calculating arc flash 2015

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C A LC U L AT I N GARC FLASH ENERGIES

and PPE for Systems <250V

For those facilities which have the majority of

equipment rated less than 250V, the standardiza-

tion of a #2 PPE level for all circuits rated less than

250V provides an easy-to-implement solution.

®

© 2005-2015 EasyPower LLC | 7730 SW Mohawk St, Tualatin, OR 97062, USA | Tel. 503-655-5059 · Fax 503-655-5542 | www.EasyPower.com2

AbstractThe 2002 IEEE-1584 Guide for the calculation of Arc Flash Hazards provides equations for the calcula-tion of incident energies for equipment rated 208V to 15kV. The guide has qualifiers for systems rated less than 240V served by transformers less than 125kVA due to the lack of relevant test data. NFPA 70E 2009 also provides qualifiers for systems rated <240V. Since neither the Standard nor the Guide address key issues relevant to providing 70E compliance for these systems, it has caused considerable confusion in industry. This paper will provide recommended calculation techniques and suggested methods for compliance with NFPA 70E, Z462-08, and NEC 110.16 for systems less than 250V.

Key Words: Arc Flash, PPE, Incident Energy, NFPA 70E, IEEE-1584

The 2002 IEEE-1584 Guide for the calculation of Arc Flash Hazards has become the industry standard for all low and medium voltage enclosed equipment. In an effort to provide essential calculation techniques for industry in a timely manner, the standards clarify that some voltage levels and circuit types could not be properly tested and modeled for the standard. Voltages less than 250V, including 240V, and 208/120V three phase did not receive sufficient testing to allow proper models to be de-veloped. The IEEE Working Group has not conducted any single-phase testing at this point; therefore, the calculations in this paper apply only to three-phase faults. The following excerpt from the IEEE-1584 tests summarizes.

“It was difficult to sustain an arc at the lower voltages. An arc was sustained only once at 208V in a 508 mm × 508 mm × 508 mm box. In all other tests with that box and the 305 mm × 368 mm × 191 mm box, the arc blew itself out as soon as the fuse wire vaporized. An arc was sustained several times at 215V in a device box (100 mm × 100 mm × 50 mm size).41 It appeared from the arc-flash photos from the 305 mm × 368 mm × 191 mm box that testing arcs usually jumped from the electrodes to the box wall and from another point on the box wall back to another electrode. The magnetic forces created by these arc currents forced them away from each other and into the box wall.

Arc faults can be sustained at 208V and have caused severe injuries with very high short-cir-cuit current applications in meter enclosures. A meter enclosure is small and tends to con-fine an arc more than laboratory test boxes with no door. Used equipment at 208V was not tested, but it is recognized that many types of equipment have relatively small open spaces between components, such as the space in a panelboard between the circuit breakers and the wall of the enclosure.

While the accuracy of the [IEEE-1584] model at 208V is not in the same class with the accuracy at 250V and higher, it will work and will yield conservative results. (Emphasis and [IEEE-1584] added) The arc-flash hazard need only be considered for large 208V systems: systems fed by transformers smaller than 125kVA should not be a concern.”


Only five (5) tests were performed at 240V and only one test at 208V. The results of the tests were insufficient to develop an accurate statistical model. Their summary statement was based more on confidence in the 400V model and the engineering knowledge that 208V systems can experience rare burn downs, than any correlation with actual <250V test results.

In an effort to provide some coverage, the 1584 Working Group deemed systems served by trans-formers 125kVA or greater and 240V or greater should use the standard low voltage equations. The Working Group also advises as noted above that 208V systems with high short circuit currents use the low voltage model even though the results will be conservative. The assumption is that the higher short circuit current of these systems regardless of voltage will sustain the arc, even though this has not been verified by test. This has left a gaping hole for many electrical systems which have 240V, and 208V and 120V circuits served by transformers less than 125kVA. These typically include 112.5, 75, 45 and other smaller kVA transformers.

Note: The IEEE-1584 provides exceptions for systems “below 240V and served by transformers, <125kVA” by stating that these systems need not be considered in the calculations. See Section 4.2. NFPA 70E 2009 provides exceptions for systems “<240V and served by transformers, <125kVA” See Article 130.3. While the inclusion rather than exclusion of 240V in 70E seems minor, this has generated significant confusion in industry.

Based on the fact that test data is extremely limited (there is only 32V difference between the two levels) and the IEEE states that the accuracy class is not the same for 250V and above, this paper will consider both 208 and 240V as the same class. This effectively re-defines a class of equipment as less than 250V, eliminating the previous confusion.

