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A historical perspective and comparative studyof the standards IEEE 1584 and NFPA 70E
B Y R A V E L F . A M M E R M A N , P . K . S E N , & J O H N P . N E L S O N
THE EXPOSURE TO
hazards associated with
electrical arcing phe-
nomena when working
on energized equipment is a topic of
significant interest to industrial plant
personnel. This article provides an
overview of the current arc-flash stan-
dards, focusing on the methods used
to calculate incident energy levels in a
system. A thorough sensitivity analy-
sis of the arc-flash hazard incident
energy calculations currently adopted
by the IEEE 1584 standard leads to
some possible conservative simplifica-
tion of the equations. These simple
equations could be used for a quick
first-cut assessment of the incident
energy levels present in a system. A
case study using data from a typical
petrochemical application provides a
comparison of the National Fire
Digital Object Identifier 10.1109/MIAS.2009.932345 © CREATAS42
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Protection Association (NFPA) 70Eand IEEE 1584 arc-flash incidentenergy equations and the results ob-tained using the proposed simplifiedcalculations.
Awareness of the various hazardscaused by an arc flash has increasedsignificantly over the past two deca-des. The regulations, standards, re-search, and application guidelinesfocus on reducing the exposure ofpersonnel to burn injuries associatedwith arc-flash events in low-voltage(LV) and medium-voltage (MV) appli-cations. Currently, the NFPA 70E—Standard for Electrical Safety in theWorkplace [1] and the IEEE 1584—Guide for Performing Arc-Flash HazardCalculations [2], [3] have the samegoal, protecting individuals who must work on or nearenergized electrical equipment. However, the philosophi-cal approaches used by these two groups to estimate thearc-flash hazards are different. For many who are requiredto apply and follow the standards, the arc-hazard calcula-tions and the interpretations, at times, can be confusing,particularly when there are discrepancies among the meth-ods being used. It has been widely accepted in the powerindustry that there is a need to perform additional researchand refine the arc-flash calculation methods to more effec-tively manage the hazard.
This article focuses primarily on the arc-flash incidentenergy calculations currently being used. Following asummary of the history of arc-flash hazard research and abrief review of the calculations, a sensitivity analysis isperformed, which leads to a potential simplification of theincident energy calculations presented in the standards. Itshould be emphasized that the sensitivity analysis and therecommendations suggested in this article are based exclu-sively on existing test data collected and made available inIEEE 1584 and the equations presented as a part of theNFPA 70E-2004 and IEEE 1584-2002 standards. Finally,calculations are provided to compare the incident energyequations and to validate the proposed simplifiedapproach for estimating energy levels. A number of graphsand charts are added to enhance the understanding andpossibly simplify the future application guidelines.
Evolution of Arc-Flash Standards
Historical Perspective of the Developmentof Arc-Flash Regulations and StandardsOn 29 December 1970, the Occupational Safety and HealthAct was signed into law. The general duty clause mandatesthat each employer ‘‘shall furnish to each of his employees,employment and a place of employment which are free fromrecognized hazards that are causing or are likely to causedeath or serious physical harm to his employees’’ [4]. TheOccupational Safety and Health Administration (OSHA),given the responsibility of providing for worker safety, ini-tiated the development of Federal regulations, includingthose that targeted identifying the electrical hazards andimplementing safe work practices.
OSHA initially used the NationalElectrical Code (NEC) as a basis forelectrical regulations. Because the NEClargely does not address employeesafety, it became apparent that a newstandard was needed. As a result, on 7January 1976, a new NFPA electricalstandards development committee wasformed. This group was given the taskof assisting OSHA in preparing stan-dards specifically addressing electricalsafety. The Committee on ElectricalSafety Requirements for EmployeeWorkplaces published the first editionof NFPA 70E in 1979. The initialedition covered installation safetyrequirements. Three subsequent edi-tions over the next decade added sec-tions on safety-related work practices
and safety-related maintenance requirements. OSHA usedthis work to create many of its regulations applying toelectrical safety.
Title 29 of the Code of Federal Regulations (CFR), Sub-part S, ‘‘addresses electrical safety requirements that are nec-essary for the practical safeguarding of employees in theirworkplaces’’ [4]. It was not until 1991 that OSHA addedwords acknowledging arc flash as an electrical hazard. Thefifth edition of the NFPA 70E, published in 1995, becamethe first standard specifically addressing the arc-flash haz-ard. This printing included requirements for protectiveclothing and defined a flash-protection boundary. The nexttwo revisions focused on detailed arc-flash hazard analysis,providing more specifications regarding the arc-flash pro-tection boundaries and incident energy calculations. NFPA70E-2004 includes sample calculations of flash protectionboundaries in Annex D. It is important to note, as quotedon page 70E-98 of the standard: ‘‘This annex is not a part ofthe requirements of this NFPA document but is includedfor informational purposes only’’ [1].
In addition to the NFPA 70E standard and the OSHATitle 29 (CFR), in 2002, the NEC started requiring theuse of labels warning workers about potential arc-flashhazards. This same year an IEEE working group com-pleted the publication of the standard IEEE 1584-2002:Guide for Performing Arc-Flash Hazard Calculations. Thenew standard presented models for estimating incidentenergy levels based on a large amount of test data. As seenfrom this brief summary, until recently, the arc-flash haz-ard has not been widely acknowledged. Extensive researchand testing performed of late has led to a better under-standing of the arc-flash hazard. The next section high-lights, chronologically, some of the most significantcontributions to the body of knowledge pertaining to arc-ing phenomena and the associated hazards.
