Arc Flash Basics: Testing Update - Mersenep-us. Flash Basics: Testing Update M. Lang and T. Neal 374MerrimacSt,Newburyport,MA01950, @ ... predicted by current IEEE 1584 equations when

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  • Arc Flash Basics: Testing Update

    M. Lang and T. Neal

    374 Merrimac St, Newburyport, MA 01950, USA mike.lang@mersen.comNeal Associates Ltd., 106 Leetes Island Road, Guilford, CT 06437, USA nealassoc@earthlink.net

    Abstract: Arc flash hazard calculations used to predict the magnitude of the heat hazard are based on tests withopen-tip vertical electrodes in enclosures. Arc ratings for PPE are developed using opposing verticalelectrodes. Tests with electrode configurations that forced the arc plasma jets outward from the boxyielded significantly higher heat energy measurements. When placed within this directional plasmaflow, protective flame resistant (FR) fabrics yielded significantly lower arc ratings.

    Keywords: arc flash hazard, current-limiting fuses, electric fuse, current limitation, arc rating, ATPV

    1. Introduction

    The last 15 years have seen tremendous progressbeing made in protecting workers against the heatenergy associated with arc flash. Research into suchinjuries in the United States during the 1990s showed that over 2,000 workers were admitted toburn centers each year. One major area ofimprovement has been the steps taken to get workersinto safer clothing. The arc rating system developedby ASTM and the development of the predictiveequations identified in NFPA 70E and IEEE 1584have been instrumental in this effort.

    At the center of these developments has been arcflash testing. The arc thermal performance value(ATPV) of electrical personal protective equipment(PPE) relies on arc flash tests performed in a highpower test lab. The IEEE 1584 equations weredeveloped empirically from arc flash tests performedin North American test labs from the late 1990sthrough 2002.

    Recent research into arc flash phenomena,however, indicates that workers could be under-protected against the heat generated during an arcflash event. Test results presented at IEEEconferences [1-3] and at the 2007 IEEE ElectricalSafety Workshop show that different configurationsof electrodes (conductors) yielded heat energy higherthan current predictions due to the directional natureof the arc development. Additionally, initial tests ofPPE, when placed within this directional plasmaflow, did not provide the level of thermal protectionpredicted by its Arc Rating as determined by theASTM test method.

    2. Directional nature of arc development

    Unrestricted high-current arcs move according tomagnetic forces to increase the area of the currentloop. Currents flowing in the opposite direction inparallel conductors give rise to forces that drive thearc away from the source to the end of the conductors

    where they typically burn off the tips of electrodes(busbars).

    The behaviour of a 3-phase arcing fault inequipment is very chaotic, involving rapid andirregular changes in arc geometry due to convection,plasma jets and electromagnetic forces. Arcextinction and re-ignition, changes in arc paths due torestriking and reconnection across electrodes andplasma parts and many other effects add to thischaotic nature and make it difficult to createequations for accurate predictions of its properties(e.g. impedance). Although it does not capture thischaotic behaviour, Fig. 1 demonstrates an arcs general directional nature. The alternating 3-phasecurrent creates successive attractive and repulsivemagnetic forces, dramatically moving the plasma jetswhich feed an expanding plasma cloud. The cloud isdriven outward, away from the tips, creating plasma dust as the highly energized molecules in the plasmacool, and recombine into various materials. Themolten electrode material ejected off the tips also isin this flow.

    Fig. 1: The general directional nature of an arc; this depictiondoes not reflect chaotic behaviour.

    3. Arc flash hazardsWhen the arc is being established, current beginspassing through ionized air generating massive

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  • quantities of heat. Large volumes of ionized gases,along with metal from the vaporized conductors, areexplosively expelled. As the arc runs its courseelectrical energy continues to be converted intoextremely hazardous energy forms. Hazards includethe immense heat of the plasma, radiated heat, largevolumes of toxic smoke, molten droplets ofconductor material, shrapnel, extremely intense lightand a pressure wave from the rapidly expandinggases.

    Recent tests have shown that an object in theexpanding plasma cloud (refer to the red object inFig. 1) is directly exposed to the highest heat of theevent. Temperatures greater than 15,000 C have beencited for this area. In addition to the convective heattransfer from the plasma, this object is directlyexposed to the molten metal ejected from theelectrode tips and radiated heat from surroundingplasma.

