Instlltion guide for hzrdous res This instlltion guide should not be used s the controlling document for the instlltion of devices in hzrdous re. This guide is NOT A CONTROL DRAWING. Any instlltion of devices within hzrdous re s defined in the NEC MUST BE in ccordnce with tht device's CONTROL DRAWING nd locl ordinnces.
Tble of contents Acronyms, terms nd definitions... 3 1.0 Introduction... 4 1.1 The fire tringle... 4 1.2 Fire loss control... 5 1.3 The Ntionl Electricl Code... 6 2.0 Hzrdous res nd clssifictions... 7 2.1 Clss, Division nd Zone... 7 2.2 Methods of protection... 9 2.2.1 Intrinsic sfety... 10 2.2.1.1 Intrinsiclly sfe pprtus entity prmeters... 11 2.2.2 Explosion proof... 13 2.2.3 Air purge... 13 2.2.4 Other methods... 13 2.3 Clss I, Division 2 loctions... 13 2.4 Wiring methods... 14 2.4.1 Intrinsiclly sfe circuits... 14 2.5 Mrking exmples... 15 3.0 Wiring Wilcoxon intrinsiclly sfe sensors... 15 3.1 Dynmic vibrtion sensor wiring... 15 3.1.1 Effects of brrier on bis output voltge (BOV)... 15 3.1.2 Troubleshooting... 17 3.2 LPS 4-20 ma sensor wiring... 18 3.2.1 Troubleshooting the 4-20 ma LPS wiring... 19 4.0 Technicl ssistnce... 20 4.1 Technicl nd ppliction ssistnce... 20 4.2 Customer service... 20 Pge 2 of 20
Acronyms CENELEC IEC NEC NFPA CSA IEPE Europen Committee for Electrotechnicl Stndrdiztion Interntionl Electrotechnicl Commission Ntionl Electric Code is registered trdemrk of the Ntionl Fire Protection Assocition Ntionl Fire Protection Assocition Cndin Stndrds Assocition Integrted Electronic Piezo Electric (trnsducer) Terms nd definitions Sensor Accelerometer or PiezoVelocity trnsducer (PVT) Power/signl The connection to n IEPE sensor crrying the power nd lso the superimposed signl Common The return side of the IEPE circuit Ω, MΩ Ohm, Meg-Ohm (million Ohms) Ignitble Any substnce tht cn be burned Flmmble Cpble of being esily ignited nd of burning quickly Combustion A usully rpid chemicl process tht produces het nd, often, light Combustible A mixture or substnce tht cn be ignited to produce combustion Dust Smll prticles of solid mteril tht cn be suspended in the ir esily Fiber Thred-like mteril Flyings Smll metl prticles produced during mchining, turning or grinding in metlworking Pge 3 of 20
1.0 Introduction The concept of intrinsic sfety in wiring recognizes tht ignitble, flmmble or combustible mterils will be present with ir, or other oxidizer, in sufficient quntity nd rtio so s to represent fire or explosion hzrd. These mixtures could esily be ignited by mtch or other open flme, but just s esily could be ignited by highenergy sprk. Consequently, the wiring used in res where these mixtures re present cn be implemented in fshion to bsolutely preclude ny possibility of igniting these mixtures. Tht is the essence of intrinsic sfety! Intrinsiclly sfe wiring will never hve enough energy vilble within the hzrdous defined re to ignite ny explosive or combustible mixture. This concept is pplied to mixtures of gsses, dusts, or metls. Where it is impossible to reduce the electricl circuit energy, s with electric motor power, then physicl isoltion of the circuits from the hzrdous tmosphere, dust or metls is implemented. This is the principle behind explosion-proof wiring. Even if the circuit did ignite quntity of hzrdous mixture tht intruded into the explosion-proof wiring, the wiring continment cn contin the resulting explosion. Then, ny escping hot gsses will be cooled sufficiently so tht they would be incpble of igniting ny of the hzrdous mixture outside of the explosion-proof conduit or fixtures. 1.1 The fire tringle Fuel nd oxidizer must be present in concentrtion pproprite to form combustible mixture. The ignition source must supply enough energy to initite combustion. It need not be sprk or flme. Temperture lone cn supply the energy of initition. The fire tringle is tool to illustrte tht ll three must be present to hve fire or explosion. If ny one of the elements of the tringle is not present in sufficient mount, then combustion cnnot occur. The vilbility of energy, by either therml or electricl mens, cn cuse the ignition of combustible mixture. The energies required to ignite vrious groups of combustible substnces hve been proven by experimenttion. Grphs of this dt hve been produced, nd cn be used to indicte sfe levels of energy. Pge 4 of 20
