875

7.2 Electrical and Intrinsic Safety

W. F. HICKES

(1969)

C. M. J. OUDAR

(1985)

B. G. LIPTÁK

(1995)

A. ROHR

(2003)

Types of Devices:

A. Intrinsic safety barriersB. Instrument housings and enclosuresC. Terminals with surge protection or suited for hostile industrial environmentsD. Protectors against transients

Partial List of Suppliers:

ABB Products (A) (www.abb.com)Allen-Bradley Co. (C) (www.ab.com.)Altech Corp. (B, C) (www.altechcorp.com)AMCO Engineering Co. (B) (www.amcoengineering.com)Ametek Inc. Panalarm Div. (A) (www.ametek.com)Bailey Controls Co. (A) (www.pmcx.com/bailey_network.htm)Bebco Industries, EPS Div. (B—purged) (www.okbebco.com)Capital Controls Co. (B) (www.capitalcontrols.com)Comark Corp. (C—NEMA 4) (www.comarkcorp.com)Contech Engineering Inc. (B) (www.contecusa.com)Contrec Inc. (A) (www.contrec.co.uk)Controlled Power Inc. (D) (controlledpowerinc.com)Eaton Corp., Cuttler-Hammer Products (C) (www.ch.cutler-hammer.com)EFI Electronics Corp. (D) (www.efinet.com/home.html)Fibox Enclosures (B) (www.fiboxusa.com)Fisher Controls International Inc. (A) (www.emersonprocess.com)Hammond Enclosures (B) (www.hammfg.com)Hardy Instruments Inc. (B) (www.hardyinstruments.com)HiTech Technologies Inc. (B) (www.hitechtech.com)Hoffman Engineering Co. (B) (www.hisoregon.com)Honeywell Industrial Controls (A) (www.acs.honeywell.com)Instrument Enclosures (B) (www.id-reps.com)Invensys (A, B) (www.foxboro.com), (www.invensys.com)Leeds & Northrup (A, B) (www.procinst.com)Moore Industries (D) (www.miinet.com)MTL Inc. (A, B, D) (www.mtlnh.com)Norstat Inc. (D) (www.norstat.com)Optima Enclosures (B) (www.optimaeps.com)Pepperl

+

Fuchs Inc. (A) (www.pepperl-fuchs.com)Phoenix Contact (C, D) (www.phoenixcontact.com)Pro-Tech (B) (www.protech1.com)Rochester Instrument Systems (B) (www.rochester.com)Ronan Engineering Co. (A) (www.ronan.com)Schroff Inc. (B) (www.pentair-ep.com)R. Stahl Inc. (A) (www.rstahl.com)Superior Electric (D) (www.superiorelectric.com)Weidmüller Inc. (C) (www.weidmueller.de)Wieland Inc. (C) (www.wielandinc.com)Y-E-P Industries Inc. (B) (www.yepind.com)Yokogawa Corp. (D) (www.yca.com)

© 2003 by Béla Lipták

876

Safety and Miscellaneous Sensors

Electrical and intrinsic safety-related worldwide standardiza-tion is still in the evolutionary process. The general conceptsof electrical safety are internationally accepted, but theirimplementation is still being modified, and the correspondingstandards are taking effect at different times in the variouscountries. It is advisable to check the prevailing regulationsin the particular country at the particular time where andwhen a particular plant is being built.

INTRODUCTION

There are three ways in which electricity can kill or injure.Two are indirect—the result of fire and the result of explo-sion. One is direct—electrocution by electric shock.

As an academic matter, a fire and an explosion are basi-cally the same, explosion being simply a very fast-spreadingform of fire. They are considered separately for two reasons;one, because the results are so different, and two, perhapsmore important, because the precautionary and preventivemeasures are quite different.

The danger of fire from electrical causes is generallyconfined to the supply side of instrumentation—from thepoint where power enters the system up to and including thepower transformer found in most process control instrumen-tation. Transformer secondary circuits within the instrumen-tation can present a potential fire hazard, but they can becontrolled with proper instrument construction. Instrumentfield wiring, employing the popular signal of 4 to 20 mA DCand lower, operates at energy levels at which fire hazard isremote. The precautions required for personnel protectionand reliability automatically result in fire safety.

Power supply to the point of entry into an instrument issubject to detailed rules and regulations. In most of theUnited States, the National Electrical Code (NEC) has beenadopted and has the effect of law. Many states and munici-palities have their own codes, which differ slightly from theNEC. These are often obsolete editions of the NEC. It isalways wise to check local rules and interpretations whenplanning an installation.

Safety depends primarily on three fundamental factors:

1. Enclosure of live parts, both to avoid personnel contactand accidental short circuiting

2. Fuses or circuit breakers to open in case of overload3. Grounding of all exposed metal

ENCLOSURES

For ordinary locations, an enclosure need only be tight enoughto prevent entrance of the human finger far enough to contactlive parts. Unless ventilation is required for cooling, it shouldalso be tight enough to prevent entrance of foreign materialand to prevent escape of sparks or hot material in case of

internal short circuit or fire. It is particularly important toprevent the escape of flaming drops from any burning insula-tion or plastics. The enclosure itself must not support combus-tion. This does not rule out plastics but does require selectivityin their use.

Because instruments differ in use, the usual rules forelectrical enclosures need modification when applied. Forordinary electrical equipment, it is assumed that only a qual-ified electrician has access to the interior, and, therefore, onlyunusual interior hazards need be guarded. However, instru-ments frequently have doors for access by other than qualifiedtechnicians for purposes such as recorder chart changing,inking of pens, and controller adjustments. No live partsoperating at voltage levels dangerous to personnel should beaccessible during operational maintenance.

NEMA Terminology

For special environmental conditions, further requirementsare imposed that usually follow the terminology establishedby the National Electrical Manufactures Association (NEMA)for motor starters and similar equipment. The following isexcerpted from NEMA ICS “Industrial Control”:

Type 1

General Purpose A general-purpose enclosure isintended primarily to prevent accidental contactwith the enclosed apparatus. It is suitable for gen-eral-purpose applications indoors where it is notexposed to unusual service conditions. A Type 1enclosure serves as a protection against dust, light,and indirect splashing, but is not dust-tight.

