DEVELOPMENT OF THE GUIDE FOR PERFORMING ARC-FLASH HAZARD CALCULATIONS

Copyright Material IEEE Paper No. PCIC-2003-01

Daniel Doan Senior Member, IEEE DuPont Co. P.O. Box 80840 Wilmington, DE 19880 USA

George D. Gregory Fellow, IEEE Square D Company 3700 Sixth St. SW Cedar Rapids, IA 52404 USA

Herman O. Kemp. Member, IEEE Tennessee Valley Authority 1101 Market St Chattanooga, TN 37402 USA Bruce McClung Fellow, IEEE Electrical Safety Consulting Services, Inc. 656 Whittington Drive Charleston, WV 25312 USA

Vince Saporita, IEEE Senior Member Cooper Bussmann P. O. Box 14460 St. Louis, MO63178 USA Craig M. Wellman Senior Member, IEEE DuPont Co. P.O. Box 80840 Wilmington, DE 19880 USA

Abstract – This paper reports on the accomplishments of the IEEE 1584TM Working Group. This working group raised money for testing, oversaw a significant amount of testing, analyzed the data, developed a new model for incident energy calculation and wrote IEEE 1584TM IEEE Guide for Performing Arc Flash Hazard Calculations. The paper discusses: the working group’s starting point; the need for laboratory testing; the test and analysis methods employed; the discovery from the testing; the development of the model and the guide; and the need for further work. IEEE 1584TM-2002 significantly updates prior information on conducting an arc flash hazard analysis in accuracy, usability, and depth. The guide offers a new empirically derived model for enclosed equipment and some open lines for voltages from 208 volt to 15 kilovolts and a theoretically-derived model for applications involving higher voltages or large gaps between conductors. Included with this guide are programs with embedded equations, which may be used to determine incident energy and the arc flash protection boundary. Index Terms – Arc flash, arc flash hazard analysis, flash protection boundary, incident energy

I. INTRODUCTION IEEE 1584TM [1] is a real breakthrough in the world of arc flash hazard analysis. Previous methods of performing arc flash hazard analysis, including those in NFPA 70E-2000 [2], were based on theoretical concepts or drawn from limited testing. The new testing concentrated on arcing faults in a variety of electrical equipment enclosures (arcs in boxes) more typical of actual work place situations. The 1584 Working Group conducted extensive testing on the arc flash hazard. They tested a wide range of voltages and currents and used a set of test enclosures designed to simulate real equipment rated from 208 V to 15 kV. They spot-checked their results with testing in actual used equipment. They then applied statistical

analysis techniques using both the new test results and previous test results to develop a model of the arc flash hazard. The model consists of a set of equations, which can be embedded in computer programs. The equations have been embedded in a spreadsheet type calculator, which is included with the guide. The model determines arc current, incident energy, and the arc flash protection boundary. It includes equations for some current limiting fuses acting within their current limiting range and a simplified method for low-voltage circuit breakers operating in their instantaneous range. Conducting an arc flash hazard analysis has been very difficult in the past. Not enough arc-flash incident energy testing had been done to allow developing models of arc-flash energy that accurately represented real applications. The algorithms that were available were difficult to solve. It must be emphasized that the 1584 guide and other means of calculating incident energy are intended for use under engineering supervision, Major electrical power system analysis software suppliers are or will be offering the 1584 models in their programs and that will greatly reduce the engineering effort needed to perform an arc flash study. For users of those programs, the guide also provides much useful information on how to conduct an arc flash hazard study and contains case history anecdotes of arc flash hazard experiences.

II. REALITY OF THE ARC FLASH HAZARD The number of injuries from arc flash accidents is likely very high, but government statistics do not allow easy or reliable documentation of the number of these injuries. And, few companies are willing to share details on accidents with serious or fatal consequences. The working group decided to simply document as many arc flash injury cases as it could find. Its first method to identify accident cases was to tap the personal knowledge of the working group members. The

second method was to search the internet for arc flash accidents. The internet search resulted in finding information from the U.S. Department of Energy, the Occupational Safety and Health Administration, and the Mine Safety and Health Administration. From these sources, fifty-four documented cases were identified. The voltage systems of these cases ranged from 480 V ac to 161,000 V ac. The cases are distributed as follows: fifty-three per cent in the utility industry, thirty-nine per cent in the industrial environment, and eight per cent in other settings.