    Objects close to the arc but outside of the plasma jets(refer to the green object in Fig. 1) are not likelysubjected to as high a quantity of heat. Exposure ispredominately radiant heat, but includes convectiveflow from the thermal expansion of the gases.Objects in line with the electrodes but distant fromthe plasma jets (refer to the blue object in Fig. 1)receive lower convective heating and less radiantheat and molten metal spray.

    The amount of heat absorbed varies with themethod of heat transfer and receiving surfaceproperties. For example, the amount of heattransferred from a mass of molten copper to a surfacearea would be greater if it adhered to the objectinstead of contacting it for a brief time.

    4. Current test setups used for standards

    Although the overriding principle of electricalsafety is to de-energize equipment and place it intoan electrically safe condition prior to work, there arenumerous cases where companies put workers in PPEto perform tasks on energized equipment. Thestandards typically utilized to predict the magnitudeof heat exposure and the protective ability of flameresistant (FR) fabric worn by exposed workers arebased upon two unique electrode configurations intheir test procedures as explained below. Heattransferred during tests with these orientations ismost likely dominated by radiant heat.

    4.1. NFPA 70E

    First issued in 1979, NFPA 70E, Standard forElectrical Safety in the Workplace [4] covers the fullrange of electrical safety issues, from work practicesto maintenance, special equipment requirements andinstallation. In the 1995 edition, arc flash hazardswere first addressed with the addition of "arc flash

    hazard boundaries," with the equations based on arcsin open air. Arc-in-a-box equations were added to the 2000 edition as options to calculate a workers potential heat energy density exposure. Theseequations came from results of arc flash tests with asteel box and vertical electrodes with open tips [5-6]as shown in Fig. 2. The 2004 edition added the IEEE1584 equations below.

    Fig. 2: Open-tip vertical electrode configuration

    4.2. IEEE 1584

    Issued in 2002, IEEE 1584TM-2002, Guide forArc Flash Hazard Calculations, [7] providesguidelines for an analysis to "identify the flash-protection boundary and the incident energy atassigned working distances throughout any positionor level in the overall electrical system." The resultsfrom over 300 arc flash tests were incorporated intothe low-voltage predictive equations for enclosedequipment contained within IEEE 1584. Threeenclosure sizes were used in these tests, but all testsalso used the vertical electrodes with open tips shownin Fig. 2.

    4.3. ASTM 1959

    The current edition of ASTM F1959, StandardTest Method for Determining the Arc Rating ofMaterials for Clothing, [8] uses a single phaseopposing electrode vertical orientation. This standardtest method determines the arc rating of material usedin arc rated PPE. The test procedure places materialsin locations surrounding the area where the open airarc is initiated. The majority of the heat transferred tothe material is likely radiated from the arc. This openair arrangement from the 1980s would simulateflashovers on overhead power systems.

    5. Effects on heat measurements withalternate test configurations

    Research performed at Ferraz Shawmuts High Power Test Laboratory has uncovered electrode

  • configurations that project significantly more heatenergy out of enclosures toward worker locationsthan currently predicted by the standards. Tosimulate components found in low-voltage electricalequipment, various setups were created for controlledtesting. Heat was measured and compared withpredictions of IEEE 1584 for switchgear. Results ofthese comparisons were published in two recentIEEE papers. [2-3] Configurations that forced thearcs plasma jets outward toward the workerproduced heat measurements nearly twice thosepredicted by current IEEE 1584 equations whenstudied at typical working distances of 18 inches.

    All arrangement described below are variations ofan arrangement described in IEEE 1584 [7]. Thesetest setups used a 508mm x 508mm x 508mm steelbox with one side open. 3-phase arcing tests wereconducted at 208V, 480 V and 600V. The gapbetween electrodes was 32mm and the distancebetween the electrodes and the back of the box was102mm. Incident heat energy was measured with anarray of 9 copper calorimeters as described in theIEEE 1584 test procedure. Photographs of the arcswere captured from video taken with a FASTCAMhigh-speed camera, at up to 10 000 frames per second.The station back up breaker was typically set at 6cycles to limit arc duration.

    5.1. Vertical Configuration

    In the vertical configuration setup used in theIEEE1584 test program, the electrodes entered thebox from the top. The electrode tips were open and254 mm from the bottom of the box. This setupsimulates equipment where bussing is vertical andopen-ended such as a main-lugs power panel.

    The arc development, similar to that described forFig. 1, will be downward toward the bottom of thebox in this case. As described in [2], there is anoutward convective flow due to the thermalexpansion of the gases and not magnetic forces.Photo of the arc development is shown in Fig.3. Mosttests resulted in heat measurements consistent withthe predicted levels of IEEE1584.

