Area classification

1. Electrical AreaElectrical Area
Classification TrainingClassification Training
Presented by:Presented by:
Vincent W. Wedelich P.E.Vincent W. Wedelich P.E.
January 8, 2008January 8, 2008

2. What will be covered in this course.What will be covered in this course.
 Introduction to area classificationIntroduction to area classification
terminology and standards.terminology and standards.
 Identifying electrical equipment that canIdentifying electrical equipment that can
be used in different hazardous areas.be used in different hazardous areas.
 Introduction to PIP ELEHA01 SpecificationIntroduction to PIP ELEHA01 Specification
 Introduction to PIP ELEHA01 Form No. 1Introduction to PIP ELEHA01 Form No. 1
Flammable Combustible.Flammable Combustible.
 Introduction to PIP ELEHA01 Form No. 2Introduction to PIP ELEHA01 Form No. 2
Sources of Release.Sources of Release.

3. Introduction to area classificationIntroduction to area classification
terminology and standards.terminology and standards.
 Standards WorldwideStandards Worldwide
 National Electric Code N.E.C.National Electric Code N.E.C.
 Class LocationsClass Locations
 LocationsLocations
 GroupsGroups
 Explosion elementsExplosion elements

5. Notable Standards For AreaNotable Standards For Area
ClassificationClassification
 lEC60079 Part 10 for the UK and Europe.lEC60079 Part 10 for the UK and Europe.
 BS5345 Part 2 for UK only.BS5345 Part 2 for UK only.
 API 500 A for USA.API 500 A for USA.

7. N.E.C. Article 500 Code SectionsN.E.C. Article 500 Code Sections
 Article 500 Hazardous LocationsArticle 500 Hazardous Locations
 Article 501 Class I LocationsArticle 501 Class I Locations
 Article 502 Class II LocationsArticle 502 Class II Locations
 Article 503 Class III LocationsArticle 503 Class III Locations
 Article 504 Intrinsically Safe SystemsArticle 504 Intrinsically Safe Systems
 Article 505 Class 1, Zone 0, 1, and 2Article 505 Class 1, Zone 0, 1, and 2
LocationsLocations
 Article 510 Hazardous Location -SpecificArticle 510 Hazardous Location -Specific
 Article 511 Commercial GaragesArticle 511 Commercial Garages
 Article 513 Aircraft HangarsArticle 513 Aircraft Hangars
 Article 514 Gasoline Service StationsArticle 514 Gasoline Service Stations
 Article 515 Bulk Storage PlantsArticle 515 Bulk Storage Plants
 Article 516 Paint Spray ApplicationArticle 516 Paint Spray Application

8. Class LocationsClass Locations
 N.E.C. Article 500.5 (B)N.E.C. Article 500.5 (B)
 An area whereAn area where
FLAMMABLE GASESFLAMMABLE GASES
or VAPORS are or mayor VAPORS are or may
be present in the air inbe present in the air in
sufficient quantities tosufficient quantities to
produce explosive orproduce explosive or
ignitable mixtures.ignitable mixtures.

9. Class I Locations (Gases)Class I Locations (Gases)
 N.E.C. Article 500.5 (B)N.E.C. Article 500.5 (B)
 An area whereAn area where
FLAMMABLE GASESFLAMMABLE GASES
or VAPORS are or mayor VAPORS are or may
be present in the air inbe present in the air in
sufficient quantities tosufficient quantities to
produce explosive orproduce explosive or
ignitable mixtures.ignitable mixtures.

10. Class I Locations (Gases)Class I Locations (Gases)
7FA GE Turbine Installation7FA GE Turbine Installation

11. Class I Locations (Gases)Class I Locations (Gases)
7FA GE Turbine Installation7FA GE Turbine Installation

12. Class I Locations (Gases)Class I Locations (Gases)
CLASS I INDUSTRIES AND APPLICATIONSCLASS I INDUSTRIES AND APPLICATIONS
 Natural or liquefied gas storage facilitiesNatural or liquefied gas storage facilities
 Chemical plantsChemical plants
 Petroleum refineriesPetroleum refineries
 Bulk handling or storage facilities for gasolineBulk handling or storage facilities for gasoline
 Dip tanksDip tanks
 Storage tanks for flammable liquids or gasStorage tanks for flammable liquids or gas
 Spraying areas for paints or plasticsSpraying areas for paints or plastics
 Aircraft fuel servicing areas or hangersAircraft fuel servicing areas or hangers
 Well drilling (oil and gas), offshore or onWell drilling (oil and gas), offshore or on
 Pipeline pumping areasPipeline pumping areas
 Printing machine areasPrinting machine areas

13. Class II Locations (Dust)Class II Locations (Dust)
 N.E.C. Article 500.5 (C)N.E.C. Article 500.5 (C)
 An area where presenceAn area where presence
of COMBUSTIBLEof COMBUSTIBLE
DUST presents a fire orDUST presents a fire or
explosion hazard.explosion hazard.

