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An arcing fault is the flow of current through the air between phase conduc-

tors or between phase conductors and neutral ground. An arcing fault can release tremen-dous amounts of concentrated radiant energy at the point of the arcing in a small fraction of a second, resulting in extremely high tem-peratures, a tremendous pressure blast, and shrapnel hurling at high velocity. An electrical safety program, which contains an arc flash study, is crucial to minimizing injury that could result from an arc flash or blast. The safety program will define the equipment the employee will wear and the manner in which he or she will safely work. This paper looks at an arc flash study that consists of short-cir-cuit analysis, protective device coordination, and arc flash analysis for meeting OSHA and NFPA 70E compliance standards.

OSHA and NFPA 70EThe Occupational Safety and Health Administration (OSHA) maintains that it is the employer’s responsibility to provide a safe work environment for its employees.1

Working near or on electrical equipment is not considered, by its nature, a safe activity. However, there is a consensus standard for electrical safety in the workplace. This stan-dard is NFPA 70E, the Standard for Electrical Safety in the Workplace, which was developed by the National Fire Protection Association (NFPA) in cooperation with OSHA.2 A good way to look at it is that the National Electric Code (NEC), also known as NFPA 70, is a code for safe design and installation,2 whereas the Standard for Electrical Safety in the Workplace (NFPA 70E) relates to safety practices after the electrical installation is complete and the personnel are in the production and mainte-nance mode of operation.

NFPA 70E, 2004 edition, is comprised of four chapters and 13 annexes. The four chap-ters are outlined below.

• Chapter 1: Safety-Related Work Prac-tices.

• Chapter 2: Safety-Related Maintenance Requirements.

• Chapter 3: Safety Requirements for Special Equipment.

• Chapter 4: Installation Safety Require-ments.3

This paper is focused on chapter 1 of NFPA 70E, specifically Article 130.3, which states that a flash hazard analysis shall be done in

order to protect personnel from the possibil-ity of being injured in an arc flash.4

The NEC, Article 110.16 Flash Protection, refers to the labeling that needs to be placed on the electrical equipment and is summa-rized in the following paragraph:

Electrical equipment, such as switchboards, panelboards, industrial control panels, meter socket enclosures, and motor control centers that are in other than dwelling occupancies, and are likely to require examination, adjust-ment, servicing, or maintenance while ener-gized shall be field marked to warn qualified persons of potential electric arc flash hazards. The marking shall be located so as to be clear-ly visible to qualified persons before examina-tion, adjustment, servicing, or maintenance of the equipment.

Furthermore, the NEC actually refers to the NFPA 70E standard in Fine Print Note (FPN) No. 1 following Article 110.16:

FPN No. 1: NFPA 70E-2004, Standard for Electrical Safety in the Workplace, provides assistance in determining severity of potential exposure, planning safe work practices, and selecting personal protective equipment.5

The labeling of panels and elec-trical equipment is required by the NEC. OSHA, in cooperation with

Arc Flash Study: The Six Steps

An arcing fault can release tremendous amounts of

concentrated radiant energy, resulting in an immense

pressure blast and hurling shrapnel. This paper

discusses arc flash analysis, short-circuit analysis, and

protective coordination studies for meeting OSHA and

NFPA 70E compliance standards.

THiS ArTiclE iS AvAilAblE ONliNE AT www.AiST.Org FOr 30 dAyS FOllOwiNg PublicATiON.

Authors

Timothy J. Lemoine (left) and Ronald G. Studtmann (right), �atri�� �e�hn�l�gie�� �n���atri�� �e�hn�l�gie�� �n��, �aumee, Ohi� (t�lem�ine�matri��ti���m, rg��tudtmann�matri��ti���m)t�lem�ine�matri��ti���m, rg��tudtmann�matri��ti���m)

112 ✦ Iron & Steel Technology

the NFPA, requires that arc flash information be determined and documented for protec-tion of personnel.6 The best way to convey this information is through the use of arc flash labels. The labels are the end product of the arc flash study projects that companies have undertaken or will undertake in the near future in order to be in compliance. The label-ing is the cornerstone of any NFPA 70E safety program; however, there are several project steps prior to printing the labels. These steps are listed below, and it should be noted that the arc flash analysis is only one of the steps of an entire arc flash study.

1. Field verification and audit. 2. Update one-line diagrams. 3. Software modeling and design. 4. Short-circuit analysis. 5. Protective device coordination

study. 6. Arc flash analysis.