Significant Milestones in Arc-Flash Research1) Lee: In 1982, ‘‘The Other Electrical Hazard: Electri-
cal Arc Blast Burns’’ [5] was published. This articleis considered by many to be one of the most impor-tant research contributions on arcing phenomena inopen air. This article was significant in that itquantified the potential burn hazards and educated
OSHA INITIALLYUSED THE
NATIONALELECTRICAL
CODE (NEC) ASA BASIS FORELECTRICAL
REGULATIONS.
43
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personnel about the safety implications. Lee estab-lished the curable burn threshold for the humanbody as 1.2 cal=cm2. Lee also published a second
very relevant article in 1987, ‘‘Pressures Developedfrom Arcs’’ [6]. The pressure effects of an arc inci-dent are quantified in this publication.
2) Doughty et al. [7]: ‘‘TestingUpdate on Protective Clothingand Equipment for Electric ArcExposure’’ [7], published in1997, details the incidentenergy levels associated withLV arc-flash events and was thefirst to describe how an eventis intensified when the arc ini-tiates within electrical equip-ment enclosures.
3) Doughty et al. [8]: ‘‘PredictingIncident Energy to Better Man-age the Electric Arc Hazard on600 V Power Distribution Sys-tems’’ [8] was published in2000. This article semiempiri-cally quantified the incidentenergy calculations for LV sys-tems and is the source of the in-cident energy calculations usedin the NFPA 70E standard.
4) Jones et al.: In 2000, ‘‘StagedTests to Increase Awareness ofArc-Flash Hazards in ElectricalEquipment’’ [9] was also pub-lished. Experimental investiga-tions, using mannequins, wereconducted to improve the un-derstanding of how humans canbe adversely affected by arc-flash incidents.
5) IEEE Standard 1584: The firstedition of IEEE Guide for Per-forming Arc-Flash Hazard Calcu-lations [2] was issued in 2002.This standard used extensivetest data to develop empiricalequations derived from statisti-cal analysis. Tests data weremade available from varioussources and are included as anappendix to the standard. Anarticle written by Gammon andMatthews, ‘‘IEEE 1584-2002,Incident Energy Factors and Sim-ple 480-V Incident Energy Equa-tions’’ [10], includes a thoroughstatistical analysis and summaryof the IEEE 1584 test data.
6) Stokes and Sweeting: ‘‘Electric Arc-ing Burn Hazards’’ [11], pub-lished in 2006, provides a criticalevaluation of the testing method-ology, in particular, the electrodeorientation used to assess thearc-flash hazard for the IEEE1584 standard development. Inaddition, this article included an
OSH Act29 December 1970
1970
1980
1990
2000
2006
NFPA 70ECommittee
Formed to Assist OSHA1976
Arc-FlashResearch
Ralph LeeArc-Blast Burns
Electric ArcResearch
NFPA 70EEdition 1Part I
Ralph LeePressure from Arc
Blasts
NFPA 70EEdition 2
Part II Added
NFPA 70EEdition 3
Part III Added
NFPA 70EEdition 4
Minor Revisions
OSHASubpart S
Arc-Flash Added
NFPA 70EEdition 5
Arc-Flash Added
Testing Update onProtective Clothing and
Equipment for Electric ArcExposure
IEEE 1584-2002IEEE 1584-2004a
Guide for PerformingArc-Flash Calculations
NEC Requires Arc-Flash Hazard Warning
NFPA 70EEdition 6Arc-Flash
RequirementsExpanded
Collaborative EffortDevelopment of Definitive Industry Standard
NFPA 70EEdition 7-2004
Predicting Incident Energy toBetter Manage the Electric
Arc Hazard on 600-V PowerDistribution Systems
DevelopmentsLeading to Regulations and Standards
Addressing the Arc-Flash Hazard
National Electrical CodeUsed for the Basis ofOHSA Regulations
NEC Does Not AddressWorker Safety
New Standard Needed
Stokes and SweetingElectric Arcing Burns
Effects of InsulatingBarriers in Arc-Flash
Testing
1800
1Historical development of arc-flash standards.44
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extensive list of literature onelectric arcs. The authors suggestthat this body of knowledge haslargely been overlooked in thedevelopment of the current IEEE1584 standard. Several discussionarticles were published whichprovided additional analysis ofthe issues being debated. ‘‘Clo-sure to Discussions of ‘ElectricArcing Burn Hazards’’’ [12] pub-lished by Stokes and Sweetingfurther documented their concerns.
7) Wilkins et al.: ‘‘Effect of Insulat-ing Barriers in Arc-Flash Test-ing’’ [13] was published in2008. The authors of this articleused vertical conductors termi-nated in insulating barriers fortheir testing methodology. Thenature of the arc is very similarto what is observed when theelectrodes are oriented horizon-tally, thus reinforcing the workof Stokes and Sweeting. Lang presented additionalinformation regarding the evaluation of alternatetest configurations in February 2007 at the 14thAnnual IEEE/IAS Electrical Safety Workshop heldin Calgary, Alberta, Canada [14].
Future Development of Arc-Flash StandardsIn 2006, the IEEE and NFPA agreed to collaborate on ajoint research initiative to increase the understanding ofarc-flash phenomena. This effort also plans to includeworking with the international community for globaladoption of such a standard. It is the hope that the part-nership between these two organizations will lead to adefinitive industry standard regarding arc-hazard analy-sis and mitigation. Figure 1 summarizes some of themore significant events in the development of arc-flashhazard standards and regulations, provided for futurereference only.