14. Class II Locations (Dust)Class II Locations (Dust)
CLASS II INDUSTRIES AND APPLICATIONSCLASS II INDUSTRIES AND APPLICATIONS
 Grain storage, handling or processing plantsGrain storage, handling or processing plants
 Coal storage, handling or processing facilitiesCoal storage, handling or processing facilities
 Metal grinding or metal powder producingMetal grinding or metal powder producing
facilitiesfacilities
 Gunpowder or explosive (fireworks) plantsGunpowder or explosive (fireworks) plants
 Sugar, cocoa, spice or starch production orSugar, cocoa, spice or starch production or
handling facilitieshandling facilities

15. Class III Locations (Fibers)Class III Locations (Fibers)
 NEC Article 500.5 (D)NEC Article 500.5 (D)
 An area made hazardousAn area made hazardous
because of the presence ofbecause of the presence of
easily ignitable FIBERS oreasily ignitable FIBERS or
FLYINGS, but in which suchFLYINGS, but in which such
fibers or flying's are not likelyfibers or flying's are not likely
to be in suspension in the airto be in suspension in the air
in quantities sufficient toin quantities sufficient to
produce ignitable mixtures.produce ignitable mixtures.

16. Class III Locations (Fibers)Class III Locations (Fibers)
 CLASS III INDUSTRIES ANDCLASS III INDUSTRIES AND
APPLICATIONSAPPLICATIONS
 Cotton, textile or flax producing orCotton, textile or flax producing or
handling facilitieshandling facilities
 Wood cutting, pulverizing or shapingWood cutting, pulverizing or shaping
plantsplants
 Clothing manufacturing facilitiesClothing manufacturing facilities

17. LocationsLocations
 Division 1Division 1
 Division 2Division 2

18. Division 1 LocationDivision 1 Location
 NEC Articles 500.5(B)(1),NEC Articles 500.5(B)(1),
500.5(C)(1) and 500.5(D)(1)500.5(C)(1) and 500.5(D)(1)
 An area where the HAZARDAn area where the HAZARD
EXIST UNDEREXIST UNDER
NORMALOPERATINGNORMALOPERATING
CONDITIONS. This alsoCONDITIONS. This also
includes locations where theincludes locations where the
hazard is caused byhazard is caused by
frequent maintenance orfrequent maintenance or
repair work or frequentrepair work or frequent
equipment failure.equipment failure.

19. Division 1 LocationDivision 1 Location
 Consider that there are 8,760 hours in aConsider that there are 8,760 hours in a
year. It is proposed that a Division I locationyear. It is proposed that a Division I location
would be one that is within the flammablewould be one that is within the flammable
range more than 0.1% of the time, that isrange more than 0.1% of the time, that is
more than 8.76 hrs/yr.more than 8.76 hrs/yr.
 From a practical viewpoint on this basis, weFrom a practical viewpoint on this basis, we
would suggest that any area in thewould suggest that any area in the
flammable range 10 hrs/yr. or more shouldflammable range 10 hrs/yr. or more should
be classified as Division 1.be classified as Division 1.

20. Division 2 LocationDivision 2 Location
 NEC Articles 500.5(B)(2) ,NEC Articles 500.5(B)(2) ,
500.5(C)(2), and 500.5(D)(2)500.5(C)(2), and 500.5(D)(2)
 An area where ignitable gases,An area where ignitable gases,
vapors, dust, or fibers arevapors, dust, or fibers are
handled, processed, or used,handled, processed, or used,
but which EXIST ONLY UNDERbut which EXIST ONLY UNDER
ABNORMAL CONDITIONS,ABNORMAL CONDITIONS,
such as containers or closedsuch as containers or closed
systems from which they cansystems from which they can
only escape through accidentalonly escape through accidental
rupture or breakdown.rupture or breakdown.
 Note: No electrically conductiveNote: No electrically conductive
dust are included in Class II,dust are included in Class II,
Division 2 atmospheres.Division 2 atmospheres.