Step 1: Field Verification and AuditThe field verification and auditing step is critical to the entire study and provides the information required to complete the other steps of the project. Prior to the engineering firm’s arrival on-site, the plant will need to supply its existing one-line diagrams and the equipment locations on a plan view drawing. Typically, the engineer will spend a few hours reviewing these to become familiar with the equipment and the plant layout. The purpose of this step is for the engineer to verify the validity of the one-line diagram and to obtain any additional information required for the computer model.

As a standard practice, the engineer will work with one or two electricians to gather and document the data. Personal protective equip-ment (PPE) will need to be worn and applied per NFPA 70E Table 130.7(C)(9)(a) and Table 130.7(C)(10).7 These NFPA tables need to be utilized, since there are no labels at this point. The purpose of the project is to produce the labels so the employees will not need an NFPA table to determine the hazard category.

The engineer will document the data as the electricians gather it and will prompt the elec-tricians for additional information. In most cases, at the higher voltages, the engineer is no longer “qualified” and will need to stay beyond the flash protection boundary (FPB). The project team will do their field work when the equipment is in an electrically safe work condition, unless the work falls into the vari-ance to this rule as discussed in NFPA 70E, Article 130.1 Justification for Work.8

Some examples of the field work data col-lection tasks are:

• Transformers: rating, impedance, volt-ages.

• Low-voltage/medium-voltage (LV/MV) switchgear: nameplate data, withstand rating of bus, interrupt rating of the switchgear circuit breakers.

• Motor control center (MCC): name-plate data, main circuit breaker data, manufacturer, model number.

• MCC circuit breakers feeding down-stream panels: manufacturer, model number.

• Distribution panels: manufacturer, model number, circuit breaker data for all panel circuit breakers.

• Circuit breakers: manufacturer, model number, settings, ratings (i.e., amper-age and interrupt rating).

• Circuit breaker trip units: type, manu-facturer, model number, settings (i.e., long time pickup and delay, short time pickup and delay, instantaneous, ground pickup and delay).

• Protective overcurrent relay: manufac-turer, model number, settings.

• Current transformers (CTs): CT ratio. • Locate equipment on a plot plan. • Conductor length where obtainable.

Step 2: Update One-line DiagramsThis step simply takes some of the informa-tion that was gathered during the field work and places it permanently on the drawing. In most cases, this is a CADD drawing and the changes normally take less than a day. This step is not crucial, but it is a good prac-tice in order to preserve the data that took significant time and effort to gather. A small amount of effort now will eliminate the need for additional field time in the future during routine maintenance of the safety program. The remaining data is normally entered into a spreadsheet for archive purposes.

In general, the update of the single-line diagrams will include the following modifica-tions:

• Update electrical equipment that is not depicted correctly.

• Add missing electrical equipment. • Update/add conductor sizing (if space

permits). • Field work details per the level of detail

of the original single-line (i.e., trans-former ratings, impedance).

Step 3: Software Modeling and DesignAlmost all the arc flash analysis projects utilize some type of software program to perform

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the calculations. These commercially avail-able programs usually consist of a graphical user interface (GUI) and add-on electrical transient analysis modules. This step utilizes the GUI to input the data. This interface will enable the user to mimic the one-line diagram in the graphical software. Each bus or node has accompanying fields to input data into the database that comes with the software. Once completed, the model will contain all information gathered from the field verifica-tion and audit.

The modeling process will uncover any missing data that is required but may not have been obtained or available during the field work. At times, another visit or a phone call to the plant may be required to track down the missing data. The electrical soft-ware package normally contains a library of manufacturer data. The make and model numbers gathered during the field work can now be matched to the make and model in the library in order to obtain all the data the program requires. At this point, research by phone or via the Internet may be required in order to track down data for older equip-ment that may not be in the library of the software program. When the missing data is obtained, it can be added to the library for future projects.

Another critical data field that will need to be entered into the model at this point is the conductor length. This is obtained through a combination of the field work and through the scaling of the plot plan. Once the equip-ment is located on this plan, the distance between equipment can be scaled through traditional measurement methods from the drawing.

Step 4: Short-circuit AnalysisIn this step, the maximum fault levels are calculated at various locations in order to identify problem areas. A problem exists if the maximum short-circuit (SC) rating of a device is less than the available SC current that could potentially flow through that device.

At this point, the short-circuit analysis soft-ware module is required. In this module, the user will select which bus to fault and the software will automatically perform the calcu-lations. The user will be warned if any devices are underrated (i.e., SC availability > device rating). This technique is implemented on each selected bus in the model. At the end of this SC analysis, the documentation that can be generated is the interrupt duty summary report, momentary duty summary report, and the recommendations to resolve any under-rated devices.

