An electrical protection program isan essential component of any power sys-tem. These programs reduce the risk ofdamage to equipment and processes, butmore importantly, they minimize yourstaff’s exposure to potentially-fatal arcflash hazards. One of the core compo-nents of such a program is the ability toidentify and analyze high-risk arc flashareas in your electrical system.

When identifying arc flash hazards,there is one critical issue that must beconsidered: In many electrical facilities,protective device trip settings have beenset only on bolted 3-phase short circuitcriteria. Yet low-voltage arc faults lessthan 1.0kV may produce a current mag-nitude significantly smaller than the cir-cuit’s maximum 3-phase bolted short cir-cuit current. While the released incidentenergy should be smaller at lower currentmagnitudes, overcurrent devices maytake longer to trip, causing the release ofincident energy up to several minutes.

Today, the process of identifying andanalyzing high-risk arc flash areas typi-cally falls under two calculation meth-ods: NFPA 70E and IEEE 1584.Although both methods consider low-current magnitude, each has a differentway of accounting for its effect in the cal-culation of incident energy.

Another significant factor to consid-er is the introduction of CSA-Z462,which has been developed by theCanadian Standards Association.Developed in parallel with the 2009Edition of NFPA 70E, CSA-Z462 will bethe primary calculation method used atCanadian facilities. Because the 2009Edition of NFPA 70E is rooted in the2004 edition, it is fundamentally impor-tant to understand NFPA 70E.

METHOD A: NFPA 70EUnder the NFPA 70E 2004 method,

it is recommended that incident energyfor equipment 600V and below be deter-mined from the “maximum” and “mini-mum” short-circuit currents. In fact, itrecommends a 62% reduction of themaximum available short-circuit currentto determine situations where theupstream overcurrent device could takeseconds or minutes to operate (NFPA

70E 2004 Annex D.6). The reduction percent corresponds

to the industry accepted minimum cur-rent level for self sustaining arc faults.Equation [D.6.2 (a)] can then be used tocalculate incident energy.

METHOD B: IEEE 1584The other method to calculate inci-

dent energy for low-voltage equipment isIEEE 1584TM-2002 and 2004a “IEEEGuide for Performing Arc-Flash HazardCalculations” (sections 5.1 to 5.5). TheIEEE 1584 empirically-derived equa-tions can predict very low arc fault cur-rent values. IEEE 1584 2002 equation 1can be used to determine the magnitudeof the actual arc fault current – unlikeNFPA 70E that uses the available shortcircuit current.

In the simple electrical systemdescribed in this article, the calculatedarcing current magnitude can be as lowas 45% of the maximum available bolted3-phase short-circuit current. The 45%value already accounts for the additional

15% reduction recommended by IEEE1584 for systems with nominal voltagesless than 1000V (section 5.2 of IEEE1584a 2004).

DETERMINING THE HIGHEST INCIDENTENERGY VALUE

One area of concern for arc flashanalysis is the lower magnitude of low-voltage arc faults. Depending on whichmethod is used to determine the incidentenergy results, the results can be very dif-ferent. Yet no matter which method isused for arc flash analysis, it may be nec-essary to run several variations in the arcfault current magnitude. This will helpdetermine with certainty the absolutehighest incident energy value that can bereleased.

It is essential that you consider allthe possible arc flash locations and theprotective devices involved for protectingthe circuit to properly identify low-volt-age arc hazards. In addition, it may be

ASSESSING LOW-VOLTAGE ARC HAZARDSBy Albert Marroquin, ETAP/Operation Technology, Inc.

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Figure 1: Arc Fault at line side of CB5 showing NFPA 70E results

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necessary to run two sets of calculations– one for maximum currents and anotherfor minimum currents.

In Figure 2, we use power systemanalysis software to perform arc flashanalysis at two locations in the system todetermine low-voltage arc hazards. Thissystem has a typical arrangement forovercurrent and short-circuit protection.The 1.5MVA transformer is fed from a177MVAsc utility connection and is pro-tected for short-circuit with a 100-Amp,15.5kV standard-speed fuse located onthe 13.8kV primary voltage side. Thetransformer feeds a 480V switchgearwith a main 2400-Amp power circuitbreaker with a solid state trip device.

The power system analysis softwareis then used to simulate an arc fault onthe switchgear bus bars at the “SWGRB” location. You can see the computerprogram results for a fault at this bususing the IEEE 1584 2004a method inFigure 2.

We also used the NFPA 70E methodto evaluate the arc fault at the same loca-tion for both maximum and minimumexpected short-circuit currents. The pro-tective device expected to trip the arcfault is the main breaker CB5. The resultsof the four different arc fault analyses arelisted in Table 1.

