1200 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 39, NO. 4, JULY/AUGUST 2003

A Summary of Arc-Flash Energy CalculationsDaniel R. Doan, Senior Member, IEEE, and Ronald A. Sweigart

AbstractRecent research has been published on the hazards ofan electrical arc flash. The calculations required for determiningarc-flash incident energy exposure for our electrical workers areinvolved and sometimes difficult. The authors have completed arc-flash energy studies for many industrial sites, from small facili-ties to large chemical plants. This paper provides a summary ofthe results of these studies, with an emphasis on the wide range ofresults that were found. Learnings about electrical equipment de-sign, installation, and operation are discussed that were found tobe associated with high arc-flash energy values. In addition, somemethods are described to evaluate the arc-flash energy values fora facility that can help the owner determine the most effective arcflash hazard management policy.

Index TermsArc flash, electrical arc flash, flame-resistantclothing, incident energy, personal protective equipment, safetyby design, safety study.

I. INTRODUCTION

RESEARCH into the nature and hazard of the electrical arcflash has been published in several papers in recent years.Lee published the first paper describing this hazard and pro-viding a theoretical incident energy equation [1]. Subsequentpapers discussed protective clothing testing and evaluations byNeal and Doughty [2], [3], and results of more recent testinghave provided additional information [4], [5]. Equations fromthese latter two papers were accepted and published by theNational Fire Protection Association (NFPA) in [6, Ch. 3 andAppendix B].

The arc flash is an explosion of heat, hot gases, and moltenmetal usually caused by a short circuit of energized conductors.A workers skin can be seriously burned, and clothing can beignited. Injury to bare skin begins at an incident energy value ofabout 1.2 cal/cm . This is the equivalent heat energy of holdingthe skin in a candle flame for about 1 s.

To protect a worker against this hazard, personal protectiveequipment is required. The recommended levels of clothing re-quired for various incident energy values is shown in Table I,which is a summary of [2, Table V].

The authors have completed arc-flash studies for many sites,from small facilities to large chemical plants. The results havebeen interesting. Many times the site engineers are surprised atthe variety of incident energy values on their plant. Often withina single building or electrical room, there is a large variation

Paper PID 0313, presented at the 2002 IEEE Petroleum and ChemicalIndustry Technical Conference, New Orleans, LA, September 2325, and ap-proved for publication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONSby the Petroleum and Chemical Industry Committee of the IEEE IndustryApplications Society. Manuscript submitted for review September 15, 2002and released for publication April 15, 2003.

The authors are with DuPont Engineering, Wilmington, DE 19880-0840 USA(e-mail: doan@ieee.org; Ronald.A.Sweigart-1@usa.dupont.com).

Digital Object Identifier 10.1109/TIA.2003.813724

TABLE IINCIDENT ENERGY RATINGS OF PROTECTIVE CLOTHING

of calculated incident energy. In addition, equipment that looksidentical may have vastly different incident energy hazards.

The goals of these studies are to:1) calculate the incident energy values for all major

equipment;2) find design and operating problems in the electrical sys-

tems that produce high incident energy values;3) help the site engineers determine the best flame resistant

clothing system that matches the exposures at the site.

II. SUMMARY OF SITES STUDIEDA variety of plant sites have been studied. The sites ranged

from small chemical facilities with 15 employees to largemulti-business plants with thousands of employees. Table IIprovides a list of the sites with the number of buses at eachsite in medium-voltage/high-voltage (MV/HV) equipment,low-voltage (LV) switchgear, and motor control centers(MCCs). Table III shows a breakdown of the equipment studiedby voltage class.

III. STUDY REQUIREMENTSA person who is responsible for electrical safety at an indus-

trial plant has a lot to accomplish to complete an arc-flash energystudy. The calculations require information that is sometimeshard to obtain. The following information must be assembled:

short-circuit study transformer information; utility short circuit values; cable and feeder lengths and specifications; system single-line drawings;

coordination study fuse information (type and amps); breaker information and settings;

0093-9994/03$17.00 2003 IEEE

DOAN AND SWEIGART: SUMMARY OF ARC FLASH ENERGY CALCULATIONS 1201

TABLE IISITES STUDIED

TABLE IIIBUSES STUDIED BY kV CLASS

system operating information plant operating methods; normal switching condition; other switching conditions; working distances for tasks (distance from workers body

to the possible arc exposure).For example, to find the correct clearing time at a certainshort-circuit fault current, the engineer would need to obtainthe right timecurrent curve for an LV circuit breaker. If thecurves are not available in the project or equipment files, themanufacturer, model, and settings of the circuit breaker mustbe determined from the field.