Step 5: Protective Device Coordination StudyThe next step in performing an arc flash study is to perform a protective device coordination study. The goal of this step is to ensure that the downstream devices (breakers/fuses) clear the short-circuit fault before the upstream devices. This effort will minimize the effect of a fault to the smallest area possible.

The computer software is utilized to gener-ate the graphical representation of the time current characteristic (TCC) curves of the protective overcurrent device under review. To begin, the TCC curves of the protective devices are inserted onto a computer-generat-ed TCC graph consisting of a log-log grid with a time (seconds) vertical axis and a current (amperes) horizontal axis at a given voltage. The TCC curves for majority of the protective overcurrent devices can be found using the computer software’s data library.

At times it will be necessary to research the manufacturer’s literature to obtain the TCC curves for obsolete or superseded protective overcurrent devices. The TCC curves for the obsolete protective overcurrent device can be manually entered into the computer soft-ware’s data library, allowing the TCC curve to be used in the current and future studies. The TCC curves for the protective overcurrent devices are shown on the TCC graph, and the curve is extended to the short-circuit fault cur-rent that was calculated as part of the short-cir-cuit analysis. Transformer damage curves and cable insulation damage curves are plotted on the TCC graph. Also, the TCC curves for pro-tective overcurrent devices at different voltage levels are plotted but are shifted automatically by the computer software by the voltage ratio. In addition, it is quite common that a simpli-fied single-line diagram will be shown on the TCC graph to aid in identifying the portion of the electrical distribution system with which the TCC graph is associated.

An area of an electrical distribution system will be selected for a particular TCC graph. One should attempt to limit the number of TCC curves on the TCC graph for presenta-tion purposes. Too many TCC curves will clut-ter the log-log grid and will make it difficult to read.

The objective of the protective device coor-dination study is to limit the area of the outage due to a short-circuit fault to the smallest prac-tical area. Selective protective coordination is achieved when the protective overcurrent device closest to the fault opens before the upstream protective overcurrent device clears the short-circuit fault. The protective overcur-rent device pickup settings must be evaluated

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with respect to the protected circuit’s ampac-ity or downstream equipment’s continuous current ratings. The fault clearing time of each protective overcurrent device for a fault at a given location must also be evaluated.

For selective coordination between protec-tive overcurrent devices, a time separation between the protective overcurrent devices’ TCC curves must be maintained. The recom-mended time separation for microproces-sor-based and electromechanical overcurrent relays is 0.2 seconds and 0.3 seconds, respec-tively, plus the circuit breaker operating time. The time separation is required to accommo-date CT error, setting errors, tolerances and relay overtravel.

The time separation for low-voltage circuit breakers — including thermal magnetic cir-cuit breakers, electronic trip circuit breakers, and low-voltage power circuit breakers — is handled differently than overcurrent relays. The objective with low-voltage circuit breakers is to avoid the crossing of the TCC curves with no intended time delay.

The time separation for fuses must be such that the total clearing time of the downstream fuse shall be 75% of the minimum melt curve of the upstream fuse.9 The time separa-tion for selective coordination of fuses with source side relays/low-voltage circuit breaker is recommended to be 0.2–0.3 seconds.10 The time separation for selective coordination of fuses with load side relays/low-voltage circuit breaker is recommended to be 0.3 seconds.11 Adjustments to the protective overcurrent device settings can be made to properly pro-tect the cable circuits and the electrical equip-ment, as well as adjusting the time separation between the protective overcurrent devices.

In addition to maintaining time separa-tion between the TCC curves, the magnetic trip setting or instantaneous setting of the protective overcurrent device should be set at the highest possible setting while maintain-ing selective coordination with the upstream overcurrent protective devices. Typically for low-voltage power circuit breakers being uti-lized as a feeder circuit breaker, the breaker trip unit is recommended to have long time, short time and instantaneous characteristics. Typically, for low-voltage power circuit break-ers being utilized as a main circuit breaker, the breaker trip unit is recommended to have long time and short time characteris-tics. In many instances, the breaker trip unit for the main circuit breakers may utilize the instantaneous function to reduce the arc flash incident energy, which will be discussed later in this paper.

Selective protective coordination is more of an art than an exact science. The electrical engineer must review the electrical distribu-

tion system and the associated TCC curves to determine if selective coordination is possible. There will be instances where two overcurrent protective devices are on the same feeder cir-cuit, and therefore selective coordination may not exist. In these instances, the lack of selec-tive coordination will not increase the area of an electrical outage for a fault located on the load side of the feeder circuit and is therefore acceptable.