Type 2

Drip-Tight A drip-tight enclosure is intended toprevent accidental contact with the enclosed appa-ratus and, in addition, is so constructed as to excludefalling moisture or dirt. A Type 2 enclosure is suit-able for application where condensation may besevere, such as is encountered in cooling rooms andlaundries.

Type 3

Weather-Resistant (Weatherproof) A weather-resistant enclosure is intended to provide suitableprotection against specified weather hazards. It issuitable for use outdoors.

Type 4

Watertight A watertight enclosure is designed tomeet the hose test described in the following note.A Type 4 enclosure is suitable for application out-doors on ship docks and in dairies, breweries, etc.

Note:

Enclosures shall be tested by subjection to a streamof water. A hose with a one-inch (25 mm) nozzleshall be used and shall deliver at least 65 gallonsper minute (246 1/min). The water shall be directedon the enclosure from a distance of not less than 10feet (3 m) and for a period of 5 min. During thisperiod it may be directed in any one or more direc-tions as desired. There shall be no leakage of waterinto the enclosure under these conditions.

© 2003 by Béla Lipták

7.2 Electrical and Intrinsic Safety

877

Type 5

Dust-Tight A dust-tight enclosure is providedwith gaskets or their equivalent to exclude dust. AType 5 enclosure is suitable for application in steelmills, cement mills, and other locations where it isdesirable to exclude dust.

Type 6

Submersible A Type 6 enclosure is suitable forapplication where the equipment may be subject tosubmersion, as in quarries, mines, and manholes.The design of the enclosure will depend upon thespecified conditions of pressure and time.

Type 7

(A, B, C, or D) Hazardous Locations Class IThese enclosures are designed to meet the applica-tion requirements of the National Electrical Codefor Class I hazardous locations that may be in effectfrom time to time.

Type 8

(A, B, C, or D) Hazardous Locations Class I OilImmersed These enclosures are designed to meetthe application requirements of the National Elec-trical Code for Class I hazardous locations that maybe in effect from time to time. The apparatus isimmersed in oil.

Type 9

(E, F, or G) Hazardous Locations Class II Theseenclosures are designed to meet the applicationrequirements of the National Electrical Code forClass II hazardous locations that may be in effectfrom time to time.

Type 10

Bureau of Mines Explosion-Proof A Type 10enclosure is designed to meet the explosion-proofrequirements of the U.S. Bureau of Mines that maybe in effect from time to time. It is suitable for usein gassy coal mines.

Type 11

Acid- and Fume-Resistant Oil Immersed Thisenclosure provides for the immersion of the appa-ratus in oil such that it is suitable for applicationwhere the equipment is subject to acid or othercorrosive fumes.

Type 12

Industrial Use A Type 12 enclosure is designedfor use in those industries where it is desired toexclude such materials as dust, lint, fibers and fly-ings, oil seepage, or coolant seepage.

Type 13

Oil-Tight and Dust-Tight Indoor A Type 13enclosure is intended for use indoors to protectagainst lint, dust, seepage, external condensation,and spraying of water, oil, or coolant.

IP Terminology

This terminology is used mostly in Europe. According toInternational Electrotechnical Commission’s (IEC) IEC60529 (2001–02), an enclosure can be defined as a combi-nation of two numerals. The first numeral describes the pro-tection provided against solid foreign objects, while the sec-ond refers to the type of protection provided against thepenetration of water.

First Numerals:0. Non-protected1. Protected against solid objects greater than 50 mm2. Protected against solid objects greater than 12.5 mm3. Protected against solid objects greater than 2.5 mm4. Protected against solid objects greater than 1.0 mm5. Dust-protected6. Dust-tight

Second Numerals:0. Non-protected1. Protected against dripping water2. Protected against dripping water when tilted up to 15

°

3. Protected against spraying water4. Protected against splashing water5. Protected against water jets6. Protected against heavy seas7. Protected against the effects of immersion8. Protected against the effects of continuous submersion

In addition, two optional letters can also be used. Thefirst letter describes the degree of protection against accessof hazardous parts. The second letter indicates the degree towhich protection is provided against special operating con-ditions. Neither of these letters is used in the instrumentationand control fields.

Typical IEC enclosure specification for indoor instrumentcabinets in a control room ranges from IP20 to IP42. For fieldmounted transmitters the housing is usually specified as IP65.

FUSES AND CIRCUIT BREAKERS

The conventional 15 or 20 A fuse or breaker in the supplywiring to an instrument is designed to protect the wiring, notthe instrument. Component failures or circuit faults withinthe instrument may result in total destruction of the instru-ment. To minimize damage and possible fire, a much smallerfuse, usually 1

/

4 to 3 A, is used in the instrument.

Grounding

A low-resistance, noncurrent-carrying metallic connection toground should be established and maintained from everyexposed metallic surface that can possibly become connectedto an electrical circuit. Electrical connection could occurbecause of a fault, such as a loose wire making electricalcontact, or as a result of leakage current through insulation.

Grounding is usually accomplished by bonding all ele-ments together in a system terminated at the ground connec-tion where power enters the premises. It may be a bare orgreen insulated wire. More often it is the conduit enclosingthe wires. It must be securely joined, electrically and mechan-ically, to each piece of equipment. It is connected at theservice entrance to the grounded circuit conductor (whitewire) and to ground. Instead of connection to a ground at the

© 2003 by Béla Lipták

878

Safety and Miscellaneous Sensors

entrance connection, other suitable earth ground connectionsare acceptable. Equipment mounted directly on the structuralsteel of a building is considered effectively grounded. Waterpipes are not effective grounds, particularly since the increas-ing use of plastic pipe for water connection.

Grounding serves two distinct purposes, both relating tosafety. First, since the ordinary power circuit has one sidegrounded, a fault that results in electrical contact to thegrounded enclosure will pass enough current to blow a fuse.Second, possibility of shock hazard is minimized since thelow-resistance path of a properly bonded and grounded sys-tem will maintain all exposed surfaces at substantially groundpotential.

