Classification of and Protection for Hazardous Locations

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramco’semployees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,or disclosed to third parties, or otherwise used in whole, or in part,without the written permission of the Vice President, EngineeringServices, Saudi Aramco.

Chapter : Electrical For additional information on this subject, contactFile Reference: EEX20801 W.A. Roussel on 874-1320

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Classification Of AndProtection For Hazardous Locations

Engineering Encyclopedia Electrical

Classification of and Protection for Hazardous Locations

Saudi Aramco DeskTop Standards

CONTENTS PAGES

PRINCIPLES OF COMBUSTION.....................................................................1

Fire Triangle ............................................................................................1

Flammable and Combustible Liquids......................................................2

Ignition Temperature ...............................................................................3

Explosive Range......................................................................................6

POTENTIAL IGNITION SOURCES IN A HAZARDOUSLOCATION ........................................................................................................9

Arcs and Sparks.....................................................................................10

Hot Surfaces ..........................................................................................11

HAZARDOUS LOCATION CLASSIFICATION SYSTEM ........................12

Factors Used To Determine The Classification Of ALocation.................................................................................................12

Specific Location Classifications ..........................................................13

Class I Hazardous Locations......................................................13

Groups ...................................................................................................13

Divisions................................................................................................15

Class I, Division 1......................................................................16

Class I, Division 2......................................................................17

Differences Between Class I, Class II, And Class IIIHazardous Locations ............................................................................17

PROTECTION METHODS FOR ELECTRICAL EQUIPMENT ....................20

Fundamental Protection Philosophy......................................................20

Confining the Explosion........................................................................20

Isolation.................................................................................................21

Energy Limitation..................................................................................22

Miscellaneous........................................................................................22

Summary of Protection Techniques and TheirApplication ............................................................................................23

GLOSSARY .....................................................................................................25



Saudi Aramco DeskTop Standards 1

PRINCIPLES OF COMBUSTION

This section provides information on the following topics that pertain to the principles ofcombustion:• Fire Triangle• Flammable and Combustible Liquids• Ignition Temperature• Explosive Range

Fire Triangle

Fires and explosions result from the chemical reaction among the three elements shown inFigure 1:• Fuel• Oxygen• Heat (Ignition Source)

(Ignition Source)

Fuel

Oxygen

Heat

Figure 1. Fire Triangle




Fires and explosions can occur only in locations in which all three elements of the fire triangleare present. In locations that Saudi Aramco classifies as hazardous, the fuel source isnormally a flammable gas, a vapor from a flammable or combustible liquid, a combustibledust, or an easily ignitable fiber. One of these fuels must then be mixed in air or oxygen in aproportion that will produce a flammable or explosive mixture. The source of oxygen isnormally the atmosphere surrounding the hazardous location. Finally, an ignition source ofsufficient energy must be present to ignite the mixture. Ignition sources are discussed in moredetail later in this module.

Flammable and Combustible Liquids

Liquids are classified as flammable or combustible on the basis of the ease with which theyrelease vapors. The ease with which a liquid releases vapor is referred to as volatility: themore volatile a liquid is, the more easily it releases vapor; the less volatile a liquid is, the lesseasily it releases vapor.

The volatility of a liquid can be defined in terms of flash point: technically, the temperature atwhich a liquid has become hot enough to release sufficient vapor to cause ignition when aflame is applied. Liquids can also be classified as flammable or combustible in relation toflash point:

• The flash point of a flammable liquid is below 100°F (37.8°C). • The flash point of a combustible liquid is at or above 100°F (37.8°C).

Figure 2 lists the flash points of the most common flammable and combustible liquids thatoccur in Saudi Aramco plants and facilities.