Arc-Flash Incident Energy CalculationsOne of the most important and essential elements of anarc-flash hazard analysis is the estimation of the incidentenergy. These calculations help predict the amount ofenergy available during an arc-flash event. Incident energyis typically expressed in J=cm2 or cal=cm2. The calcula-tions detailed by NFPA 70E-2004 and IEEE 1584-2002are used to establish the flash protection boundary, i.e.,the distance from an arc source that would cause the onsetof a second-degree burn. The energy required to produce acurable, second-degree burn on unprotected skin has beenestablished as 5.0 J=cm2 (or 1.2 cal=cm2Þ.
Factors Influencing Incident Energy LevelsTo help the reader fully comprehend the complexity ofthese types of calculations, a comprehensive list of the fac-tors influencing the incident energy is provided below.This has been known and well recognized by researchersover the years.
n System conditionsn available short-circuit currentn X/R ration prefault voltagesn loading.
n Protective devices (time–currentcharacteristic)n the first upstream devicen the second upstream device.
n System groundingn Electrical electrodes and potential
arc lengthsn spacing between phasesn spacing between phases and
groundn orientationn insulated versus noninsulated.
n Size and shape of enclosuresn Atmospheric conditions
n ambient temperaturen barometric pressuren humidity.
n Arc conditionsn randomness of the arc
n interruption of the arcn arc plasma characteristicsn other unidentified factors.
n Dissipation of energyn heatn latent heat of vaporizationn lightn soundn pressure wave.
n Other miscellaneous factors.Reviewing this long list of variables, it is obvious that
determining the precise arc-flash incident energy to whicha worker may be exposed is extremely difficult, if notimpossible. Only an estimate of a worker’s potential inci-dent energy exposure can be established. Consequently, itis wise to have the calculations be on the conservative orsafer side when protecting personnel.
NFPA 70EThe sixth edition of NFPA 70E, Standard for ElectricalSafety Requirements for Employee Workplaces [15], includes aset of equations used to calculate the available incidentenergy for LV systems (600 V and below). The seventhedition, NFPA 70E-2004, Standard for Electrical Safety inthe Workplace, moved the incident energy calculations toAnnex D. The IEEE 1584-2002 methods for computingincident energy are also included in the annex. The calcula-tions are used to establish the personal protective equipment(PPE) required for a worker. The incident energy calcula-tions, based on fault current, working distance, and protec-tive equipment clearing times are as follows:
EMA ¼ 5, 271 D�1:9593A
3 tA ½0:0016 F2 � 0:0076 F þ 0:8938�, (1)
EMB ¼ 1038:7 D�1:4738B
3 tB ½0:0093 F2 � 0:3453 F þ 5:9675�, (2)
IN 2006, THE IEEEAND NFPAAGREED TO
COLLABORATEON A JOINTRESEARCH
INITIATIVE TOINCREASE THE
UNDERSTANDINGOF ARC-FLASHPHENOMENA.
45
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where EMA, maximum open air incident energy (cal=cm2);EMB, maximum 20 in cubic box incident energy (cal=cm2);DA and DB, distance from arc electrodes (in) (for distances 18in and greater); tA and tB, arc duration (s); F, short-circuitcurrent (kA) (for the range of 16–50 kA).
IEEE 1584The IEEE 1584-2002 standard was developed using testdata compiled from several laboratories. The calculations,which were derived statistically, are used to predict the inci-dent energy an employee could experience when workingon energized equipment. These equations also help estab-lish the boundary distances for workers not wearing theproper PPE. This article does not address the charts andsimplified equations that were developed for Class L andRK1 LV fuses or for the materials presented that deal withcertain types of LV circuit breakers.
This article focuses specifically on the incident energy equa-tions that are described later. Compared with the NFPA 70Ecalculations, the IEEE 1584 equations are more complicated,involving an increased number of variables. It is also apparentthat the IEEE 1584 calculations accommodate a wider rangeof voltage and bolted fault current levels. The equations forthe incident energy calculations are summarized as follows:
Arcing Current Calculationsn System voltage less than 1,000 V:
lg (Ia) ¼ K þ 0:662 lg(Ibf )þ 0:0966 V
þ 0:000526 Gþ 0:5588 V lg (Ibf )
� 0:00304 G lg (Ibf ): (3)
n System voltage more than 1,000 V:
lg (Ia) ¼ 0:00402þ 0:983 lg (Ibf ), (4)
Ia ¼ 10lg (Ia), (5)
where Ia, arcing current (kA); K, �0.153 for open configura-tions and �0.097 for box configurations; Ibf , bolted 3/ faultcurrent [symmetrical rms (kA)]; V, system voltage (kV); G,gap between conductors (mm) (Table 1); lg, log with a base 10.
Incident Energy Calculations
lg (En) ¼ K1 þ K2 þ 1:081 lg (Ia) þ 0:0011 G (6)
En ¼ 10lg (En), (7)
where En, normalized incident energy ( J=cm2); K1,�0.792 for open configurations and �0.555 for box con-figurations; K2, 0 for ungrounded and high-resistancegrounded systems and �0.113 for grounded systems; G,gap between conductors (mm) (Table 1).
