Arc Flash Papeper

8/13/2019 Arc Flash Papeper

1/7134 Iron & Steel Technology

The electrical power system at a plant isoften taken for granted. Unlimited poweris assumed to be available, and the infrastruc-ture of the power system is assumed to last for-ever. Even as new equipment is added or exist-ing equipment is idled, little thought is givento the overall power system.

Another overlooked area is arc flash and arcblast safety. Until recently, electrical safety was

often limited to shock hazards and lock-out/tagout programs. Most safety programsfocused on a worker coming in contact withenergized parts.

Recent changes to the National FireProtection Act (NFPA 70E) and the National

Electric Code (NEC) have compelled compa-nies to address these areas by performing anarc flash hazard analysis.12 The purpose ofthis analysis is to determine the potential riskof arc faults and arc blasts at every industrialpanel to which a worker may be exposed.Based on this analysis, appropriate work prac-tices and personal protective equipment(PPE) must be utilized to reduce the risksassociated with arc flashes and arc blasts.

A conservative method based on voltagelevel, type of task being performed, and stan-dardized tables can be used to conduct this

analysis. This method is simple and inexpen-sive in the short term, but often results inoverprotection. When a worker is overlyencumbered by PPE, productivity and moraleare negatively impacted. This method alsodoes not address ways of reducing PPErequirements through engineered solutions,such as using current-limiting fuses, usingsmaller transformers, etc.

Another method of arc flash hazard analysisrequires an engineered study. The availablefault current is determined for each industrialpanel based on voltage, overcurrent protec-tion settings, utility contribution and equip-ment ratings. This type of analysis is moreexact and often leads to less-restrictive PPErequirements. It also identifies problem areasand points within an electrical system that canbe modified to further reduce PPE require-ments by reducing the available fault current.

This paper seeks to help companies mini-mize the cost of performing engineered stud-ies for arc flash hazard analysis. Each steprequired to perform an arc flash hazard analy-sis will be detailed. The level of expertiserequired for each step will also be discussed(i.e., electrician, plant engineer, or licensedpower system PE). Typical areas of concernfor steel mills will also be addressed.

Electrical Single LinesThe basis for an engineered study is the elec-trical single lines. These drawings typicallyshow the distribution of electrical power fromthe utility company down to the switchboard,motor control center (MCC) or panel level.Large-horsepower (hp) motors or MG sets arealso included. Each item is assumed to bethree-phase AC unless otherwise noted. Allthe breakers, fuses and protection relays thatprovide protection for the system are shown.Any switches, disconnects and tie-breakers arealso shown (Figure 1).

The vast majority of engineering studiesinput a plants single lines into a software pro-gram for analysis. Since the study is only asaccurate as the information put into this pro-gram, many of the single lines need to be fieldverified. Most plants do not have up-to-date

single lines. As a result, the first step in per-forming an arc flash hazard analysis is walkingthrough a plant and correcting the single linediagrams. Depending on the size of the plant,this can be a significant effort.

In addition to verifying the actual connec-tions shown on the single line, an engineeredstudy requires the following information to befield verified:

Minimizing the Cost of ArcFlash Evaluation Studies

This paper examines the criteria required to perform

an arc flash analysis on a steel producing facility,

including what information can be gathered in-house

and what must be completed by a licensed power

systems engineer. A strategic program for minimizing

outside costs is presented.

AuthorDan Laird, associate professor, Youngstown State University, Youngstown, Ohio ([email protected])

THIS ARTICLE IS AVAILABLE ONLINE AT WWW.AIST.ORG FOR 30 DAYS FOLLOWING PUBLICATION.


2/7April 2007 135

Utility fault contribution and protectivedevice information.

Transformer nameplate information,including impedances and, if possible,

X/R ratios. Nameplate fuse ratings. Nameplate breaker ratings and settings. Protective relay settings. Equipment ratings (symmetrical and

asymmetrical ratings, voltage ratings,etc.).