III. NEED FOR ARC FLASH HAZARD ANALYSIS A review of many arc flash hazard studies showed the importance of conducting such a study. [3] The results of arc flash hazard analysis studies covering thirty-three plants have been combined and reviewed. These studies covered 4940 cases -- buses or switching points -- rated 600 V and lower. Figures 1 and 2 show the number of cases at each level of incident energy. While the median is low, a significant number of cases had very high incident energy levels, with the potential for devastating injuries. The review found similar results for medium voltage buses. After an arc flash hazard analysis has been completed, engineering revisions can be made to the system to reduce the incident energy to manageable levels. Usually adequate changes can be made to the protective devices for a nominal or modest cost. But only a study can identify the buses with high incident energy levels and only a study can determine the rating of the personal protective equipment needed at each location in a facility. Based on this study, it is clear that it is worth while to do an arc flash hazard analysis. NFPA 70E offers default tables as a means of determining incident energy quickly and easily. These tables are suitable for their intended use, providing an immediately available answer, but are not a substitute for performing an arc flash hazard analysis. The default tables lists typical equipment and tasks and provide estimated incident energy, assuming a particular available or bolted fault current and a particular protective device fault clearing time. However, if the bolted fault current or protective device clearing time are different than shown in the 70E table, the incident energy will be quite different.

IV. NEED FOR LABORATORY TESTING The 1584 Working Group wanted to be able to calculate possible incident energy over the full range of commercial, industrial, institutional, and utility applications. But, useable test data was available only for testing at 600 Volts and 2400 Volts. It covered only two box sizes and arcs in open air and it covered only a limited range of currents. Experience had shown that 120 V was not a concern for the arc flash hazard but a 208 V arc could cause serious injuries, so 208 V was accepted as the minimum voltage of concern. The most commonly found voltages of interest included 208 V, 400 V, 480 V, 4160 V, and 13,800 V. The currents of interest ranged from under 1 kA to over 100 kA.

The boxes used for testing had been relatively large compared to the size of MCC units and panelboards, so testing in smaller boxes was needed. The relationship between incident energy testing using simulated enclosures (test boxes) and real equipment had not

been established, so the working group wanted to undertake the task of determining it. Figure 1 Chart of incident energy levels, 600V

Figure 2 Chart of incident energy levels, 600 V, over 10 cal/cm2

V. RAISING OF SUPPORT

The IEEE Standards Association has a process for soliciting donations and managing the business side of a special effort in support of a standard. The 1584 Working Group in concert with IEEE-SA staff held a fund raising program in support of our test program and the special staff effort needed to complete the standard with an accelerated schedule. IEEE set up a web site and sent out letters to address lists provided by volunteers. Volunteer members of the WG then followed up with phone calls and email notes. This joint effort raised $75,000 in cash, several in-kind contributions of unpublished data, test laboratory time and donations of used electrical equipment (typical of that in operating service throughout the petroleum and chemical industry). Tests in the donated equipment were instrumental in verifying arcing faults in empty enclosures (except for the arc terminals) are more intense than arcing faults in actual equipment with devices and buses and insulators, etc.

Buses 600V and Under

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VI. VARIABLES CONSIDERED AND DESIGN OF

EXPERIMENTS The working group decided to apply the test method developed and refined by R.L Doughty, T. E. Neal, and others and published in several papers in the 90s [4], [5], [6]. This test method paralleled the test method in ASTM F-1959/F1959M-99 Standard Test Method...For Clothing [7]. By using a very similar test setup and instrumentation, we avoided the many potential sources of error that would have to be considered if a more fundamental test method was employed. It was determined that the Design of Experiments method would allow a significant improvement in the scientific credibility of the results over simple curve fitting methods. A list of variables that might affect the incident energy are shown below as the X variables. The result we are trying to find, which may be called the process output or the Y variable, is the incident energy.

Y = Incident Energy X1 = Time X2 = Working Distance X3 = Open Circuit Voltage X4 = Bolted Fault Current X5 = System X/R X6 = Electrode Gap X7 = Number of phases X8 = Grounded or Ungrounded System X9 = Box or No Box X10 = Box Size X11 = Box Shape X12 = Electrode Configuration (Delta or In-line) X13 = Electrode to Box Spacing X14 = Frequency

Of these, previous research by Doughty et al tested X1, X2 and X4 fairly completely, and X3, X7, X8, X9, and X12 in a limited fashion. The most efficient way to understand and develop the equations for this is a method called the ‘Design of Experiments’. This is used in process industries to learn the interactions of all the variables and to help the engineer understand the process. By testing all factors in a designed experiment, this method is able to identify all factors and combinations of factor that affect the process output. If every variable is adjusted over its range while the others are kept fixed, the affects of the interactions will not be discovered and the resulting data will not enable a comprehensive solution to arc flash calculation. Figure 3 shows this effect for a system with two X variables. Separate testing of the variables would miss the interaction of low X1 with high X2.

Figure 3. Interaction of variables.

