03 - Calculating Arc Flash Energy for Systems Upto 250V

and PPEfor Systems

The 2002 IEEE-1584 Guide for the calculation of Arc Flash Hazards provides equations for the calculation ofincident energies for equipment rated 208V to 15kV. The guide has qualifiers for systems rated less than 240Vserved by transformers less than 125kVA due to the lack of relevant test data. NFPA 70E 2009 also providesqualifiers for systems rated

Only five (5) tests were performed at 240V and only one test at 208V. The results of the tests were insufficientto develop an accurate statistical model. Their summary statement was based more on confidence in the 400Vmodel and the engineering knowledge that 208V systems can experience rare burn downs, than anycorrelation with actual

3DJH(DV\3RZHU//&_&DOFXODWLQJ$UF)ODVK(QHUJLHVDQG33(IRU6\VWHPV9

Figure 1

As can be seen in Figure 2, and 3, these requirements lead to long arcing times (2.2 and 10 sec) until theprimary fuse/breaker can trip. It should also be noted that this same problem exists with varying degrees for allLV step-down transformers, regardless of the transformer size, because the primary protective device must besized for normal operation. Correction of this problem requires primary breakers with adjustable short time tripsand/or maintenance switch functions to effectively see the low arcing current and clear the fault. This in turnrequires more expensive equipment for these numerous low voltage circuits, making it cost prohibitive for smalltransformers.

The calculated incident energies using the 1584 equations on typical 208V circuit designs provide extremelyhigh incident energy values compared with the actual short circuit capacity of the circuit or measured 480Venergies on the same type of circuit.

Incongruity and/or inconsistency in the calculations, destroys the confidence in arc flash hazard study resultsfor facility managers and reduces compliance from the electricians performing day-to-day tasks.When workerswith years of electrical experience review calculations promoting 12 calories of energy on a 45kVA transformeror 83 calories on the secondary of a 112kVA transformer, as compared to lower values on their 480V systems,it minimizes their acceptance of the results and therefore the critical nature of the overall safety message.

Figure 2

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Figure 3


NFPA 70E 2009, Article 130.1 requires a work permit, arc flash hazard incident energy calculations (or theequivalent hazard risk category [HRC] from Table 130.7), and the arc flash hazard boundary (AFHB) for allenergized work above 50V. The only exceptions to this rule are for diagnostics as defined in Article 130.1(3).However, the incident energy or HRC, and AFB are still required for diagnostic applications so the workerknows the PPE requirements. NFPA 70E, Article 130.3(C) also requires the incident energy or HRC on AFHlabeling for all electrical equipment above 50V.

Since the majority of arc flash hazard studies are performed using software that creates the one-line diagram ofthe facility power system, the preferred method is to include the 240V and 208V circuits as part of the analysis.This provides the user with an NFPA 70E Article 250.2 compliant one-line diagram as well as greatly simplifyingthe analysis and labeling aspects of the study.

Based on NFPA 70E requirements and the lack of applicable IEEE-1584 equations for circuits

Figure 4

These same tests with butted electrodes yielded sustained arcs lasting up to 300ms (18 cycles). This correlateswith historical events where 208V equipment burn downs have occurred. However, the exact circumstances ofthese events are not well known. For this type of scenario to occur, the initial arc blast may very well need tomove or position the bus/electrodes close enough together to maintain a sustained arc for equipment meltdownto occur.

Recent testing by a Ferraz Shawmut1 has shown that with the electrodes terminated in a barrier, a sustainedarc can be maintained for several seconds with as little as 4 KA of bolted fault current. This is the equivalent ofa 45-75kVA transformer. The sustained arc is not a sustained arc blast with dozens of calories of incidentenergy. The initial arc blast where the air is superheated and expelled carrying the vaporized metal, is over inseveral cycles leaving the conductive atmosphere and sustained arc to burn away the electrodes/equipment.The actual energy transfer at 208V while important, is less than at 480V, and is not properly accounted for inthe existing IEEE-1584 equations.

1 Effect of Insulating Barriers in Arc Flash Testing. R.Wilkins, M. Lang and M. Allison, IEEE, 2006 PCIC.


Even though there is still much more to learn about 208V arcing faults, several important facts come to lightwith this recent data.

1. Arc duration is difficult to maintain for more than 10 cycles without some external influence such as abarrier, or butted electrodes.