Grounding is effective against hazard from leakage cur-rents. All electrical insulation is subject to some electricalleakage. This may rise to a significant level as insulationdeteriorates with age, or as layers of conductive dust accu-mulate in the presence of high humidity. A proper groundingsystem with low electrical resistance will conduct leakagecurrents to ground without developing significant potentialon exposed surfaces.

Grounding of exposed metal surfaces is distinct from thegrounded conductor of the ordinary power wiring. The latteris a current-carrying ground that is capable of developingsignificant potential, particularly on long lines, and withsurge or even short-circuit currents. Grounding systems aresubstantially noncurrent carrying, except for possible leakagecurrents. Potential can build up only during the time requiredfor a fuse to blow as a result of a specific fault that resultsin direct contact between power and grounding systems.Grounding is customarily not required for signal circuitswhere either maximum voltage is 30 V, or maximum currentunder any circumstance cannot exceed 5 mA.

Personnel Safety

The electrical energy necessary to kill or injure a personvaries widely with conditions of exposure, especially withcontact conditions (i.e., wet or dry skin, contact area, and thepath the current takes through the body). A few millivoltsapplied directly to the heart can cause fibrillation and death.Yet it is a common, though not approved, practice amongelectricians to ascertain, if a 120-V (or even a 240-V) circuitis energized by putting two fingers of the same hand incontact with the two conductors.

If the person is insulated from the ground, for instance,by standing on a dry wood floor, the current path is throughthe fingers and the hazard is nil. If the person is standing inwater or has a firm grasp on a water pipe, the current pathwould be through the central nervous system, and it mightwell be fatal. Much depends also on the nature of the contact.An electrical shock causes muscles to contract, thus a finger-tip contact can be broken, but if the live part is gripped, itmight be impossible to let go.

In addition to guarding against possible lethal shocks, thesurprise factor should not be overlooked. An unexpected, but

harmless, shock may induce a sudden reaction and expose aworker to an entirely different hazard, such as falling off aladder.

Energy Levels

This discussion of energy levels hazardous to humans is limitedto those circumstances where the whole body is involved. Formedical electronics, where electrodes may be implanted withinthe body, a much lower set of numbers apply. Considerationof such equipment is outside the scope of this volume.

The significant factor is current through the body. Acurrent of less than 1 mA is imperceptible to a normal man.If it is above 3 mA, it becomes unpleasant. If it is above 10mA, the victim is unable to let go, and above 30 mA, asphyx-iation may result. Still higher levels lead to heart stoppageand death. These values are for sustained contact. Muchhigher levels can be tolerated for a fraction of a second.

To relate this information to circuit voltage requiresknowledge of body resistance. Internal body resistance canbe as low as 100

Ω

, but the resistance of the whole body isprimarily in the skin and in skin contact. Dry fingers graspinga wire or small terminal will have a resistance in the orderof 100,000

Ω

. Wetting the fingers would lower this. To lowertotal body resistance to 1000

Ω

would require immersion ofhand or foot in water and a solid grip on a large object. Asa practical working figure, the National Electrical SafetyCode requires guarding above 50 V. A measurement of 30 Vis considered safe for general use, even in children’s toys,though surely not for swimming pools.

The NEC permits circuits up to 150 V if they are inca-pable of delivering more than 5 mA without special require-ments for wire insulation.

If all line voltage circuits are enclosed in properly fusedand grounded enclosures, and signal circuits not meetingeither the 30 V or the 5 mA criteria are guarded, personnelsafety will be assured.

EXPLOSION HAZARDS

Areas in which combustible gas, vapor, or dust may bepresent in explosive proportions are called

hazardous loca-tions

. Special precautions must be taken with electrical equip-ment in hazardous locations to eliminate a source of ignitionthat could touch off an explosion. The specific precautionsvary with the nature of the combustible material and theprobability of its presence. An atmosphere is considered tobe explosive if the concentration of the explosive vapors iswithin its lower and upper explosive limits (LEL and UEL).Some properties of flammable and explosive vapors and gasesand their flammability limits or LEL and UEL values arelisted in Tables 7.2a and 7.2b. Note that the data provided bythe two tables is

not

identical, because they report the resultsof different sets of tests.

© 2003 by Béla Lipták

7.2 Electrical and Intrinsic Safety

879

NEC Definition of Hazardous Locations

The first step in deciding what equipment to use is to determinethe nature and the degree of hazard. The NEC

1

describes haz-ardous locations by class, group, and division. The

class

definesthe physical form of the combustible material mixed with air:

Class I—Combustible material in the form of a gas orvapor

Class II—Combustible material in the form of a dustClass III—Combustible material in the form of a fiber,

such as textile flyings

The

groups

are subdivisions of the

classes

:

Group A—Atmospheres containing acetyleneGroup B—Atmospheres containing hydrogen, gases or

vapors of equivalent hazard, such as manufacturedgas

Group C—Atmospheres containing ethyl ether vapors,ethylene, or cyclopropane

Group D—Atmospheres containing gasoline, hexane,naphtha, benzine, butane, propane, alcohol, acetone,benzol, lacquer, solvent vapors, or natural gas

TABLE 7.2a

Properties of Some Flammable Liquids and Gases

MaterialChemical Formula

Specific Gravity Air

=

1

Ignition Temperature in AirFlamability Limits

in Air (% vol.)

(

°

F) (

°

C) Lower Upper

Methane CH

4

.55 1193 645 5.3 15.0

Natural gas Blend .65 1163 628 4.5 14.5

Ethane C

2

H

6

1.04 993–1101 534–596 3.0 12.5

Propane C

3

H

8

1.56 957–1090 514–588 2.2 9.5

Butane C

4

H

10

2.01 912–1056 489–569 1.9 8.5

Toluene C

7

H

8

3.14 1026–1031 552–555 1.3 6.7

Gasoline A blend 3–4.00 632 333 1.4 7.6

Acetone C

3

HO 2.00 1042 561 2.6 12.8

Benzene C

6

H

6

2.77 968 520 1.4 6.7

Carbon monoxide CO .97 1191–1216 644–658 12.5 74.0

Hydrogen H

2

.07 1076–1094 580–590 4.0 75.0

Hydrogen sulfide H

2

S 1.18 655–714 346–379 4.3 45.0

TABLE 7.2b

Properties and LEL and UEL Values of Explosive Materials

Product Formula LEL vol% UEL vol%

Boiling Point Ignition Temperature

°

C

°

F

°

C

°

F

Hydrogen H

2

4.00 75.00

−

252.7

−

482.9 560 1040

Ethylene C

2

H

4

2.70 36.00

−

103.9

−

155.02 425 797

Cyclopropane C

3

H

6

2.40 10.40

<

0

<

32 498 928.4

Hexane CH

3

(CH

2

)