A complete and correct study of the arc-flash energy cannotbe done without this information. Guessing or using typicalnumbers can result in incorrect energy values. Values that aretoo low will mean that the workers may be wearing inadequateprotective equipment for the hazard. Values that are too high willmean that the workers may be wearing more protective equip-ment than is really needed; this creates hazards such as heat ex-haustion and lack of visibility.

Fig. 1. Typical single-line diagram of a large site.

IV. SUMMARY OF INCIDENT ENERGY VALUES

A typical single-line diagram of a site, showing the connec-tion of HV, MV, substation transformers, and LV equipment, isshown in Fig. 1. The HV transformers in a large site can be sizedfrom 10 to 40 MVA, and the typical substation transformer is2500 kVA.

At transmission voltages (over 34.5 kV), the incident energylevels were usually based on working distances of 60 in ormore. This reflects the fact that the equipment is normallyoutside, in a power yard, and the worker is a farther distanceaway during switching operations, as compared to low voltagework. The total energy in a fault may be higher than most LVfaults, but the calculated incident energy at the longer workingdistance was usually within a reasonable range for availableprotective clothing. At these voltages, the incident energy levelswere found to range from 15 to 200 cal/cm , with an averageof 60 cal/cm .

At distribution voltages (from 11 to 34.5 kV), the incidentenergy levels were usually based on working distances of 36 in,which is the typical distance between a worker and the ener-gized equipment when throwing a switch or when using a shorthot-stick for placing ground clusters. At these voltages, incidentenergy levels were found to range from 1 to 200 cal/cm , withan average of around 32 cal/cm .

At voltages of 2.4 and 4.16 kV, the typical working distanceused in the calculations was 24 in, and the resulting energy levelswere from 1 to 55 cal/cm , with an average of about 7.5 cal/cm .

At utilization voltages (208600 V), the typical working dis-tances used was 18 in. This describes the reach of a workers armduring troubleshooting or switching operations. The incidentenergy levels at these voltages were found to range from below 1to over 400 cal/cm with an average of about 15 cal/cm . Sincethis is such a wide range of values, it is helpful to divide thepieces of equipment into MCC and switchgear groupings.

The MCCs had energy levels that ranged from 1 to130 cal/cm with an average of 5 cal/cm . The LV switchgear,which would include substation circuit breaker lineups, powerdistribution panels, and other LV equipment, had energy levelsfrom 1 to over 400 cal/cm , with an average of 21 cal/cm .

1202 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 39, NO. 4, JULY/AUGUST 2003

TABLE IVINCIDENT ENERGY VALUES OF SELECTED SYSTEMS

Fig. 2. Timecurrent curves for primary fuse example.

V. DESIGN ISSUES FOUND DURING STUDIES

With some careful review of the complete study information,it is possible to learn something about the equipment that woulddescribe the variations in incident energy calculated. Table IVdescribes some of the values found for equipment of variousvoltage levels.

Bus differential protection and current-limiting fuses reducedthe energy for many buses. Often, very high values of energywere found when the system had higher than needed primaryfuses, or circuit breakers with maximum settings for instanta-neous trips.

In the case of the 480-V switchgear, primary fusing can havea large impact on the incident energy. A primary fuse that issized for higher current than is required, or a slow-type fuse,

TABLE VRESULTS OF PRIMARY FUSE CHANGE

Fig. 3. Timecurrent curves for circuit-breaker example.

TABLE VIRESULTS OF CIRCUIT-BREAKER SETTING CHANGE

can be changed out to reduce incident energy. Fig. 2 and Table Vshow a real example of a primary fuse problem that was discov-ered during the study. Changing to a faster type fuse reduced theenergy from an unmanageable 430 cal/cm to 54 cal/cm . Thesystem coordination was checked, and in this case the faster fusetype did not impact the coordination with downstream protec-tion devices. Before making any change of this type, the usermust be careful to avoid other problems.

An LV power circuit breaker, when set up with instantaneoustrip, can protect downstream equipment very well, as long as thearcing fault current is above the instantaneous setting. Severalcases were found where the fault current was below the instanta-neous setting, usually because of long cables to an MCC or dis-tribution panel. One example with a 700-ft-long feeder is shownin Fig. 3 and Table VI. Adjusting the instantaneous trip from12 to 8 was enough to reduce the energy from 46 cal/cm tobelow 1 cal/cm . The workers would then need no fire-resistantclothing.

DOAN AND SWEIGART: SUMMARY OF ARC FLASH ENERGY CALCULATIONS 1203

Fig. 4. Number of buses by incident energy level.