21. Division 2 LocationDivision 2 Location
 A Division 2 location would be one that isA Division 2 location would be one that is
within the range more than 0.01% and upwithin the range more than 0.01% and up
to 0.1 % of the time (0.876 hours to 8.76to 0.1 % of the time (0.876 hours to 8.76
hours).hours).
 From a practical viewpoint on this basis,From a practical viewpoint on this basis,
we would suggest that any area in thewe would suggest that any area in the
flammable range classified as Division 2,flammable range classified as Division 2,
would be in the range between 1-10would be in the range between 1-10
hrs/yr.hrs/yr.

22. Comparison of Div 1 and Div 2 forComparison of Div 1 and Div 2 for
GasesGases

23. Groups A, B, C, and D (Gases)Groups A, B, C, and D (Gases)
NEC Article 500.6(A)NEC Article 500.6(A)
 Groups indicates the DEGREE OF THEGroups indicates the DEGREE OF THE
HAZARD.HAZARD.
 GROUPS A, B, C and D are classified byGROUPS A, B, C and D are classified by
chemical families as shown in NFPA 497M-chemical families as shown in NFPA 497M-
1986 and 325M-1984.1986 and 325M-1984.
 The important factor in classifying a gas orThe important factor in classifying a gas or
vapor by Group is how much PRESSURE isvapor by Group is how much PRESSURE is
created during an explosion. Group Acreated during an explosion. Group A
(Acetylene) creates the most pressure, with(Acetylene) creates the most pressure, with
Group B (Hydrogen) next.Group B (Hydrogen) next.

24. Groups A, B, C, and D (Gases)Groups A, B, C, and D (Gases)

25. Groups A, B, C, and D (Gases)Groups A, B, C, and D (Gases)
Relative speed and maximum pressure of five test
gases: acetylene, hydrogen, ethylene, propane, and methane.

26. Groups E, F and G (Dust)Groups E, F and G (Dust)
NEC Article 500.6(B)NEC Article 500.6(B)
 Groups indicates the DEGREE OF THEGroups indicates the DEGREE OF THE
HAZARD, based on Electrical ResistivityHAZARD, based on Electrical Resistivity
from Table on right (Source: ANSI/ISA-from Table on right (Source: ANSI/ISA-
S12.10-1988)S12.10-1988)
 GROUP E -Atmospheres containingGROUP E -Atmospheres containing
combustible METAL DUST.combustible METAL DUST.
 GROUP F -Atmospheres containingGROUP F -Atmospheres containing
CARBON BLACK, CHARCOAL, COAL,CARBON BLACK, CHARCOAL, COAL,
or COKE DUST.or COKE DUST.
 GROUP G -Atmospheres containingGROUP G -Atmospheres containing
AGRICULTURAL and other dustsAGRICULTURAL and other dusts

27. Summary of Classes and GroupsSummary of Classes and Groups
CLASS I:FLAMMABLE VAPORS & GASSESCLASS I:FLAMMABLE VAPORS & GASSES (Volatile gas or(Volatile gas or
vapor present in sufficient quantity to produce ignition or explosion).vapor present in sufficient quantity to produce ignition or explosion).
GROUP AGROUP A:ACETYLENE:ACETYLENE
GROUP B:GROUP B:HYDROGENHYDROGEN
GROUP C:GROUP C:ETHYLENEETHYLENE
GROUP D:GROUP D:GASOLINEGASOLINE
CLASS II:COMBUSTIBLE DUSTSCLASS II:COMBUSTIBLE DUSTS (Combustible dusts present in(Combustible dusts present in
sufficient quantity to present a fire or explosion hazard).sufficient quantity to present a fire or explosion hazard).
GROUP E:GROUP E:METAL DUSTSMETAL DUSTS
GROUP F:GROUP F:CARBON DUSTS -COALCARBON DUSTS -COAL
GROUP G:GROUP G:GRAIN DUSTSGRAIN DUSTS
CLASS III:FIBERS & FLYINGSCLASS III:FIBERS & FLYINGS (Easily ignitable fibers or flyings(Easily ignitable fibers or flyings
present but not likely to be suspended in the air).present but not likely to be suspended in the air).