4

CH

3

1.20 7.50 68.7 155.7 233 451.4

Benzine 0.70 5.90 30–210 86–410 280 536

Butane C

4

H

10

1.50 8.50

−

0.6 31 372 701

Propane CH

3

CH

2

CH

3

2.10 9.50

−

42.2

−

43.96 470 878

Ethylic alcohol C

2

H

5

OH 3.50 15.00 78.4 173.1 423 793.4

Acetone CH

3

COCH

3

2.5 13.00 56.5 133.7 535 995

Natural gas 3.93–6.60 13.20–17.50

<

0

<

32 482 899.6

Methane CH

4

4.40 17.00

−

161.4

−

258.5 537 998.6

Acetylene C

2

H

2

2.3 100

−

85

−

121 305 581

Ethylene oxide CH

2

OCH

2

3.00 100 13.5 56.3 435 815

Source:

The Italian Electrotechnical Committee, CEI 31–35: Electrical Apparatus for Explosive Atmospheres—Guide for Classification of HazardousAreas. With permission.

© 2003 by Béla Lipták

880

Safety and Miscellaneous Sensors

Group E—Atmospheres containing metal dust, includingaluminum, magnesium and their commercial alloys,and other metals of similarly hazardous characteristics

Group F—Atmospheres containing carbon black, orcoal or coke dust

Group G—Atmospheres containing flour starch or graindusts

The

division

defines the probability of an explosive mix-ture being present. Only the breakdown for Class I is givenbecause it is the one most often encountered. Classes II andIII are similarly subdivided.

Class I, Division 1—Locations (1) in which hazard-ous concentrations of flammable gases or vaporsexist continuously, intermittently, or periodicallyunder normal operating conditions, (2) in whichignitable concentrations of such gases or vaporsmay exist frequently because of repair or mainte-nance operations or because of leakage, or (3) inwhich breakdown or faulty operation of equip-ment, or processes which might release ignitableconcentrations of flammable gases or vapors,might also cause simultaneous failure of electricalequipment.

Class I, Division 2—Locations (1) in which volatile flam-mable liquids or flammable gases are handled, pro-cessed, or used, but in which the hazardous liquids,vapors, or gases will normally be confined withinclosed containers or closed system which they canescape only in case of accidental rupture or breakdownof such containers or systems, or in case of abnormaloperation of equipment, (2) in which ignitable con-centrations of gases or vapors are normally preventedby positive mechanical ventilation, but which mightbecome hazardous through failure or abnormal oper-ation of the ventilating equipment, or (3) that are adja-cent to Class I, Division 1 locations, and to whichignitable concentrations of gases or vapors mightoccasionally be communicated unless such commu-nication is prevented by adequate positive-pressureventilation from a source of clean air, and effectivesafeguards against ventilation failure are provided.

As a rule of thumb, any atmosphere tolerable for a personto breathe is not within the explosive range. Except for meth-ane and hydrogen, gases and vapors become toxic or irritatingwell below their lower explosive limit. An explosive mixtureof dust limits visibility to a few feet.

Economic reasons (i.e., cost of lost product) tend to limitDivision 1 locations to an area within a few feet of probableleaks such as pump glands and valve stem packing. TheAmerican Petroleum Institute has developed a series ofdetailed criteria for classifying areas at such distances intypical situations.

2

IEC Definition of Hazardous Locations

Hazardous area definitions by the IEC can be found in IEC60079–10 (2002–06). The IEC approach is to define thenature of the source, which releases the explosive substance.This can be continuous, primary or secondary, which corre-spond to a zone definition of zone 0, 1, or 2.

The extension and the shape of the zone is affected bythe release rate (the higher the release, the greater the zone), thelower explosive limit (LEL; the lower the LEL, the greaterthe zone), the ventilation, and the specific gravity ofgas/vapor when released. A gas/vapor heavier than 1.2 SG

air

originates a hazardous zone with the shape lying on theground, while if lighter than 0.8 SG

air

it originates a hazard-ous zone of vertical shape. For specific gravities between 0.8and 1.2, the hazardous zone encompasses the zones for lighterand heavier gas/vapors.

In Europe, the majority of the countries derived theirnational regulations from the IEC or have adopted the IECregulations and although their timing varies, the regulationsare consistently applied. In Canada, the IEC standards arecompulsory. In the United States, the IEC standards havebeen accepted on an optional basis.

Explosions

An explosion is dependent upon the simultaneous presenceof three conditions (Figure 7.2c). An oxidizer in the form ofthe oxygen in air is ordinarily present. Fuel, a gas, vapor, orfinely divided solid is normally kept confined for economicreasons, if not for safety. However, by definition, a hazardouslocation is a place where fuel and oxidizer are present incombustible proportions, at least at times. Ignition of thisdangerous combination must not be permitted. Electricalequipment must be built and operated in a manner to preventits becoming a source of ignition. It could ignite this hazard-ous atmosphere in either of two ways: by surface tempera-tures in excess of the ignition temperature or by sparks. Somesparks are incidental to normal operation, as in the operationof switches, while some are accidental, as in faulty connec-tions. Both must be guarded against.

FIG. 7.2c

Prerequisites for an explosion.

Ignition Source

Fuel Oxidizer

© 2003 by Béla Lipták

7.2 Electrical and Intrinsic Safety

881

PROTECTION METHODS

There are several approaches to safety:

1. Confine explosions so they do no damage (explosionproof)

2. Keep atmosphere away from ignition source (pressur-ization or ventilation, oil immersion, sealing, or potting)

3. Limit energy to levels incapable of ignitions (intrinsicsafety)

4. Miscellaneous (sand filling, increased safety, dustignitions-proof, nonincendive)

Explosion-Proof (“Flameproof” in Britain)—All equip-ment is contained within enclosures strong enough towithstand internal explosions without damage, andtight enough to confine the resulting hot gases so thatthey will not ignite the external atmosphere. This isthe traditional method and is applicable to all sizesand types of equipment.

