device protection in the danger zone

7/28/2019 Device Protection in the Danger Zone

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I N P U T O U T P U TT E S W I N A L S i O C K S R E M O T E l / o B C t f t C E K C T W O R K S S i 6 ti A t C 0 S @ t T I O R S C A 0 t t S G / W t H i R G C O N N C E t S R S M O O $ I R G S T E R M I A t 8 t. Q C K S

Devce Protection in theDangerZoneIntrinsic Safety SystemsAre Emergng Sowyas anAlternative toExposion-ProofSolutions inNorthAmerica. It's Tmeto BeCear AboutHow ItWorks

B Y I A N V E R H A P P E N A

F L A S K '^jm: We celebrated our 15th anniversary last year by republishing some of the moretimeless content we'd produced over the years. They were really well received, sowe'll do it again this year from time to time. Here's an article from September 2005 that explains intrinsic safety (IS) as itwas beginning to receive more consideration in North America. I n an accompanying sidebar, the author updates us on a fewnoteworthy events that have developed since then.

intrinsic safety s t i l l is not widely understood in NorthAmerica. Unt i l recently, explosion-proof practices were

Jcommonly used in areas classified as requiring protection. I The need for that protect ion is based on the l i k e l i -hood of a potentially hammable atmosphere being present,which, in turn, determines the class in the North Americanarea classification system.The experience of North American industrial machine

builders that sell into hazardous environment markets overwhelmingly has been based on explosion-proof methods.

Instrument manufacturers for industries with these environments, typically hydrocarbon processing-related industries such asrefining and chemicals, design their instruments to be both explosion-proof and intrinsically safe ( IS) . This allows manufacturers tosell the same device anywhere in the world, regardless of the areaFIGURE 1: BREAK UP THE TEAM

Fuel Energy

OxygencA leastonesdeo thewel-known fire triange mus be removedtopreven firesorexposons

classification and protection system used by the facility.Regardless of the method used to prevent fires or explosionsin a fac i l i ty, all methods are designed to remove one of thesides of the "f ire triang le" shown in Figure I .

Explosion-proof and intrinsic safety systems remove (morecorrectly, manage) or l i m i t the energy level released to theenvironment. Encapsulation and potting, on the other hand,keep oxygen away from the energy source.

Each gas has Its own range of concentrations over whichits stoichiometric ratio allows it to burn. Outside this range,combustion, and hence an explosion, w i l l not occur. The extreme example of this; I f a device is placed in a 100% methaneenvironment, it w i l l not burn or explode because there is nooxygen present to complete the react ion.

Similarly, every gas has a different temperature at which itignites. The concept of divis ions is based on the type of gaspresent, while the "T" or temperature rating is based on gasign i t ion temperatures.A l l these chemical factors must be kept in mind when se

lecting equipment to be used to prevent explosions.

Prevent or Disperse Explosions?A s indicated, both explosion-proof enclosures and intrinsicsafety prevent explosions by l i m i t i n g the amount of energy inthe explosive environment. An explosion-proof enclosure usesits mass and design to disperse the energy to a low level beforeit escapes the enclosure. Intrinsic safety systems are designedto prevent the energy level in the hazardous area f rom beingabove the explosive-limit conditions.It is important to realize that intrinsic safety is a system. A l l

the components of the system need to be considered in the design, including not only the IS device used to l im i t the energyavailable to the hazardous area, but cable and remote devices as

C 0 N T R 0 L D E S I G N . C O M A M A R C H 2 0 1 3


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w e l l . Passive devices that do not store energy, such as terminalblocks, normally are not an issue and need not be considered.T h e capacitance of a cable, which is used when calculating theenergy stored in a cable, is considerably affected by the presence of a screen or shield. It is important to use the correctcapacitance value for the cable type installed.

Intrinsic safety systems aredesigned to prevent the energy

level in the hazardous area,typically the field, frombeing above

the explosive limit conditions.IS devices, the key components in intrinsic safety systems,

are available in two distinct formats: safety barriers and galvanic isolators.