If you can imagine the above plot with 14 variables, instead of only 2, you can understand the complexity of our problem. The test program should recognize this and address all the variables and their interactions. Ideally we would determine the maximum range of each variable, select one or more intermediate points for each and do a full factorial set of tests. That is how we would test all of the possible combinations of the variables. We would do repeat testing of each combination perhaps three times to assure the test results were consistent. The test order would be as random as possible to minimize affects of other unknown factors. Given three points for each variable, that would be 3 raised to the 14th power, or almost 4 800 000 different tests. Obviously we could not begin to afford that kind of a test, but we did apply the method with a minimum of test points and a minimum of combinations. Let’s consider the range and number of test points needed for each. We will consider low voltage (LV) cases first. Time, as a variable, was found to be independent in several sets of tests designed to examine the affect of time as a variable. Incident energy was found to be linear with time. Therefore, the formula for incident energy can have time in seconds as a multiplier. All test results can be normalized for a set time (0.20 seconds [200 milliseconds] was chosen), and this variable can be removed from the list. Working distance will be discussed with the variables of box size and shape Open circuit voltage has been specified already to be 208 V, 400 V, 480 V and 600 V. We need only test at 208, 400, and 600 volts, as those voltages are spaced well apart to show the interactions of the other variables. (In fact we did test at 450 and 480 V but it was not necessary.) For bolted fault current we would like to consider a range from 0.75 kA to 100 kA: we would like to go as low as possible, certainly to below 5 kA and as high as possible. Most bolted fault currents are in the range of 5 to 50 kA, so we should have some intermediate points. Suggested test points: are 0.75, 5, 33 and 100 kA. We could limit the testing at .75 kA to a single test vs the multivariable tests at the other levels. We will still be able to get a polynomial function for bolted fault

current. That would leave us with one test point at 0.75 kA and 3 points for the multivariable tests. A working group member reviewed several large studies and analyzed the X/R of about 1100 buses, from 208V to 13.8kV. The results are shown in Table 1. This data could be used in future testing. Voltage Ave Std Deviation Range . 208V 2.9 1.3 0.4 to 5.2 480V 2.6 2.3 0.2 to 12.6 2-14 kV 10.2 4.9 4 to 25

Table 1 X/R values in large studies The electrode gap (Ge) has been set at 1.25” or 32 mm for most 600 V testing to date. A check of a NEMA 600 V MCC unit confirms that the arc gap between phases in the bucket is typically about 1.25”. In an inspection of 400 V IEC equipment, a working group member found the phase-to-phase gap to be as small as 9 mm. Instead of testing a range of gaps for each voltage, we could possibly test each voltage at only two gaps as shown in Table 2. Voltage 208V 400V 600V Ge (mm) 9 and 18 9 and 18 18 and 32 Table 2 Test range for electrode gap That is a total 6 combinations. With two gap points at each voltage we will be able to generate a linear function for arc gap. With three points for voltage we can generate a polynomial function for voltage. Previous testing at single phase gave varied results, and usually the arc would quickly go out. The working group decided to test only 3-phase systems in this effort. Box size and shape significantly affected incident energy in previous studies, and changed the relationship of working distance to incident energy. We should consider the proposed tests with boxes and variables as shown in Table 3 to find the correct relationship between distance and energy for each box type. Abbreviations include G for grounded and UG for ungrounded. Boxes: A B C . Dimensions (in.) 20 cube 14 x 12 x 7.5 4 x 4 x 4 (mm) 508 cube 368 x 305 x 191 102 cube G/UG system UG UG G Working Distance (in.) 12, 36, 72, 120 Same 12, 24

Table 3 Variables for box type testing. For the A and B boxes a single open circuit voltage and bolted fault current can be used in each setup. For the C box a range of currents must still be tested, but that test series could be independent. For spacing between electrodes and box, most equipment has larger spacing to grounded metal; so spacings of 9, 32, and 100 mm were thought reasonable.

Electrode configuration was tested in Doughty’s research, and there was little variation due to the flat vs. delta arrangement of electrodes. Therefore it was felt reasonable to do all these tests with a flat arrangement of electrodes. Frequency was assumed to be not significant. Almost all tests were run at 60 Hz, but several sets of tests were run at 50 Hz. These tests matched the 60Hz test setups in every aspect but frequency and proved the assumption was true, within the range of 50 to 60 Hz. Table 4 is the total list of variables. The 600V test range for Doughty et al. is also shown. Xs Doughty Tests 1584 Tests . Time 6 cycles Adjust as needed Working distance (mm)

457 -- 1520 305, 457, -- 3050 OC Voltage 600 208, 400,600 V Bolted fault current 16-100 kA .75, 5, 33, 100 kA X/R unknown 2, 5, 10 Electrode gap(mm) 19 --76 9, 18, 32 # of phases 1, 3 3 Grounded system vs. ungrounded

G, UG G, UG Box /No box B, NB B Box size A A, B, C Box shape Cubic Cubic, rectangular Electrode configuration

Delta, flat Flat Spacing electrodes to box(mm)

102 9, 32, 100 Frequency (Hz) 60 50, 60 see note

Table 4 Low Voltage Variables and values selected.