Purging, Pressurization, Ventilation—This depends uponthe maintenance of a slight positive pressure of air orinert gas within an enclosure so that the hazardousatmosphere cannot enter. Relatively recent in generalapplication, it is applicable to any size or type ofequipment.

Oil Immersion—Equipment is submerged in oil to adepth sufficient to quench any sparks that may beproduced. This technique is commonly used forswitch-gears but it is not utilized in connection withinstruments.

Sealing—The atmosphere is excluded from potentialsources of ignition by sealing them in airtight con-tainers. This method is used for components suchas relays, not for complete instruments.

Potting—Potting compound completely surroundingall live parts and thereby excluding the hazardousatmosphere has been proposed as a method of pro-tection. There is no known usage except in combi-nation with other means.

Intrinsic Safety—Available energy is limited under allconditions to levels too low to ignite the hazardousatmosphere. This method is useful only for low-power equipment such as instrumentation, commu-nication, and remote control circuits.

Sand Filling—All potential sources of ignition are bur-ied in a granular solid, such as sand. The sand acts,in part, to keep the hazardous atmosphere away fromthe sources of ignition and, in part, as an arcquencher and flame arrester. It is used in Europe forheavy equipment. It is not used in instruments.

Increased Safety—Equipment is so built that thechance of spark or of dangerous overheating is nil.In practice, this means rugged construction, widespacings between parts of opposite polarity, extrainsulation, nonsparking fans and good mechanical

protection. Widely used in Europe for heavy equip-ment such as large motors. It is also recognized forinstruments particularly in Germany. It is not rec-ognized in the United States.

Dust Ignition-Proof—Enclosed in a manner to excludeignitable amounts of dust or amounts that mightaffect performance. Enclosed so that arcs, sparks,or heat otherwise generated or liberated inside ofthe enclosure will not cause ignition of exterioraccumulations or atmospheric suspensions of dust.

Nonincendive—Equipment which in normal opera-tions does not constitute a source of ignition (i.e.,surface temperature shall not exceed ignition tem-perature of the specified gas to which it may beexposed, and there are no sliding or make-and-breakcontacts operating at energy levels capable of caus-ing ignition. Used for all types of equipment inDivision 2 locations. Relies on the improbability ofan ignition-capable fault condition occurring simul-taneously with an escape of hazardous gas.

A summary of the various protection methods is givenin Table 7.2d.

Advantages and Disadvantages of Protection Methods

Table 7.2e attempts to rate the various protection methodsused for instrumentation. Methods are rated from A to C.

Safety

All methods are safe if the equipment is properlyinstalled, maintained, and protected by a correctly selectedmethod that is consistent with the area classification. Intrinsicsafety is rated A because recognized standards are most con-servative and it is less dependent on day-to-day usage. Ordi-nary carelessness does not make intrinsically safe equipmentunsafe. Explosion-proof equipment is worthless if the coveris left off, and purging is dubious under the same circum-stances. Purging is also dependent on reliability of purge airsupply.

Cost of the Instrument

Purging is usually lowest in costbecause it requires no special construction for hazardouslocation use, except for an air inlet. Since an intrinsicallysafe instrument does not require the special housing of anexplosion-proof instrument, there is a possible saving. Thecost difference between an explosion-proof housing and arugged weatherproof housing is too small to justify two sep-arate designs for small devices, such as field-mounted trans-mitters. Cost of review and listing by a testing agency, suchas Underwriters Laboratories or Factory Mutual, add to thecost of intrinsic safety.

Cost of Installation

Intrinsic safety is at least potentially lowest in cost. The NECpermits wiring for approved intrinsically safe equipment in

© 2003 by Béla Lipták

882

Safety and Miscellaneous Sensors

hazardous locations to be the same as in ordinary locations, suchas multiwire cables without special protection. For explosion-proof or purged equipment, all wiring must be in rigid con-duit, all fittings must be explosion-proof, and conduits mustbe sealed. Purged equipment also requires an air supply sys-tem with purge failure alarms and in some cases automaticshutdown.

Maintenance

Intrinsic safety is rated A because the equip-ment is accessible for routine calibration checks and adjust-ments. Explosion-proof equipment must be deenergizedbefore being opened or maintenance must be deferred untilthe area is known to be safe. Purging is similarly limited andalarms and interlocks must also be maintained.

Flexibility

Purging is rated A because essentially any stan-dard or special instrument with reasonably tight housing canbe readily adapted to purging. Explosion-proofing is limitedto instruments available in that construction or that can be fittedinto a standard, explosion-proof box. The need for externaladjustments and indication can make this very expensive.Intrinsic safety must be evaluated as a system. In the past a

serious problem has been the inability to interconnect appa-ratus of different manufacturers. Now this problem is over-come through the use of properly selected safety barriers andcompliance with the constraints mentioned in the certificationsissued by the official bodies.

The marking scheme described in paragraph A-4–2 ofthe National Fire Protection Association’s (NFPA) NFPAStandard 493 provides a convenient way to assess the com-patibility of apparatus of different manufacturers with respectto intrinsic safety. This concept facilitates the connectionbetween two-terminal devices such as a two-wire transmitterand barrier.

3

Purging, Pressurization, or Ventilation

Any reasonably tight enclosure housing electrical equipmentcan be made safe by providing a continuous flow of air orinert gas. The enclosure can be of any size from a smallinstrument case or fractional horsepower motor to an entirebuilding, such as control houses.

The essentials are:

1. A source of clean air2. Sufficient initial flow to sweep out gas that may have

been present3. Sufficient pressure building in the enclosure to prevent

entrance of combustible atmosphere4. Suitable alarms and interlocks

A typical purge assembly is illustrated in Figure 7.2f.

