One of the most important stepsin the process of communicat-ing safety to electrical person-nel about the dangers

associated with different energized equip-ment tasks, is to classify the tasks usingthe category levels described in NFPA70E Table 130.7(C)(11). This table de-scribes five different hazard/risk categorylevels (0 to 4) which are used to describethe hazard/risk level of different tasks.

The difficult part of an arc flash hazardevaluation, however, is to determine howto classify the tasks to be performed fordifferent types of equipment that are op-erating at different voltage levels. For thispurpose, NFPA 70E 2004 provides twomethods for the classification of the risksassociated with working on energizedequipment.

One alternative is to use Table130.7(C)(9)(a) to determine the arc flashrisk level associated with a particular en-ergized equipment task. This is a“lookup” table that lists different tasks tobe performed on energized equipment andtheir associated risk levels. The other al-ternative is to perform a more labor inten-sive flash hazard analysis using the morerigorous calculation guidelines providedby the IEEE 1584 2002 Standard and/orNFPA 70E 2004 Annex D.

The purpose of this article is to helpshed some light into the assumptions andgeneralizations utilized by the NFPA 70ETable 130.7(C)(9)(a) and to describe itslimitations and shortcomings which maylead to an improper selection of the Per-sonal Protective Equipment (PPE) re-quired for the task.

When to Use the PPE Tables?Ever since NFPA 70E 2004 was first

published, many discussions have takenplace about the validity of the use of Table130.7(C)(9)(a); however, it is clear thatthere are situations which require the useof this table. Such situations include the

an arc fault simulation at bus MCC-1.The simulation is performed using typ-

ical values for the gaps (distance betweenenergized bare conductors in mm) as pub-

lished in IEEE 1584. The working dis-tance has been assumed to be 24 inches(the NFPA 70E Table does not provideany flash protection boundary or workingdistances).

The MCC has a maximum availablebolted short-circuit current of 25 kA. As itmay be seen from the displayed results onthe one-line diagram of Figure 1, the pro-gram determined a hazard/risk CategoryLevel 3 for the faulted location (MCC-1).This category level agrees with the onelisted in the table.

A different simulation was performedfor the same MCC, but this time using themaximum available bolted short-circuitcurrent of 65 kA.

Evaluating NFPA 70E ArcFlash Hazard Category Tables

need to perform emergency work on en-ergized equipment without proper arcflash hazard labels.

The table is also useful for determiningthe PPE required for performing equip-ment inspection (as is the case when aflash hazard analysis needs to be per-formed). Arguably, the tables may also beutilized as part of a simplified safety pro-gram for facilities with small number ofbuses (simple radial systems with lessthan 20 buses).

Limitations of Table130.7(C)(9)(a) for MCCs

Table 130.7(C)(9)(a) lists and classifiestasks to be performed on equipment likepanelboards, switchboards, Motor ControlCenters (MCCs), switchgears and motorstarters. As an example of these tasks, thetable classifies the “insertion or removalof individual starter “buckets” fromMCC” (600 V class) as a task with haz-ard/risk Category 3 (8 < cal/cm? < 25).

This task is accompanied by footnote 4which indicates that the maximum avail-able bolted short-circuit current limit is 65kA and that the maximum fault clearingtime (arcing time) should be about 0.33seconds or 20 cycles. The category levelprovided by the table can be validated byusing power systems analysis software.The program has been configured to usethe IEEE 1584 2002 method. Figure 1shows a typical MCC configuration with

By Albert Marroquin

12 Electrical Products & Solutions • September 2007

Case ID Ibfat Fault Loca-

tion (kA)

Main BreakerFault Clearing

Time(sec)

Working Distance

(inch)

Incident En-ergy at MCC-1 (cal/cm?)

Hazard/RiskCategory

Case 1 23.3 0.300 24 8.61 3

Case 2 23.3 0.300 18 13.81 3

Case 3 64.8 0.300 24 24.31 3

Case 4 64.8 0.330 18 38.98 4

Note: Ibf denotes a bolted 3-phase short-circuit current.

Figure 1:Arc flash simulation for a typical MMC

(Continued on page 15)

Table 1: Incident Energy for a fault at MCC-1 for different working distances

The use of the maximum values of short-circuit current reveals that the incident en-ergy level could easily reach Category 4or higher. Table 1 lists the results of thesimulations performed for the MCC. Table1 also shows the working distances and thefault clearing time for each simulated case.

The results of Table 1 show that theNFPA 70E Table 130.7(C)(9)(a) may sug-gest the use of a PPE rating which maynot be adequate for the task. The typicalworking distance according to IEEE 15842002 is 18 inches for a 0.480 Volt MCC;however, Case 4 shows that the incidentenergy level for an 18 inch working dis-tance is close to exceeding the limit ofCategory 4 (40 cal/cm?).

In this case, the main problem of thetable is that it does not list the working dis-tance which is one of the most dominantfactors in the calculation. This means thateven if the available bolted fault currentand the fault clearing time stay within thenoted limits, the table may not provide ad-equate protection since the working dis-tance is not specified. The table results aregreatly generalized in this case and it maynot be prudent to use the table to recom-mend PPE for this task on MCCs.

Limitations of Table130.7(C)(9)(a) for MV Switchgear

We can take a second example for an-other very common task. Table130.7(C)(9)(a) describes tasks for ener-gized work on metal clad switchgearabove 1 kV. In this case, we can take a4.16 kV switchgear with typical workingdistance and conductor gaps based onIEEE 1584 2002 (WD =18 inches andGap =102 millimeter). Table130.7(C)(9)(a) recommends PPE rated forCategory 4 for the insertion or removal of

circuit breakers from cubicles with thedoors open. Figure 2 shows a diagram fortypical medium voltage switchgear andthe simulated arc flash results for two dif-ferent locations within the equipment.