Flash PointMaterial oF oC

Acetone -4 -20Benzene 12 -11Di-isobutylene 23 -5Fuel Oils 100 to 270 38 to 132Gasoline -45 -43Heptane 25 -4Jet Fuels -- -- A and A-1 110 to 150 43 to 66 B and JP4 -10 to 30 -23 to -1 JP5 95 to 145 35 to 63 JP6 100 38Methanol 52 11Toluene 40 4Xylenes 81 27

Figure 2. Flash Points of Common Flammable andCombustible Liquids

Ignition Temperature

The ignition temperature of a solid, a liquid, or a gaseous substance is the minimumtemperature that is required to initiate self-sustained combustion. Ignition temperature ismore correctly referred to as auto-ignition temperature (AIT) or spontaneous ignitiontemperature (SIT) because a substance will, at its AIT or SIT, “automatically” or“spontaneously” ignite due to the heat energy that is present.

The published ignition temperatures for materials are extremely conservative because theyhave been determined under ideal conditions. The ignition temperature of a substance canchange substantially due to the following variables:

• The percentage of the vapor or gas in the air.

• The shape and the size of the space where the ignition occurs.

• The type and the temperature of the ignition source.

• The catalytic or other effects of the materials that may be present.




• The oxygen concentration.




Figure 3 shows the ignition temperatures for common vapors and gases found in SaudiAramco plants and facilities.


Material oF oC

Acetone 869 465Acetylene 581 305Benzene 928 498Butane 550 288Carbon Monoxide 1128 609Di-isobutylene 736 391Ethane 882 472Fuel Oils - - No. 1 410 210 No. 2 494 257 No. 3 505 236Gasoline 536 to 880 280 to 471Heptane 399 204Hexane 437 225Hydrogen 968 520Hydrogen Sulfide 500 260Jet Fuels - - JP4 464 240 JP5 475 246 JP6 446 230Methane 999 630Methanol 725 385Naphtha 550 288Propane 842 450Toluene 896 480Xylenes 986 530

Figure 3. Ignition Temperature of Common Vapors and Gases




The AITs (SITs) of agricultural and chemical dusts are similar to the AITs (SITs) offlammable vapors and gases. Figure 4 shows the ignition temperatures for common chemicaldusts.


Chemical Dust Material oF oC

Acetoacetanilide 824 440Adipic Acid 1022 550Benzoic Acid 824 440Dicyclopentadiene dioxide 784 420Hydroxyethyl cellulose 770 410Nitrosoamine 518 270Sorbic acid 860 460Stearic acid, aluminum salt 572 300Sulfur 428 220

Figure 4. Ignition Temperatures of Common Chemical Dusts

Explosive Range

All flammable gases, and all flammable and combustible liquids that are heated above theirflash point are potentially ignitable. However, ignition will only occur when the fuel-airconcentration (vapor/air mixture) is in the explosive range. The explosive range for aflammable gas encompasses all fuel-air concentrations between the lower flammable orexplosive limit (LFL or LEL) and the upper flammable or explosive limit (UFL or UEL). Forfuel-air concentrations below the LFL (LEL) and above the UFL (UEL), no sustained ignitionwill occur when the fuel-air mixture is exposed to an ignition source. The LFL (LEL) and theUFL (UEL) for a flammable gas are usually expressed in terms of percentage by volume ofgas or vapor in air under normal ambient conditions.

The explosive ranges of materials are highly variable as demonstrated in Figure 5. Acetylene,an example of a material with a broad explosive range, has an LFL (LEL) of 2.5% by volumein air and an UFL (UEL) of 100%. This explosive range shows that no air or oxygen isneeded for acetylene to ignite explosively since a volume consisting of 100% acetylene willignite. Gasoline, an example of a material with a narrow explosive range, has a LFL (LEL) of1.4% by volume in air and an UFL (UEL) of 1.6%. This explosive range shows that arelatively small amount of gasoline vapors in air is required to form an explosive mixture.