Air Supply

For a single instrument or other small device,the instrument air system is the best source of air for pres-surization since it is clean and dry. Where a large number of

TABLE 7.2d

Reference for Protection Methods

Class I Gas/Vapor Group A, B, C, D

Class II Dust Group E, F, G

Class III Flyings and Fibers

Protection Method Div. 1 Div. 2 Div. 1 Div. 2 Div. 1 Div. 2

Explosion-proof housings OK Required only for sparking or hot devices

Not applicable unless also dust ignition-proof

OK if tightly enclosed and no overheating when covered with flyings

Dust ignition-proof Not applicable Dust-proof and no overheating when dust covered

Not applicable

Intrinsic safety OK OK OK OK OK OK

Purging OK OK Subject of work by NFPA ? ?

Potting ? OK if no overheating

Hermetic sealing ? OK if no overheating

Oil immersion Acceptable but not convenient to use for instruments ? ?

Nonincendive Not applicable OK Not applicable unlessdust ignition-proof

OK OK if tightly enclosed andno overheating whencovered with flyings

TABLE 7.2eRating the Protection Methods

SafetyCost of

InstrumentCost of

Installation Maintenance Flexibility

Intrinsic safety

A C A A C

Explosion-proof

B B B B B

Purging C A C C A

© 2003 by Béla Lipták

7.2 Electrical and Intrinsic Safety 883

units are involved or an entire control house, the large vol-umes required make this impractical. The best solution varieswith individual plant conditions.

Finding a safe place for an air intake requires carefulstudy. An intake 25 ft (7.5 m) off the ground and not withina 45° shadow cone of any potential source of vapor is gen-erally considered adequate for a refinery handling vaporsheavier than air. For gases nearly as dense or lighter than air,there is no definitive answer except distance and an upwindlocation. In any event, the suction line must be of substantialconstruction and free of leaks where it passes through apotentially hazardous area.

Initial Purging Pressurized apparatus, when put in serviceafter having been open, may contain combustible mixture.Before power may be turned on, sufficient air must passthrough it to sweep out the combustible gas or at least reduceit to a harmless concentration. This is usually achieved byrequiring a time interval to elapse after closing up the appa-ratus and starting purge flow before circuits are energized. Aflow of four times the internal volume of the case is adequatefor the usual instrument housing. Large or compartmental-ized enclosures require special consideration.

If natural leakage does not provide the necessary purgevolume in a reasonable length of time, auxiliary vents maybe provided to accelerate the operation.

Pressure During operation the enclosure must be maintainedunder a pressure of at least 0.1 in. of water column (25 Pa) toprevent influx of combustible mixture. This figure is equivalentto the wind pressure at 15 mi/h (24 km/h). While higher windvelocities might force outside air into the housing, the potentialhazard is considered negligible since at these velocities anycombustible vapors would be very rapidly dispersed.

Alarms and Interlocks While some warning of pressuriza-tion failure is needed, the specific requirement depends onthe nature of the enclosure’s content and the degree of hazardoutside.

Classification of Purging Systems The Instrumentation,Systems, and Automation Society (ISA)4 and the NFPA5 clas-sify purging as follows:

Type X purging reduces the classification within an enclo-sure from Division 1 to non-hazardous. Type X purgingwould permit an arcing switch in a general purpose hous-ing located in a truly hazardous, Division 1 location. Sincefailure of the purge air supply would soon lead to disaster,immediate automatic shutdown by pressure switch or flowdetector is required.

Type Y purging reduces the classification within an enclo-sure from Division 1 to Division 2. Type Y purged equip-ment within an enclosure does not normally constitute asource of ignition, hence purge failure present no imme-diate hazard. Only a visible or audible indication isrequired.

Type Z purging reduces the classification within an enclo-sure from Division 2 to non-hazardous. Type Z purgedequipment, used in Division 2, where the atmosphere isnot normally hazardous, again presets no immediate haz-ard and visible or audible indication is sufficient.

Temperature of all parts exposed to the atmosphere, inhazardous locations, must not exceed 80% of the ignitiontemperature of the combustible material.

Explosion-Proof Components

Article 100 of the National Electrical Code defines explosion-proof apparatus as:

Apparatus enclosed in case which is capable of withstand-ing an explosion of a specified gas or vapor which mayoccur within and of preventing the ignition of a specifiedgas or vapor surrounding the enclosure by sparks, flashes,or explosion of the gas or vapor within, and which operates

FIG. 7.2f Packaged purge systems are marketed for Class 1 area and are provided with rapid exchange purging capability. (Courtesy of Bebco Industries.)

Optional Pressure Loss Alarm Switchor Electrical Power Control UnitSupplier

Installer

PurgingGas Supply

Venturi Orifice

Rapid ExchangePressure Gauge

Rapid Exchange PressureControl Filter Regulator

Rapid ExchangeControl Valve

EnclosurePressure Indicator

Enclosure PressureControl Valve

Required EnclosureProtection Vent

Reference Out

EnclosureWarningNameplate

Supply InProtectedEnclosure

© 2003 by Béla Lipták

884 Safety and Miscellaneous Sensors

at such an external temperature that a surrounding flam-mable atmosphere will not be ignited thereby.

Explosion-proof apparatus is not intended to be gas tight.It is assumed that no enclosure that may have to be openedfrom time to time for inspection or maintenance can practi-cally be maintained gas tight. Hence, if the surrounding atmo-sphere is hazardous, the atmosphere within will also becomehazardous, and an internal explosion may result. If the boxholds together and the only openings are long, narrow, andpreferably crooked paths, the escaping gases will not be hotenough to ignite the external atmosphere. The British term,flameproof, is perhaps more descriptive of the function thatthe American term, explosion-proof.

There are two broad approaches to construction: a rela-tively tight box with broad flanges and tightly fitted, threaded(5 thread minimum) or rabbet joints, or a relatively loose boxwith many small passages designed to minimize pressurebuildup. The first requires a very strong box to withstand fullexplosion pressure, up to 175 PSIG (1208 kPa) for Group Cor D, and 1000 PSIG (6900 kPa) or greater for Group B orA. This is usually cast iron or cast aluminum. Plastic is usedin Europe for smaller boxes.