Safety barriers use zener diodes and current-limiting resistors to l i m i t the current and voltage availabl e at the hazardousarea ter mi nal s. A fuse, i f used in the barrier, restricts the faultpower; the zeners restr ict the voltage; and the resistor restrict sthe cm rent. Figure 2 is a simplified schem atic of a safety barrier. The excess energy f r o m a barrier is routed to ground,normally through a low -i mpe dance bus bar.

O n the other hand, a galvanic isolator (Figure 3), as t hename implies, breaks any direct connection between the safea n d hazardous area circuits by int erpos ing a layer of insulat i o n between the tw o areas. The power transfer to the field-important to maintain loop-powered devices normally isv i a some f o r m of transformer, while the return signal f r omthe device in the hazardous area is tra nsm it ted across thehazardous area/safe boundary via an optocoupler. transformer or relay.F I G U R E 2 : S I M P L I F I E D SAFETY B A R R I E R

Resistor Resistor Fuse

Hazardous area


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The final power l im i t a t i o n to the hazardous area Is accomplished wi th a diode and resistor network similar to that ofthe safety barrier. Because galvanic isolators have differentmethods of forwarding the return signal to the safe area, theymust be matched to the application.

Because a galvanic isolator removes any direct connectionbetween the hazardous and safe areas, safety barriers requirea good path to ground. That makes the factory ground system the predominant potential source for signal noise, wi ththe result that proper grounding or earthing techniques mustbe followed. The two main reasons for grounding instrumentsystems are to minimize interference while providing a signal reference, and to segregate and define the fault path requirements for safe dispersion of excess energy. Rapid energydissipation is required to prevent a fire or explosion.

The standard industry practice of grounding instrumentcircuits at only one point is critical to the success of i n t r ins i -cally safe circuits. In addition, the JS circuits should be isolated to withstand a 500 V insulation test. However, the useof galvanic isolators as an interface reduces the cr i t i ca i i ty ofa well-designed and functioning ground grid wi th minimalpotentials across the plant. Therefore, if some remote apparatus requires a separate power supplyi.e., they are notloop-powered, four-wire devicesthen the preferred solutionto maintain an IS circuit is galvanic isolators at either end ofthe cable.

It is worth notine the isolators form the boundaries betweenFIGURE 3: POSITIVELY GALVANIC

Gavanc isoaors breakanydrect connecon beweenthesaeand hazardous areacrcutsby inerposnga layero insuaion beweenthe twoareas

C 0 N T R 0 L D E S I G N . C O M A M A R C H 2 0 1

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HMore machines.L e s s labor.C u t c o n s t r u c t i o n time,generate more r ev enue .B y r e d u c i n g th e l a b o r an dh a r d w a r e to c o n s t r u c t equipment,m a c h i n e b u i l d e r s c a n s e e moretime a va i la b le on their a s s e m b l yfloor. E x t r a time on the floor m e a n smore m a c h i n e s . C o n t r o l s c a b i n e t sc an be a major h a r d wa r e , l a b o r an dtime I n v e s t m e n t for any c o m p a n y ,o u t s o u r c e d or not. By s w i t c h i n g todistributed modular I/O p r o d u c t s ,m a c h i n e b u i l d e r s c an s i g n i f i c a n t l yr e d u c e th e s i z e an d c o s t of theirc o n t r o l s c a b i n e t while r e d u c i n gthe time to a s s e m b l e I/O on them a c h i n e . O n e c u s t o m e r s a v e d 60%o v e r their p r e v i o u s I/O a r c h i t e c t u r e .It's time to look at the p o s s i b i l i t i e s .

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the two safe areas and the single hazardous area. The safe area at the controlsystem end oft en is a rack roo m or unclassified area, while the safe area at theremote device end may have to be created through the use of either explosion-proof or purged housings.