E ¼ Cf En (t=0:2) (610x=Dx), (8)
where E, incident energy (cal=cm2); Cf , calculation factor1.0 for voltages above 1 kV and 1.5 for voltages below1 kV; En, normalized incident energy ( J=cm2); t, arcingtime (s); D, distance from the possible arc point to the per-son (mm); x, distance exponent (Table 1).
Lee MethodFor cases outside the ranges established for use in both theNFPA 70E and IEEE 1584 standards, the Lee method is tobe used. The Lee model is presented below:
E ¼ 5:12 3 105 V Ibf (t�
D2), (9)
where E, incident energy (cal=cm2); V, system voltage(kV); t, arcing time (s); Ibf , bolted 3/ fault current (kA);D, distance from the possible arc point to the person (mm).
Figure 2 provides a comparison between the NFPA70E and IEEE 1584 standards. This side-by-side appraisalof the standards includes a summary of variables needed tocalculate the incident energy and the conditions for whichthe calculations are applicable.
Sensitivity Analysis of IEEE 1584 IncidentEnergy CalculationsMany companies follow the IEEE 1584 methodology whencalculating incident energy levels, because it was developedusing a large number of test data and encompasses a widerrange of voltage and current. This method is believed by
many to provide more accurateresults. On the other hand, theincreased complexity of the re-quired calculations suggests that acomputer program should be usedto manage the equations effec-tively. The IEEE 1584-2002 stan-dard comes equipped with a set ofspreadsheet calculators to assistwith an arc-flash study. These cal-culators are not always easy to fol-low and at times can be confusing.Some companies rely heavily oncommercial software packages tohelp estimate incident energylevels within their facility.
In an effort to provide a clearunderstanding of the IEEE 1584incident energy calculations, thesensitivity of the equations to the
TABLE 1. FACTORS FOR EQUIPMENT AND VOLTAGE CLASSES.
System Voltage(kV) Equipment Type
Typical Gap BetweenConductors (mm)
DistanceExponent (x)
0.208–1 Open air 10–40 2.000
Switchgear 32 1.473
MCC and panels 25 1.641
Cable 13 2.000
>1–5 Open air 102 2.000
Switchgear 13–102 0.973
Cable 13 2.000
>5–15 Open air 13–153 2.000
Switchgear 153 0.973
Cable 13 2.00046
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2
Incident Energy Equations (Incident Energy in cal/cm2)
Required Variables and Constants
NFPA 70E - 2004
Standard for Electrical Safety in the Workplace
IEEE 1584 - 2002 Guide for Performing Arc-Flash Hazard Calculations
System Application Limits
Parameter NFPA 70E-2004 IEEE 1584-2002 Working Distance 457 mm (18 in) or more 457 mm (18 in) or more
Type of Installation Open Air or Cubic Box Open Air, Cubic Box and Cable Bus Voltage Level 208–600 V 208 V–15 kV
Range of Current 16–50 kA 0.7–106 kA Time of Arc Exposure Unlimited Unlimited
System Voltage Under 1,000 V
lg Ia = K + 0.662 lg (Ibf) + 0.0966 V + 0.000526 G + 0.5588 V (lg (Ibf)) − 0.00304 G (lg (Ibf))
1,000 V < System Voltage < 15 kV
lg (Ia) = 0.00402 + 0.983 lg (Ibf)Ia =10 lg(Ia)
lg (En) = K1 + K2 + 1.081 lg (Ia) + 0.0011 G
En = 10 lg(En) Normalized incident energy (J/cm2)E = Cf En (t / 0.2) (610x/ Dx )
Constants
K: − 0.153 for open configurations − 0.097 for box configurations
K1: − 0.792 for open configurations (no enclosure) − 0.555 for box configurations (enclosed equipment)
K2: 0 for ungrounded and high-resistance grounded systems− 0.113 for grounded systems
C f: calculation factor 1.0 for voltages above 1 kV1.5 for voltages below 1 kV
Required Variable NFPA70E
IEEE1584
V: System Voltage (kV) Ibf: Fault Current (kA)
t: Arcing Time (sec) D: Working Distance (mm) Open/Enclosed Equipment G: Conductor Gap (mm) K: Grounded, Ungrounded,and High-ResistanceGrounded Systems
Cf: Calculation Factor
x: Distance Factor
Distance Factors and Typical Conductor Gaps
SystemVoltage
(kV)
EquipmentType
Typical GapBetween
Conductors(mm)
Distance ×Factor
Open Air 10–40 2.000Switchgear 32 1.473MCC and Panels
25 1.6410.208–1
Cable 13 2.000Open Air 102 2.000
Switchgear 13–102 0.973>1–5Cable 13 2.000
Open Air 13–153 2.000Switchgear 153 0.973> 5–15
Cable 13 2.000
Lee Method: E = 5.12 × 105V Ibf (t / D2)
Used to predict the open-air incident energy levels in cases where working voltages or conductor gapsfall outside of the range of the NFPA 70E and IEEE 1584 Standards
Maximum open-air incident energy (cal/cm2)
E = 5271 D−1.9593 t [0.0016 Ibf − 0.0076 Ibf + 0.8938]
Maximum 20” cubic box incident energy (cal/cm2)
E = 1038.7 D −1.4738 t [0.0093 Ibf − 0.3453 Ibf + 5.9675]where D is in inches
2
2
Summary of incident energy calculations.47
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assorted variables was investigated. Thisis to be emphasized; no effort has beenmade in this article to validate the equa-tions presented in the IEEE Standard1584. The approach is demonstrated onthe equations derived for LV and MVsystems. A discussion of three cases fol-lows: 1) system voltages 480 V andbelow, 2) system voltages below 600 V,and 3) system voltages over 1,000 V.These voltage values were selected sothat some comparison could be madewith the experimental results providedin the IEEE Standard 1584.