A very smll mount of energy is required to cuse n ignition. For exmple, mixture of hydrogen in ir requires only 2O µjoule of energy. In electricl circuits the mechnism for the relese of this ignition energy is often sprk from circuit wiring fult tht cretes gp in the wire llowing sprk to form. Electricl components nd equipment with hot surfces lso cn cuse ignition. The fuel component cn be lmost ny substnce. Most mterils will burn if given the right temperture nd pressure conditions. Fire is simply n oxidtion process. Some oxidtion processes proceed t slow pce while fire is rpid oxidtion process. Steel rusts over period of dys, weeks, nd months. If steel is finely grnulted, plced in pure oxygen environment, nd then exposed to high tempertures, it will burn rpidly; lmost like it ws col. Flour in bkery cn do the sme thing s cn the dust produced from grin stored in grin elevtor. While the oxidizer in most fires is oxygen, other chemicls my be oxidizers. For exmple, elementl mgnesium will rect violently with wter to relese het. The mgnesium is the fuel nd wter provides the oxidizer. Certin fuels, when combined with ir, cn form n explosive mixture. The min difference between rpid fire nd n explosion is tht n explosion cretes pressure wve due to the rpid production of hot gs volume. Tht pressure wve is wht is responsible for the "bng" ssocited with n explosion. The explosive pressure wve cn cuse serious dmge to fcilities nd humns. 1.2 Fire loss control The concepts embodied in the fire tringle hve been codified by vrious orgniztions. In the United Sttes, one of the erliest orgniztions estblished ws the predecessor of FM Globl. In 1835 Zchrih Allen, Rhode Islnd ntive nd prominent textile mill owner, set out to reduce the insurnce premium on his Rhode Islnd mill by mking property improvements tht would minimize the chnce of fire loss 1. Although widely ccepted tody, the concept of loss control ws virtully unherd of t the time, but to Allen, proctive pproch to preventing losses before they occurred mde good 1 http://www.fmglobl.com/corporte_info/history.html, April 2003 Pge 5 of 20
economic sense. As Allen predicted, proper fire prevention methods, monitored by regulr fire inspections for mill policyholders, resulted in fewer losses. As time went on, more nd more compnies relized the finncil benefits of hving their properties defined by their insurers s highly protected risks, or HPR properties. The mutul insurnce compnies worked with their policyholders to help them reduce their fire risks. One of the core methods used to do this ws n nlysis of the cuse of the loss. Lessons lerned were then pplied to fcilities of other policyholders to help them lessen the risk of fires. Eventully, the mutul insurnce originlly provided just to textile mills owners ws expnded to include most other types of industril fcilities. As other types of industril fcilities becme insured, the principles of loss control were pplied to those compnies. Insurers, tody, regulrly require fcilities to use equipment tht hs been certified to comply with generlly ccepted stndrds for risk reduction. Crrying one or nother of mny such mrks identifies equipment meeting such stndrds. Exmples re Underwriters Lbortories, Inc. (UL), Fctory Mutul (FM), nd Cndin Stndrds Orgniztion (CSA). 