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Intrinsic safety 101

Protection methods, containment, segregation, and prevention

By Robert Schosker ' This article answers the question, "Why worry about hazardous locations?" The area classifica-|tions. for the U.S. and the European (the International Electrotechnical Commission [IEC] stan-|dard) approaches are described and contrasted, including protection methods, containment,

segregation, and prevention

What is intrinsic safety?Intrinsic safety (IS) has been around a long time. It is the concept of limiting ignition-capable energyto Below that of the hazardous material a process may be working with. IS, as most people callit, wasborn out of the ashes of a horrible roining accident in the U.K. in 1913. The explosion was caused by

Substitute l! Operat°r 1the dangerous Residual training/substances protection risk qualification

Minimize the Risk reducing bytertiary explosion

protection

54 INTECH SEPTEMBER/OCTOBER 2013 WWW.ISA,ORG

AUTOMATION BASICS

a low-voltage signaling system that was used toadvise the surface crew that coal cars were readytO be brought to the surface. The ensuing re-search revealed that the most important factorin determining the safety of, an electrical circuitis the energy stored in the circuit. The conceptof intrinsic safety was born.

So why do we worry about hazardous loca-tions? Many explosions are the result of lim-

ited knowledge by employees, which can bedirectly related to inadequate training, lackof documentation, and questionable safetymeasures. These disastrous results may have

been prevented with proper training, new in-strumentation, inspection, maintenance, orrevised safety procedures. Of course, even ifthe plant takes these steps, an explosion couldstill happen; however, every effort to reduce"residual risk" is a benefit. Residual risk is therisk left Over after all other forms or mechani-cal means of risk reductions have been taken.

A hazardous area is an area containing (orlikely to contain) an ignitable concentration offlammable gas, vapor, or dust, where an electri-cal spark of sufficient power will cause an ex-

plosion. However, an explosion can only occurwhen an oxidizer, the hazardous substance (gas,

vapor, or dust), and energy (thermal or electri-cal) are present at the same time. These threegroups comprise what is known as the hazard-

ous location ignition triangle.

who defines a hazardous location? What is thedifference between zones and divisions? Theseare all very good questions, and ones that areoften asked. We already know that a hazardouslocation is an area that contains of is likely tocontain an ignitable concentration of flammablegas, vapor, or dust. Once we have determinedthat a hazardous area exists, then we classify thatarea using either zones or divisions. Although thephysical principles of explosion protection arethe same worldwide and are not differentiated,the procedures determined by national legisla-tion in the approximately 100-year history of ex-plosion protection have resulted in various solu-

tions. Each solution has a unique perspective onhow to classify a hazardous area.

Division . .

I Class ; Division; Groups; T__

Class (t-lit)(1-2)ÿe-.----

-Gas groups (x--y)

Temp codes (T1-T6) ,ÿ

Oxidizer Fuel(air) (gas, vapor or dust)

If any one of these groups is removed, anexplosion cannot happen. As we will discussthroughout this article, intrinsic safety is amethod of protection that removes one of thesegroups from the ignition triangle.

The article will first answer some basic ques-tions about hazardous locations. We already

know why we worrÿ but what are we worryingabout? What is a hazardous location? What or

Energy(thermal or electrical)

Divisions, mainly a North American-approvedmethod of classifying an area, use four catego-

ries: class, division, group, and temperature.They are commonly seen like this:

Hazardous due to the presence of flammable substances such as gases or vapors

Hazardous due to the presence of flammable substances such as dusts orpowders. -

Hazardous due to the presence of flammable substances in a fiber.or flying state.

The categorization of these areas is carriedout in North America in accordance with theNational Electrical Code NFPA 70, article 500.Class I, II, or III represent the form of the haz-ardous material you must deal with.

Danger can be present during normal functioning, during repair or main-tenance, or where a fault may cause the simultaneous failure of electricalequipment.

Combustible material is present but confined to a closed container orsystem, is normally vented or in an area adjacent to a'Division I location.

Division is a categorization by the probabilitythat these materials will be present in a poten-tially hazardous quantity (Division 1 and Divi-sion 2). The division is commonly the perimeteror designated area of the hazardous material.

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÷

Class I i GroupAGroup B

Iÿ Fÿrouÿ C

i: Group D

\

Class Ii I Group E

iGroup F

Atmospheres containing acetylene

Atmospheres containing hydrogen and flammableprocess gasses with more than 30 percent hydro-gen by volume, or gases or vapors posing a similarrisk level such as butadiene and ethylene oxide

The graphical representation below illustrates the con-cept of division-based areas.