System Voltages 480 V and BelowEquations (3) and (5) were evaluatedfor different conductor gap distanceson a 480-V system with a box configu-ration as shown in Figure 3.
Figure 3 reveals that a worst caserelationship between the arcing current and the bolted
fault current, for voltages less than480 V, in closed configurations, couldbe approximated by the simplifiedequation of a straight line given by (asshown in Figure 3)
Ia ¼ 0:6 Ibf : (10)
Using information extracted fromthe IEEE 1584 test results database,arcing current and three-phase boltedfault current data corresponding tovoltages less than 480 V are plotted(scatter plot) in Figure 4.
A line representing (10) is super-imposed over the IEEE 1584 testdata. Test points on the scatter plotare grouped into three shaded areas asidentified. Group 3 is of particularinterest because of the wide variationin arcing current levels observed for
fault currents more than 100 kA. This is attributed tothe test procedure, which investigated a broad range ofconductor gap distances varying from 7 to 32 mm. Fig-ure 3 reveals that the incident energy equation modelsthe increasing influence of conductor gap distance athigher fault current levels. The simplified approach doesnot account for this because only the worst case was con-sidered, i.e., a conductor gap of 10 mm. Nevertheless, allof the data points fall beneath this line, confirming thatthis simple linear equation could be used as a worst caseapproximation of the relationship between the arcingcurrent and bolted fault current, for voltages less than480 V, in closed configurations. It will be interesting tosee whether this simplification could be proven by addi-tional testing.
Next, the sensitivity of the normalized incident energyto the conductor gap distance was evaluated using (6) and(7). Results for ungrounded or high-resistance groundedsystems are shown in Figure 5. Ungrounded systems arefeatured in this study because our investigation con-firmed other literature, which states that ‘‘typically an
70
G = 10 mm
la = 0.6 lbf
G = 25 mm G = 40 mm
Arcing Versus Bolted Fault Current(480 V, Box Configuration)
60
50
40
30
20
10
00 20 40 60 80
Bolted Fault Current (kA)
Arc
ing
Faul
t Cur
rent
(kA
)
100
3Sensitivity to gap between conductors: 480 V and below.
60
50
40
30
20
10
00
1
2
3
20 40 60 80Bolted Fault Current (kA)
Arc
ing
Faul
t Cur
rent
(kA
)
100
la = 0.6 lbf
Arcing Versus Bolted Fault CurrentIEEE 1584 Test Data Under 480 V
Sample Size: 48
4Scatter plot of IEEE 1584 LV test data: 480 V and below.
30
25
20
15
10
5
00 10 20 30 40 50
Arcing Current (kA)
Nor
mal
ized
Ene
rgy
(J/c
m2 )
60
G = 10 mm G = 25 mm G = 40 mm
Normalized Incident Versus Arcing Current(480 V, Ungrounded)
G = 10 mm G = 25 mm G = 40 mm
5
En = 0.43 la
Sensitivity to gap between conductors: 480 V and below.
ONE OF THE MOSTIMPORTANT AND
ESSENTIALELEMENTS OF AN
ARC-FLASHHAZARD
ANALYSIS IS THEESTIMATION OFTHE INCIDENT
ENERGY.
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ungrounded system results in the incident energy levelthat’s about 30% greater than that of a solidly groundedsystem’’ [16]. Taking the worst case, as depicted on thegraph later, in a similar approach as before, a simplifiedequation for the incident energy is derived as follows:
En ¼ 0:43 Ia: (11)
Last, the incident energy is calculated using (8). Con-tinuing to simplify the equations using the worst caseapproach, the working distance (D) is set to 457 mm(18 in) and the value of 1.641 is selected for the distanceexponent (x) for G ¼ 25 mm. It should be noted thatselecting the worst case condition for the distance expo-nent could be problematic. It has been shown that theprocedure adopted to calculate the incident energy using adistance exponent can give anomalous results [17], and itshould be further investigated. Because this is a LV systembeing evaluated, 1.5 is used for the calculation factor (Cf ).The incident energy calculation shown with the valuesselected for the variables follows:
E ¼ 1:5 En (t=0:2) (610=457)1:641: (12)
Combining (10) to (12) results in a simplified form ofthe incident energy equation:
E ¼ 3:11 (Ibf ) (t), (13)
where E, incident energy (cal=cm2); Ibf , bolted 3/ faultcurrent (kA); t, arcing time (s).
System Voltages 600 V and BelowA similar procedure was applied for different conductorgap distances on a 600-V system with a box configuration.Figures 6–8 summarize the information. As before, thedata are grouped into three distinct regions. A scatter plotof the data confirms that as a worst case the relationshipbetween the arcing current and bolted fault current, forvoltages less than 1,000 V in a closed configurations,could be approximated by the simplified equation
Ia ¼ 0:8 Ibf : (14)
Taking the worst case shown on the graph, as before, asimplified expression for the incident energy is derived:
En ¼ 0:43 Ia: (15)
Finally, using the procedure previously described, theincident energy calculation derived for this case is sum-marized as
E ¼ 4:14 Ibf (t): (16)
System Voltage More Than 1,000 VA similar approach was used to evaluate the incidentenergy calculations for voltages more than 1,000 Vusing the IEEE 1584 equations listed as (4) and (5). Fig-ures 9–11 detail the data used to develop the simplifiedequations.