With this reduction of variables, there are still 4 x 3 x 4 x 3 x 3 x 2 x 3 x 2 x 3 tests, or 15,552 different test points. According to the labs scheduled to run the testing, on our budget we would have time for 60 +/- tests. So now we need to use some engineering judgement to reduce this to a workable set of tests. As mentioned above, the box shapes and working distances could be done separately, with the 10 tests in Table 3. That also covers the cubic vs rectangular box shape variable. Now we are left with: voltage, bolted fault current, X/R, gap, grounded vs ungrounded, electrode gap and electrode-box spacing. These 6 variables are the core of the testing that is needed. Another part of the Design of Experiments method is called factorial testing. There are full factorial and partial factorial tests. Considering a 3-variable example, full factorial testing is done at all corners of the diagram shown in Figure 4. There would be 8 tests in this case.

Figure 4. Example of full factorial testing for 3 variables. Partial factorial testing is a way to use statistics to understand the variable interaction, but while limiting the number of tests. Figure 5 shows an example of a one-half factorial test selection, with the 3-variable system.

Figure 5. Example of partial factorial testing.

A one-quarter factorial method is also available for more than 4 variables. This method was used to determine the final set of tests for the 6 variables above.

VII. Test results and preliminary discovery

Photo 1 – Front view of Photo 2 – Side view of 20”

20” box test setup box during flash Photos 1, 2, and 3 show testing in a box that is 20” on a side. The front of the box is open. This box simulates a low voltage

switchgear circuit breaker space. Photo 4 shows a test in a motor control center (MCC.)

Photo 3 – Front view of 20” Photo 4 – MCC before box following flash test The test videos for arc-in-a-box tests showed the arc current always flowed from electrode to box wall, through the box wall, and back to one or two other electrodes. See photos 5a and 5b for testing in the 14” x 12” x 7.5” box. These photos are consecutive shots of the same test showing phase rotation of the arc. They show the arcs from different electrodes repel each other and go from electrode to box wall or box bottom, through the wall and back to one or two other electrodes. In videos the arcs can be seen dancing wildly within the box.

Photos 5a and 5b Arcs in box

This arc behavior made the combined average arc length much longer than the distance between electrodes, in fact many times greater. We were not able to estimate or predict arc length, but believe the arcs were as long as permitted by the box size, electrode arrangement and spacing and system parameters. We consider the arc lengths with the almost empty test boxes to be the worst case relative to real equipment with all its complexity and variation. As metal parts are added in a real enclosure, the possible arc lengths become smaller. Arcs are known to be almost pure resistance and arc energy is a function of arc length. Therefore, longer arcs allow higher energy than shorter arcs, other things being equal and assuming the arc is sustained. On this basis, the nearly empty test boxes allow conservative incident energy results compared to real equipment. In real equipment with all its complexity and variation, it is impossible to predict the minimum arc length that may occur. But, the minimum is not important because relatively small arc lengths will allow relatively small arc energy. So, the arc-in-a-box method allows a conservative solution to the incident energy problem, based on the large arc length that occurs in the almost empty test boxes. We were able to sustain an arc at 208 V in only one case. It may be that the arc length was too great to allow a 208 V system to sustain the arc. From incidents in the field, we know arc injuries have occurred in 208 V equipment. It appears that

to sustain the arc at lower voltages, the arc must be confined more tightly. Future testing at 208V with smaller boxes, similar to meter boxes would be worthwhile. Initial testing at 277 V with a device box, a 4” x 4” x4” box, showed fault energy was very low. Follow-up calculations showed that in any practical installation, incident energy would be so low that it need not be considered. Even a short length of #10 or smaller wire supplying the box greatly limits the available fault current. Based on this analysis, we dropped further consideration of device boxes and 120 V and 277 V circuits.