For the second approach, the pressure rise can be keptdown, to 20 PSIG (138 kPa) or less, allowing lightweightconstruction. Though officially recognized, the latter approachhas been little used because of the possibility of vent passagesbecoming plugged by dirt or by injudicious use of a paintbrush.

INTRINSIC SAFETY

The NEC defines intrinsic safety as follows:

Intrinsically safe equipment and wiring shall not be capableof releasing sufficient electrical or thermal energy undernormal or abnormal conditions to cause ignition of a spe-cific flammable or combustible atmospheric mixture in itsmost easily ignitable concentration.

Abnormal conditions shall include accidental damage toany field-installed wiring, failure of electrical components,application of over-voltage and maintenance operations,and other similar conditions.

A quantity of a combustible mixture must be heated toits ignition temperature for an explosion to occur. A weakspark heats so little mixture that heat loss exceeds heat supplyand the incipient explosion dies out. A large spark heatsenough mixture for combustion to become self-sustainingand an explosion propagates. If energy is kept at a low level,ignition will not occur. This is the basis of intrinsic safety. Itis not sufficient that energy be low in normal operation. Itmust also be low under any conceivable abnormal operationor fault condition.

Energy Levels

Safe energy levels cannot be defined in any simple form.Ignition depends on specific gas, gas concentration, voltage,current, energy storage elements, contact material, contactsize, and speed of opening or closing of contacts. Ignition ofhydrogen (one of the most easily ignited gases) has beenachieved under laboratory conditions (high voltage) withenergy as low as 20 µJ. For common hydrocarbons and thevoltages actually encountered in instrumentation, energiescirca 0.2 to 0.3 mJ are required.

Curves such as Figure 7.2g show limiting circuit param-eters that provide ignition energy for a particular gas. Theycan be used safely only after careful examination of thespecific equipment by one skilled in the art and by applicationof an adequate safety factor. Actual ignition testing of thespecific apparatus is the preferred practice.

System Approach

In evaluating equipment for intrinsic safety, it is always nec-essary to look at all elements of the complete loop. Forexample, an ordinary thermocouple (TC), by itself, is unques-tionably safe. Add a simple millivolt indicator and it is stillsafe. Connect the couple to a recorder, powered from a 120 Vline, and the question of safety arises. What could happen ifthe recorder allowed dangerous amounts of energy to reachthe TC leads? The problem is easily solved by good recorderdesign and construction, but it cannot be overlooked.

Thus, there is no such thing as an intrinsically safe instru-ment unless it operates with a self-contained, low-energypower source, such as a small battery or solar cell that isisolated from all other power sources. Anything that is con-nected or wired to that instrument must be viewed as com-ponents of the system. This requires a careful study of allcomponents of the complete loop and of the relevant con-straints given by the official bodies. Someone must evaluatethe combination and assume responsibility for its safety.

Certification of Intrinsic Safety

The practical answer for the instrument buyer and user is tolook for certification by a qualified testing agency. In theUnited Sates there is the Underwriter Laboratories or FactoryMutual Research Corporation. It is the Canadian StandardsAssociation in Canada, Physikalische-Technische Bundesan-stalt in Germany, and the British Approvals Service for Elec-trical Equipment in Flammable Atmospheres (BASEEFA) inBritain. Many other countries have their own agencies—these are the most well known.

While each testing agency has its own detailed rules andtraditions, NFPA 493–1978, “Intrinsically Safe Apparatus forUse in Division 1 Hazardous Locations,” outlines a reason-ably typical evaluation. As it says, however, “it is not aninstruction manual for untrained persons but is intended topromote uniformity of practice among those skilled in theart.” The following outline of the procedure will not make

© 2003 by Béla Lipták

7.2 Electrical and Intrinsic Safety 885

the reader an expert but will indicate the conservative natureof the approach.

Three steps are involved:

1. Circuit analysis—to determine worst possible faultconditions

2. Evaluation—to ensure a margin of safety under thecondition found above

3. Construction review—to ensure that critical componentsare reliable and that circuit will function as planned

Circuit Analysis Circuit analysis is a review of the circuit,component by component, considering the possible mode offailure of each and its effect on energy levels available in thehazardous area. It starts where line power enters the systemand includes all parts of the interconnected instrument systemwherever located—in the control house or in the field. Theobject is to pinpoint the fault condition or combination ofconditions allowing highest energy in the field circuits.

Evaluation This is the next step in the procedure. The pur-pose is to ascertain whether or not each fault condition con-stitutes a possibility of ignition. With the use of the actualcircuit, the faults are produced by short- or open-circuitingcomponents with the field leads connected to a test apparatus.The test apparatus consists of pair of contacts operating in achamber filled with the most readily ignited mixture of asuitable combustible gas with air. The contacts simulate a bro-ken wire, a wire dragging on a surface, or the short-circuitingof a pair of leads.

An internationally accepted form of test apparatus is ofWest German origin. In July 1967, this test apparatus wastentatively accepted by the IEC meeting in Prague and waslater adopted.

It is also possible to use measured voltages, currents,inductance, capacitance, etc., and compare them with theresults of previous tests with similar circuit parameters. Asillustrated in Figure 7.2g, suitable curves exist,3,4 but theymust be interpreted with care and a suitable margin of safety.

FIG. 7.2g Left: Inductance circuits (L > 1 mH). Minimum igniting current at 24 V (1 = igniting current). Applicable to all circuits containing cadmium,zinc, or magnesium. Right: Inductance circuits (L > 1 mH). Minimum igniting current at 24 V. Applicable only to circuits where cadmium,zinc, or magnesium can be excluded. (Reprinted with permission from NFPA 493–1978. Standard for Intrinsically Safe Apparatus andAssociated Apparatus for Use in Class I, II, and III, Division 1 Hazardous Locations, Copyright 1979, National Fire Protection Association,Quincy, MA 02269. This reprinted material is not the complete and official position of the NFPA on the referenced subject, which isrepresented only by the standard in its entirety.)