Many power system engineers have tried to use the standard equations for these circuits with poor results. One of the major problems is that the equations assume that once the arc is initiated, it can be sustained indefinitely until an upstream breaker/fuse trips. For typical 240V and 208V circuits as shown in Figure 1, this leads to extremely high arc flash incident energies at the equipment. This is because the transformer primary device, either fuse or breaker, must be sized to carry the transform-er full load amperes and magnetizing inrush for normal operation.


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

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

Incongruity and/or inconsistency in the calculations destroys the confidence in arc flash hazard study results for facility managers and reduces compliance from the electricians performing day-to-day tasks. When workers with years of electrical experience review calculations promoting 12 calories of energy on a 45kVA transformer or 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 1


Figure 2


Figure 3


NFPA 70E RequirementsNFPA 70E 2009, Article 130.1 requires a work permit, arc flash hazard incident energy calculations (or the equivalent hazard risk category [HRC] from Table 130.7), and the arc flash hazard boundary (AFHB) for all energized work above 50V. The only exceptions to this rule are for diagnostics as de-fined in Article 130.1(3). However, the incident energy or HRC, and AFB are still required for diagnos-tic applications so the worker knows the PPE requirements. NFPA 70E, Article 130.3(C) also requires the incident energy or HRC on AFH labeling for all electrical equipment above 50V.

Since the majority of arc flash hazard studies are performed using software that creates the one-line diagram of the 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 dia-gram as well as greatly simplifying the analysis and labeling aspects of the study.

Based on NFPA 70E requirements and the lack of applicable IEEE-1584 equations for circuits <250V, the system study consultant is left without clear guidance on how to calculate incident energy and label equipment for this voltage range.

NFPA 70E Compliance OptionsNFPA 70E Article 130.3 requires an arc flash hazard analysis for the calculation of the arc flash bound-ary and personal protective equipment (PPE) for each location in the facility. The latter requirement corresponds directly to the incident energy level in calories calculated with the IEEE-1584 equations. The energy in calories can then be matched to a manufacturer’s equivalent FR rated clothing and PPE equipment, or cross referenced in 70E Table 130.7(C)(11) Protective Clothing Characteristics to deter-mine the PPE level.

Using the modified IEEE-1584 calculations provides the most comprehensive approach. When cal-culations cannot be performed, NFPA 70E Table 130.7(C)(9), (10), and (11) can be adopted, or a stan-dardized conservative level of PPE required for all equipment rated <250V, though these options have their limiting drawbacks. Each option will be explained in detail along with the proper AFH labeling method so an intelligent choice can be made for any safety program.

Option 1: Modified IEEE-1584 Calculations (Recommended Option)IEEE-1584, 2002, created an empirically derived model for arc flash calculations between 208V and 15kV and bolted fault currents between 700A and 106,000A. The model is based on statistical analysis, test results and curve fitting programs. Details of the model can be found in sections 5.1, 7.5, and clause 9 of the 1584 guide. As stated above, testing for the 2002 guide provided minimal statistical data for voltages <250V.

Recent AFH testing by a large utility has yielded a significant amount of data for systems rated <250V. In Figure 4 below, the results of over 100, 208V tests with bolted short circuit currents ranging from 18-40 kA, and a gap as small as 1/2” indicates that the arc does not self sustain past 10 cycles using an open tip model. As the gap increases, arc time diminishes.


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

Recent testing by a Ferraz Shawmut1 has shown that with the electrodes terminated in a barrier, a sustained arc can be maintained for several seconds with as little as 4 KA of bolted fault current. This is the equivalent of a 45-75kVA transformer. The sustained arc is not a sustained arc blast with dozens of calories of incident energy. The initial arc blast where the air is superheated and expelled carrying the vaporized metal is over in several 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 in the existing IEEE-1584 equations.

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

Figure 4


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

1. Arc duration is difficult to maintain for more than 10 cycles without some external influence such as a barrier, 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 equip-ment configurations 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 480V blast, 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 energy transfer. See footnote 1.

7. Gaps larger than 12.7 mm (0.5”) such as 32 mm were self extinguishing for lower levels of cur-rent even with a barrier.

From this information, we can draw several conclusions. The first is that 208V circuits rated <125kVA have inherently low X/R ratios, typically in the region of 0.3–2.0 with the highest X/R being close to the transformer secondary terminals. As conductor impedance is added to the circuit (say to a downstream panel or machine), the X/R ratio drops significantly and the ability of the circuit to maintain an arc drops significantly also.