Flammable Limits % by Volume

Material Lower Limit Upper Limit

Acetone 2.5 13Acetylene 2.5 100Benzene 1.3 7.9Butane 1.6 8.4Carbon Monoxide 12.5 74Di-isobutylene 0.8 4.8Ethane 3 12.5Fuel Oil No. 1 0.7 5Gasoline 1.4 7.6Heptane 1.05 6.7Hexane 1.1 7.5Hydrogen 4 75Hydrogen Sulfide 4 44Jet Fuel (JP4) 1.3 8Jet Fuel (JP6) 0.6 3.7Methane 5 15Methanol 6 36Naphtha 1.1 5.9Propane 2.1 9.5Toluene 1.2 7.1Xylenes 1.1 7

Figure 5. Upper and Lower Flammable (Explosive) Limits

for Common Vapors and Gases




Figure 6 shows curves for two flammable gases, propane and hydrogen, with ignition energyin joules plotted against volume concentration in air. Ignition energy is the energy that isneeded to ignite flammable and combustible materials. It is expressed as a finite quantity thatdepends on:

• Material.

• Concentration.

• Time period over which the energy release occurs.

The parabolic shape is characteristic of all gases and vapors. This figure is useful indemonstrating some of the terminology used in the text as well as reminding everyone that themere presence of a flammable material does not make an area hazardous from an explosiveperspective.

Volume Concentration (%)

Propane-air, 1 ATM

Ignition Energy (mj)

100.01

0

0.1

20 4030

1.0

Hydrogen-air, 1 ATM

6050 70 9080 100

Figure 6. Ignition Energy vs. Volume Concentration forPropane and Hydrogen




POTENTIAL IGNITION SOURCES IN A HAZARDOUS LOCATION

A finite quantity of energy is needed to ignite flammable and combustible materials. Theenergy that is necessary depends upon the particular material involved, the concentration ofthis material in the air or with an oxidizing agent, and the time period over which the energyrelease occurs. Much laboratory experimentation has been conducted to determine theminimum ignition energy required to ignite many materials. Figure 7 shows the results ofsome of this work for a few common materials. It should be noted that these energy levelswere recorded under the most ideal conditions. For every day conditions found in the refineryenvironment, the energy levels required for ignition of any of these materials is considerablyhigher - in excess of 1 watt.

Considerably more energy is required to cause ignition of finely divided dusts, and even moreenergy is needed for solid materials. This need for greater energy is due to the fact that thesame homogeneous mixture which occurs with gases and vapors does not occur with dustsand solids, making the ignition process much less efficient.

Flammable MaterialMinimum Ignition Energy,

Millijoules (Milliwatt-Seconds)

Acetylene 0.017Hydrogen 0.017Ethylene 0.08Methane 0.3

Figure 7. Minimum Energy Needed to Ignite CommonFlammable Gases

This section provides information on the following potential ignition sources that areassociated with electrical equipment:

• Arcs and Sparks

• Hot Surfaces

Additional information on these subjects is found on pages 43 through 48 of the supplementaltextbook, "Electrical Installations in Hazardous Locations," by Peter J. Schram and Mark W.Earley.




Arcs and Sparks

Electrical arcs and sparks are potential ignition sources because they produce heat. Thesesources can be described as follows:

• An electrical arc is a sustained luminous discharge of electricity across a gap in a circuitor between electrodes.

• An electrical spark is a luminous electrical discharge of very short duration between twoconductors that are separated by a gas, such as air.

The duration of the luminous discharge of electricity is the only major difference between anelectrical arc and an electrical spark.

Electrical arcs and sparks can be generated from a number of sources including the following:

• Arcs due to intermittent contact in electrical equipment.

• Static electricity (except lightning).

• Lightning.

The latter two are caused by natural phenomena and are not included in this discussion.Electrical equipment such as motor controllers, circuit breakers, switches, and relays produceelectrical arcs as part of their normal operation. These electrical arcs occur each time a motorcontroller, circuit breaker, switch, or relay performs a switching function. Except for certainsignaling, communications, and data processing circuits where the energy released at thecontacts is maintained below the energy required to cause ignition, the arc at the contacts ofthe switching equipment is capable of causing ignition. The contacts of such equipment mustbe protected to prevent the propagation of explosions to the surrounding flammableatmosphere.