Interpreting the information on the graphs gives theequations that follow:
Ia ¼ 0:95 Ibf , (17)
En ¼ 0:60 Ia: (18)
To conclude this process, the incident energy for anungrounded system is calculated using (8). The working
70
80
90G = 10 mm
la = 0.8 lbf
G = 25 mm G = 40 mm
Arcing Versus Bolted Fault Current(600 V, Box Configuration)
60
50
40
30
20
10
00 20 40 60 80
Bolted Fault Current (kA)
Arc
ing
Faul
t Cur
rent
(kA
)
100
G = 10 mm
lall = 0.8 lbfll
G = 25 mm G = 40 mm
6Sensitivity to gap between conductors: 600 V and below.
G = 10 mm
En = 0.43 la
G = 25 mm G = 40 mm
Normalized Incident Energy Versus Arcing Current(600 V, Ungrounded)
40
35
25
15
5
30
20
10
00 10 20 30 40 50 60 70 80
Arcing Current (kA)
Nor
mal
ized
Ene
rgy
(J/c
m2 ) G = 10 mm
EnEE = 0.43 lall
G = 25 mm G = 40 mm
8Sensitivity to gap between conductors: 600 V and below.
60
70
80
50
40
30
20
10
00
1
2
3
20 40 60 80Bolted Fault Current (kA)
Arc
ing
Faul
t Cur
rent
(kA
)
100
la = 0.8 lbf
Arcing Versus Bolted Fault CurrentIEEE 1584 Test Data Under 1 kV
Sample Size: 166
0
1
2
3
20 40 60 80 100
lall = 0.8 lbfll
7Scatter plot of IEEE 1584 LV test data: 600 V and below.
49
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distance (D) is set to 457 mm (18 in),and the value of 2.000 is selected forthe distance exponent (x). Becausethe voltage of the system being eval-uated is greater than 1 kV, 1.0 is usedfor Cf. Equation (19) summarizesthis approach:
E ¼ 1:0 En(t=0:2) (610=457)2:000:
(19)
Combining (17)–(19) results in asimplified form of the incident energyequation:
E ¼ 5:1 (Ibf ) (t): (20)
Verification of Results Derived for the LV CaseTo check the validity of the simplified approach presentedin this article, average incident energies and bolted faultcurrents data from the IEEE 1584 test database are plot-ted for voltages under 1,000 V. The values selected corre-spond to arc durations of approximately six cycles (100ms). If the time in (13) and (16) is set equal to 100 ms
(0.1 s), then the expressions becomeas follows:
E ¼ 0:31 (Ibf ), (21)
E ¼ 0:41 (Ibf ): (22)
Figure 12 shows that all the avail-able data points fall below the linesrepresenting the equations derivedfor the 480- and 600-V cases, indi-cating that the simplified approachresults in conservative estimates forthe incident energy levels in a LVsystem when compared with theavailable test data.
As an additional means of verification, the simpli-fied version of the equations presented in this articleare compared with the results derived from the NFPA70E and IEEE 1584 incident energy equations. NFPA70E incident energy levels were derived using (2) andIEEE 1584 (3) and (5)–(8) were used to calculate theincident energy levels. Figure 13 provides a summaryof the calculations in a graphical form, confirming that
60
50
40
30
20
10
00 20 40 60 80
Arcing Current (kA)100
En = 0.6 la
Normalized Incident Energy Versus Arcing current(Undergrounded)
Nor
mal
ized
Ene
rgy
(J/c
m2 ) G = 23 mm
G = 103 mmG = 53 mmG = 153 mm
EnEE = 0.6 lall
G = 23 mmGG = 103 mmG
G = 53 mmGG = 153 mmG
11Sensitivity to gap between conductors: More than 1 kV.
40
35
30
25
20
15
10
5
00 20 40 60
Bolted Fault Current (kA)80 100
Inci
dent
Ene
rgy
(cal
/cm
2 ) E = 0.41 lbf
E = 0.31 lbf
(600 V)
(480 V)
Incident Energy Versus Bolted Fault Current
E = 0.41 lbfll
E = 0.31 lbfll
(600 V)
(480 V)
12Validation of simplified approach.
4540353025
20
15105
00 5 10 15 20 25 30 35 40 45
Bolted Fault Current (kA)
Arc
ing
Faul
t Cur
rent
(kA
)
la = 0.95 lbf
Arcing Versus Bolted Fault CurrentIEEE 1584 Test Data Under 1 kV
Sample Size: 148
lall = 0.95 lbfll
10Scatter plot of IEEE 1584 MV test data.
ONE OF THEDRAWBACKS OFUSING THE NFPA
70E APPROACH ISTHE POTENTIAL TO
OVERPROTECTTHE WORKERS.
45
40
35
30
25
20
15
10
5
00 5 10 15 20 25 30 35 40 45
la = 0.95 lbf
Arcing Versus Bolted Fault Current
Bolted Fault Current (kA)
Arc
ing
Faul
t Cur
rent
(kA
)
9System voltage more than 1 kV.
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the simplified approachgives conservative val-ues in most cases. TheNFPA hazard or risk cat-egories are also shown onthe figure for reference.