VIII. ANALYSIS OF DATA AND DEVELOPMENT OF MODEL

The results of the testing were analyzed using a statistical program. All of the available test data that fit the list of variables was put into a database. Since the final energy calculations will require use of the arcing current, that intermediate variable was measured and entered in the data. The data was normalized for time to 0.2 seconds duration, so the data would be comparable. A linear regression analysis was performed on the data, using the X variables and their interactions, such as X1 multiplied by X2. The resulting output equation for arcing current was not very good at describing the data and energy. Other forms of equation were considered, such as squares and square roots of the variables, and log functions of the variables. The best fit that was found for the data is an equation that included the log function of the bolted fault current and arcing current. A copy of the statistical analysis report that was used to find log(Ia) as a function of the X variables is presented in Program Output 1 on the following page. This report was generated after significant analysis was completed, and the variables that had insignificant effect on the arcing current were eliminated. The equation can be extracted from this program output. The terms of the equation are in the “Term” column and coefficients are in the “Coef ”column. This model had an R-square of 98.3%. R-square is a measure of the accuracy of the model, with 100% being perfect. The result is shown in Equation 1. log (Ia) =K +0.662 log(Ib) +0.0966 Voltage +0.000526 G +0.5588 log(Ib) Voltage – 0.00304log(Ib) G Equation 1 where: log (Ia) is log in base 10 of arcing current (kA) K is –0.153 for open configuration or –0.097 for box configuration log (Ib) is log in base 10 of symmetrical RMS bolted fault

current (kA) Voltage is system voltage (volts) G is distance between buses (mm) A similar method was used to determine the equation for log(Ia) for medium voltage applications. In the MV case,

bolted fault current was the only significant variable. This model had an R-square of 99.8%. The Incident energy equation is expected to be in the form of Equation 2, with some function of the list of the variables, and time as a linear factor. The distance from the arc would reduce the energy based on an exponent. E = f(variables) t / DX Equation 2 For open air cases, we expect the exponent x would equal two, since physics suggests a distance squared reduction. For cases where the arc occurs in equipment, the exponent is not two. The exponent is derived by applying a curve-fitting program to the test data for each type of equipment. The low voltage (LV) switchgear distance exponent is based on the 20” box test data described in Doughty, et al. The low voltage motor control center (MCC) and panelboard distance exponent is based on test results with the 12” x 14” x 7.5” as shown in Figure 6.

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Control Centers and Panelboards Using these exponents, the data was normalized to the distance of 610 mm. In this way, all the data could be used for the incident energy equation analysis. With a method identical to that used for the arcing current analysis, an equation was found for the log to the base 10 of incident energy normalized for an arc duration of 200 ms and a distance from arc to calorimeters of 610 mm. This equation is shown in Equation 3. The R-square is 89%, which is satisfactory.

log(En) = K1 + K2 + 1.081 log(Ia) + 0.0011G Equation 3 where K1 is -0.792 for open configurations (no enclosure) or -0.555 for box configurations (enclosed equipment) K2 is 0 for ungrounded and high resistance grounded systems, or -0.113 for grounded systems.

PROGRAM OUTPUT 1. REGRESSION ANALYSIS OF ARC CURRENT AT LOW VOLTAGE Program Output for: logIarc versus Variables for LV Data Points Analysis of Variance for logIarc, using Adjusted SS for Tests Source DF Seq SS Adj SS Adj MS F P . Open/Box 1 0.5510 0.0319 0.0319 8.30 0.005 logIbf 1 14.3291 0.2046 0.2046 53.28 0.000 Voltage 1 0.4683 0.0005 0.0005 0.13 0.716 logIbf*Voltage 1 0.0295 0.0438 0.0438 11.40 0.001 ElectrGap 1 0.0941 0.0004 0.0004 0.10 0.749 logIbf*ElectrGa 1 0.0242 0.0242 0.0242 6.30 0.014 Error 70 0.2688 0.2688 0.0038 Total 76 15.7650 Term Coef SE Coef T P . Constant -0.1249 0.1486 -0.84 0.404 Open/Box 0.028274 0.009814 2.88 0.005 logIbf 0.66200 0.09069 7.30 0.000 Voltage 0.0966 0.2642 0.37 0.716 logIbf*Voltage 0.5588 0.1655 3.38 0.001 ElectrGap 0.000526 0.001637 0.32 0.749 logIbf*ElectrGa -0.003043 0.001213 -2.51 0.014 Unusual Observations for logIarc Obs logIarc Fit SE Fit Residual St Resid 1 1.66257 1.80071 0.01652 -0.13814 -2.31 R 17 1.09945 0.93623 0.02730 0.16322 2.93 R 18 0.64994 0.81099 0.01973 -0.16106 -2.74 R 32 1.07041 1.14803 0.03508 -0.07763 -1.52 X 68 0.25840 0.28119 0.03766 -0.02280 -0.46 X 70 0.27753 0.41699 0.01652 -0.13946 -2.33 R 72 0.27257 0.29872 0.03829 -0.02616 -0.54 X 76 1.31917 1.36642 0.03337 -0.04725 -0.90 X 77 1.81398 1.93033 0.02758 -0.11635 -2.10 R R denotes an observation with a large standardized residual. X denotes an observation whose X value gives it large influence.