In addition to the anticipated arcs at the contacts of switching equipment, arcs can occur as aresult of insulation breakdown between live parts at different polarities, or between live partsand grounded metal. Arcs also can occur when wires break or fuses open. For example, anelectrical conductor can burn open when subjected to a high level of current, such as a groundfault or short circuit. If the protective system does not isolate power, and if the voltage is highenough, an arc can occur across the open space.




Hot Surfaces

A hot surface refers to any surface at an elevated temperature in electrical equipment that isexposed to the surrounding atmosphere. A hot surface is another potential source of ignition.Certain types of electrical equipment can produce high surface temperatures during operation.If the external surface temperature of an explosionproof enclosure is above the ignitiontemperature, the external surface itself can act as an ignition source such that the protectiveenclosure is inconsequential.

High surface temperatures can be created by the normal operation of electrical equipmentsuch as a resistance heater or an incandescent lamp in a light fixture. High surfacetemperatures can also be created by the abnormal operation of electrical equipment such aswhen a rotor stalls or when an arcing fault occurs inside a motor enclosure.

Electrical equipment that is certified for use in explosive atmospheres has been examined andtested to assure that the potential for ignition due to high surface temperature has beenprevented or minimized.




HAZARDOUS LOCATION CLASSIFICATION SYSTEM

Factors Used To Determine The Classification Of A Location

For a description of non-hazardous and hazardous locations read the section titled "WhatHazardous Locations Are Not" on page 11 and “What Hazardous Locations Are” on page 15of the supplemental textbook "Electrical Installations in Hazardous Locations" by Peter J.Schram and Mark W. Earley.

A location needs to be classified relative to the presence of flammable gases, vapors and thelike, only if electrical equipment is to be installed in that location. The classification of alocation will serve as the basis for the minimum requirements that the electrical equipmentmust meet to be installed in the location. The factors to be considered in determining whetheror not a location is classified as hazardous are as follows:

• The likelihood that flammable gases will be present; the quantity of flammable gasesexpected; the rate at which flammable gases and vapors could be released.

• The adequacy of the ventilation in the location under consideration.

• The total volume of the location under consideration.

These three factors are interdependent and must be considered as a group when determining ifa location need be classified as hazardous.

Certain flammable material containment systems are considered not subject to failure;therefore, the surrounding location is not classified as hazardous. Such systems include:

• All-welded closed piping systems without valves, flanges, or similar devices.

• Closed piping systems constructed with continuous metallic tubing without valves,fittings, flanges, or similar devices.




Specific Location Classifications

One of the most common hazardous location classification systems used, and the one used atSaudi Aramco facilities, identifies the type of material present, the specific category ofmaterial present, and the probability of its presence. This is stated using the class, group, anddivision designations, respectively.

Class I Hazardous Locations

Class I hazardous locations are defined as locations in which flammable gases or vapors areor may be present in quantities sufficient to produce explosive or ignitable mixtures. Thissection further describes the system used to define Class I hazardous locations into groups anddivisions and the importance of group and division designations to equipment protectiontechniques.

Groups

Class I hazardous locations are further defined into groups according to the type of flammablegas or vapor present in an atmosphere. The groups were originally established based on teststo determine gap width and lengths required to prevent transmission of the explosion to thesurrounding atmosphere. These tests led to the determination of explosion pressure andpressure rise times which affected the flame path lengths and widths required to preventexplosion transmission. The wide range of data also led to the concept of having differentgroupings of gases based on their explosion characteristics.

This test work relates directly to the explosionproof housing protection technique which relieson enclosures designed to be strong enough to withstand internal explosion pressures and withflame path lengths and gap widths conservative relative to the results obtained in the above-described tests. The groupings allowed different design criteria based on the same testresults and the grouping of gases as described below.