1.3 The Ntionl Electricl Code By the end of 1895, there were five different recognized stndrds in the United Sttes tht ddressed the sfe use of electricl equipment. Five different codes ment five different sets of rules for mking n electricl instlltion. This, of course, creted significnt confusion nd controversy. Something hd to be done to produce ntionl code on ntionl scle. On quest for solidrity, severl ntionl orgniztions held meeting in New York on Mrch 18, 1896, nd nmed itself the "Joint Conference of Electricl nd Allied Interests", to be chired by W. J. Hmmer. At this conference the five Americn Codes, together with the Germn Code, the Code of the British Bord of Trde, nd the Phoenix Rules of Englnd, were discussed nd referred to committee with Professor Frncis B. Crocker of Columbi University ppointed s the chir. The committee selected the most suitble criteri from ll the vrious codes, nd fter printing drft code, it ws sent to 1,200 interested individuls in North Americ nd Europe for comment. The conference met gin in My nd June of 1897 nd estblished n electricl code tht ws unnimously pproved t the June meeting s the "Ntionl Code." Becuse it ws so fir nd brod in its ppliction, it ws dopted without dely by the Ntionl Bord of Fire Underwriters in lieu of its own, nd then Pge 6 of 20
issued by them s the "Ntionl Electricl Code of 1897". And thus, the "NEC " ws born. 2 In the United Sttes tody, the Ntionl Fire Protection Assocition (NFPA) publishes NFPA 70, Ntionl Electricl Code, lso known s the NEC. The NFPA does not police or enforce complince with the NEC. Neither do they certify, test or inspect products, designs, or instlltions for complince. However, most sttes nd loclities within the U.S. cite the NEC s the uthority controlling electricl instlltions. 2.0 Hzrdous res nd clssifictions 2.1 Clss, Division nd Zone NEC Article 500 is entitled "Hzrdous (Clssified) Loctions, Clsses I, II, nd III, Divisions 1 nd 2." Articles 500 through 516 enumerte the vrious clssifictions nd stndrds pplicble to hzrdous loctions in the United Sttes. Article 505, in prticulr, llows the use of the "Zone" system for flmmble gsses, vpors, or liquids. Consequently, the Zone system does not pply to dust, fiber, or flyings. Tble 2.1.1 - Clss loction definitions Tble 2.1.2 Are loction comprison, Clss I NEC Zone versus Division loctions 2 http://www.nfp.org/itemdetil.sp?ctegoryid=524&itemid=18295&url=publictions/necdigest/about%20the %20NEC /History, April 2006 Pge 7 of 20
Hzrdous loctions re determined by the type of combustible mteril present, the extent of time it is present, nd the physicl construction of the re where such mteril is present. The presence of flmmble gsses or vpors in quntities sufficient to produce n explosive or ignitble mixture constitutes Clss I loction. A Clss II loction is chrcterized by combustible dust. Clss III loctions hve esily ignitble fibers or flyings, but not suspended in the ir in quntities sufficient to produce n ignitble irborne mixture. Tble 2.1.3 Apprtus grouping Division 1 loctions, in generl, re those res where ignitble or flmmble concentrtions of combustible mterils exist continuously or repetedly during norml opertions. Division 2 loctions, in generl, re those res where such mterils exist in ignitble or flmmble concentrtions only during periods of bnorml operting conditions. Zone 0 loctions, in generl, re those res where ignitble or flmmble concentrtions of combustible mterils exist continuously or for long periods of time. Zone 1 loctions, in generl, re those res where ignitble or flmmble concentrtions of combustible mterils re likely to or frequently exist during norml opertions. Zone 2 loctions, in generl, re those res where ignitble or flmmble concentrtions of combustible mterils re not likely to occur during norml opertions or will exist for only brief period of time. For ll of these loctions there re lso vrious Tble 2.1.4 Temperture code/clss for pprtus Pge 8 of 20
groups of gsses, vpors, dusts or fibers. The groups hve been estblished bsed on the ignition energy required for ech of the constituents within tht group. The tble below illustrtes typicl mterils within ech group nd the group identifiction. All flmmble mterils hve n ignition temperture. Even if the mteril is not exposed to n open flme or sprk of certin temperture, they will ignite if they re exposed to n object whose temperture exceeds the ignition temperture for tht mteril. All pprtus designed for instlltion in hzrdous res re rted for their mximum surfce temperture. Consequently, ll pprtus hve temperture code ssocited with their hzrdous re clssifiction. The tble below reflects the vrious temperture codes. 