When using the maximum short-cir-cuit current to determine the incidentenergy, the results reveal that because ofthe fast action of the instantaneous partof the solid state trip device in CB5, theincident energy released at the bus is 2.69cal/cm_ with a hazard category of 1,based on NFPA 70E-2004, Table130.7(C). However, if you use the mini-mum short-circuit current, the resultingincident energy can reach as high as 25cal/cm_ (category 4). This is caused bythe much longer clearing time of CB5.

IEEE 1584 predicts hazard category3 results (12.5 to 14.51 cal/cm_) as theworst-case scenarios. The IEEE 1584method provides the more accurateresults in this case since it is using theactual arcing current (Ia) to determine thetime it takes the CB5 breaker to operate.

ESTABLISHING WORST-CASE INCIDENTENERGY

The preceding simulation may notbe adequate to determine the worst-caseincident energy for this low-voltage

equipment. If you simulate an arc fault atthe main breaker compartment, as shownin Figure 1, the incident energy releasedat this location can be much larger sincethe primary protective device would beFuse2 with a longer clearing time.

In Table 2, the results indicate thatthe incident energy released for a faultlocated at the line (incoming) side of thecircuit breaker CB5 is more threateningdue to the longer operating time of thefuse. The Time Current Characteristic

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Arc HazardsContinued from Page 26

Figure 2: Arc Fault at Bus SWGR B showing IEEE 1584 results

Table 1: I.E. for a fault at Bus “SWGR B” @ 18.0 inch working distance e

Table 2: I.E. for a fault at CB5 @ 18.0 inch working distance

Note 1: Ibf or Ia denotes whether the bolted 3-phase short-circuit (Ibf) or the arcing current(Ia) were used to determine the fault clearing time.Note 2: The Fuse total clearing time was determined from the current at the 13.8 kV base.(see Figure 3)

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(TCC) of Fuse2, along with the expectedfault clearing times for the minimum,maximum and arcing fault values, can beseen in Figure 3.

It should be noted that a small reduc-tion in the fault current leads to a muchlonger total clearing time. In fact, therehave been several documented arc flashincidents in low-voltage equipment thathave lasted for several seconds or evenminutes due to slow response ofupstream protective devices.

An effective method to minimize thehazard associated with low-current mag-nitude arc faults in low-voltage equip-ment is to change the settings of the pro-tective devices to decrease the arc faultclearing time. In general, main power cir-cuit breakers do not have their instanta-neous response enabled because of coor-dination with devices downstream.

In the case of the arc faults at thebus, temporarily setting the instanta-neous pickup of the main power circuitbreaker to the left of the lowest expectedarc fault current value should significant-ly reduce the fault clearing time. Some

devices on the marketplace are availablewith “Maintenance” modes that automat-ically override normal protective devicecoordination settings and introduce aninstantaneous pickup setting.

This instantaneous pickup setting islow enough to pick up the arc fault cur-rent magnitude. When the energizedelectrical work or maintenance is com-plete, the main protective device can beset back to normal operation settings.The maintenance mode settings and thefault arrow marked as “Minimum ArcingCurrent” shows the absolute lowest arc-ing current magnitude in Figure 4.

Although adding and reducinginstantaneous pickup settings is one wayto reduce the hazard associated with low-voltage arcs, you may also consider usinglight detecting relays or “Arc FlashSensors”. These devices detect lightemitted by the arc fault. If there is an arc,the light sensors will send a trip signal torelay, which can take less than 2 cycles totrip the breaker. Arc sensors can also beused with overcurrent relays. The arcsensor relay would only send the tripping

signal if both overcurrent and light sen-sors detect the presence of an arc fault. Aset-up like this will also reduce nuisancetrips caused by light sources not relatedto arc faults.

MINIMIZING RISKOn a final note, you should give seri-

ous consideration to not performing ener-gized work in high-risk locations thatdepend on upstream overcurrent protec-tive devices to trip the fault unless youminimize the hazard in some way. Thisapproach for reducing incident energy isone more method to significantly reducethe risk of arc flash incidents and mini-mize your staff’s exposure to potentiallylethal arc flash occurrences.

In short, whichever analysis methodis used (IEEE 1584 or NFPA 70E, or acombination of both) it is imperative thatyou consider extremely low magnitudesof the arc faults in low-voltage equip-ment. Both the maximum and minimumarc fault current levels must be analyzedto accurately assess the hazard of ener-gized electrical work.

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Figure 3: Fuse TCC showing long times at steep portions ofthe curve

Figure 4: TCC showing Maintenance mode for CB5

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