Class I hazardous locations are divided into four groups, with each group containing materialsof similar explosion characteristics. The characteristic gas for each group was chosen basedon the most commonly available commercial materials at that time:

• Group A hazardous locations have atmospheres containing acetylene.

• Group B hazardous locations have atmospheres containing hydrogen and other gasesand vapors with equivalent characteristics.

• Group C hazardous locations have atmospheres containing diethyl ether and other gasesand vapors with equivalent characteristics.

• Group D hazardous locations have atmospheres containing gasoline and other gases andvapors with equivalent characteristics.




Many flammable gases and vapors were placed in one of the four chosen groups. A study ofthe chemical characteristics (such as the structure, weight, and other factors) of theseflammable gases and vapors showed that the gases and vapors in different groups weredistinctly different from each other, but that flammable gases and vapors within a particulargroup were similar to each other. This finding led to the current practice of groupingunknown gases and vapors by their chemical characteristics rather than by a formal testprogram which was found to be unnecessary for many materials.

The original work was based on establishing requirements for flame paths in explosionproofenclosures by determining maximum experimental safe gaps (MESG) for many materials.Later work in determining minimum ignition energy (MIE) of materials from electrical arcingand sparking showed a close correlation to the MESG data. This correlation allowedresearchers and practitioners to conclude that the gas groupings were valid whether explosionprotection of electrical parts was by means of containment (explosionproof enclosures) or bylimiting available electrical energy in the circuits (intrinsic safety) located in the classifiedlocation.

Figure 8 lists the group classifications of the common flammable gases and vapors found inSaudi Aramco plants and facilities. Additional information on groups can be found on pages33 through 37 of the supplemental textbook, "Electrical Installation in Hazardous Locations",by Peter J. Schram and Mark W. Earley.




Material Group Designation

Acetone DAcetylene ABenzene DButane DCarbon Monoxide CEthane DEther CFuel Oils DGasoline DHeptane DHexane DHydrogen BHydrogen Sulfide CMethane DMethanol DNaphtha DPropane DToluene D

Figure 8. Group Classifications of CommonFlammable Gases and Vapors

Divisions

The criterion for designating a location as a Class I hazardous location is the probablepresence of flammable gases or vapors in a location where electrical equipment is intended tobe installed. Once the decision is made to classify a location, the location is assigned a groupdesignation according to the specific gas or vapor that is present in the location. The finaldescription of a Class I hazardous location is to designate the location Division 1 or Division2. This final classification must consider the likelihood or probability of the presence of acombustible gas or vapor during normal operations.




Class I, Division 1

As defined in API Recommended Practice 500 (RP 500), Class I, Division 1 locations arelocations that meet one or more of the following criteria:

• Ignitable concentrations of flammable gases or vapors exist under normal operatingconditions.

• Ignitable concentrations of flammable gases or vapors may exist frequently because ofrepair or maintenance operations or because of leakage.

• Breakdown, faulty processes, or faulty operation of equipment might lead to the releaseof ignitable concentrations of flammable gases or vapors and might also be the cause ofsimultaneous failure of electrical equipment that could become a source of ignition.

Additionally, API Recommended Practice 500 (RP 500) provides the following examples oflocations that would be classified as Class I, Division 1:

• Locations where volatile flammable liquids or liquefied flammable gases are transferredfrom one container to another.

• Interior locations where volatile flammable solvents are used.

• Locations containing open tanks of volatile flammable liquids.

• Inadequately ventilated locations where flammable gases accumulate or volatileflammable liquids stand or collect.

• Interiors of refrigerators and freezers in which volatile flammable materials are stored inopen, lightly stoppered, or easily ruptured containers.

• All other locations where ignitable concentrations of flammable gases or vapors arelikely to occur during normal operation.

For additional Class I, Division 1 considerations, read the section of API RecommendedPractice 500 (RP 500) titled "Classification Criteria" (part 4.3).