Figure 7 Program output used to develop equation for log of arc current For example, in the case of MCC and Panels, if the normalized incident energy is 16.0 joules/cm2, time is 0.1 second, and the working distance is 1000 mm, then E = 16.0 * 0.1 / 0.2 * 6101.641 / 10001.641 or E = 3.6 joules/cm2 . The basic model developed in the statistical analysis was further analyzed to compare the PPE required for each test point based on calculated incident energy vs. the PPE required for each test point based on measured incident energy. This was a test of the model. The test data was normalized to 0.2 seconds duration and distance of 610 mm, so variations in time and distance were not factors. For this analysis, a set of incident energy levels was chosen as 1.2,8,25,40,and 100 cal/cm2. Based on the test results, a calculation factor multiplier was included in the equations to allow a choice of most appropriate calculation factor. A calculation factor of 1 means no change in incident energy. The resulting table for low voltage is shown below as Table 5.

The full data set was then divided into boxes based on the difference between PPE levels as determined by calculation and measurement, with various calculation factors.

Table 5 Calculated versus actual PPE required for LV data

IX. Fuse Testing and Analysis Research for the paper, “The use of low voltage current limiting fuses to reduce arc flash energy” [6] provided test data to show how the current limiting characteristic of Class L and RK1 fuses could be utilized to reduce the arc-flash hazard. Five available short-circuit currents were utilized with six different size fuses.

Calculation factor

Two high

One high Same One low Two low

1 1 10 129 25 0

1.25 1 30 113 21 0

1.5 2 49 106 8 0

1.75 2 75 86 2 0

1.9 2 82 79 2 0

Based upon these tests, preliminary curves were developed which showed incident energy versus three phase available bolted short-circuit current.

Fig. 8 Incident Energy – 600A Class RK1 Fuse An example of those curves is shown in Figure 8. It shows the “knee” of the curve where the fuse becomes current limiting. For available short-circuit currents above the “knee”, the arc-energy is in the range of 0.1 cal/cm2, or less. For available short-circuit currents below the “knee”, the fuses were not in their current limiting range, so the arc-energy increased significantly as the opening time increased. Unfortunately, the amount of testing was limited, so ideal curves could not be drawn for all the tested fuses and no testing was done on fuses below 400 amperes. With incomplete information, equations for arc-energy could not be developed. As part of the research for the P1584 Standard, additional testing was completed to better define the “knee” of fuses already tested, and to develop required data for smaller ampere fuses. This additional test data resulted in the development of charts and equations for 800 A, 1200 A, 1600 A, and 2000 A Class L fuses, and for 100 A, 200 A, 400 A, and 600 A Class RK1 Fuses. Equations were derived for the tested fuses and then inserted into the P1584 spreadsheet. Then, for available short-circuit currents within the range of the testing, the spreadsheet calculates the arc-energy for the given available short-circuit current. The spreadsheet can also be used to calculate arc energies for other, untested fuses, as long as the arcing current and the time to open for that arcing current are known. Since the spreadsheet calculates the arcing current if the available short-circuit current is known, the user can utilize the calculated arcing current from the spreadsheet along with the fuse’s time-current curve to determine an opening time. Estimating an opening time for fuses within their current limiting range can be difficult, so the P1584 document provides a method, based upon the current at which the fuse opens at .01 seconds. A comparison between this calculation method based upon the time to open for a particular arcing current and the calculation method based upon the actual

fuse equations shows the actual fuse equations to be more accurate, as they should be, because they were derived from actual tests with fuses.

X. SIMPLIFIED CALCULATIONS FOR CIRCUIT BREAKERS

With the availability of equations for performing arc flash analysis, a method is available to calculate the flash protection boundary and incident energy for any low-voltage system. However, the method requires availability of the time-current trip curve for the overcurrent protective device in order to perform the calculation. Obtaining the curves for low-voltage circuit breakers could be difficult, especially where manufacturers have changed ownership or products have been made obsolete. In order to overcome this unnecessary obstacle, a short cut method is provided that applies a generic solution applicable for any low-voltage circuit breaker. This method is presented in more detail in a reference paper presented at the 2002 PCIC Conference. [8] The method applies to molded-case, insulated-case and low-voltage power circuit breakers. It is necessary for the calculator to know which of these three types of circuit breaker is installed. In addition, it is necessary to know the system voltage, the circuit breaker continuous current rating and the type of trip unit, thermal-magnetic, all magnetic, electronic with instantaneous tripping or electronic with short-time delay. It is desirable to know the instantaneous trip setting, but if that is not known, a default value is available. To arrive at a generic solution, trip curves were collected for as many circuit breakers as could be found. From these, calculations for incident energy and flash protection boundary were performed for a range of potential bolted fault current levels. The results were separated by classes by circuit breaker type, current rating and trip unit type. Each class is represented by one characteristic representing the highest values for that class. The results are presented in easy to use form with one variable, namely bolted fault current available at the work location. An example set of equations for a 1200-ampere, thermal magnetic circuit breaker is: E = 0.377 Ib + 1.36 Equation 4 D = 12.5 Ib + 428 Equation 5 where E = Incident energy in Joules/cm2 Ib = Bolted fault current in kA

D = Arc flash boundary distance in mm (For E = 5 Joules/cm2) These short cut equations are suitable for use only where the circuit breaker is operating within its instantaneous trip range. For operation below that range, the short cut method does not apply and the longer method must be applied. The equations are included in the spreadsheet calculator supplied with IEEE 1584 as an option.