2. Short duration arcs transfer some energy and should be considered a potential hazard.3. Longer arcing times (sustained arc) at this voltage can be maintained under the right equipmentconfigurations such as barriers. A barrier may be as simple as a breaker termination.

4. The initial arc blast and resulting energy (heat) transfer at 208V is limited and much less than a 480Vblast, indicating energy transfer is a function of power not just current.

5. Barriers can increase the arc energy up to 30% at 208V.6. Low X/R ratios significantly reduce the sustainability of the arc as well as the arcing current and energytransfer. See footnote 1.

7. Gaps larger than 12.7 mm (0.5) such as 32 mm were self extinguishing for lower levels of current evenwith a barrier.

From this information, we can draw several conclusions. The first is that 208V circuits rated

Recalculating Figure 1 using a 0.5s and 1.0s arc duration yielded the following incident energy results at 208V.

Table 1208V MCCBTrip Comparison of IEEE-1584 with Modified 1584 Calculations

Table 2208V FuseTrip Comparison of IEEE-1584 with Modified 1584 Calculations

Calculation Notes for Tables 1 and 2:2* is used based on the worst case requirements.ED = Extreme Danger. Incident energy >40 Cal/cm2Trip: Incident energies were calculated on trip times from properly sized MCCBs and FusesArc = 0.5s: Incident energies were calculated on a fixed arc duration of 0.5 seconds without trip devices.Arc = 1.0s: Incident energies were calculated on a fixed arc duration of 1.0 seconds without trip devices.

FaultedBus

XfmrkVA

SCkA

TripDevice

IeCal/cm2 PPE

IeCal/cm2 PPE

IeCal/cm2 PPE

Fuse Trip Arc = 0.5s Arc = 1.0s

PNL-1 15 1471 25A 0 #0 1.6 #0 3.1 #1

PNL-2 45 3909 80A 1.7 #1 3.3 #1 6.6 #2

PNL-3 75 5329 125A 11.7 #3 4.2 #2 8.4 #3

DPB-D 112 6696 175A 39 #4 5 #2 10 #3

DPB-E 125 6974 200A 66.2 ED 5.1 #2 10.3 #3

DPB-F 300 14647 500A 414.5 ED 9 #3 18 #3

FaultedBus

XfmrkVA

SCkA

TripDevice

IeCal/cm2 PPE

IeCal/cm2 PPE

IeCal/cm2 PPE

Arc = 0.5s Arc = 1.0s

PNL-1 15 1471 25A 3.9 #1 1.6 #1 3.1 #1

PNL-2 45 3909 80A 12.6 #3 3.3 #1 6.6 #2

PNL-3 75 5329 125A 17.8 #3 4.2 #2 8.4 #3

DPB-D 112 6696 175A 81.1 ED 5 #2 10 #3

DPB-E 125 6974 200A 103 ED 5.1 #2 10.3 #3

DPB-F 300 14647 500A 704.4 ED 9 #3 18 #3


Several things stand out about these calculations. First are the extremely high incident energies when actualbreaker or fuse trip times are used without a MAX time limit.When MAX arc duration limits are used, more realis-tic energy levels are developed.These levels range from 0.0 to 5.1 calories for a 30 cycle arc duration and can bean order of magnitude less than MCCB and fuse trip times. Figure 5A and 5B below illustrates the comparison.

Figure 5A (208V)

Figure 5B (208V)

1

10

100

1000

0 2000 4000 6000 8000 10000 12000 14000 16000

Cal

/cm

2

Bolted Fault kA

IEEE-1584 MCCB Trip

IEEE-1584 Fuse Trip

IEEE-1584 Arc Duration 0.5s


1

10

100

1000

15 45 75 105 135 165 195 225 255 285

Cal

/cm

2

Transformer kVA

IEEE-1584 MCCB Trip

IEEE 1584 Fuse Trip




Figure 6 compares 208V and 480V incident energies using the IEEE-1584 equations and calculated MCCB triptimes. 480V incident energy information is provided in Tables 3 and 4 below. As can be seen, the 208V incidentenergies are nearly an order of magnitude higher than the 480V energies. This is because the 1584 equationsrely only on current magnitude and arc duration only, and do not account for voltage (power) or if the arc can besustained. Note: The discontinuities associated at 75kVA are due to the instantaneous trip characteristics of theprotective device.