Motor/generator nameplate informa-tion (motors larger than 50 hp or largegroups of motors that run simultane-ously).

Regenerative drive information. Conductor sizes, type and number per

phase. Relative length of cable runs (50 feet). Notation if cable is overhead (in free

air) or in raceway/conduit. Bus duct ratings, lengths and loads.

Gathering this information is the most criti-cal part of the study. It is also the biggestopportunity for saving money by doing asmuch in-house as possible. Providing detailedinformation means that the power systemsengineer needs to make fewer assumptions.When making engineering assumptionsregarding life safety, an engineer will tend to

be conservative. These conservative estimates

can lead to overprotection one of the thingsto be avoided by doing an engineered study.

The optimal condition is to have up-to-datesingle lines. If there is equipment missing or

no longer in use, these drawings need to beupdated. If there is too much informationmissing, an engineering firm cannot evenquote the cost of performing an arc flash haz-ard analysis. The most cost-effective way of ver-ifying the information is to have the plant elec-trician(s) red-line the single line drawings byhand. These red-lined drawings can be updat-ed in electronic format at a later time. Whilethe electrician is verifying these connections,nameplate data can be gathered on the trans-formers, motors, generators and other equip-ment. This can often be done by an engineer-

ing intern who has the appropriate safety train-ing and the guidance of an engineer. A goodsafety practice is to always have the internescorted by a qualified electrician at all times.(Note: Always follow the prescribed boundarydistances for voltage rated equipment, even ifescorted by qualified personnel.)

Some information can be gathered safelyonly if the electrical equipment is de-ener-gized. This includes conductor sizes, numberof conductors, some relay settings, somebreaker settings and fuse information. Theideal situation is to gather this informationduring a planned shutdown of the plant elec-

trical system. Most of this information can be

Electrical single line drawing.

Figure 1



gathered by an engineering intern under theguidance of an engineer and with the assis-tance of a qualified electrician.

If an outage is not possible, the next bestsolution for saving money is to spend time inthe maintenance library. Any accurate cableand conduit schedule can be used to mark

conductor size and number of conductors onthe single line drawings. Previous calibrationreports for protective relays and breakers canbe used to gather nameplate ratings for eachdevice and give the engineering consultingcompany a checklist from which to work. Thiswill save time when collecting data and reducethe overall cost of the project. If digital tripunits have been added to power circuit break-ers, the settings for these devices can be tabu-lated and provided to the outside consultingcompany. Any records regarding fuse infor-mation, such as infrared studies, can also be

useful. Keep track of any current-limiting-typefuses that have been installed so that the engi-neering firm does not need to conservativelyuse a non-current-limiting type in the com-puter model.

Another crucial cost-saving opportunity is inobtaining an electronic copy of the computermodel from the engineering consulting firmthat performs the study. These firms typicallyprovide a hard copy of their report and analy-sis, but most will provide it electronically aswell, if requested. Constructing the electricalsystem database is the most time-consumingtask for the engineering firm. Once the system

is built into software, it is very easy to updatethe study as fuses are changed, lines are addedor equipment is removed. Having an electron-ic copy of the database ensures that a com-plete database reconstruction will not have tobe paid for a second time, even if a new con-sulting firm is chosen. Since the coordinationof overcurrent protection devices in a plantshould be examined at least every five to 10years, it makes sense to have a model of thepower system available.

Many plants purchase their own copy of thismodeling software (i.e., SKM, ETAP, etc.) to

examine different scenarios, such as upgrad-ing to current-limiting fuses, opening tie-breakers or replacing existing equipment.This software can be expensive and requirestraining to understand how changes areimplemented, so this may or may not be agood value.