Incident Energ y 18 Inches Fro m3-P hase Arc D ow nstream O f

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XI. MODELS IN GUIDE

The empirically derived model is provided for applications within its range. For applications beyond its range, a theoretically derived model is offered. Each model allows calculating arc current, incident energy, and flash protection boundary. The empirically derived model is based upon laboratory testing followed by statistical analysis and application of curve fitting programs as described above. It is applicable for systems within the range shown in Table 6.

Voltages in the range of 208 V -15 000 V, three-phase. Frequencies of 50 Hz or 60 Hz. Bolted fault current in the range of 700 A -106 000 A. Grounding of all types and ungrounded. Equipment enclosures of commonly available sizes Gaps between conductors of 13 mm -153 mm (0.5” to 6”) Faults involving three phases.

Table 6 – Range of empirically derived model While not all tests covered the extremes of the range shown, analysis has concluded that the results are reasonable. The theoretically derived model, based on the Lee method, is recommended for applications where the voltage is over 15 kV or where the gaps are over 153 mm (6”). These models are intended for applications where faults will escalate to three-phase faults. Where single-phase systems are encountered or escalation to a three-phase fault is not possible, these models will give relatively conservative results. Recognize that real arc exposures may be more or less severe than indicated by these models. The models give only the estimated maximum incident energy and estimated arc-flash boundary distance. Refer to IEEE 1584TM [2] for more information on these models, on the automatic calculators, and on performing an arc flash hazard analysis.

XII. PROCESS FOR DETERMINING ARC FLASH

INCIDENT ENERGY AND ARC FLASH BOUNDARY The way to perform an arc flash hazard analysis is discussed at length in the guide and much helpful information is provided. The guide includes sample data sheets for field investigation and guidance on determining clearing times for circuit breakers, instantaneous relays, and fuses. The steps in the process are: • Calculate available fault currents (bolted fault currents) at

points of interest. If there are multiple modes of operation with parallel feeders, consider all likely cases separately. Determine the amount of this current that flows through the protective device vs. the portion that comes from an alternate supply or from large motors feeding power into the fault.

• Calculate possible arc fault currents at the points of interest using the appropriate equations below.

• Determine the clearing time for the upstream protective device based on the amount of the arc fault current in that protective device.

• For low voltage cases, determine a second clearing time at 85% of the calculated possible arc fault current.

• Determine the expected distance from an employee to the possible arc location. This distance may extend inside equipment and it depends on tasks performed and tools used.

• Calculate the estimated maximum incident energy levels and the estimated arc-flash boundary distance using the appropriate equations. Select the highest numbers for the two low voltage calculations.

The second calculation is a means of compensating for the fact that a small error in determining the arcing current can cause a big error in protective device operating time.

XIII. APPLICATION TOOLS

Included with the guide are two worksheet-based programs, the IEEE 1584 Arc Flash Hazard Calculator and the IEEE 1584 Bolted Fault Calculator. The Arc Flash Hazard Calculator is an easy means of applying the models in the guide. It includes equations embedded in the worksheet for:

Arc fault current Incident energy Arc flash boundary Current limiting fuse cases Simplified circuit breaker cases The bolted fault calculator is a tool for calculating bolted fault current for simple radial systems. It is intended for use where a more sophisticated program is not available.

XIV. CONCLUSIONS The 1584 test program together with previous published and unpublished testing involved tests of arcs in air and tests in boxes that simulated classes of equipment by physical size: LV MCCs and panelboards, LV switchgear, and medium voltage switchgear. This testing covered wide ranges of voltages, currents, gaps, frequencies, and grounding types. The range for low voltage is shown in Table 4. Refer to the guide for the medium voltage testing range. The test setups were based on the ASTM test methods using calorimeters to determine incident thermal energy. This approach negated many sources of error that would have to be considered if more fundamental and necessarily more theoretical methods of analysis were used. Statistical analysis tools have been applied to develop the empirically derived model. They included design of experiments, regression analysis, and curve fitting. Many possible factors were tried and the best fits were selected for the model. For LV applications, users can select a calculation factor to shift the incident energy calculated to be more or less conservative, i.e. they can increase or decrease the likely