    Fig. 3: Front view of arc development from vertical test 3msinto event.

    5.2. Barrier Configuration

    In the barrier configuration, the electrodes of thevertical setup are terminated into a block of

    insulating material (barrier) as shown in Fig. 4. Thissetup represents conductors connected to equipmentfed from the top.

    Fig. 4: Barrier test configuration simulating the line sideconnections of top fed equipment.

    With the barrier in place, the arcsdownwardmotion is halted and plasma jets are formed along theplane of the barrier top surface (i.e. perpendicular tothe plane of the electrode). [3] This significantfinding is demonstrated in Fig. 5. The photo on thetop shows a side view of arc development along theplane of the barrier in a setup without side panels.The photo at the bottom shows the same test butrecessed in the box. This test shows the possibility ofhigher convective heat transfer toward workers thanthe open vertical setup. The barrier configuration alsoejected significantly more molten electrode material.

    Fig. 5: Side view of arc development from barrier tests4ms into a 42kA, 480V event. Open configuration(Top) and in a enclosure (Bottom))

  • Chart 1 compares heat measurements with thebarrier setup to standard predictions. The black linerepresents predictions of IEEE 1584 equations forswitchgear (508mm cubic box) for the available faultcurrents with a fixed 6-cycle clearing time.Alarmingly, the barrier test results almost alwaysrose above the line--sometimes more than twice theprediction.

    Chart I: Comparison of barrier results to predictions of IEEE1584 equations for switchgear. The black line representsprediction; the blue line is 167% of the prediction.

    5.3. Horizontal Configuration

    Another configuration that deserves seriousconsideration is the horizontal electrode configuration. This setup simulates equipment where bussing is open-ended, but pointing toward thefront of the enclosure, like that in the equipmentshown in Fig. 6.

    Fig. 6: The horizontal test setup was designed to simulate rear-fedequipment like this unit.

    When the electrodes are horizontal and fed fromthe back, the arc development is very similar to thatdescribed for Fig. 1 and is shown in Fig. 7. In the toppicture of Fig 7 the electrodes were brought to thefront of the box, clearly showing the plasma jetsformed on the tips of the electrodes. In the bottompicture, the electrodes were moved to 104 mm fromthe back of the box. Although the plasma jets are notvisible there is a greater outward flow, since thewalls of the enclosure give a more focused expansionof the plasma.

    Fig. 7: Side view of plasma flow at 8 milliseconds into a 44kA600V arcing event for horizontal electrodes. Electrode tipsare flush with the front of the box (T) and recessed to104mm from the back of the horizontal setup test box (B).

    Like the barrier configuration, all tests resulted inheat measurements significantly above the predictedlevels. In some case, the incident heat energy densitywas more than three times that of the vertical tests.

    Of equal concern is the fact that arcing currentswere below predicted levels for this configuration(see Chart II). In some applications, clearing timeswill be significantly longer than expected if thearcing current is too low to operate the short circuitelement of the upstream overcurrent protectivedevice (OCPD). In these applications, the increase inarc flash heat energies will be far greater than thedifferences obtained in tests with a fixed clearingtime of six cycles.

    Chart II: Comparison of arc currents from horizontal tests topredictions of IEEE 1584 equations for switchgear.Note that some of the test results were below even the85% value recommended by the standard.

  • 5.4. Tests with current-limiting fuses

    IEEE 1584 has equations to predict incident energyfrom equipment when protected by UL Class RK1fuses [9] and UL Class L fuses [10] with ampereratings of 2000A and less. These equations werederived from 600V tests using the verticalconfiguration of Fig. 2 [7, 11]. Plots of the predictedincident energy for two fuses are shown in Chart III.

    If the arc fault current is large enough for these fusesto be in their current-limiting ranges, the fuses willdramatically reduce the electrical energy delivered tothe arc. Extensive testing has shown that Class RK1fuses and Class L fuses of 1600A and less can limitincident energies to below the 1.2 cal/cm2 (5 J/cm2)critical value accepted for a second degree burn.

    Chart III: Plot of predicted incident energies.

    A number of tests with current-limiting fusesshowed, with proper fuse selection, that workerswould be exposed to far less heat energy even whenstanding in locations subject to the plasma flow fromthe alternate configurations (see Fig. 8). The heatmeasured during tests with all configurations wasvery close to those predicted by c...

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