TABLE VIIANNUAL EXPOSURES FOR EACH PIECE OF EQUIPMENT

VI. EVALUATION FOR CLOTHING SELECTION

Additional review of all the data for a facility gives someinteresting information. A histogram showing all the busesgrouped by their incident energy value is shown in Fig. 4. Thenumber of buses at certain energy levels could be helpful forchoosing a garment system for a site or company. In this case,a large portion of the buses have incident energy levels below 5cal/cm , and the next major step occurs at 15 cal/cm .

The data can also be manipulated to give some idea of thenumber of times each year that workers are exposed to the en-ergy levels. Table VII describes the typical number of annualexposures for each piece of equipment, based on the typical op-eration of that equipment. These numbers can vary by companyand site.

When each bus or piece of equipment is assigned an esti-mated annual exposure from Table VII, the resulting number ofexposures can be added up and graphed in a histogram. Fig. 5

shows the annual exposures of all workers in the sites that werestudied, graphed by energy level. A chart of this type may behelpful in choosing the best clothing selection. In this example,a large quantity of exposures were found in the range of 1.25cal/cm . A base set of clothing rated for up to 5 cal/cm , suchas a flame-resistant shirt and pants for everyday wear, may be agood choice. The next obvious step is at 15 cal/cm , and a cov-erall for each worker may be best as a second layer. The less-fre-quent exposures above 15 cal/cm could be covered by higherrated suits for occasional use.

VII. CONCLUSION

Careful review of the results of the arc-flash incident energycalculations for a multiple-site company shows interesting in-formation. The wide range of results shows the importance of adetailed analysis of the incident energy.

Arc flash energy calculations help site owners identify prob-lems in their operation, including primary fuses of the wrongsize and breaker settings that may be incorrect. In some casesa small change in breaker setting, such as changing an instan-taneous setting from 12 to 8 , makes a big difference in theincident energy value.

A graphical summary of the arc flash energy results for busesand exposures helps the engineer understand the electrical haz-ards on the site. Knowing, for example, that most of the buses

1204 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 39, NO. 4, JULY/AUGUST 2003

Fig. 5. Number of annual exposures by incident energy level.

have energy values under 4 cal/cm can help the engineer decideon the selection of a certain clothing type for the workforce.

This analysis provides a sound engineering basis for makingfact-based decisions in managing arc-flash hazards.

ACKNOWLEDGMENT

The authors thank R. L. Doughty and H. L. Floyd fortheir suggestions and encouragement; and R. Jones, D. Pace,W. Jordan, K. White, and D. Zipse for their comments.

REFERENCES[1] R. H. Lee, The other electrical hazard: Electric arc blast burns, IEEE

Trans. Ind. Applicat., vol. 1A18, p. 246, May/June 1982.[2] T. E. Neal, A. H. Bingham, and R. L. Doughty, Protective clothing

guidelines for electric arc exposure, IEEE Trans. Ind. Applicat., vol.33, pp. 10411054, July/Aug. 1997.

[3] R. L. Doughty, T. E. Neal, T. A. Dear, and A. H. Bingham, Testingupdate on protective clothing and equipment for electric arc exposure,IEEE Ind. Applicat. Mag., vol. 5, pp. 3749, Jan./Feb. 1999.

[4] R. L. Doughty, T. E. Neal, and H. L. Floyd, II, Predicting incident en-ergy to better manage the electric arc hazard on 600-V power distributionsystems, IEEE Trans. Ind. Applicat., vol. 36, pp. 257269, Jan./Feb.2000.

[5] R. L. Doughty et al., The use of low voltage current limiting fusesto reduce arc flash energy, IEEE Trans. Ind. Applicat., vol. 36, pp.17411749, Nov./Dec. 2000.

[6] Electrical Safety Requirements for Employee Workplaces, NFPA 70E,2000.

Daniel R. Doan (S81M81SM00) received theB.S.E.E. and M.S.E.E. degrees from the Massachu-setts Institute of Technology, Cambridge.

He is a Consultant for DuPont Engineering,Wilmington, DE, where he is the Chair of DuPontsElectrical Standards Committee.

Mr. Doan has coauthored IEEE papers on captivemotor transformers, power system reliability, andmotor control networks, and has coauthored tutorialson power system reliability and arc flash hazardsfor the Petroleum and Chemical Industry Committee

(PCIC) of the IEEE Industry Applications Society. He is the Chair of the PapersReview Subcommittee of the PCIC and a Registered Professional Engineer inthe Commonwealth of Pennsylvania.

Ronald A. Sweigart received the Associate ofApplied Science in Electrical Engineering Tech-nology degree from Pennsylvania State University,University Park.

He is a Division Engineering Designer for DuPontEngineering, Wilmington, DE, with project designresponsibilities in chemical, textile fibers, andpolymer processes worldwide.

Mr. Sweigart is a Registered Professional Engineerin the State of Delaware and the Commonwealth ofPennsylvania.

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