2.2 Methods of protection There re four bsic principles to provide protection for hzrdous re electricl circuit wiring. Prevent rcs, sprks nd hot surfces; prevent the combustible mteril from entering the spce; contin ny explosion of combustible mteril within the electric enclosure; or limit the energy vilble for sprks nd hot surfces. Methods of protection nd their permitted use res re summrized in tble 2.2.1. Tble 2.2.1 Protection methods nd permitted use for NEC These protection methods re the methods permitted. NEC Article 500.8 (A) (1) (1) sttes tht the suitbility of identified equipment shll be determined from (1) the equipment listing or lbeling, (2) evidence of equipment evlution from qulified testing lbortory or inspection gency concerned with product evlution, or (3) evidence cceptble to the uthority hving jurisdiction such s mnufcturer's self-evlution or n owner's engineering judgment. Subprgrph (2) sttes tht equipment tht hs been identified for Division 1 loction shll be permitted in Division 2 loction of the sme clss nd group. Pge 9 of 20
2.2.1 Intrinsic sfety The Ntionl Electric Code defines n intrinsiclly sfe circuit s circuit in which ny sprk or therml effect is incpble of cusing ignition of mixture of flmmble or combustible mteril in ir under prescribed test conditions. It lso defines n intrinsiclly sfe system s one tht is n ssembly of interconnected intrinsiclly sfe pprtus, ssocited pprtus, nd interconnecting cbles in tht those prts of the system tht my be used in hzrdous (clssified) loctions re intrinsiclly sfe circuits 3. Intrinsiclly sfe pprtus hve been tested to meet these requirements. The testing of the pprtus is designed to verify the operting prmeters of the device nd set the limits for its use in hzrdous res. In the typicl intrinsiclly sfe system for vibrtion trnsducers, the trnsducer is n pproved device with n intrinsiclly sfe circuit nd is wired through n pproved zener brrier device to the non-hzrdous re. All wiring between the trnsducer nd the brrier must lso meet the requirements for hzrdous (clssified) re instlltion. Zener brrier devices limit the vilbility of energy to the wiring in the hzrdous re. The figure here illustrtes the internl schemtic of bsic zener brrier. The zener diode in the center of the circuit cts to clmp the voltge vilble to the circuit in the hzrdous re. Zener diodes re used becuse they hve very high resistnce until the voltge rises to their conduction voltge. Once in conduction the diode "clmps" the voltge to mximum vlue. It is this property of the zener diode tht is exploited for use in Figure 2.2.1 - Bsic zener brrier schemtic instrumenttion circuits in hzrdous res. The resistor, R, limits the mximum current vilble to the hzrdous re circuit wiring. Since the voltge nd the current re limited, the power is, therefore, lso limited. 3 NEC Article 504.2, NFPA, 2002 Pge 10 of 20
As n dditionl sfety method, the brrier device lso contins fuse. The fuse will ct to limit the current through the brrier circuit in the event tht either the resistor or the zener should fult. WARNING: Wilcoxon trnsducers tht crry pprovls for intrinsiclly sfe circuits must lwys be instlled with sfety brriers in the circuits for Clss I Division 1, Zone 0 or Zone 1 res. Simply buying trnsducer tht crries n intrinsiclly sfe rting is not sufficient for permnently instlled vibrtion trnsducers. Portble vibrtion nlyzers or dt collectors used within hzrdous (clssified) res must lso be rted for use within those res. 2.2.1.1 Intrinsiclly sfe pprtus entity prmeters Erlier it ws explined tht n intrinsiclly sfe system is composed of ssembly of interconnected intrinsiclly sfe pprtus, ssocited pprtus, nd interconnecting cbles. In the cse of vibrtion sensors, the intrinsiclly sfe pprtus is the sensor itself. The ssocited pprtus is the sfety brrier. When the sensor is pproved s intrinsiclly sfe for use in hzrdous res, there re certin limittions plced upon it. The sensor cnnot be connected to "ny old" sfety brrier. Rther, the brrier must hve intrinsiclly