Short-circuit AnalysisOnce the power system database has beenconstructed, a short-circuit analysis can beperformed. This analysis determines the mag-nitude of available current throughout the

power system at various time intervals follow-ing a fault. The computer model is used to

determine the bolted three-phase short-circuitcurrent at each point in the system. Once thefault levels have been calculated, they arecompared to the withstand rating of theequipment in the electrical distribution sys-tem. For example, if a breaker is rated for80,000 amps asymmetrical/50,000 amps sym-

metrical, it can withstand a first1

/2 cycle of80,000 amps and clear a fault of 50,000 amps.If the equipment is underrated, it can fail tooperate properly and cause an even greaterhazard than if it did not operate at all.

Most software packages will generate a listof equipment that is not adequately braced aspart of an equipment evaluation. Since allpanels and breakers do not list the bracinginformation, the software may estimate typicalratings based on voltage and amperage.Paying to have the engineering consultingfirm conduct an equipment evaluationimplies they will track down the true ratingsfor all panels that are flagged by the softwareas not being adequately braced. It may be acost savings to simply ask for a list of equip-ment that does not pass the equipment evalu-ation and verify the actual ratings using in-house resources. If the information is notfound on equipment nameplates, this meansthe equipment manufacturer should be con-tacted in order to obtain the true ratings.

Protective Device CoordinationAnalysisA coordinated system is one in which theprotective devices (circuit breakers, fuses,relays, etc.) operate at the proper times toeliminate or minimize the damage to a system.In the worst case, no protective devices willoperate, resulting in risk to both equipmentand personnel. If set too aggressively, the pro-tective devices may operate for normal condi-tions, such as transformer magnetization andmotor inrush current. It takes a significantdegree of skill and experience to set the over-current protection properly. Significantchanges in loading or equipment requirerevisiting this analysis every few years.

In general, coordination studies are con-ducted from the loads to the source as identi-fied below:

Individual loads. Conductors. Distribution equipment: breakers, dis-

connect switches, etc. Transformers. High-voltage distribution to transform-

ers.

The above process is continued for eachsuccessive voltage level until the utility-con-

trolled devices are reached. This processneeds to performed by an experienced power


4/7April 2007 137

systems engineer. The adjustable settings onthe overcurrent protection devices have a pro-found effect on both equipment protectionand arc flash boundaries.

Consider the coordination curve shown in

Figure 3. Notice that all the protective relaysare not set to trip before transformer T1reaches full load current. All three relays willtrip before thermal or mechanical damageoccurs to the transformer. Only the relay onthe secondary side of the transformer is set totrip faster than the transformer inrush. This isan example of a coordinated system. Copies ofthese coordination curves should come withthe arc flash hazard analysis, as they providethe basis for setting available fault current.

Arc Flash Hazard Analysis

The next step in the study is to determine theincident energy at each point in the power sys-tem based on the previous information. Theseenergy levels are only as accurate as the infor-mation garnered in the previous steps. Thereis also some latitude for engineering adjust-ments in performing the arc flash hazardanalysis.

A typical working distance of 18 inches isoften used to give an average arms lengthdistance for the worker. The idea is to protectthe face and torso, since burns over less than25 percent of the body are less likely to befatal (Figure 4). This is a sound estimate forworking inside a panel or doing voltage

checks. However, some tasks may not requirethe worker to be so close to the energizedequipment. For example, a worker changingfuses in a fused disconnect cubicle may bemore than 18 inches away from the main bus-

bar, which carries a higher PPE rating. Whenswitching operations are performed, it may bepossible to use extended racking handles orhook sticks to increase the working distance. Ifareas in a plant show up on the arc flash studyas having a PPE rating higher than Class 4, thestudy should be rerun with a working distanceof 36 inches in these areas. If the result is lessthan a PPE rating of Class 4, a worker can per-form those tasks beyond a working distance of36 inches in PPE while the equipment is ener-gized. This is particularly true for work doneon medium- or high-voltage systems, where

the only work performed while energized isswitching or racking breakers in and out.Work that cannot be done from beyond 36inches will have to be done when the system isde-energized.