30. Identifying electrical equipment that can be used inIdentifying electrical equipment that can be used in
different hazardous areas.different hazardous areas.
 UL Explosion proof Apparatus Definition NEMAUL Explosion proof Apparatus Definition NEMA
7 Enclosures.7 Enclosures.
 Purging/Pressurization Systems.Purging/Pressurization Systems.
 Intrinsically Safe Equipment.Intrinsically Safe Equipment.

32. (NEMA 7 Enclosures.(NEMA 7 Enclosures.
 In Class I, Division 1 and 2 locations, conventionalIn Class I, Division 1 and 2 locations, conventional
relays, contactors, and switches that have arcingrelays, contactors, and switches that have arcing
contacts must be enclosed in explosion-proof housings,contacts must be enclosed in explosion-proof housings,
except for those few cases where general-purposeexcept for those few cases where general-purpose
enclosures are permitted by theenclosures are permitted by the NEC.NEC.
 By definition, enclosures for these locations mustBy definition, enclosures for these locations must
prevent the ignition of an explosive gas or vapor thatprevent the ignition of an explosive gas or vapor that
may surround it. In other words, an explosion inside themay surround it. In other words, an explosion inside the
enclosure must be prevented from starting a largerenclosure must be prevented from starting a larger
explosion on the outside.explosion on the outside.

33. (NEMA 7 Enclosures.(NEMA 7 Enclosures.
 Adequate strength is one requirement for such anAdequate strength is one requirement for such an
enclosure. For explosion-proof equipment, a safetyenclosure. For explosion-proof equipment, a safety
factor of 4 is used; i.e., the enclosure must withstand afactor of 4 is used; i.e., the enclosure must withstand a
hydrostatic pressure test of four times the maximumhydrostatic pressure test of four times the maximum
pressure from an explosion within the enclosure.pressure from an explosion within the enclosure.
 In addition to being strong, the enclosure must beIn addition to being strong, the enclosure must be flameflame
tight.tight. This term does not imply that the enclosure isThis term does not imply that the enclosure is
hermetically sealed but rather that the joints or flangeshermetically sealed but rather that the joints or flanges
are held within narrow tolerances. These carefullyare held within narrow tolerances. These carefully
machined joints cool the hot gases resulting from anmachined joints cool the hot gases resulting from an
internal explosion so that by the time they reach theinternal explosion so that by the time they reach the
outside hazardous atmosphere, they are not hot enoughoutside hazardous atmosphere, they are not hot enough
to cause ignition.to cause ignition.

35. UL Explosion proof Apparatus DefinitionUL Explosion proof Apparatus Definition
(NEMA 7 Enclosures)(NEMA 7 Enclosures)
 Class I, Division 1Class I, Division 1
 An enclosure that willAn enclosure that will
withstand an internalwithstand an internal
explosion of gases orexplosion of gases or
vapors and preventvapors and prevent
those gases or vaporsthose gases or vapors
from igniting gases orfrom igniting gases or
vapors in thevapors in the
surrounding atmospheresurrounding atmosphere
outside of the enclosure.outside of the enclosure.

37. Explosion proof Equipment -JointsExplosion proof Equipment -Joints

39. Explosion proof Equipment -JointsExplosion proof Equipment -Joints

40. Explosion proof Equipment -JointsExplosion proof Equipment -Joints

41. What is the max. surface temperature of theWhat is the max. surface temperature of the
VMVC-100-S712?VMVC-100-S712?

42. Explosion elementsExplosion elements

43. SEVERAL FIRE HAZARD PROPERTIES OF THE MATERIALSSEVERAL FIRE HAZARD PROPERTIES OF THE MATERIALS
SHOULD BE KNOWN BEFORE CLASSIFICATION IS DONE:SHOULD BE KNOWN BEFORE CLASSIFICATION IS DONE:
 Flash PointFlash Point - The flash point of a liquid is the minimum- The flash point of a liquid is the minimum
temperature at which the liquid gives off sufficient vaportemperature at which the liquid gives off sufficient vapor
to form an ignitable mixture with air near the surface.to form an ignitable mixture with air near the surface.
 Auto ignition TemperatureAuto ignition Temperature -The minimum temperature-The minimum temperature
required to initiate or cause self-sustained combustion ofrequired to initiate or cause self-sustained combustion of
a solid, liquid or gas independently of the heating ora solid, liquid or gas independently of the heating or
heated element.heated element.
 Vapor DensityVapor Density - The vapor density of a substance is the- The vapor density of a substance is the
ratio of the weight of a volume of pure vapor or gas toratio of the weight of a volume of pure vapor or gas to
the equal volume of dry air at the same temperature andthe equal volume of dry air at the same temperature and
pressure.pressure.