Class I, Division 2

As defined in API Recommended Practice 500 (RP 500), Class I, Division 2 locations arelocations that meet one or more of the following criteria:

• Locations where volatile flammable liquids, vapors or gases are handled, processed orused, but are normally confined in closed containers or closed systems from which theycan only escape in cases of accidental rupture or breakdown of such containers or incases of abnormal equipment operation.

• Locations where ignitable concentrations of flammable gases or vapors are normallyprevented by positive ventilation but might become ignitable through failure orabnormal operation of the ventilating equipment.

• Locations adjacent to Class I, Division 1 locations (and not separated by a vaportightbarrier) and to which ignitable concentrations of gases or vapors might occasionally betransferred unless such transfers are prevented by adequate positive pressure ventilationfrom a source of clean air, and effective safeguards are provided to prevent ventilationfailures.

Class I, Division 2 locations usually include locations where volatile flammable liquids orflammable gases or vapors are used in containment and would not pose a risk of explosionunless there was a malfunction of both the containment of the flammable materials and anysuitable (for Division 2 locations) electrical equipment.

For additional Class I, Division 2 considerations, read the section of API RecommendedPractice 500 (RP 500) that is titled "Classification Criteria" (part 4.4).

Additional information on divisions can be found on pages 31 and 32 of the supplementaltextbook, "Electrical Installation in Hazardous Locations", by Peter J. Schram and Mark W.Earley.

Differences Between Class I, Class II, And Class III Hazardous Locations

The differences between Class I, Class II, and Class III hazardous locations are best describedby their associated definitions. The definition for a Class I hazardous location is given herefor comparison with the definitions for Class II and Class III locations as follows:

• Class I locations are those locations in which flammable gases or vapors are or may bepresent in quantities sufficient to produce explosive or ignitable mixtures.

• Class II locations are those locations that are hazardous because of the presence ofcombustible dusts.




• Class III locations are those locations that are hazardous because of the presence ofeasily ignitable fibers or flyings that are not likely to be in suspension in the air insufficient quantities to produce ignitable mixtures.

These definitions point out that the only major difference between the different classes ofhazardous locations is the type of fuel that is present.

For further information on the differences between Class I, Class II, and Class III hazardouslocations, read the following sections of the supplemental textbook, "Electrical Installations inHazardous Locations", by Peter Schram and Mark W. Earley:

• Section 2-4, Class I Locations pages 16-32

• Section 2-5, Class II Locations pages 32-40

• Section 2-6, Class III Locations pages 40-41

The table in Figure 9 summarizes the characteristics of Class I, Class II and Class IIhazardous locations.




Divisions

Classes Groups 1 2

I - Gases,Vapors, andLiquids

A: Acetylene

B: Hydrogen, etc.

C: Ether, etc.

D: Hydrocarbons, fuels,solvents, etc.

Explosiveconcentrations maybe normally presentand are hazardous.

Explosiveconcentrations arenot normally present(but may accidentallyexist).

II - Dusts E: Metal dusts(conductive* andexplosive)

F: Carbon dusts (some areconductive, * and all areexplosive)

G: Flour, starch, grain,combustible plastic orchemical dusts(explosive)

Ignitable quantities ofdust normally is ormay be insuspension, orconductive dust maybe present.

Dust is not normallysuspended in anignitableconcentration (butmay accidentallyexist). Dust layersare present.

III - Fibers andFlyings

Textiles, woodworking,etc. (easily ignitable, butnot likely to be explosive)

Ignitableconcentrations maybe present as a resultof fibrous materialbeing handled orused inmanufacturing.

A location whereflyings or fibers arestored or handled instorage (exclusive ofmanufacturing).

* Note: Electrically conductive dusts are dusts with a resistivity less than 105ohm-centimeter.

Figure 9. Summary of Class I, II, III Hazardous Locations




PROTECTION METHODS FOR ELECTRICAL EQUIPMENT

This section provides information on the various methods of protection for electricalequipment for use in hazardous locations as follows:

• Fundamental protection philosophy

• Confining the explosion

• Isolation

• Energy limitation

• Miscellaneous

• Summary of protection techniques and there application

• Comparison of the three major techniques

Fundamental Protection Philosophy

The fundamental protection philosophy has two parts: the first concerned with location andthe second with precautions.

