger spacings among wiring
connections) are taken to prevent the possibility of high temperatures
or sparks. A minimum IP rating of IP 54 is required." (See UL 2279)
Encapsulation: "Arcing contacts are completely surrounded by an
encapsulating material." (See UL 2279)
Powder filling: "Arcing contacts are surrounded by a filling
material (glass or quartz powder)." (See UL 2279)
Class I, Zone 2
A
Class I, Zone 2 location is a location:
-
where ignitable concentrations of
flammable gases, vapors or liquids are not likely to occur in normal
operation or, if they do occur, will exist only for a short period;
-
where volatile flammable liquids, or
flammable gases or vapors exist, but are normally confined within
closed containers
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where ignitable concentrations of gases,
vapors, or liquids are normally prevented by positive mechanical
ventilation;
-
adjacent to a Class I, Zone 1 location
from which ignitable concentrations could be communicated.
Equipment that is intended for use in a Class I, Zone 2 area is usually of
the nonincendive, non-sparking, restricted breathing, hermetically sealed
or sealed device type.
Nonincendive: See definitions under Class I, Division 2
Non-sparking: See definition under Class I, Division 2
Restricted breathing: "The enclosure relies on tight seals and
gaskets to prevent diffusion of the explosive atmosphere into the
equipment enclosure. Provision for checking that the restricted
breathing properties of the enclosure are maintained is provided." (See
UL 2279)
Hermetically sealed: See definition under Class I, Division 2
Sealed device: "A device that is constructed so that it cannot be
opened and is sealed to restrict entry of an external atmosphere. The
device may contain arcing parts or internal hot surfaces." (See UL 2279)
Class II, Division 1
A Class II, Division 1 location is a location where:
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ignitable concentrations of combustible
dust can exist in the air under normal operating conditions;
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ignitable concentrations of combustible
dust may exist because of equipment breakdown that simultaneously
causes the equipment to become a source of ignition; or
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electrically conductive combustible dusts
may be present in hazardous quantities.
Equipment intended for use in a Class II, Division 1 area is usually of
the dust-ignition-proof, intrinsically safe, or pressurized type.
Dust-ignition-proof : "Enclosed in a manner that will exclude
dusts and, where installed and protected in accordance with the NEC,
will not permit arcs, sparks or heat otherwise generated or liberated
inside of the enclosure to cause ignition of exterior accumulations or
atmospheric suspensions of a specified dust on or in the vicinity of the
enclosure." (See NFPA 70)
Intrinsically safe: See definitions under Class I, Division 1.
Pressurization: See definition under Class I, Division 1.
Class II, Division 2
A
Class II, Division 2 location is a location where:
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combustible dust is not normally in the
air in ignitable concentrations;
-
dust accumulations are normally
insufficient to interfere with normal operation of electrical
equipment;
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dust may be in suspension in the air as
the result of infrequent malfunctioning of equipment; or
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dust accumulation may be sufficient to
interfere with safe dissipation of heat or may be ignitable by
abnormal operation.
Equipment intended for use in a Class II, Division 2 area is usually of
the dusttight, nonincendive, non-sparking, or pressurized types.
Dusttight: "Constructed so that dust will not enter the enclosing
case under specified test conditions. An example of such conditions
would be a UL Type 12 enclosure." (See NFPA 70)
Nonincendive: See definitions under Class I, Division 2
Non-sparking: See definition under Class I, Division 2
Pressurization: See definition under Class I, Division 1.
Class III, Division 1
A
Class III, Division 1 location is a location where easily ignitable fibers
or materials producing combustible flyings are handled, manufactured or
used.
Equipment intended for use in a Class III, Division 1 area is usually of
the dusttight or intrinsically safe type (both previously defined).
Class III, Division 2
A
Class III, Division 2 location is a location where easily ignitable fibers
are stored or handled.
Equipment intended for use in a Class III, Division 1 area is usually of
the dusttight or intrinsically safe type (both previously defined).
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Interpreting Rating Nomenclature:
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Classifications Inside North
America
Classes
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Class I |
Flammable gases or vapors are present in the
air in quantities sufficient to produce explosive or ignitable mixtures. |
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Class II |
Combustible or conductive dusts are present. |
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Class III |
Ignitable fibers or flyings are present, but
not likely to be in suspension in sufficient quantities to produce ignitable
mixtures. (Group classifications are not applied to this class.) |
Divisions
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Division 1 |
The substance referred to by
class is present during normal conditions. |
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Division 2 |
The substance referred to by
class is present only in abnormal conditions, such as a container failure or
system breakdown. |
Groups
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Group A |
Acetylene |
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Group B |
Hydrogen (or gases of
equivalent hazard) |
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Group C |
Ethylene (or gases of
equivalent hazard) |
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Group D |
Gasoline (or gases of
equivalent hazard) |
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Group E |
Metal Dust |
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Group F |
Coal Dust |
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Group G |
Grain Dust |
Classifications Outside North
America
Zones
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Zone 0 |
Area in which an explosive gas-air mixture
is continuously present or present for long periods. |
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Zone 1 |
Area in which an explosive gas-air mixture
is likely to occur in normal operation. |
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Zone 2 |
Area in which an explosive gas-air mixture
is not likely to occur, and if it occurs it will only exist for a short
time. |
Protection
Types
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Zone |
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d |
Flameproof (Explosion proof) Enclosure |
1,2 |
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e |
Increased Safety |
1,2 |
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ia |
Intrinsic Safety |
0,1,2 |
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ib |
Intrinsic Safety |
1,2 |
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o |
Oil Immersion |
2 |
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p |
Pressurized Apparatus (Purged Apparatus) |
1,2 |
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q |
Powder Filling (Sand Filling) |
2 |
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m |
Encapsulation |
1,2 |
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n |
Normally Nonsparking and/or Nonincendive
Circuits) |
2 |
Temperature Codes
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° F |
° C |
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T1 |
842
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450
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T2 |
572
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300
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T3 |
392
|
200
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T4 |
275
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135
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T5 |
212
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100
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T6 |
185
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85
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Groups
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Group I |
For application in below ground
installations (mines) where methane (firedamp) and coal dust may be present. |
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Group IIA |
For application in above ground installation
where hazards due to propane may exist. This group most closely matches the
North American Group D. |
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Group IIB |
For application in above ground
installations where hazards due to ethylene may exist. This group most
closely matches the North American Group C. |
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Group IIC |
For application in above ground
installations where hazards due to hydrogen or acetylene may exist. This
group most closely matches the North American Groups A and B. |
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