Another area of flexibility is in setting themaximum arcing duration in the arc flash haz-ard analysis. This permits the setting of a max-imum (trip time + breaker time) for the inci-dent energy and flash boundary calculations.IEEE 1584 Annex B.1.2 states, If the time islonger than 2 seconds, consider how long aperson is likely to remain in the location ofthe arc flash. It is likely that the personexposed to arc flash will move away quickly if

Single line as drawn in modeling software.

Figure 2



it is physically possible, and 2 seconds is a rea-sonable maximum time for calculations. Aperson in a bucket truck or a person who hascrawled into equipment will need more timeto move away.

This 2-second approximation has causedmuch debate among power systems engi-neers. Many engineering firms will run thestudy with a longer maximum clearing timefirst. If the analysis shows some restrictive PPErequirements in an area of frequent work, thestudy may be rerun at the 2-second clearingtime. In this case, it should be stressed to theowner of the electrical system that the arcmay in fact last longer than 2 seconds andthat not every task is suitable for this assump-

tion.

PPE RequirementsOnce the incident energy is calculated at the

working distance for each point in the system,suitable PPE can be prescribed for that level ofenergy. Table 1, from NFPA 70E, shows thePPE hazard category and minimum arc ther-mal performance exposure values. Many com-panies provide their workers with a uniformthat meets Class 1 requirements and haveClass 2 and Class 4 PPE kits available for allother tasks. In this case, a worker will wearClass 4 PPE for hazard categories 3 and 4.

A common mistake is to purchase PPEbefore the study is completed, only to find outthat it is inadequate or overkill for most tasksperformed in the plant. There is also the pos-

sibility that PPE requirements can be relaxed

Transformer protection curve.3

Figure 3


6/7April 2007 139

if fuses or breaker settings are changed, moreinformation becomes available at the nextplanned outage, or equipment is replaced. Ifa plant has only a few Class 4 locations, it maynot make sense to purchase Class 4 PPE forevery maintenance worker. The application ofarc flash labels, employee training and issuing

of PPE should ideally happen at the sametime, but all these tasks should be based onthe results of the study. Otherwise, it may benecessary to do these tasks twice and incuradditional costs.

Items Particular to the SteelIndustryElectrical power systems for steel producershave some unique features. Below are somecommon concerns for arc flash hazard analysis:

DC Systems There is currently no arc flash

standard for DC systems. The IEEE 1584method currently used to perform most arcflash evaluations is based on experiments con-ducted in high-power laboratories on AC sys-tems. Until enough experimental data existsfor DC systems, arc flash analysis must stop atthe rectifiers converting AC to DC. When astandard is developed, it will pay to have theelectronic copy of the database on hand toquickly rerun the arc flash hazard analysis forthe plant.

Regenerative Systems Bridle and winder

motors are often running in regenerationmode, i.e., they are sending energy back tothe incoming line during normal operation.On a fault condition, this added energy fromregenerating motors must be applied to theshort-circuit study. The engineering consult-

ing firm should be assisted in identifying

which motors operate in regeneration, espe-cially if the firm is not familiar with the steelindustry.

Open Commutators Many commutators onlarge motors and generators in a steel mill areopen to ambient air. This creates a situationwhere entire motor rooms are in the flashzone of the motor or generator at all times.Since these rooms often house variable speeddrives, MCCs or switchgear, restrictive PPEmust be worn even for routine tasks. It may benecessary to partition motor rooms to avoid

cumbersome PPE for everyday tasks.

Exposed Busbar Many steel mills haveexposed busbar on slate panels, crane rails orprocesses like electrolytic cleaners. Again,these conductors are considered exposed at

Probability of survival after a burn injury.