Methane

Group D

Group C

Methane

Group D

Group C

GroupsA and B

GroupsA and B

100 µH

200 µH

500 µH

1 mH

2 mH

5 mH

10 mH

20 mH

50 mH

100 mH

200 mH

500 mH

1H

100 µH

200 µH

500 µH

1 mH

2 mH

5 mH

10 mH

20 mH

50 mH

100 mH

200 mH

500 mH

1H

LL

10 mA 20 mA 50 mA100 mA 200 mA 500 mA 1A 2A 10 mA 20 mA 50 mA100 mA200 mA 500 mA 1A 2A 3A 5AI

© 2003 by Béla Lipták

886 Safety and Miscellaneous Sensors

Construction Review This is the final step in the procedure.A circuit component must be of a reliable form of construc-tion if it is to be depended upon for safety. A transformerthat will withstand a high potential (1480 V applied primaryto secondary) immediately after being deliberately burnedout is considered reliable. This presupposes that the trans-former is so constructed that consistent performance of thisnature can be anticipated. A resistor must withstand grossoverloading without significantly changing in value. Liveparts, such as terminals, must be so separated that an acci-dental short circuit is essentially impossible. If an instrumentsystem survives this kind of examination, its safety is assured.

International Regulations

The international approach distinguishes two categories offield mounted instruments:

1. Simple apparatus, such as thermocouples, resistancetemperature detectors or contacts that do not requirecertification, but do need to be connected to other instru-ments (across barriers).

2. Intrinsically safe apparatus, such as transmitters, I/Pconverters and solenoid valves, which require instal-lation certification for the specified hazardous area,category of gas, and temperature class. These appara-tus must be connected to the associated instruments(across barriers).

The barriers, installed in a safe area, provide the separa-tion between the intrinsically safe devices in the field and thenonintrinsically safe devices (distributed control systems,programmable logic controllers, recorders, indicators, etc.)in the control room. The barriers must also be certified andmust be suitable for being connected to field mounted appa-ratus, which are installed in a particular hazardous area andexposed to a particular group of gases and temperature class.The nameplate and the certificate both must give the follow-ing data:

1. Maximum open circuit voltage2. Maximum short circuit current3. Maximum allowable external capacitance4. Maximum allowable external inductance5. Maximum transferred power

The safety barriers serve to send on the signals betweenthe control room and the field. They must be suitable tohandle the maximum voltage (Um) that is present in thecontrol room. This voltage currently in the United States is240 V. The above considerations are the basic philosophy,while all other mandatory details can be found in the IECregulations.

Grounding In order to prevent the formation of sparks withenergy contents that exceed the intrinsically safe limit or the

developing of over-voltages, the selection of the safety bar-riers must also consider the grounding of the field instruments.

If the grounding is dedicated, equipotential, and the fieldmounted device is isolated from the ground with an electricalstrength of at least 500 V, Zener diode type barriers can beused. If the grounding is not equipotential, galvanic insulationis required. If the circuitry in the field mounted instrumentis grounded, the barriers must be provided with galvanicinsulation.

Barriers The Zener barriers are inexpensive and reliablepassive circuits that consist only of some resistors and Zenerdiodes. Unfortunately, they do require equipotential ground-ing, which can make the overall installation expensive. Theintrinsically safe apparatus must be connected to the equipo-tential ground at a properly identified single point and via anisolated conductor having a resistance of not more than 1 ohm.

The intrinsically safe side of the galvanically isolatedbarriers includes an energy limiting circuit with Zener diodesand limiting resistors. It transfers the signals to and from thehazardous area via such isolating components as transform-ers, opto-couplers, and relays. The Zener diodes in the bar-riers are duplicated or tripled to maintain the protectiontoward the external circuit also in case of failure of one ortwo of them.

The certificate provided for the barriers must show the totalinductance limit, the maximum allowable ratio of inductance/resistance for the wiring, and the capacitance of the circuit(field instrument plus wiring) in the classified area.

Cables and Their Installation In case of a short circuit or thebreaking of a circuit, the wiring between the barrier and thefield apparatus must not store dangerous quantities of energy,which might cause a spark. The screens must be groundedat only one point and in a safe area. The cables connectingto intrinsically safe apparatus must be identified in a consis-tent and easily recognizable manner. If the method of iden-tification is by color, all intrinsically safe cables must be lightblue and no other cable in the plant can have light blue color.Multicore cables can carry only intrinsically safe signals.

In connection with cable trays, one option is to lay theintrinsically safe cables in different cable trays than the onescarrying nonintrinsically safe cables. Another option is toprovide physical separation between the two types within thesame cable tray. A third option is to provide armoring, sheath-ing, or screening for one of the cable types (intrinsically safeor nonintrinsically safe) in the tray. Clearly marked and sep-arate field junction boxes must also be provided for intrinsi-cally safe and for nonintrinsically safe circuits.

Terminations Reliable separation must be provided betweenintrinsically safe and nonintrinsically safe circuits. This canbe done by providing a separating panel or by keeping aminimum of 2 in. (50 mm) distance from other circuits. Ifthis distance method of separation is used, it should be

© 2003 by Béla Lipták

7.2 Electrical and Intrinsic Safety 887

checked to make sure that even if another wire is discon-nected, it cannot contact the intrinsically safe terminals. Inaddition, all intrinsically safe terminals must be so identified.

References

1. National Fire Protection Association, NFPA 70–2002 (ANSI CI-1981),“National Electrical Code 2002 edition,” Canadian Equivalent, CSAStandard C22.1–1982, “Canadian Electrical Code Part I,” Quincy,MA/Rexdale, Ontario, 1981, 1982.

2. American Petroleum Institute 500, “Recommended Practice for Clas-sification of Areas of Electrical Installations in Petroleum Refineries,”2nd ed., Washington, D.C., November 1997.

3. National Fire Protection Association, NFPA 493–1978, “IntrinsicallySafe Apparatus and Associated Apparatus for Use in Class I, II andIII, Division Hazardous Location,” Quincy, MA, 1978.

4. Instrumentation, Systems, and Automation Society, ISA-RP12.4,“Pressurized Enclosures,” Research Triangle Park, NC, 1996 (coversonly small enclosures, such as instruments).

5. National Fire Protection Association, NFPA 496, “Standard for Purgedand Pressurized Enclosures for Electrical Equipment,” Quincy, MA,1998 (similar to ISA RP12.4, but expanded to cover large switchgearand complete control houses).

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