The second conclusion is that equipment construction plays a significant role in whether a 208V arc can be maintained. If a barrier does not exist for the arc to terminate on, it will typically self extinguish in less than ten cycles. If the arc is sustained, at the end of a barrier, it may generate sufficient heat to cause equipment burn down. The latter arcing phenomena is not considered to be the same magnitude of danger to the worker as the initial arc blast.

We cannot re-write the IEEE-1584 equations to obtain realistic results for <250V circuits, however, we can suggest modifications based on current guide lines already adopted by the IEEE Working Group. By taking the IEEE-1584 two second concept (Annex B.1.2)2 and using an arc duration with an applied safety margin that is reasonable by engineering standards, realistic results are obtained. For the pur-poses of this paper, a safety margin of 3-6 times the 10 cycle arc duration time (0.5s–1.0s) can be backed up with sound engineering rationale for circuits rated <250V. A 0.5 second arc time essentially splits the difference between a typical self extinguishing arc and a sustained arc duration. In most cases it will err on the conservative side. Because the actual arc blast at this voltage is short, and the energy transfer small compared to 480V, the 0.5s arc time using the standard equations is a conservative energy calcu-lation as shown below.

2 The IEEE 2 second rule is based on the worker removing themselves from the initial working distance within 2 seconds or being blown back from the initial working distance from the arc blast. It is not an arc duration limit. However, the concept can be easily applied in Easy-Power® at different voltage levels or specific equipment for accurate answers.


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

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 MCCB’s 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.

Table 1208V MCCB Trip Comparison of IEEE-1584 with Modified 1584 Calculations

Table 2208V Fuse Trip Comparison of IEEE-1584 with Modified 1584 Calculations


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

Figure 5A (208V)

Figure 5B (208V)


Figure 6 compares 208V and 480V incident energies using the IEEE-1584 equations and calculated MCCB trip times. 480V incident energy information is provided in Tables 3 and 4 below. As can be seen, the 208V incident energies are nearly an order of magnitude higher than the 480V energies. This is because the 1584 equations rely only on current magnitude and arc duration, and do not ac-count for voltage (power) or if the arc can be sustained.

Note: The discontinuities associated at 75kVA are due to the instantaneous trip characteristics of the protective device.

Figure 6

Table 3480V MCCB Trip Comparison of IEEE-1584 with Modified 1584 Calculations


Table 4480V Fuse Trip 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 MCCB’s 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.

Figures 7A and 7B compare both 208V and 480V incident energies at bolted fault currents and each transformer kVA using the modified IEEE-1584 arc duration times. This graph clearly illustrates the 2 sec-ond rule applied at 480V and the proposed 0.5 second (30 cycles) rule for voltages less than 250V. For transformers less than 125kVA, the incident energies remain below 5 cal/cm2 equating to a maximum #2 PPE level. This is conservative compared to NFPA 70E Table 130.7(C)(9), but realistic considering the need for a safety factor to be applied. Future testing may yield sufficient information to eliminate the 3X arc duration safety factor.


More conservative results could be obtained by using a 1.0 second (60 cycles) arc duration; however, this would 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.

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 MAX Times (sec) section, select 0.5 seconds for the <250V entry field. This is a global function that limits the arc duration if the protective device does not trip within the 0.5 second limit. This is the pre-ferred option because it allows a protective device to clear the fault first, before the limit is imposed.

The second option is found in each equipment dialog box. See Figure 9. Under Trip Times for this Bus section, select User-defined Times in the drop down menu, then in the Upstream Devices: row, select 0.5 seconds for the Time (s) field. This option should be used to “fix” the arc duration to a spe-cific time for the particular equipment selected. Using this function overrides any protective device that may trip for the selected equipment.

Figure 7B


Figure 9

Figure 8


Both options allow the EasyPower® user unlimited flexibility in modeling circuits rated <250V, and <125kVA. By modifying the IEEE-1584 method to conform to recent testing, several advantages are obtained in the arc flash study process. They include:

• A complete EasyPower one-line diagram and software model of the facility’s electrical sys-tem can be developed, providing AFH calculations and labeling for all equipment through 208/120V circuits. This eliminates the confusion of calculations for 480V and above equip-ment, and then implementing the NFPA 70E tables for circuits less than 250V.

• An EasyPower one-line diagram and software model conforms more strictly to NFPA 70E 205.2, and 120.2(F)(1) requirements and is easily modified when the system changes.

• An EasyPower one-line diagram and software model provides work permits and safety pro-gram information directly from the one-line.

• The modified IEEE-1584 arc duration times yield more realistic results based on recent test data, yet remain conservative as compared to Table 130.7(C)(9).

• EasyPower provides asset management capability in the software one-line model.