Figure 6

Table 3480V MCCBTrip Comparison of IEEE-1584 with Modified 1584 Calculations

FaultedBus

XfmrkVA

SCkA

TripDevice

IeCal/cm2 PPE

IeCal/cm2 PPE

IeCal/cm2 PPE

Arc = 0.5s Arc = 1.0s

PNL-1 15 1471 25A 1.3 #1 1.3 #0 3.3 #1

PNL-2 45 3909 80A 1.7 #1 1.7 #0 8.2 #3

PNL-3 75 5329 125A 0.2 #0 0.2 #0 11 #3

DPB-D 112 6696 175A 22.3 #3 13.4 #3 13.4 #3

DPB-E 125 6974 200A 28.5 #4 13.9 #3 13.9 #3

DPB-F 300 14647 500A 167.7 ED 27.6 #4 27.6 #4

0.1

1

10

100

1000

0 2000 4000 6000 8000 10000 12000 14000 16000

Cal

/cm

2

Bolted Fault kA

1584 (208V) MCCB Trip

1584 (480V) MCCB Trip

1584 (480V) 2 Sec time


Table 4480V FuseTrip Comparison of IEEE-1584 with Modified 1584 Calculations

Calculation Notes for Tables 3 and 4:2* is used based on the worst case requirements.ED = Extreme Danger. Incident energy >40 Cal/cm2Trip: Incident energies were calculated on trip times from properly sized MCCBs and Fuses2 Sec Limit: Incident energies were calculated based on actual trip times with a 2 sec MAX.Arc = 2.0s: Incident energies were calculated on a fixed arc duration of 2.0 seconds without trip devices.

Figure 7A and 7B compares both 208V and 480V incident energies at bolted fault currents and each transformerkVA using the modified IEEE-1584 arc duration times. This graph clearly illustrates the 2 second rule applied at480V and the proposed 0.5 second (30 cycles) rule for voltages less than 250V. For transformers less than125kVA, the incident energies remain below 5 cal/cm2 equating to a maximum #2 PPE level. This isconservative compared to NFPA 70E Table 130.7(C)(9), but realistic considering the need for a safety factor tobe applied. Future testing may yield sufficient information to eliminate the 3X arc duration safety factor.

Figure 7A

0

5

10

15

20

25

30

35

40

0 50 100 150 200 250 300 350

Cal

/cm

2

Transformer kVA

4. 1584 (208V) 2.0 Sec

3. 1584 (480V) 2.0 Sec

2. 1584 (208V) 1.0 Sec

1. 1584-(208V) 0.5 Sec

FaultedBus

XfmrkVA

SCkA

TripDevice

IeCal/cm2 PPE

IeCal/cm2 PPE

IeCal/cm2 PPE

Arc = 0.5s Arc = 1.0s

PNL-1 15 1471 25A 0 #0 0 #0 3.3 #1

PNL-2 45 3909 80A 0 #0 0 #0 8.2 #3

PNL-3 75 5329 125A 0.3 #0 0.3 #0 11 #3

DPB-D 112 6696 175A 1.4 #1 1.4 #1 13.4 #3

DPB-E 125 6974 200A 2.9 #1 2.9 #1 13.9 #3

DPB-F 300 14647 500A 17.9 #3 17.9 #3 27.6 #4


More conservative results could be obtained by using a 1.0 second (60 cycles) arc duration; however, thiswould require a #3 PPE for many 208V circuits which is not warranted by experience or by test results.

Figure 7B also shows energy results obtained in the Ferraz testing with a 0.1 second trip time for comparison.

Figure 7B

There are two techniques in EasyPower in which the 0.5 second method can be implemented.

The first is through the Arc Flash Controls Dialog in the Short Circuit focus. See Figure 8. Under the MAXTimes (sec) section, select 0.5 seconds for the

Figure 8

Figure 9


ted.balderreeTypewritten Text

Both options allow the EasyPower user unlimited flexibility in modeling circuits rated

by NFPA 70E Article 205.2 and 120.2(F)(1) for LOTO procedures, etc., would have to be drawn using a CADsystem or hand drawings.

Option 3: NFPA 70ETable 130.7NFPA 70E Article 130.3 Exception No.2 allows Tables 130.7(C)(9), 130.7(C)(10), and 130.7(C)(11) to be usedfor determination of PPE requirements in lieu of a detailed incident energy analysis. Based on recent

Three methods have been presented to handle arc flash hazard incident energy calculations for circuits rated

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03 - Calculating Arc Flash Energy for Systems Upto 250V