When planning any facility, every effort should be made to locate electrical equipmentoutside of the hazardous location. This approach has several benefits, with the primary onebeing a reduction of the risk of explosion to nearly zero regardless of the circumstances.

There are, however, many occasions when protection by location is not possible. When thiscondition exists, then extraordinary precautions are required to reduce the risk of explosionthat may be caused by electrical equipment to an acceptable level. The acceptable level oracceptable risk is addressed by the second part of the fundamental philosophy: that is, safetyexperts worldwide agree that any protection technique applied to electrical equipment shouldrequire two independent events, each of low probability, between safe operation and apotential explosion. All protection techniques in use today meet this fundamental criterion aswill be demonstrated in the following discussion.

Confining the Explosion

This method has one technique - the explosionproof housing. In this case, it is fully expectedthat ignitions or explosions will occur within the enclosure because no attempt is made toprevent entrance of flammable materials. However, the enclosure is designed to withstand theexplosion pressure from any internal explosions for the highest gas rating of the enclosure(Groups A, B, C, or D), and any openings in the enclosure are tight enough and long enoughto sufficiently cool any products of combustion to a temperature that will not ignite theexternal atmosphere. Such enclosures are gas group dependent, with the design requirementsfor Group D materials being the least restrictive.




The “two independent event” criterion applied to this technique is subtle, but effective as thefollowing indicates:

1. The first level of protection obviously consists of the design requirements to be appliedto the enclosure.

2. The second level of protection is more subtle and relates to the tightness of the enclosureand the nature of the electrical parts within. Most enclosure designs are relatively tight,designed for outdoor use to prevent entrance of the elements. Such enclosures have atleast restricted breathing and would require presence of flammable materials for longperiods of time to enter the enclosure. Assuming that flammable materials do gainentrance, either of two conditions would exist: (1) if there are ignition-capable arcs andsparks occurring in normal service, any entering flammables will simply burn off beforethey reach an explosive concentration; or (2), if there are no normally arcing or sparkingparts, the mixture will sit harmlessly unless there is a concurrent equipment failureresulting in an ignition-capable energy release. If the latter should occur, then theenclosure would be called upon to perform its intended function - to confine theexplosion. In either event, the requisite two fault criterion is satisfied.

Isolation

There are several equipment design techniques that may be used to satisfy the objective of theisolation method: to isolate the ignition source from the flammable mixture. Techniquesinclude oil immersion, sealing and restricted breathing, potting, and encapsulation. The mostcommon technique is pressurization, also known as purging. This technique is based onmaintaining a pressure within the enclosure using a protective gas, normally air, at a levelhigher than the external pressure. Any flow, therefore, is from within to the outsideatmosphere such that no externally present flammables can enter the enclosure. The threelevels of protection are as follows:

1. The application of the pressurizing gas to maintain the internal pressure.

2. Automatic shutdown at loss of pressure, activated by a pressure sensitive cut-off switchthat is applied when there are normally ignition-capable arcing parts or hot surfaces inthe enclosure.

3. A visible or audible alarm at loss of pressure that is activated by a pressure sensingdevice when the internal parts are non-sparking and that is suitable for use in a Division2 hazardous location.

The faults as described above make such equipment suitable for use in a Class I, Division 1hazardous location without regard to the gas group present.




Energy Limitation

The energy limitation method has two techniques that may be applied: intrinsic safety andnon-incendivity. The former is intended for use in Division 1 hazardous locations while thelatter is intended for use in Division 2 hazardous locations. For intrinsic safety, two levels ofprotection are provided if we assume that two faults must occur within the circuitry to causean increase in the energy levels present. But even after two faults, or one fault with theapplication a factor of safety of 1.5 to voltage, current, or a combination of both asappropriate, no ignition of the test gas is allowed. The intrinsic safety technique is sensitive togroup classifications of hazardous locations since it is based on ignition energy, whichincreases from Group A to Group D.