Discussion of Resultsfor the LV CaseAs observed in the pre-ceding analysis, the sim-plified approach providesconservative estimates ofarcing fault current andthe incident energy lev-els. Caution applying thissimplified approach isadvised at bolted faultsbelow 20 kA, because arcsustainability issues areprobable at these currentlevels.
Validation of the sim-plified approach in thissection focused on the rela-tionship observed betweenincident energy and bolted fault current. It is well docu-mented that the available fault current and time–currentcharacteristics of the protective devices have the mostsignificant effect on arc-flash hazard incident energy levels.Therefore, the next section, which features a case study, pro-vides an analysis of the relationship between the incidentenergy and the arc duration.
Petrochemical System: A Case StudyData from a typical petrochemical power distributionnetwork were used to perform an arc-flash comparativestudy using one of the commercial software packagesavailable. Nominal voltages of 480 V, 4.16 kV, and12 kV are present in the system, providing a good oppor-tunity to compare both the LV and MV incident energycalculations. Figure 14 shows some of the results of thestudy. Recall that the NFPA 70E incident energy calcu-lations are valid for voltage levels up to 600 V and forbolted fault currents between 16 and 50 kA. Therefore,the Lee method was used to estimate the energy valuesfor the 12-kV bus. A working distance of 18 in was usedfor all the calculations.
Using the calculations described in the arc-flash stan-dards, the NFPA 70E approach typically produces moreconservative estimates (higher values) for the incidentenergy in the cases explored for this study. One of thedrawbacks of using the NFPA 70E approach is the poten-tial to overprotect the workers. Using a sensitivity analy-sis, the alternative method proposed in this articleproduces more conservative estimates of the potentialincident energy exposure than the IEEE 1584-2002equations. This is to be expected as the method is basedon some simplifications of the IEEE 1584-2002 equa-tions. However, the estimates for incident energygleaned from the approximations are less conservativethan the NFPA 70E calculations.
35
Incident Energy Versus Bolted Fault CurrentIEEE 1584, NFPA 70E, and Simplified Method
Calculation Comparison
30
25
20
15
10
5
00 20 40 60
Bolted Fault Current (kA)80 100
Hazard Risk Category 4
Hazard Risk Category 3
Hazard Risk Category 2
Hazard Risk Category 1
NFPA 70E
IEEE 1584
E = 0.31 lbf
Inci
dent
Ene
rgy
(cal
/cm
2 )
E = 0.41 lbf(600 V) (480 V)Hazard Risk Category 4
Hazard Risk Category 3
Hazard Risk Category 2
Hazard Risk Categog ry y 1
NFPA 70E
IEEE 1584
E = 0.31 E lbfllE = 0.41 lbfll
(600 V) (480 V)
13Comparison of various results.
1,000
900800
700
600500
400
300200
1000
0 20 40 60 80 100 120
Arc Duration (Cycles)
0 20 40 60 80 100 120
Arc Duration (Cycles)
Incident Energy Versus Arc Duration(480 V Bus)
Incident Energy Versus Arc Duration(12 kV Bus)
500
450 NFPA 70E
NFPA 70E
IEEE 1584
IEEE 1584
Simplified (480 V)Simplified (600 V)
Simplified (>1 kV)
400
350300
250200
Inci
dent
Ene
rgy
(cal
/cm
2 )In
cide
nt E
nerg
y (c
al/c
m2 )
150100
500
NFPA 70EIEEE 1584Simplified (480 V)Simplified (600 V)
NFPA 70E
IEEE 1584Simplified (>1 kV)
14Comparison of incident energy calculations.
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ConclusionsThis article has reviewed the incidentenergy equations used in the NFPA 70Estandard as well as the empirically basedequations of the IEEE 1584 document. Asimplified quick assessment approach hasbeen proposed for performing incidentenergy calculations.
The question is then, ‘‘whichmethod should be used?’’ This issue iscomplex, because research focused onmodeling arc-flash events and predict-ing incident energy levels is still in itsinfancy. As emphasized in this article,real arc-flash exposures are very diffi-cult to predict because of their ran-dom complex nature and the largenumber of variables involved. Furthercomplicating matters are the variedworking conditions and actual equipment configurationsencountered.
The incident energy information derived from an arc-flash study is used to help develop strategies for minimiz-ing burn injuries. Effectively modeling a large-scale powersystem, analyzing and accurately calculating the energyreleased during an arc-fault event is the cornerstone of anarc-flash hazard analysis. NFPA 70E incident energy cal-culations are based on theoretical concepts and from mod-els derived using very limited test data. Similarly, IEEE1584 includes a theoretically derived model developed forthree-phase, open-air systems, applicable for any voltage.In other words, both standards use Lee’s article [5] as thetheoretical basis for understanding the electrical arcingphenomena. Lee’s research includes many simplifyingassumptions, most notably that the shape of the arc is notimportant [5]. Certainly, a methodology developed for openair is not suitable for situations where the arc initiateswithin an enclosure or in cases where the system buses aretightly spaced. In an attempt to fill in the obvious gaps,the IEEE 1584 standard also featured statistically derivedmodels for incident energy calculations based on a signifi-cant amount of test data. Test results obtained for theIEEE 1584 Standard were compiled using a vertical orien-tation of the three-phase arcing electrodes. The effect ofdifferent electrode orientations and the use of insulatingbarriers have been investigated, and the results indicatethat a horizontal electrode configuration produces higherincident energy levels [11]–[14]. A number of specificitems were presented at the 2007 IEEE/IAS ElectricalSafety Workshop [14] recommending ways to incorporatethis research into subsequent versions of the arc-flash haz-ard standards.