After this step in the arc flash study, the fol-lowing deliverables are generated. Protective Device Setting sheets document the ratings and settings of fuses, thermal magnetic circuit breakers, electronic circuit breakers, low-volt-age power circuit breaker trip units and protec-tive overcurrent relays. The Protective Device Setting sheets will also indicate changes to the original settings of the protective overcurrent devices. An additional deliverable includes TCC graphs that were generated as part of the protective device coordination study.

Step 6: Arc Flash AnalysisIn this step, the incident energy levels will be determined. The incident energy level is then utilized to define the flash protection bound-ary and the hazard/risk category. In certain instances where the hazard/risk category is elevated, the protective device coordination study may need to be revisited and readjusted in order to lower the hazard/risk category.

Based on the results of the short-circuit analysis and the protective device coordina-tion study, an arc flash analysis is performed on the electrical distribution system under consideration. The arc flash calculations are based on the IEEE-1584 method,12 which is an acceptable method of arc flash calculations per NFPA 70E-2004.13 The computer software calculates the bus arcing current based on the bus short-circuit fault current calculated from the short-circuit analysis. The computer software then utilizes the bus arcing current to determine the fault clearing time of the protective overcurrent device. Based on equa-tions derived from empirical data, the com-puter software calculates the incident energy, the flash protection boundary and the associ-ated hazard/risk categories. It is important to note that the arc flash equations derived from empirical data are dependent on infor-mation entered into the computer model, such as the type of electrical equipment, gap between conductors/buses, distance X factors and working distances.

The hazard/risk categories are based on the hazard/risk categories listed in Table 130.7(C) (11) of the 2004 edition of NFPA 70E, Standard of Electrical Safety in the Workplace.14 The hazard categories are as follows:

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• Category 0 (0.0–2.0 cal/cm2). • Category 1 (2.0–4.0 cal/cm2). • Category 2 (4.0–8.0 cal/cm2). • Category 3 (8.0–25.0 cal/cm2). • Category 4 (25.0– 40.0 cal/cm2). • Beyond category 4 (> 40.0 cal/cm2).

The initial results of the arc flash analysis are reviewed to determine if the calculated hazard/risk categories can be reduced by mak-ing adjustments to the protective overcurrent device settings that were generated as part of the protective device coordination study. It is common that the hazard/risk category can be reduced by decreasing the instantaneous trip setting of the protective overcurrent device

to a value below the bus arcing current. The adjustments are made while maintaining selective coordination between the protective overcurrent devices.

According to Standard IEEE 1584, the 208V and 240V systems supplied by transformers smaller than 125 kVA do not require an arc flash analysis15 due to empirical data docu-menting that the bus arcing current cannot sustain itself under these operating condi-tions. For this situation, it is recommended that the hazard/risk category of 0 be used.

Once the final arc flash analysis is complete, the incident energy reports and the arc flash labels for the electrical equipment are gener-ated. The arc flash labels are affixed to the

Figure 1

E��ample �f a time �urrent �hara�teri��ti� (�CC) �urve�

116 ✦ Iron & Steel Technology

electrical equipment only after the study’s rec-ommendations have been implemented and the facility has established a safety and PPE program for the plant personnel to follow.

The results of arc flash analysis are recom-mended to be documented on the single-line diagrams, along with the calculated short-circuit current values. As a minimum, the

hazard/risk category should be documented on the single-line diagrams. Optionally, the calculated incident energy and the flash pro-tection boundary can also be documented on the drawings.

With the results of the arc flash analysis, the plant personnel will need to adhere to the personal protection equipment that is