44.  Specific GravitySpecific Gravity - The specific gravity of a substance is- The specific gravity of a substance is
the ratio of the weight of that substance to the weight ofthe ratio of the weight of that substance to the weight of
an equal volume of another substance (usually water).an equal volume of another substance (usually water).
 Flammable (Explosive) LimitsFlammable (Explosive) Limits - In popular terms, a- In popular terms, a
mixture below the lower flammable limit (LFL) is too leanmixture below the lower flammable limit (LFL) is too lean
to burn while a mixture above the upper flammable limitto burn while a mixture above the upper flammable limit
(UFL) is too rich to burn.(UFL) is too rich to burn.
 Vapor PressureVapor Pressure - Vapor pressure is the measure of the- Vapor pressure is the measure of the
force exerted by molecules evaporating from the surfaceforce exerted by molecules evaporating from the surface
of the liquid. Vapor pressure is a direct indication of theof the liquid. Vapor pressure is a direct indication of the
volatility of a liquid.volatility of a liquid.
SEVERAL FIRE HAZARD PROPERTIES OF THE MATERIALSSEVERAL FIRE HAZARD PROPERTIES OF THE MATERIALS
SHOULD BE KNOWN BEFORE CLASSIFICATION IS DONE:SHOULD BE KNOWN BEFORE CLASSIFICATION IS DONE:

45. FLAMMABLE LIQUIDS DEFINED BY NFPAFLAMMABLE LIQUIDS DEFINED BY NFPA
 Class IA -Liquids with flash points below 73°F and boiling point below 100°F.Class IA -Liquids with flash points below 73°F and boiling point below 100°F.
 Class IB -Liquids with flash points below 73°F and boiling points at or above 100°F.Class IB -Liquids with flash points below 73°F and boiling points at or above 100°F.
 Class IC - Liquids with flash points at or above 73°F and below 100°F.Class IC - Liquids with flash points at or above 73°F and below 100°F.
 A combustible liquid is a liquid whose flash point is at or above 100°F.A combustible liquid is a liquid whose flash point is at or above 100°F.
 The combustible liquids have been defined by NFPA as:The combustible liquids have been defined by NFPA as:
 Class II -Liquids with flash points at or above 100°F and below 140°F.Class II -Liquids with flash points at or above 100°F and below 140°F.
 Class IIIA -Liquids with flash points at or above 140°F and below 200°F.Class IIIA -Liquids with flash points at or above 140°F and below 200°F.
 Class IIIB -Liquids with flash points at or above 200°F.Class IIIB -Liquids with flash points at or above 200°F.
 A flammable liquid is defined as one having a flash point below 100°F and having aA flammable liquid is defined as one having a flash point below 100°F and having a
vapor pressure not exceeding 40 psi at 100°F.vapor pressure not exceeding 40 psi at 100°F.

46. NFPA has considered a new definition of aNFPA has considered a new definition of a
flammable gasflammable gas
 For purposes of Electrical Classification, a flammable gas shall beFor purposes of Electrical Classification, a flammable gas shall be
considered to have:considered to have:
1) A vapor pressure exceeding 40 psia (25.3 psig) at 100 OF.1) A vapor pressure exceeding 40 psia (25.3 psig) at 100 OF.
2) A lower flammable limit of less than 7% by volume (in air) by ASTM2) A lower flammable limit of less than 7% by volume (in air) by ASTM
E681-79 or Bureau of Mines flammability tube method and have eitherE681-79 or Bureau of Mines flammability tube method and have either
(a) A minimum ignition energy (MIE) of <10 millijoules (in air),(a) A minimum ignition energy (MIE) of <10 millijoules (in air),
oror
(b) Have a minimum ignition current (MIC) of <0.5 ampere.(b) Have a minimum ignition current (MIC) of <0.5 ampere.