In the meantime, the quick first-cut approach devel-oped in this article represents an effective way to estimatethe arc-flash hazard incident energy levels based on thecurrent IEEE 1584 standard. Available fault current andthe clearing time of the protective devices, as known, havethe greatest impact on the potential arc-flash hazard. Thesimplified versions of the IEEE 1584 incident energyequations, proposed in this article, emphasize these rela-tionships. Furthermore, it has been demonstrated thatconsistent results are obtained using this method.
AcknowledgmentsThe authors wish to acknowledge thePower Systems Energy Research Cen-ter (PSerc) for helping to support thisresearch. PSerc is an Industry Univer-sity Cooperative Research Center (pre-viously a National Science FoundationCenter).
References[1] Standard for Electrical Safety in the Workplace,
NFPA 70E-2004.
[2] IEEE Guide for Performing Arc-Flash HazardCalculations, IEEE 1584-2002.
[3] IEEE Guide for Performing Arc-Flash HazardCalculations-Amendment 1, IEEE 1584a-2004.
[4] Title 29 of the Code of Federal Regulations
(29 CFR). Part 1910: General Industry;
Safety Standards for Electrical Systems and
Safety-Related Work Practices, Occupa-
tional Safety and Health Administration
(OSHA) Standards, Washington, DC.
[5] R. Lee, ‘‘The other electrical hazard: Electrical arc blast burns,’’ IEEETrans. Ind. Appl., vol. IA-18, no. 3, pp. 246–251, May/June 1982.
[6] R. Lee, ‘‘Pressures developed from arcs,’’ IEEE Trans. Ind. Appl.,vol. IA-23, no. 4, pp. 760–764, July/Aug. 1987.
[7] R. L. Doughty, T. E. Neal, T. A. Dear, and A. H. Bingham, ‘‘Testing
update on protective clothing and equipment for electric arc expo-
sure,’’ in IEEE PCIC Conf. Rec., 1997, pp. 323–336.
[8] R. L. Doughty, T. E. Neal, and H. L. Floyd, ‘‘Predicting incident
energy to better manage the electric arc hazard on 600 V power
distribution systems,’’ IEEE Trans. Ind. Appl., vol. 36, no. 1,
pp. 257–269, Jan./Feb. 2000.
[9] R. A. Jones, D. P. Liggett, M. Capelli-Schellpfeffer, T. Macalady, L.
F. Saunders, R. E. Downey, L. B. McClung, A. Smith, S. Jamil, and
V. J. Saporita, ‘‘Staged tests to increase awareness of arc-flash hazards
in electrical equipment,’’ IEEE Trans. Ind. Appl., vol. 36, no. 2,
pp. 659–667, Mar./Apr. 2000.
[10] T. L. Gammon and J. H. Matthews, ‘‘IEEE 1584-2002, incident
energy factors and simple 480-V incident energy equations,’’ IEEEInd. Appl. Mag., vol. 11, no. 1, pp. 23–31, Jan./Feb. 2005.
[11] A. D. Stokes and D. K. Sweeting, ‘‘Electric Arcing Burn Haz-
ards,’’ IEEE Trans. Ind. Appl., vol. 42, no. 1, pp. 134–141, Jan/
Feb 2006.
[12] A. D. Stokes and D. K. Sweeting, ‘‘Closure to discussions of electric
arcing burn hazards,’’ IEEE Trans. Ind. Appl., vol. 42, no. 1,
pp. 146–147, Jan./Feb. 2006.
[13] R. Wilkins, M. Lang, and M. Allison, ‘‘Effect of insulating barriers
in arc flash testing,’’ IEEE Trans. Ind. Appl., vol. 44, no. 5,
pp. 1354–1359, Sept./Oct. 2008.
[14] M. Lang and K. Jones, ‘‘An evaluation of alternate test configurations
for future arc flash models,’’ Presented at the 14th Ann. IEEE IAS
Electrical Safety Workshop, ESW2007-14, Calgary, Alberta, Canada,
2007.
[15] Standard for Electrical Safety Requirements for Employee Workplaces,NFPA 70E-2000.
[16] C. St. Pierre, ‘‘Putting arc-flash calculations into perspective,’’ Elec-tric. Constr. Mainten., vol. 103, no. 6, pp. 48–58, June 2004.
[17] R. Wilkins, M. Allison, and M. Lang, ‘‘Calculating hazards,’’ IEEEInd. Appl. Mag., vol. 11, no. 3, pp. 40–48, May/June 2005.
Ravel F. Ammerman ([email protected]) and P.K. Senare with the Colorado School of Mines, Golden, Colorado.John P. Nelson is with NEI Electric Power Engineering, Inc.,in Arvada, Colorado. Ammerman is a Member of the IEEE.Sen is a Senior Member of the IEEE. Nelson is a Fellow ofthe IEEE. This article first appeared as ‘‘Arc Flash HazardIncident Energy Calculations, A Historical Perspective andComparative Study: IEEE 1584 and NFPA 70E’’ at the2007 Petroleum and Chemical Industry Conference.
THECALCULATIONS
ARE USED TOESTABLISH THE
PERSONALPROTECTIVEEQUIPMENT
REQUIRED FORA WORKER.
52
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