Hazardous locations

Maximum temperature(°C)450r

30O280260230215200180i65160'

13512010085

(°F)842572

, 536

500446419392356329320275248212185

Temperature classin North America

TIT2

T2AT2BT2CT2DT3

T3AT3BT3CT4

T4AT5T6

Furthermore, classes, of hazardous areas are divided into

subgroups depending i3n the type of flammable gas or vaporpresent..

Finallyÿ apparatus installed directly in a hazardous areamust be' classified for themaximum surface temperature thatthe device will produce.under normal operation or in theevent of a fault, The maximum surface temperature must be

below the minimum ignition temperature of the gas present,

Atmospheres such as ether, ethylene, or gases orvapors posing a similar risk level

Atmospheres such as acetone, ammonia, benzene,butane, cyclopropane, ethanol, gasoline, hexanemethanol, methane, natural gas, naphtha, propane,or gases or vapors posing a similar risk level

Atmospheres containing combustible metal dusts,including aluminum, magnesium, and their com-mercial alloys, or other combustible dusts whoseparticle size, abrasiveness, and conductivity presentsimilar hazards in the use of electronic equipment

Atmospheres containing combustible carbona-ceous dusts including carbon black, charcoal,coal, or coke dusts that have more than 8 percenttotal entrapped volatiles, or dusts that have beensensitized by other materials so that they presentan explosion hazard

Atmospheres contai.ning combustible dusts notincluded in Group E or Group F, including flour,grain, wood, plastic, and chemicals

: i GroupG

KeyDivision I

Division 2

GradeSump or trenchÿ

In Europe, the zone method of area classification is used.The European zone practice is described in IEC/EN 60079-10. In accordance with this standard, any area where there is

a probability that flammable gas or dispersed dust may existmust be classified into one of the following areas:

Zone 0

Zone 1

Zone 2

Zone 20

Zone 21

Zone 22

An area !n which an explosive airlgas mixture is continu-ously present or present for long periods.

An area in which an explosive airlgas mixture is likely tooccur in normal operation.

An area in which an explosive airlgas mixture is unlikelyto occur, but, if it does, only for short periods of time.

• An area in which a combustible dust cloud is part of theair permanently, over long periods of time, or frequently.

• An area inwhich a combustible dust cloud in the air islikely to occur In normal operatlon.

An area in which a combustible dust cloud in the air mayoccur briefly or during abnormal operation.

The European zones then require that the apparatus besubdivided into two groups, according to IEC/EN 60079-0.These two groups in general terms indicate whether the ap-paratus is located ,above or below ground.

Group I

Group II

Apparatus to be used in mines where the danger is repre-sented by methane gas and coal dust.

Apparatus to be used in surface industries where the dangeris represented by gas and ÿapor that has been subdividedinto three groups: A, B, and C. These subdivisions are basedon the maximum experimental safe gap (ME SG) for anexplosion-proof enclosure or the rai!!mum ignition current(MIC) for intrinsically safe electrical apparatus.

And as with divisions and gas groups, there are sub-groups of Group I and II within the zones. Each is associ-ated with a certain number of gases having an ignition

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energy in the value reported and is represented by the gasreferenced in the table below. Included in this compari-son are the Class I and II division-based classifications.

Boundaries. Zone method - Boundaries (Designated areas)

Material Apparatus classification

Europe (* IEC) North America

Methane Group I (mining) Class I, Group D

Acetylene Group IIC Class I, GroupA > 20 IJJ

Hydrogen Group IIC Class t, Group B > 20 IJi

Ethylene Group lib Class I, Group C > 60 IJJ

Propane Group IIA Class I, Group D > 180 pJ

Conductive dust(metal) Group IIIC* Class 11, Group E

Nonconductive dust Group IIIB* Class II, Group F(carbon)

Cereal/flour Group IIIB* Class II, Group G

Fibers/suspended Group IliA* Class IIIparticles

Ignitionenergy

.......... ; Zone 0

Zone 1Iÿ Zone 2

A temperature code is also required just like divisions; how-ever,' the zone method is simplified. With both divisions andzones, the maximum surface temperature must be below

the minimum ignition temperature of the gas present.

• T3

T4

T5T6

The graphical representation in the upper right illustratesthe concept of zone-based areas.

The article has explained hazardous areas and how thoseareas are classified, but what methods of protection exist foryour hazardous areas? As previously mentioned, to reducethe risk of explosion, you must eliminate one or more of thecomponents of the ignitiontriangle. There are three basicmethods of protection: explosion containment, segrega-

tion, and prevention.