The standard industrypractice of groundinginstrument circuits at

only one point is criticalto the success of

intrinsically safe circuits.Most industrial instrumentation ca

bles include a ground wire as part of thewires w i t h i n the overall insulated product. Many cables also include a screenor shield to l i m i t the effects of nearbycables. It is especi ally imp orta nt to useindividually shielded conductors for anytype of fieldbus installation.

Screens or shields n orm all y are terminated in junction boxes without bonding the m to the structure. The shieldsthen are connected through the terminalto the home-run cable and the host system and its associated ground point. Forthe same reason, unused condu ctors i n acable should be terminated in a terminal so that, if used i n the futu re, they already are connected, and to ensure theyare not an inadvertent source of a spark,short circuit or ground loop.

When screens/shields are used toguard against p ick up of high frequencies, they usuall y are earthed at a nu mber of points to prevent the screen f r o mpresenting a tuned aerial to the highfrequency. For IS circuits w i t h thisproblem, the acceptable solu tio n is toinclude 1,000 pF capacitors to groundat convenient points such as junctionboxes. This effectively detunes thescreen, but does not provide a path for

the low-frequency currents, which cancause interference problems to f low offthe screen or shield.

Ground the BusesMost new installations and many existing transmitters use smart devicescapable of some f o r m of digital communic atio n. Th is can be as simpl e as theH A R T proto col that is superimposedon the 4-20 mA sig nal, or a f u l l fieldbussolution such as Foundation fieldbus orProfibus. A ny circuit that has a fieldbussignal must use galvanic isol ation . Thi sis because the grounding required bysafety barriers w i l l route the signal itself to ground as w e l l . Those isolatorshave to be designed to operate at thespecific frequency transmitted. Thatfollows f r o m the discussion above abouteach isolator type being matched to itsservice and the frequency "tuning."

The lSA-50, lEC 61158 standard-basedfieldbuses also must adhere to the energylimitations as dictated above. That is whyintrinsically safe Foundation fieldbusnetworks normally are restricted to approximately four devices per network.Since the original justification, at least toproject managers and others focused onthe upfront or construction costs of a p r o j -ect, invariably included easily identifiedways to at least break even, the reductionin w i r i n g was an obvious target. Fieldbusdevices typically consume about 20 mA .So i f a segment can support 80 mA , theresult is that IS circuits significantly reduce the benefits of netw orki ng by reducing the number of devices on a networkf r om 8-12 devices down to 4- 6 devices.

True to f o r m , industrial connectorsuppliers found several innovative waysaround this problem. The most com mon ,and the one inc orporate d into exi stin gstandards, is the Fieldbus IntrinsicallySafe Concept (PISCO), based on workby Physikalisch-Technischen Bunde-sanstalt (www .ptb .de) . It demonstratesthat if the inductanc e and capacitanceper unit length of f ie ld cables are w i t h -in defined l i m i t s , then the risk of spark

A M A R C H 2 0 1 3 C 0 N T R 0 L D E S I G N . C O M


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i g n i t i o n does not increase w i t h totallength. The safe operating levels of thepower supplies w i t h electronic currentl i m i t i n g also were established, which al lowed the use of higher currents on thenetwork, typically 130 mA.

The result is that 6- 8 devices now canbe installed on a FISCO network. Otherbenefits of FISCO systems:

The system can be created by anycombination of apparatus that arecertified as FISCO apparatus.

No analysis of the inpu t capacitanceand input inductance parameters isnecessary.

The documentation requirement isreduced to a l i s t of apparatus used.

The key is the devices must be FISCO-app.-oved and, at present, there are few ofthose- alth ough many manufacturers nowarc obtaining this certification.

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BaeFNCOreqre onyasaety faoo 1.1. v thi5saey faofoISaFSCOsystems, it can providemore energy to the network, typically180 m.\. Figure 4 shows the power supp ly design l i m i t s for various area classifications. Thi s additional energy worksout to allow a FN I CO system to more

than double the number of devices on anIS network , yet it provides the same f lex-i b i l i t y to work on a l i v e system.