Figure 4

NFPA 70 E PPE Hazard Category and Minimum Arc Thermal Performance Exposure Values

Minimum arc thermal performanceexposure value (ATPV)* or

Hazard/risk Clothing description Total weight breakopen threshold energy (EBT)*Category (number of clothing layers in parentheses) (oz./yd.2) rating of PPE cal/cm2

0 Untreated cotton (1) 4.57 N/A

1 FR shirt and FR pants (1) 4.58 5

2 Cotton underwear plus FR shirt and FR pants (2) 912 8

3 Cotton underwear plus FR shirt and FR pants 1620 25plus FR coverall (3)

4 Cotton underwear plus FR shirt and FR pants 2430 40plus double-layer switching coat and pants (4)

*ATPV is defined in the ASTM P S58 standard arc test method for flame-resistant (FR) fabrics as the incident energy that would just cause the onset ofa second-degree burn (1.2 cal/cm2). EBT is reported according to ASTM P S58 and is defined as the highest incident energy which did not cause FRfabric breakopen and did not exceed the second-degree burn criteria. EBT is reported when ATPV cannot be measured due to FR fabric breakopen.

Table 1



all times, and workers would be within theirflash zones under normal circumstances. Thesolution may not be PPE, but finding creativeways to either contain the energized parts orkeep workers at a safe distance.

Arc Furnaces The process of making steel

can require very high-powered electrical sys-tems and large transformers. Working in theseareas may affect not only electrical mainte-nance workers, but production workers aswell. The approach boundaries for unquali-fied individuals can be prohibitive. Engi-neering creativity is again needed to shieldworkers from the exposed energized equip-ment at greater distances than currently main-tained in most steelmaking facilities.

SummaryCompanies are required to perform an arc

flash hazard analysis by NFPA 70 E. Choosingto do an engineered study will provide a safeand accurate assessment of arc flash hazardsand provide options for making a plant a saferplace to work. These options can include PPE,design changes or equipment changes. Thesestudies should be performed by an experi-enced power systems engineer. Much of theinformation required by the power systemsengineer can be obtained using in-houseresources as outlined above. The cost of

future studies can be minimized by obtainingan electronic copy of the modeling database.The cost of equipment evaluation can be min-imized by acquiring equipment bracing infor-mation using in-house resources. Calculatingthe working distance based on the task per-formed can reduce PPE requirements.

Creativity can be applied to increase the safeworking distance and ultimately reduce PPErequirements. Care should be taken to avoidordering the incorrect PPE. It is recommend-ed that a PPE plan be developed based on theresults of the engineered study. The steelindustry has its own unique concerns relativeto arc flash. They can be considered and actedupon prior to performing an engineeredstudy in many cases.

References

1. National Fire Protection Association Inc., NFPA

70E: Electrical Safety Requirements for EmployeeWorkplaces, 2004 Edition, 2004.

2. National Fire Protection Association Inc.,National Electric Code 2005, Article 110.16, 2005.

3. Barr, Edward, Coordination of the Kraft Facility Philadelphia, Pa., Reuter Hanney Inc., May 2005.

4. American Burn Association, National BurnRepository 2002 Report: Percent Survival Versus AgeRange, March 2002.

5. IEEE, IEEE Standard 1584-2002 Guide forPerforming Arc-flash Hazard Calculations, 2002.

This paper was presented at AISTech 2006 The Iron & Steel Technology Conference andExposition, Cleveland, Ohio, and published in the AISTech 2006 Proceedings.

December 2006 Steel Shipments Down 11.8 Percent From Previous Year

The American Iron and Steel Institute reported that, for the month of December 2006, U.S.steel mills shipped 7,609,000 net tons, an 11.8 percent decrease from the 8,513,000 net tonsshipped in December 2005, and a 5.0 percent decrease from the 7,991,000 net tons shippedin the previous month, November 2006. A year-to-year comparison of year-to-date shipments

shows the following changes within major market classifications: service centers and distribu-tors, up 2.1 percent; automotive, up 7.5 percent; construction and contractors products, up10.6 percent; oil and gas, up 19.6 percent; machinery, industrial equipment and tools, up 4.6percent; appliances, utensils and cutlery, down 6.0 percent; containers, packaging and shippingmaterials, up 1.2 percent; and electrical equipment, up 12.8 percent.

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