sfe rtings tht re comptible with those of the sensor. Apprtus hve four rting prmeters: Voltge, Current, Cpcitnce, nd Inductnce. The open-circuit voltge vilble t the terminls of the brrier is V oc, or V t. The shortcircuit current vilble t the terminls of the brrier is I sc, or I t. The mximum cpcitnce tht cn be connected to the brrier pprtus is C while the mximum inductnce tht cn be connected is L. The vibrtion sensor voltge rting, V mx, is the mximum voltge tht cn be pplied to the terminls of the sensor. The current rting, I mx, is the mximum current tht cn be pplied through the terminls of the sensor. The vlue of internl cpcitnce, C i, nd inductnce, L i, re lso stted. When the sensor nd brrier re connected together in system, the cble cpcitnce, C cble, nd inductnce, L cble, must lso be considered in the system. Pge 11 of 20
By compring the rtings of the vibrtion sensors with those of the brrier nd tking the cble vlues into ccount, n pproprite sfety brrier cn be selected. As long s the rtings stisfy the following equtions, the system will meet the requirement for n intrinsiclly sfe system. V oc (or V t ) must be equl to or less thn V mx I sc (or I t ) must be equl to or less thn I mx C must be greter thn or equl to C i + C cble L must be greter thn or equl to L i + L cble V oc V mx I sc I mx C C i + C cble L L i + L cble Exmple The rtings for the 793E intrinsiclly sfe rted ccelerometer re: V mx = 30 Vdc, I mx = 180 ma, C i = 0.03 µf, nd L i = 0.00 mh nd it limits the cble cpcitnce to 0.07 µf. An MTL brrier such s the 7087 hs the following prmeters: V oc = 28 Vdc, I sc = 92 ma, C = 0.13 µf, nd L = 4.2 mh. If these devices re connected using 200 feet of the Wilcoxon J9T2A cble, it hs cpcitnce of 30 pf per foot. The cble cpcitnce is then 200 times 30 pf, equling 0.006µF. Now, let's put the numbers into the equtions to see if these two devices re comptible for use s n intrinsiclly sfe system. The equtions, gin, re stted here nd the numbers substituted into the eqution. If ll the equtions re stisfied s "True" the two devices cn be used s n intrinsiclly sfe system. V oc V mx 28 V 30 V True I sc I mx 92 ma 180 ma True C C i + C cble 0.13 µf 0.03 µf + 0.006 µf True L L i + L cble 4.2 mh 0.00 mh + 0.00 mh True Pge 12 of 20
2.2.2 Explosion proof The principle behind explosion-proof trnsducers nd wiring is tht ny ignition of flmmble mteril tht occurs within the trnsducer or wiring will be contined. The hot gsses nd flmes will not be llowed to escpe into the hzrdous re nd further propgte the fire or explosion. All circuit wiring is run in conduit nd junction boxes pproved for explosion-proof instlltion. Explosion proof trnsducers nd wiring must be instlled ccording to ANSI/UL 1203-1994, Explosion-Proof nd Dust-Ignition-Proof Electricl Equipment for Use in Hzrdous (Clssified) Loctions. 2.2.3 Air purge When instlltions re not explosion proof or intrinsiclly sfe, pressuriztion is often used to mintin the clssified re sfety. Wiring nd enclosures re protected using positive ir pressure mintined within the enclosure, junction boxes nd conduit. The concept of pressuriztion protection is covered under ANSI/NFPA 496-1998, Purged nd Pressurized Enclosures for Electricl Equipment. 2.2.4 Other methods Flmeproof (d), incresed sfety (e), encpsultion (m), oil immersion (o), dust ignition proof, dust tight, powder filled (q), non-sprking (n), non-incendive, nd hermeticlly seled re other levels of protection. Ech is used where pproprite for electricl circuit wiring. Mny of them pply to AC powered circuits, but few re used for instrumenttion circuit wiring. 2.3 Clss I, Division 2 loctions In the United Sttes, section 500 of the NEC specificlly cites hermetic seling s protective method llowed for Clss I, Division 2 res. Hermeticlly seled ccelerometers cn be instlled in Clss I, Division 2 loctions. Accelerometers do not hve to be specificlly mrked s suitble for Division 2 loctions. Generl purpose equipment tht is operted in Clss I res must be mrked with temperture code if they hve surfces tht rise bove 100 C when operting in n mbient temperture of 40 C. Since Wilcoxon ccelerometers do not rise bove 100 C in n mbient temperture of 40 C, they do not require mrking. Pge 13 of 20