FNICO networks also have the benefits o f FISCO, relative to documentationand calculation requirements, while notrequiring any special certification beyond the IS approval s already requ iredby most instruments.

lEC standards suggest that l i v e working is permi tt ed in Zone 2 instal lations i fit can be demonstrated that an incendivespark or hot spot cannot be caused by theactivity. This implies that working on al i v e instrument or circuit is possible w i t ha gas clearance/hot work permit.

Inst rument circu it s, however, run th erisk that a fault injected at one pointmight create a hazard at another mier-connect ed piece of equipment . For example, in a temperature-sensing loop, asignal injected at the thermocouple headcould manifest as an unsafe energy level at the transmitter, local indicator or

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FIGURE 4: PERMITTED OUTPUT OF POWER SUPPLIES

500 -I IB/Div. 2 Group B

cr 400 - Non-incendie (FNiCO)i iB/Div. 2 Group B

O 300 - intrinsically Safe (FiSCO)u IIC/Div. 2 Group C1 200 - Non-incendive (FNiCO)

i iC/Div. 2 Group Cintrinsically Safe (FiSCO)100 -

1 1 1 11 1 1 112 14 16 18Open Circuit Voltage [v]B e c a u s e F N I C O o n l y r e q u i r e s a s a f e t y f a c t o r o f 1.1 , v s . t h e 1 .5 s a f e t y f a c t o r l o r I S a n d F I S C O s y s t e m s , i t c a n p r o v i d e m o r ee n e r g y t o t h e n e t w o r k , t y p i c a l l y 1 8 0 m A . T h i s a d d i t i o n a l e n e r g y a l l o w s a F N I C O s y s t e m t o m o r e t h a n d o u h l e t h e nbiiiber ofdevces so as IS setaniU

IS UPDATE 2013this article was publlslE C 61158 networks and IS equivalent offerings.

Th e most significant change to the lEC 60079-11 standards is replacemeiitof the FNICO standard with the FISCO ic rating. FNICO installations a|e"grandHflvered" and the new rating is effectively equivalent, but is incorporatedin aa^6M-c|i^g^rna|^^l^pm^glP^ 1

One of the "knocks" against FISCO was that it wasrFtioessible^J purchyaseredundant power supplies, whicti could result in a single point of t a i i u B ^ andadoption. With advances in electronics that enaWe rapidswitching of circuits within the time period of a singl^nessage pactet, MTL

;0 installations affet.'BUtis inJcwporated

loped and released a ReduinlBlvt FISCO product. fcirctritry, Pepperl+Fuchs and PTB developed dyiiamic arc

ion and termindt^diTf&ART) a s w IS equivalen^^Mi^d|Me Qb.orQvid^^^benefits of IS without the energy restrictionswith which it is comm3ft^rin|sociate9t5i.

Another technology that supports the IS Enti ty concept while allowing higherlevels of energy in the conTplet^vsteny^h^Moo^^|te RouteMfe^architecture model that e l fSW^ ^ p n t ^ n^^^^^^Pwork miWiaginq the %energy levels in the system between both ends of the cable*- w

-Ian Verhappen

computer interface. So the gas test associated w i t h the hot w o r k permit is requiredat all three locations.

T h e m a j o r i t y of new installations are atleast considering the use o f d i g i t a l communications protocols in the design. Despitebeing around fo r almost 10 years, fieldbussystems s t i l l are r e l a t i ve l y new, and manufacturers continue to develop innovative

ways to provide the m a x i m u m flexibilityan d return on a f a c i l i U y investment. A

I a n V e r h a p p e n is an ISA F e l l o w , C e r t i f i e d A u t o m a t i o n P r o f e s s i o n a i , and a r e c o g n i z e d a u t h o r i t y on I n d u s t r i a l c o m m u n i c a t i o n s t e c h n o l c ) g l e s w i t h 25-r y e a r se x p e r i e n c e . He c a ' i i be r e a c h e d at I v e r h a p -p e m S 'l n du s t ' - ; i a l a u t o m a t l o n n e t w o r k s . c o m .

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