Figure 2

E��ample �f an ar� fla��h label�

Example of a Simplified PPE Table

Incident energy level 2 cal/cm2 4 cal/cm2 8 cal/cm2 8 cal/cm2 25 cal/cm2

Hazard/risk category 0 1 2 2* 3

Non-melting or untreated natural fiber

�-��hirt Y Y Y

L�ng-��leeve ��llared ��hirt Y

L�ng pant�� Y Y

Flame-retardant clothing

C�verall�� Y Y Y

Fla��h ��at Y

Other flame-retardant equipment

Hard hat Y Y Y Y

Safety gla����e�� Y Y Y Y Y

Fla��h ��hield Y

Fla��h ��uit h��d Y Y

Hearing pr�te�ti�n Y Y Y

Leather gl�ve�� Y Y Y Y

Leather w�rk ��h�e�� Y Y Y Y

Table 1

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outlined in NFPA 70E-2004, Table 130.7(C) (10). This table lists the protective clothing and personal protective equipment for the various hazard/risk categories. For example, the required protective clothing and PPE for hazard/risk category 2 can be satisfied as fol-lows: flame-retardant long-sleeve shirt (mini-mum arc rating of 8), worn over untreated cotton T-shirt with flame-retardant pants (minimum arc rating of 8), hard hat, safety glasses or goggles, arc-rated face shield or flash suit hood, hearing protection, leather gloves and leather work shoes. The table allows for various combinations of protective clothing and PPE to meet the requirements of a given risk/hazard category. The table should be consulted any time qualified plant personnel are expected to be exposed to bare energized electrical conductors.

In the authors’ experience, the NFPA table can be confusing to some employees and can reference personal protective equipment that may not be provided by the employer. The recommendation to all facilities is to publish their own custom table that can be laminated and put in various locations throughout the facility, such as on electrical equipment, safety boards, hallway walls, etc. This custom table should follow the rules set forth by NFPA 70E and should also reflect the facility’s safety pro-gram that may be more conservative than the NFPA standard. This table can also reflect the PPE that the employer provides and has on hand for the employee, and it can change as new PPE is developed. An example of a simpli-fied PPE table is shown in Table 1. This table eliminates hazard/risk category –1 because the employer wants the employee to wear cat-egory 0 as a minimum. The simplified table also eliminates category 4 because the employ-ees are not allowed to work in areas where the potential incident energy level is greater than 25 cal/cm2.

SummaryAn entire arc flash study is made up of several steps, as identified in this paper. The goal of an arc flash study is to provide correct data

for appropriate labeling of the electrical equipment in a facility. This labeling dictates the level of personal protective equipment that must be worn when working on exposed live parts. The arc flash study and the labels are the cornerstone of a complete electri-cal safety program; however, there is still a significant amount of additional effort that needs to be expended to make the program fully functional.

References1. O��upati�nal Safety and Health A�t, “Se�� 5�

Dutie��,” 29 U�S�C� 654(a)(1)�2. Nati�nal Fire Pr�te�ti�n A������iati�n, “F�rew�rd

t� NFPA 70E,” NFPA 70E, Standard for Electrical Safety in the Workplace, 2004 editi�n, p� 1�

3. Nati�nal Fire Pr�te�ti�n A������iati�n, “Arti�le 90 �ntr�du�ti�n,” NFPA 70E, Standard for Electrical Safety in the Workplace, 2004 editi�n, p� 6�

4. Nati�nal Fire Pr�te�ti�n A������iati�n, “Safety-Related W�rk Pra�ti�e��,” NFPA 70E, Standard for Electrical Safety in the Workplace, 2004 editi�n, p� 25�

5. Nati�nal Fire Pr�te�ti�n A������iati�n, “Arti�le 110�16 Fla��h Pr�te�ti�n,”, NFPA 70: National Electric Code, 2008 editi�n, pp�70–34�

6. Nati�nal Fire Pr�te�ti�n A������iati�n, “Safety-Related W�rk Pra�ti�e��,” NFPA 70E, Standard for Electrical Safety in the Workplace, 2004 editi�n, p� 18�

7. �bid�, pp� 29–33�8. �bid�, pp� 23–24�9. C��per P�wer Sy��tem��, “Pr�te�tive Equipment

Appli�ati�n�� and C��rdinati�n,” Electrical Distribution — System Protection, third editi�n, 1990, p� 83�

10. �bid�, p� 121�11. �bid�, p� 117�12. �he �n��titute �f Ele�tri�al and Ele�tr�ni���

Engineer�� �n��, IEEE Std. 1584-2002, Sept� 23, 2002�

13. Nati�nal Fire Pr�te�ti�n A������iati�n, “Anne�� D,” NFPA 70E, Standard for Electrical Safety in the Workplace, 2004 editi�n, p� 101�

14. Nati�nal Fire Pr�te�ti�n A������iati�n, “Safety-Related W�rk Pra�ti�e��,” NFPA 70E, Standard for Electrical Safety in the Workplace, 2004 editi�n, p� 34�

15. �he �n��titute �f Ele�tri�al and Ele�tr�ni��� Engineer�� �n��, IEEE Std. 1584-2002, Sept� 23, 2002, p� 6� ✦

This paper was presented at AISTech 2007 — The Iron & Steel Technology Conference and Exposition, Indianapolis, Ind., and published in the AISTech 2007 Proceedings.

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