• An EasyPower one-line diagram and software model provides the easiest way to update incident energy calculations and labeling

Option 2: Level #2 PPEThe easiest method to implement is the adoption of a standard daily wear #2 PPE level. This level (min 8 cal/cm2) provides compliance with Table 130.7(C)(9), as well as the modified IEEE-1584 calculations up to a 125kVA transformer size. This method lends itself well to those companies standardizing on a Level 2, 4, and Extreme Danger PPE and labeling system.

An advantage of this method is that a safety program can be developed where only 480V and higher voltage equipment is labeled, and equipment rated <480V is not labeled. The safety program then dictates that all electrical equipment not labeled is a minimum #2 PPE with specific limited, restricted, prohibited, and arc flash hazard approach boundaries as defined in 70E. Workers can carry a card refer-encing this information as needed. This method eliminates the time consuming and expensive task of labeling all equipment rated 50V-240V.

NFPA 70E Article 130.3(C) Equipment Labeling requires that all equipment shall be field marked with a label containing the available incident energy or the required level of PPE. The Level #2 PPE method meets this requirement when used in conjunction with the safety program and training of all em-ployees and contractors that non-labeled equipment carries an understood standardized label with a specific PPE requirement (#2), and specific limited, restricted, prohibited, and arc flash hazard approach boundaries.

OSHA defines NFPA 70E as the minimum acceptable safety requirements. Going above and beyond the 70E requirement to increase the safety of the worker is always acceptable practice.


Drawbacks to this option: It may be difficult to get compliance in very hot, humid climates, even though loose fitting 11 Cal/cm2 coveralls are efficient and cool in most areas. Also, this method does not pro-vide the detailed one-line modeling associated with calculations performed with analysis software. One-line diagrams required by NFPA 70E Article 205.2 and 120.2(F)(1) for LOTO procedures, etc., would have to be drawn using a CAD system or hand drawings.

Option 3: NFPA 70E Table 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 used for determination of PPE requirements in lieu of a detailed incident energy analysis. Based on re-cent <250V test information, Table 130.7(C)(9) should provide adequate protection for circuits served by transformers rated <125kVA. This is based on the limited short circuit current available and the limited arc duration shown in the tests. For circuits served by transformers larger than 125kVA, the 1584 equa-tions should be used with or without a modified arc duration time to insure an adequate PPE level.

Table 130.7(C)(9) for Panelboards and Other Equipment rated 240V and lower is limited by Note 1 on page 34 of NFPA 70E. Note 1 states that this section applies only to maximum short circuit currents of 25 kA with a duration of 2 cycles (0.033 sec) or less. This effectively prohibits the application of the table to all equipment fed by a transformer and protected on the primary side of the transformer as shown in Figure 10, or any application where the trip time could be over 2 cycles.

Drawbacks to this option: For the reasons presented here and others, the NFPA 70E tables can provide non-conservative or inadequate protection as compared to the calculated incident energy require-ments of IEEE-1584, and should be used with caution.

© 2005-2015 EasyPower LLC | 7730 SW Mohawk St, Tualatin, OR 97062, USA | Tel. 503-655-5059 · Fax 503-655-5542 | www.EasyPower.com

SummaryThree methods have been presented to handle arc flash hazard incident energy calculations for circuits rated <250V and served by transformers <125kVA. All three methods meet compliance objec-tives found in NFPA 70E, and Z462, as well as OSHA requirements; however, using the tables or stan-dardized PPE has limiting drawbacks. Determining which method is best suited should be dictated by the end user objectives.

Modifying arc duration limits using the IEEE-1584 equations has many advantages for both the study consultant and the facility maintenance department as itemized in this paper. Selection of a 0.5 sec-ond arc duration time should provide a balance between conservative yet realistic incident energies in all circumstances based on recent test data.

For those facilities which have the majority of equipment rated less than 250V, the standardization of a #2 PPE level for all circuits rated less than 250V provides an easy-to-implement solution. Facility managers should be aware that detailed one-line diagrams are still required for 70E and Z462 com-pliance to provide safety information for LOTO procedures and to identify voltages, back feeds, and energy storage devices such as capacitors. The NFPA-70E Table method should be used with caution since many 208V circuit protective devices may have extended tripping times.

This paper also presented standardization of the voltage range used between the IEEE-1584 guide and the NFPA 70E 2009 Standard for circuit served by transformers <125kVA. In order to minimize confusion and bring the calculations in line with recent testing, this author suggests that the IEEE-1584 guide should consider a wording change to <250V, rather than the current <240V for circuits fed by transformers <125kVA.

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calculating arc flash 2015

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