Miscellaneous

Several miscellaneous protection techniques are used which do not fit into any of the abovecategories. These techniques include:

• Sand filling (a type of isolation)

• Increased safety

• Dust-ignitionproof

• Continuous dilution

Each of these protection techniques is used in specific locations. For example, increasedsafety is a popular European technique used for non-sparking motors and lighting fixtures.Such designs are suitable for international Zone 1, as shown in Figure 10, which includes theinternational classifications of hazardous locations.




Summary of Protection Techniques and Their Application

Figure 10 provides a summary of the various protection techniques and locations where theymay, and may not be used.

External Location Classification

Type Of Protection Non-Haz Zone 2 Zone 1 Zone 0

Special ProtectionEx s

X X X Some

Intrinsic SafetyEx ia

X X X X

Intrinsic SafetyEx ib

X X X

FlameproofEx d

X X X

PressurizedEx p

X X X

Increased SafetyEx e

X X X

EncapsulationEx m

X X X

Hermetic SealEx h

X X X

Type N, Non-incendiveEx n

X X

Restricted BreathingEx n

X X

Unprotected ElectricalEquipment

X Some

Figure 10. Summary of Protection Techniques




The table in Figure 10 lists the international designations for the types of protection and thehazardous locations. An “X” in a box means that pertinent type of protection is generallypermitted in the X'ed zone classification. The Special Protection, Ex s (special constructions),category can include double protection techniques, such as an explosionproof enclosure thatis also pressurized. This use of double protection techniques would be acceptable for use in aZone 0 location.




GLOSSARY

AIT Auto- (Apparent) ignition temperature. The ignition temperature of asubstance required to initiate or cause self-sustained combustion.

combustibleliquid

A liquid with a flash point at or above 100°F (37.8°C); a liquid thatreleases ignitable concentrations of vapor less easily than a flammableliquid. (See flammable liquid.)

explosionproof Pertaining to electrical equipment that is designed so that an explosion offlammable gas within the enclosure of the equipment will not igniteinflammable gas outside the enclosure.

flammable liquid A liquid with a flash point below 100°F (37.8°C); a liquid that releasesignitable concentrations of vapor more easily than a combustible liquid.(See combustible liquid.)

flying An airborne combustible material such as jute, hemp, rayon, and similarmaterials.

grade level A datum or reference level such as ground level.

hermeticallysealed

An enclosure sealed through use of a fusion joint rather than through useof a gasket joint. Sealed by welding, silver soldering, brazing, or similartechniques).

lower explosivelimit (LEL) orlowerflammability limit(LFL)

The minimum concentration of a material in air that will allowpropagation of a flame when the gas/air mixture is in contact with anignition source.

MESG Maximum experimental safe gap. The maximum gap between matingsurfaces that just prevents explosion propagation determined byexplosion test experimentation.

MIE Minimum ignition energy. The least amount of energy required to ignitea gas-air mixture at its most easily ignited concentration.

propagation Spreading out and affecting a greater area.

pyrophorics Materials that ignite spontaneously in contact with air.

SIT Same as AIT above.




upper explosivelimit (UEL) orupperflammability limit(UFL)

The maximum concentration of a material in air that will allowpropagation of a flame when the gas/air mixture is in contact with anignition source.

vapor pressure The pressure measured in psia (kilopascals) that is exerted by a volatileliquid.

vapors Common term for use in referring to the gas emitted from the surface ofa flammable or combustible liquid.

volatile Descriptive of a liquid that releases vapor with relative ease.




ADDENDUM A

TABLE OF CONTENTS

API Recommended Practice 500 (RP 500) Section 4 - Classification CriteriaAPI Recommended Practice 500 (RP 500) Section 5 - Extent of a Classified Location

Documents

Classification of and Protection for Hazardous Locations