The incident energy has been calculatedfor two different cubicles. One of them isa load CB cubicle and for this location the

incident energy is within Category 3 lim-its. However, for the Main CB cubicle,the potential incident energy release is al-most beyond that of Category 4. These re-sults are in agreement with thoserecommended by Table 130.7(C)(9)(a);however, any variation in the clearingtime or bolted

September 2007 • Electrical Products & Solutions 15

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Evaluating NFPA 70E... (Continued from page 12)

Figure 2: Arc flash simulation for a typical MVswitchgear

(Continued on page 16)

short circuit current may cause the inci-dent energy to exceed Category 4 levels.

In this case, the table fails to establishthe limits for the bolted fault current andclearing time since there is no footnotewhich defines these limits for this task.Again, it is not a good idea to use the tableto determine the PPE required for this task.

Summary of Task CategoryComparisons

The previous examples illustrated thepotential differences between the tablerecommended PPE rating and the one de-termined from the flash hazard analysis.Table 130.7(C)(9)(a) lists several othertasks and their hazard/risk categories.Table 2 presents a brief summary of com-parisons made between the NFPA 70Etable categories vs. those obtained by theusing the IEEE 1584 2002 method. Thistable is not comprehensive and its onlypurpose is to show a summary of a few ofthe comparisons made for tasks at differ-ent voltage levels.

As it can be seen from the resultsshown in Table 2, the hazard/risk cate-gories obtained using Table130.7(C)(9)(a) may be less conservativefor tasks expected to be within Category 3& 4 levels. For tasks involving Categories0, 1 & 2, it may be possible to use the ta-bles as long as the available bolted short-circuit current and the arcing time arewithin the limits specified in the footnotes.The comparisons presented in Table 2help to demonstrate why it is preferableto perform a flash hazard analysis instead

of using Table 130.7(C)(9)(a) exclusively.The previous analysis of the table cate-

gory levels suggest that they may not beadequate for tasks which may involveCategories 3 & 4, but that it may be con-servative enough for tasks listed with Cat-egory 0, 1 & 2; however, a check of theTime Current Characteristic curves(TCCs) of the protective device which isexpected to clear the arc fault should bedone regardless of the expected task haz-ard level since variations in the short-cir-cuit current levels and clearing times canresult in higher arc flash energies.

Quick Check MethodFigure 3 depicts a quick method to

check if a combination of bolted fault cur-rent and arcing time yield incident energylevels below the PPE rating.

The green and purple curves in the

TCC view of Figure 3 represent an inci-dent energy level of 25 cal/cm? for 600 Vclass MCCs and switchgear. The curveswere generated using typical working dis-tances of 24 and 18 inches and it was alsoassumed that the systems were un-grounded (this yields more conservativeresults). The curves show the relationshipbetween bolted fault current and the faultclearing time. If the bolted fault currentincreases, then the clearing time requiredshould be less so that the incident energylevel is not exceeded.

The right hand side end of the curvesrepresents the maximum bolted fault cur-rent and corresponding arcing time com-bination that yields 25 cal/cm? (onset ofCategory 4 level).

An overcurrent relay curve is shownbelow the incident energy curves. Thebolted fault current is shown below as anarrow. As long as the fault current and theTCC curve are below the incident energycurve, then the incident energy should beless than the allowable limit.

A set of curves of this type can be de-veloped for each of the types of equip-ment listed in Table 130.7(C)(9)(a).

Arc Flash Hazard Analysisis Imperative

From the previous discussion, we mayconclude that the best approach for deter-mining the rating of the PPE to be usedfor working on energized equipment is toperform an actual flash hazard analysis be-cause the NFPA 70E tables do not con-tain enough details about severalimportant parameters like working dis-tances and equipment configuration(grounding and gaps between conductors).

Variations in these parameters maycause the incident energy exposure to bemuch larger than the level suggested bythe table. Since it is necessary to continueto use the NFPA 70E tables under somecircumstances, then care must be taken tomake sure that they are only applied underthe conditions and limitations stated in thetable footnotes and it is also important toconsider the effect of the variables notlisted in the table(s). o

Albert Marroquin is a senior electricalengineer and testing manager for

Operation Technology, Inc., developerof ETAP Arc Flash analysis software.For more information, visit etap.com.

16 Electrical Products & Solutions • September 2007

Evaluating NFPA 70E.. (Continued from page 15)

Task EquipmentType

VoltageLevel

NFPA 70ETable

130.7(C)(9)(a)Category

IEEE 15842002

CalculatedCategory

Max Ibf at Fault Location

(kA)

Fault ClearingTime(sec)

Removal of bolted covers (to expose bare energized parts)

Panelboard <240 1 0 25 0.03

Work on energized parts, including voltage testing

PanelboardSwitchboard

> 240 2 0 &1 25 0.03

insertion or removal of individualstarter “buckets” from MCC”

MCC < 600 3 3, 4, & >4 65 0.330

inserting or removal (racking) ofCB from cubicles, doors open2 Switchgear < 600 3 3, 4, & >4 65 1.000

inserting or removal (racking) ofCB from cubicles, doors open1 Switchgear > 1000 4 3, 4, & >4 25 0.530

Table 2: Summary of comparisons made between Table 130.7(C)(9)(a) and IEEE 1584 results

Note: The IEEE 1584 2002 results were obtained using only typical gaps and working distancesfor the type of equipment being modeled. The maximum bolted fault current and clearing timesare taken from notes 1, 2, 3, 4 & 5.Note 1: 36 inch working distance. Note 2: 24 inch working distance.

Figure 3: Incident Energy curves for typical LVMCC and Switchgear