Temperature classin Europe

TIT2

Maximum surface temperature '(°c)45O3OO2802602302152OO18016516013512010085

® Explosion containment: the only method that allows the ex-plosion to occur, but confines it to a well-defined area, thusavoiding the propagation to the surrounding atmosphere.Flameproof and explosion-proof enclosures are based on

this method.o Segregation: a method that attempts to physically separate

or isolate the electrical parts or hot surfaces from the ex-plosive mixture. This method includes varimis techniques,such as pressurization and encapsulation.

e Prevention: a method that limits the energy, both electri-cal and thermal, to safe levels under both normal operationand fault conditions. Intrinsic safety is the most representa-

tive technique of this method.The protection methods, based on the containment and

segregation concepts, are methods that contain the explosionin order to prevent the energy source--electrical or thermal--

from coming in contact with the potentially explosive mixture.In both cases, the use of appropriate enclosures and specificwiring and installation systems is required. The intrinsic safe-ty method prevents the ignition of the exp!osive atmosphere,while simplifying the installation and use of the required ap-paratus that is connected to the electrical circuits directly lo-cated in a hazardous location.

The choice of a specific protection method depends on thedegree of Safety needed for the type of hazardous area. Theidea is to determine the best protection method based on thelowest probable degree of the simultaneous presence of anadequate energy source and a dangerous concentration levelof an air/gas mixture. Of course, other important factors to

consider include the size of the apparatus to be protected, theflexibility of the system, the possibility of needing to performmaintenance, and the installation cost. Respective of these

factors, intrinsic safety has many, advantages.So what is intrinsic safety? Intrinsic safety is based on the

.principle of limiting energy to a level below that required toignite the hazardous gas or dust. The energy limitation als0pertains to thermal energy as well. Therefore, in normal op-eration or in the event of a fault, no sparks or thermal effects

may occur that could lead to the ignition of a potentially ex-plosive atmosphere.

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o Intrinsic safety "i," example Ex ia- Divisions 1, 2/Zones 0, 1, 2

Barrier Isc/Io Cable Hazardous area

Associatedapparatus

It is one thing to understand intrinsic safety as a concept,but it has to be applied correctly to w6rk. As shown in the

• illustration, there are three components to an intrinsicallysafe circuit:

• The field device--intrinsically safe equipment locatedin the hazardous location

o Intrinsically safe barrier orassociated apparatus lo-cated in nonhazardous location

° Interconnecting wire between the two apparatusThe.example here is simple, because the fault combina-

tions are few, and the:knowledge of the apparatus safety

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parameters and cable characteristics

are sufficient to verify the safety of thesystem, A more complex system (com-binations of barriers or using multiplecables) requires a more detailed analy-sis because there are many fault com-binations to veriÿ and they are not aPways evident.

As mentioned above, more complex analysis may berequired with intrinsic safety, but with this in mind,intrinsic safety has several advantages over the other •methods of protection. They include allowing trainedpersonnel to connect and disconnect circuits under live

operation. They are guaranteed to be safe during shortcircuits or lead breakage. Intrinsic safety is the onlyignition protection class that allows connect'ors to beopened and intrinsically safe apparatus to be removedand replaced by an equivalent device in a hazardousarea. Intrinsic safety is also the only method that allowsgeneral-purpose wiring methods to be used in the haz-

ardous area. These simple advantages also generate costsavings through installation and maintenance, becausethis method allows live maintenance with no need forplant shutdown. Intrinsic safety is also more reliabledue to the use of infallible and derated components asprescribed by the standards.

Intrinsic safety was born from the ashes of a horrificaccident to become the most widely accepted methodof protection. Not only does' it limit voltage and current,

but it provides a 50 percent safety margin even duringfault conditions; no other method of tÿrotection canclaim that. In the end, it is the end users who must de-

termine the area of classification, the required processequipment, and the method of protection they want tojuse. If they decide to use intrinsic safety, I think they willfind that it provides the best mix of an affordable systemand the safety required.

ABOUT THE &UTHOR

Robert Schosker is the product manager for intrinsic safety, HART,signal conditioners, and surge protection for Pepperl+Fuchs.Since joining Pepperl+Fuchs in 1995, he has been focused ontechnology and product-related support, and is involved in, awide range of activities and roles including certifications, sales,and marketing. He has been the key lead in many IS and HARTprojects resulting in the developmenlÿ of new products for in-trinsic safety and HART infrastructure. Schosker holds a BSEEfrom the University of Akron. Direct questions or comments topa-info@us.pepperl-fuchs.com.

58 INTECH SEPTEMBER/OCTOBER 2013 WWW.ISA.ORG