47. PROPERTIES OF FLAMMABLE LIQUIDS THATPROPERTIES OF FLAMMABLE LIQUIDS THAT
INCREASE THE RISKINCREASE THE RISK
1. Lower Flammable Limit (LFL)1. Lower Flammable Limit (LFL)
2. Wider Flammable Limit2. Wider Flammable Limit
3. Lower Flash Point3. Lower Flash Point
4. Lower Auto ignition Temperature4. Lower Auto ignition Temperature
5. Lower ignition Energy5. Lower ignition Energy
6. Higher Vapor Pressure6. Higher Vapor Pressure

48. PROPERTIES OF DUST THAT INCREASEPROPERTIES OF DUST THAT INCREASE
THE RISK.THE RISK.
1. Low Cloud ignition Energy1. Low Cloud ignition Energy
2. Lower Minimum Explosive Concentration2. Lower Minimum Explosive Concentration
3. Higher Maximum Pressure Produced3. Higher Maximum Pressure Produced
4. Lower ignition temperature of Cloud4. Lower ignition temperature of Cloud
5. Lower ignition Temperature of Layer5. Lower ignition Temperature of Layer
6. Entrained Volatiles6. Entrained Volatiles

49. Electrical Area Classification StudyElectrical Area Classification Study
quick review.quick review.
1.1. Selection of Project Specific ReferenceSelection of Project Specific Reference
Material (in our case it is the PIP ELEHA01Material (in our case it is the PIP ELEHA01
2.2. Data Requirements ( conduct a site surveyData Requirements ( conduct a site survey
determine if the area is hazardous)determine if the area is hazardous)
3.3. Material and Properties (fill in PIP ELEHA01Material and Properties (fill in PIP ELEHA01
Form 1.Form 1.
4.4. Release Sources (fill in PIP ELEHA01 Form 2)Release Sources (fill in PIP ELEHA01 Form 2)
5.5. Create a Classification Document (a drawingCreate a Classification Document (a drawing
showing the different areas and theirshowing the different areas and their
hazardous classifications).hazardous classifications).

50. Introduction to PIP ELEHA01Introduction to PIP ELEHA01
SpecificationSpecification

57. Introduction to PIP ELEHA01 Form No. 1Introduction to PIP ELEHA01 Form No. 1
Flammable Combustible.Flammable Combustible.

58. Introduction to PIP ELEHA01 Form No. 2 SourcesIntroduction to PIP ELEHA01 Form No. 2 Sources
of Release.of Release.

59. Lower Explosive Limit (LEL)Lower Explosive Limit (LEL)
 When gasses and vapors are present in air theWhen gasses and vapors are present in air the
resulting mixture may be flammable. Not allresulting mixture may be flammable. Not all
gases and vapors produce flammable mixtures.gases and vapors produce flammable mixtures.
Imagine a flammable gas or vapor slowly leakingImagine a flammable gas or vapor slowly leaking
into a confined volume of air that is notinto a confined volume of air that is not
replenished. Initially the concentration of the gasreplenished. Initially the concentration of the gas
or vapor in the mixture will be too low to supportor vapor in the mixture will be too low to support
combustion. As the concentration increases acombustion. As the concentration increases a
critical point will be reached when combustioncritical point will be reached when combustion
will be possible. This is called the Lowerwill be possible. This is called the Lower
Explosive Limit (LEL).Explosive Limit (LEL).

60. Upper Explosive Limit (UEL)Upper Explosive Limit (UEL)
 If the concentration is increased beyond this point by aIf the concentration is increased beyond this point by a
significant amount then a second critical point will besignificant amount then a second critical point will be
reached. At this point the mixture will not containreached. At this point the mixture will not contain
sufficient oxygen to enable combustion to occur. This issufficient oxygen to enable combustion to occur. This is
called the Upper Explosive Limit (UEL), above which nocalled the Upper Explosive Limit (UEL), above which no
combustion or explosion is possible. In terms ofcombustion or explosion is possible. In terms of
hazardous area analysis the UEL is not normally ofhazardous area analysis the UEL is not normally of
practical significance. Various institutions havepractical significance. Various institutions have
determined the LEL for most of the regularlydetermined the LEL for most of the regularly
encountered gases and vapors. Both the LEL and UELencountered gases and vapors. Both the LEL and UEL
are expressed in percentage of volume. Article 500 ofare expressed in percentage of volume. Article 500 of
the NEC gives comprehensive listings of LEL, UEL andthe NEC gives comprehensive listings of LEL, UEL and
other important data.other important data.