The governing judgment s to the cceptbility of ccelerometers for instlltion is in section 500.8. It cites the following s n item of guidnce for determining the suitbility of equipment for instlltion, "Evidence cceptble to the uthority hving jurisdiction such s mnufcturer's self-evlution or n owner's engineering judgment." ANSI/ISA 12.12.01 2000, Nonincendive Electricl Equipment for Use in Clss I nd II, Division 2 nd Clss III, Divisions 1 nd 2 Hzrdous (Clssified) Loctions sttes in the Scope, prgrph 2.5, "This stndrd is not intended to cover equipment for use in Clss I nd Clss II, Division 1 loctions, such s equipment constructed to be intrinsiclly sfe, dust ignition-proof, or explosion-proof. Such equipment is, however, suitble for use in Clss I nd Clss II, Division 2 loctions in the sme group for which it is suitble in Division 1." 2.4 Wiring methods 2.4.1 Intrinsiclly sfe circuits The NEC Article 504 controls the wiring of intrinsiclly sfe circuits. It generlly requires tht intrinsiclly sfe circuits be physiclly seprted from nonintrinsiclly sfe circuits. Conductors nd cbles of intrinsiclly sfe circuits not in rcewys or cble trys shll be seprted t lest 50 mm (2 inches) nd secured from conductors nd cbles of ny non-intrinsiclly sfe circuits. Conductors of intrinsiclly sfe circuits cn only be plced in the sme rcewy or cble try with non-intrinsiclly sfe circuits when the intrinsiclly sfe circuits re seprted by distnce of 50 mm (2 inches) or by grounded metl prtition or pproved insulting prtition. The 50 mm seprtion of circuits lso pplies to the wiring within enclosures. Intrinsic sfety brriers hve their input nd output terminls seprted by 50 mm becuse of the spcing requirement. The input wiring of the enclosure where the brriers re instlled must mintin the required 50 mm physicl seprtion between the intrinsiclly sfe circuits nd the non-intrinsiclly sfe circuits. Most brrier mnufcturers offer specil mounting hrdwre to lbel the circuits nd keep the isoltion required between the input nd output wiring. Pge 14 of 20
2.5 Mrking exmples Intrinsiclly sfe rted trnsducers will hve the informtion regrding the Clss, Division or Zone, Group, nd Temperture rtings engrved directly on the sensor cse. Illustrted here is the engrving from Wilcoxon Reserch 793E ccelerometer. The engrving lso indictes the instlltion control document number (WR 90546) shipped with ll intrinsiclly sfe sensors. 3.0 Wiring Wilcoxon intrinsiclly sfe sensors 3.1 Dynmic vibrtion sensor wiring The 793-series, 797-series, nd 766-series vibrtion sensors hve pprovls for hzrdous re intrinsiclly sfe instlltion. Accelertion output, velocity output, nd low frequency cpbility sensors re vilble within these series of sensors. These sensors re ll considered Integrted Electronic Piezoelectric (IEPE) sensors. They re powered using constnt-current diode or similr Field Effect Trnsistor (FET) bsed powering system. 3.1.1 Effects of brrier on bis output voltge IEPE sensors hve n output tht is combintion of the DC bis output voltge (BOV) nd the superimposed vibrtion signl. The BOV is controlled by the sensor design nd exists s long s the current source provides dequte current. The sensor generlly needs t lest 1 ma of current. It is common to use current vlues of 2 to 4 ma for the constnt-current source. When IEPE sensors re used in intrinsiclly sfe circuits, the effects of the zener sfety brrier must be considered in the sensor operting circuit. It is possible to use brrier tht provides sfe circuit, but precludes the proper opertion of the sensor. Figure 3.1.1.1 - Brrier schemtic Illustrted here is the pckge outline for the MTL 7778c brrier. It hs four wiring connections. Terminls 1 & 2 re the sfe re connections nd terminls 3 & 4 re the hzrdous re terminl connections. An intrinsiclly sfe rted ccelerometer, such s the Wilcoxon 793E, would be connected Pge 15 of 20
