Intrinsic Safety
How are Hazardous Locations Defined?
Answer: According to the National Electrical Code,
Article 500, hazardous locations are defined by Class,
Group and Division. Differentiation by Class and Group
is in accordance with the laws of physics, while Division
classification is based on environmental and physical
plant conditions.
Relative to the application of Intrinsic Safety, it is
important to define the actual Class, Group and Division
into which any proposed Intrinsically Safe electrical
circuits are to be installed. As shown by the ignition
curves, all flammable mixtures do not require the same
energy levels to ignite. Because Intrinsic Safety
requires maintaining an energy level lower than that
required to ignite a specific hazardous mixture, it is
important to know what the energy allowances are for
operational and safety considerations.
The Definition of Intrinsic Safety Identifies Both
Electrical and Thermal Energy as Potential Causes
of Ignition. How Does Thermal Energy Relate to the
Ignition of a Specific Flammable or Combustible
Mixture?
Answer: There are temperatures at which a flammable
or combustible mixture will ignite. The minimum
temperature at which ignition takes place is called the
“Auto-Ignition Temperature.” Intrinsically Safe systems
will not allow thermal energy to reach levels at which a
specific flammable or combustible mixture will autoignite.
Figure 1 identifies common hazardous mixtures and
their auto-ignition temperatures.
Hazardous
Mixture
Typical Resistance Circuit Ignition Currents Identify
Only Four Hazardous Substances: Hydrogen,
Ethylene, Propane and Methane. Aren’t There More
Flammable or Combustible Materials Than That?
Answer: Yes, but those four hazardous mixtures
represent the basis for all flammable or combustible
mixtures subject to ignition from electrical sources. All
are found, as shown in the Hazardous (Classified)
Locations chart following, in Class I, with Hydrogen
identified as Group B; Ethylene identified as Group C;
Propane being Group D and, as a separate curve within
Group D, Methane.
Acetone
Acetylene
Ammonia
Benzene
Benzol
Butane
Butylalchohol
Carbon Disulphide
Carbon Oxide
Cyclohexane
Diesel Fuel
Ethane
Ethylacetate
Ethylalchohol
Ethylchloride
Ethylene
Ethylether
Ethyl Glycol
Fuel Oil
Hexane
Hydrogen aeroxide
Hydrogen disulphide
Methane
Methanol
Methyl chloride
Naphthalene
Phenol
Propane
Tetraline
Toluol
Acetylene: Group A and Hydrogen: Group B share the
same required energy levels relative to ignition. They
require less energy for ignition than does Group C,
which requires less energy for ignition than Group D.
Within Class II Group E, metal or electrically conductive
dusts, Group F, Coal Dust and Group G, electrically
nonconductive dusts, generally grain or agricultural
dusts are identified. As Groups A and B share the same
ignition curve, Group C, Ethylene, and Group E, metal
or electrically conductive dusts, share the same ignition
curve. Groups D, Propane, F, Coal Dust, and G,
electrically nonconductive dusts, share the same
ignition curve.
A complete listing of hazardous mixtures defined by
Group can be found in National Fire Protection
Association document NFPA 497 M.
Autoignition
Temperature
°C
°F
540
305
630
220
555
365
340
95
605
430
220 to 300
515
460
425
510
425
180
235
220 to 300
240
560
270
595
455
625
520
595
470
425
535
1004
581
1166
428
1031
689
644
203
1121
806
428 to 572
959
860
797
950
797
356
455
428 to 572
464
1040
518
1103
851
1157
968
1103
878
797
995
Figure 1: Autoignition temperatures of some hazardous mixtures.
Reproduced with permission of R. Stahl.
Z-128
Z
Intrinsic Safety Cont’d
Hazardous (Classified) Locations in Accordance with Article 500, National Electric Code-1990
Class I
Flammable Gases or Vapors
Division 1
• Exists under normal conditions
• May exist because of:
- repair conditions
- maintenance operations
- leakage
• Released concentration
because of:
- breakdown of equipment
- breakdown of process
- faulty operation of equipment
- faulty operation of process
which causes simultaneous
failure of electrical equipment
Group A:
Division 2
• Liquids and gases in closed
containers or the systems are:
- handled
- processed
- used
• Concentrations are normally
prevented by positive
mechanical ventilation.
• Adjacent to a Class I, Division 1
location
Atmospheres containing Acetylene
Group B:
Atmospheres such as Butadiene, ethylene oxide,
Propylene Oxide, Acrolein, or Hydrogen (or gases
or vapors equivalent in hazard to hydrogen such
as manufactured gas)
Group C:
Atmospheres such as Cyclopropane, Ethyl Ether,
Ethylene, or gases or vapors equivalent in hazard
Group D:
Atmospheres such as Acetone, Alcohol,
Ammonia, Benzene, Benzol, Butane, Gasoline,
Hexane, Lacquer Solvent vapors, Naphtha,
Natural Gas, Propane or gases or vapors
equivalent in hazard
Z-129
Class II
Combustible Dusts
Division 1
• Exists under normal conditions
• Combustible mixture
produced by:
- mechanical failure of
equipment or machinery
- abnormal operation of
equipment and provide
source of ignition from:
- simultaneous failure of
electrical equipment
- simultaneous failure of
operation of protection devices
- other causes
• Electrically conductive dusts
may be present
Division 2
• Not normally in the air
• Accumulations normally
sufficient to interfere with
normal operation of electrical
equipment or other apparatus.
• In the air as a result of
infrequent malfunctioning of:
- handling equipment
- process equipment
• Accumulations are sufficient to
interfere with the safe
dissipation of heat from
electrical equipment
• Accumulations may be ignitible
by abnormal or failure of
electrical equipment
Group E:
Atmospheres containing combustible metal dusts
(regardless of resistivity), dusts of similarly
hazardous characteristics ( < 100 kΩ/cm) or
electrically conductive dusts
Group F:
Atmospheres containing combustible Carbon
Black, Charcoal or Coke Dusts which have > 8%
total volatile material or if these dusts are sensitized
so that they present an explosion hazard and
having a resistivity > 100 kΩ/cm but ≤ 100 MΩ/cm
Group G:
Atmospheres containing combustible dusts having
a resistivity > 100 kΩ/cm or electrically
nonconductive dusts
Class III
Ignitable Fibers or Flyings
Division 1
• Fibers or materials producing
combustible flyings are
manufactured, stored or
handled
Not Grouped
• Manufacturers such as textile
mills, cotton-related mills or
clothing plants
• Fibers and flyings including
Rayon, Cotton, Sisal, Hemp,
Jute and Spanish Moss
Z-130
Division 2
• Fibers are handled except
during the process of
manufacture or are stored
except during the process of
manufacture
Z
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