| Specifications | Microsoft Word - icefa2007_paper_FINAL.doc mlang |
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| Specifications | Microsoft Word - icefa2007_paper_FINAL.doc mlang |
| Business section |
| Specifications | Microsoft Word - icefa2007_paper_FINAL.doc mlang |
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| Content | quantitiesofheat.Largevolumesofionizedgases, alongwithmetalfromthevaporizedconductors,are explosivelyexpelled.Asthearcrunsitscourse electricalenergycontinuestobeconvertedinto extremelyhazardousenergyforms.Hazardsinclude theimmenseheatoftheplasma,radiatedheat,large volumesoftoxicsmoke,moltendropletsof conductormaterial,shrapnel,extremelyintenselight andapressurewavefromtherapidlyexpanding gases. Recenttestshaveshownthatanobjectinthe expandingplasmacloud(refertotheredobjectin Fig.1)isdirectlyexposedtothehighestheatofthe event.Temperaturesgreaterthan15,000Chavebeen citedforthisarea.Inadditiontotheconvectiveheat transferfromtheplasma,thisobjectisdirectly exposedtothemoltenmetalejectedfromthe electrodetipsandradiatedheatfromsurrounding plasma. Objectsclosetothearcbutoutsideoftheplasmajets (refertothegreenobjectinFig.1)arenotlikely subjectedtoashighaquantityofheat.Exposureis predominatelyradiantheat,butincludesconvective flowfromthethermalexpansionofthegases. Objectsinlinewiththeelectrodesbutdistantfrom theplasmajets(refertotheblueobjectinFig.1) receivelowerconvectiveheatingandlessradiant heatandmoltenmetalspray. Theamountofheatabsorbedvarieswiththe methodofheattransferandreceivingsurface properties.Forexample,theamountofheat transferredfromamassofmoltencoppertoasurface areawouldbegreaterifitadheredtotheobject insteadofcontactingitforabrieftime. 4.Currenttestsetupsusedforstandards Althoughtheoverridingprincipleofelectrical safetyistode-energizeequipmentandplaceitinto anelectricallysafeconditionpriortowork,thereare numerouscaseswherecompaniesputworkersinPPE toperformtasksonenergizedequipment.The standardstypicallyutilizedtopredictthemagnitude ofheatexposureandtheprotectiveabilityofflame resistant(FR)fabricwornbyexposedworkersare basedupontwouniqueelectrodeconfigurationsin theirtestproceduresasexplainedbelow.Heat transferredduringtestswiththeseorientationsis mostlikelydominatedbyradiantheat. 4.1.NFPA70E Firstissuedin1979,NFPA70E,Standardfor ElectricalSafetyintheWorkplace[4]coversthefull rangeofelectricalsafetyissues,fromworkpractices tomaintenance,specialequipmentrequirementsand installation.Inthe1995edition,arcflashhazards werefirstaddressedwiththeadditionof arcflash hazardboundaries, withtheequationsbasedonarcs in open air. “Arc-in-a-box” equations were added to the 2000 edition as options to calculate a worker’s potentialheatenergydensityexposure.These equationscamefromresultsofarcflashtestswitha steelboxandverticalelectrodeswithopentips[5-6] asshowninFig.2.The2004editionaddedtheIEEE 1584equationsbelow. Fig.2:Open-tipverticalelectrodeconfiguration 4.2.IEEE1584 Issuedin2002,IEEE1584TM-2002,Guidefor ArcFlashHazardCalculations,[7]provides guidelinesforananalysisto identifytheflash- protectionboundaryandtheincidentenergyat assignedworkingdistancesthroughoutanyposition orlevelintheoverallelectricalsystem. Theresults fromover300arcflashtestswereincorporatedinto thelow-voltagepredictiveequationsforenclosed equipmentcontainedwithinIEEE1584.Three enclosuresizeswereusedinthesetests,butalltests alsousedtheverticalelectrodeswithopentipsshown inFig.2. 4.3.ASTM1959 ThecurrenteditionofASTMF1959,Standard TestMethodforDeterminingtheArcRatingof MaterialsforClothing,[8]usesasinglephase opposingelectrodeverticalorientation.Thisstandard testmethoddeterminesthearcratingofmaterialused inarcratedPPE.Thetestprocedureplacesmaterials inlocationssurroundingtheareawheretheopenair arcisinitiated.Themajorityoftheheattransferredto thematerialislikelyradiatedfromthearc.Thisopen airarrangementfromthe1980swouldsimulate flashoversonoverheadpowersystems. 5.Effectsonheatmeasurementswith alternatetestconfigurations ResearchperformedatFerraz Shawmut’s High PowerTestLaboratoryhasuncoveredelectrode |
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