to terminls 3 & 4. This digrm illustrted the erth ground of the brrier in the center of the rry of Zener diodes. The 793E will be connected cross terminls 3 & 4 in the circuit. The mnufcturing specifiction of the 793E clls for its BOV not to exceed 13.5 volts. If the 793E is powered using constntcurrent diode vlue of 4 ma, then the resistors will ech cuse voltge drop of 2.4 volts (600 x.004). The totl voltge drop cross the brrier nd the 793E could then be s high s 18.3 volts (13.5+2.4+2.4) nd will pper to be the BOV of the ccelerometer s viewed cross terminls 1 & 2 of the brrier. This is the effective mximum Figure 3.1.1.2 - Circuit voltges BOV tht would be seen by n nlyzer connected to this circuit nd using powering current of 4 ma. Since the ccelerometer should llow for mximum voltge to be 2 volts less thn the open-circuit supply voltge, BOV of 18.3 volts mens tht the dynmic signl cn only go 3.7 volts positively before entering this "forbidden zone" of opertion. Tht mens the effective positive dynmic signl is limited to 37g's (3.7 volts @ 100 mv/g). If the 793E hd its nominl BOV of 12 volts, then the BOV seen t the output of the MTL7778c would be bout 16.8 volts nd the positive-going dynmic rnge would be 5.2 volts, or bout 52g's. The digrm illustrted here shows the wiring for the 793E ccelerometer. Shielded, twisted pir wire is used throughout. The connector t the ccelerometer must connect Figure 3.1.1.3-793E wiring exmple the shield of the shielded, twisted pir wire to the cse of the 793E. However, the shield Pge 16 of 20
should not be connected to ground t the brrier or n electricl ground loop will be creted in the shield circuit. The shield on the output of the brrier should lso not be connected t the brrier for the sme reson. The brrier, itself, will be connected to n erth ground s required by the NEC. 3.1.2 Troubleshooting Improper wiring of Wilcoxon sensors will result in loss of signl or excessive noise in the signl. There re some simple steps tht cn verify proper instlltion. All troubleshooting described here using n ohmmeter is conducted with the circuit "deenergized" so s not to confuse the redings. If the proper reding is not indicted, the user must tke ction to determine why tht connection is not correct. The most common wiring fults re reversed wires nd ground loops. Reversed wires re where the power/signl nd the common wire get reversed. Ground loops re formed when there is more thn one circuit pth to ground. The shields should connect to ground t only one point nd tht must be verified. Wilcoxon hs specil connectors tht will connect the shield to the cse of 2-pin sensor. These re the R6Q nd the R6SL connectors. These connectors re designed with conductive spring inside the shell of the connector tht mkes contct with the neck of the connector to provide n electricl connection of the shield to the cse of 793-series or 797-series sensor. The 766-series sensors hve third pin on their connector (pin "C") tht internlly connects to the cse of the 766. Wilcoxon cbles re wired with blck wire in ll shielded, twisted pir cbles. The blck wire is the common in the circuit nd connects to pin "B" of the connector of the sensor. The wire for the "power/signl" connection will either be red or white nd connects to pin "A" of the sensor. Both the "A" nd "B" pins of the sensor re isolted from ground. Ohmmeter redings between either pin "A" nd the cse or pin "B" nd the cse should give high ohmic reding (over 10 MΩ). On the 766, the reding between pin "C" nd the cse should be less thn 1Ω. With the sensor instlled on the mchine, the cble connected nd the output terminted in the junction box or dt cquisition system, the integrity of the shield/ground connection cn be tested. The sfety brrier will be grounded to good erth ground (s required by the NEC ) nd the shields cn be tested there to verify they connect to ground. The resistnce between ech wire's shield nd the ground t the brrier should be less thn 10Ω. Verify tht the shields re not connected to ny circuit or ground t the Pge 17 of 20
brrier. Then, go to the sensor nd remove the connector. The resistnce between the shield nd the mchine cse should be over 10 MΩ. Re-connect the cble to the sensor. At the junction box or dt cquisition equipment, disconnect the wire (plug) from the input pnel. The resistnce between the shield nd the ground connection should be over 10 MΩ. Re-connect the cble to the input. 3.2 4-20 ma loop powered sensor (LPS) wiring The wiring for intrinsiclly sfe loop powered sensors (such s Wilcoxon models PC420Axx-yy-IS nd PC420Vxx-yy-IS) will be the sme. The connector t the LPS should be n isolted type of connector where the shield does not connect to the cse. The Wilcoxon R6W, R6QI, or R6SLI cn be used. They ll hve the shield isolted from the trnsducer cse t the sensor connection. Figure 3.2 LPS Series wiring In the instlltion of the LPS intrinsiclly sfe units it is importnt to void ground loops in the shield circuit wiring. The illustrtion here shows how the shields should be connected to void the possibility of ground loops. The shield t the LPS should be isolted from connecting to the cse. The shield of the cble from the LPS should connect to ground t the brrier. The signl output cble from the brrier should not connect to ground t the brrier, but must be connected to ground t the PLC or DCS. Pge 18 of 20
Wilcoxon recommends n MTL7087+, or equivlent, sfety brrier. The MTL7087+ brrier hs the following prmeters: V oc = 28 Volts, I sc = 93 ma. The LPS Series trnsducers hve the following prmeters: V mx = 30 Vdc, I mx = 106 ma. The LPS Series trnsducer nd the ssocited sfety brrier must stisfy the following equtions to be used together. Eqution LPS compred to 7087+ Result V oc V mx 28 V 30 V True I sc I mx 93 ma 106 ma True Since the reltionship between the prmeters stisfy the equtions, the MTL7087+ cn be used with the LPS Series trnsducers in Clss 1, Division 1 (or Zone 0), Group A gs hzrdous re instlltion. However, the user must lso follow the guidelines for the 7087+ s to the mximum llowble cpcitnce in the cble. The 7087+ lists the mximum cpcitnce s 0.13 µf. The Wilcoxon J9T2A cble hs 0.00003 µf per foot of cble. Therefore it would tke more thn 4,000 feet of J9T2A cble to exceed the cpcitnce limits imposed by the 7087+. 3.2.1 Troubleshooting the 4-20 ma LPS Wiring WARNING: All troubleshooting work performed on n intrinsiclly sfe wiring instlltion in hzrdous re should be conducted under "Hot Work" permit under the guidelines for permits in your plnt. If dignostics re performed using "Cold Work" permit, the Multimeter used for circuit mesurements MUST BE rted for use in hzrdous re. An exmple of the loop wiring using n MTL7087+ is illustrted here in figure 3.2.1.1. The shield of the hzrdous re wiring connects to terminl "6" of the 7087+. Check the shielding in mnner similr to tht in the dynmic sensor section of this document. Figure 3.2.1.1 - MTL7087+ wiring illustrtion Figure 3.2.1.2 illustrtes tht terminl "6" is directly connected to the erth ground through the 7087+ brrier. The +24 Volt DC power for the LPS trnsducer connects to Pge 19 of 20
terminl "1" nd the power connection to the trnsducer if off terminl "4". The return signl from the LPS trnsducer connects to terminl "5". Between terminl "5" nd "2" is diode tht restricts the signl to llow current flow only from the hzrdous re, but not to flow to the hzrdous re. If the power nd return were ccidentlly switched t the input to the brrier (power to "2" nd return to "1", there would be no current flowing in the circuit. The loop current would be "zero." Likewise, if the wiring to the LPS is reversed (power to pin "B" nd return to pin "A") there will be no current llowed to flow in the circuit due to the protective diodes instlled in the LPS Series intrinsiclly sfe trnsducers. Figure 3.2.1.2-7087+ schemtic 4.0 Technicl ssistnce 4.1 Technicl nd ppliction ssistnce For technicl or ppliction ssistnce, plese contct Wilcoxon s Appliction Engineering t 301-330-8811, FAX to 301-330-8873, or emil to techsst@wilcoxon.com. 4.2 Customer service To obtin dditionl technicl reports or sles support, plese contct customer service t 301-330-8811, or fx to 301-330-8873. Pge 20 of 20