percentage of cases where workers are under-protected vs. overprotected. The 1584 guide has greatly expanded the range of cases for which an empirically derived model can be applied. For cases outside that range, the guide offers a Lee model. Each model allows calculation of the arc current, input of the portion of that current that will flow through the protective device that will interrupt the arcing fault, incident energy at a user specified distance from the fault, and the arc flash boundary. The guide provides techniques for designers and facility operators to apply in determining the arc flash thermal energy to which employees could be exposed during their work on or near electrical equipment. It covers the analysis process from field data collection to final results, presents the equations needed to find incident energy and the flash boundary, and discusses software solution alternatives. While the 1584 model offers good solutions to many arc flash hazard calculation problems, more development work is needed to allow a better understanding of the arc flash hazard. The 1584 Working Group is planning to proceed with another round of testing and a second edition. Those interested in contributing to the test program are invited to contact one of the authors or IEEE. The working group’s web site lists working group members and test program contributors. It can be found at: http://grouper.ieee.org/groups/1584/index.html. Of course, an arc flash hazard analysis is only part of an electrical safety program. See the IEEE Yellow Book [9] and NFPA 70E for information on setting up an electrical safety program.

XV. REFERENCES [1] IEEE Std.1584TM-2002 IEEE Guide for Performing Arc Flash Hazard Calculations, Institute of Electrical and Electronics Engineers, Piscataway, NJ, USA, Copyright 2002. [2] NFPA 70E Standard for Electrical Safety Requirements for Employee Workplaces, 2000 Edition, National Fire Protection Association, Quincy, MA, USA [3] D.R. Doan and R.A. Sweigart, "A Summary of Arc Flash Hazard Calculations", Record of Conference Papers, IEEE Petroleum and Chemical Industry Technical Conference, 2002, Paper No. 34. [4] R. Doughty, T. Neal, T. Dear, A. Bingham, “Testing Update on Protective Clothing & Equipment for Electric Arc Exposure,” IEEE Industry Application Magazine, Vol. 5, Issue 1, Jan./Feb. 1999, pp 37-49. [5] R. L. Doughty, T. E. Neal, and H. L. Floyd II, "Predicting incident energy to better manage the electric arc hazard on 600 V power distribution systems," IEEE Transactions on Industry Applications, Vol. 36, No. 1, Jan/Feb 2000.

[6] R. L. Doughty, T. E. Neal, T. L. Macalady, V Saporita, and K. Borgwald, "The Use of Low-Voltage Current Limiting Fuses to Reduce Arc-Flash Energy," IEEE Transactions on Industry Applications, Vol. 36, No. 6, Nov/Dec 2000, pp. 1741-1749. [7] ASTM F-1959/F1959M-99, Standard Test Method for Determining the Arc Thermal Performance Value of Materials for Clothing. [8] G. D. Gregory, I. Lyttle and C. Wellman, “Arc Flash Energy Limitations Using Low-Voltage Circuit Breakers,” Record of Conference Papers, IEEE Petroleum and Chemical Industry Technical Conference, 2002, Paper No. 35. [9] IEEE Std 902-1998, IEEE Guide for Maintenance, Operation, and Safety of Industrial and Commercial Power Systems (IEEE Yellow Book).

XVIII. VITA Daniel R. Doan is a Consultant for DuPont in Wilmington, Delaware, where he is the Chair of DuPont’s Electrical Standards Committee. Dan received the BSEE and MSEE degrees from the Massachusetts Institute of Technology. He has co-authored four previous papers at PCIC and co-authored a PCIC Tutorial on Arc Flash Hazards. Dan is a senior member of the IEEE and is a registered Professional Engineer in the Commonwealth of Pennsylvania. George D. Gregory graduated from the Illinois Institute of Technology with BSEE (1970) and MSEE (1974) degrees. He serves as Manager, Industry Standards with Schneider Electric / Square D Company in Cedar Rapids, Iowa. He is a Fellow Member of IEEE and a frequent author in IAS Conferences. He is a registered PE in Illinois, Iowa and Puerto Rico. Herman O. (Buddy) Kemp graduated from the University of Mississippi in 1969 with a BSEE degree and from Texas A&M University in 1973 with a MEIE degree. He has been a safety manager for the Tennessee Valley Authority in Chattanooga, TN since 1978. He has been a member of IEEE for 39 years; is a member of the IEEE Industry Applications Society; and was a member of the IEEE P1584 Working Group. L. Bruce McClung, IEEE Fellow. He has a B. S. in Electrical Engineering from West Virginia University. In 1960 he joined the Union Carbide Corporation, from which he retired in 2001. He is now a Principal Consultant with Electrical Safety Consulting Services, Inc. Mr. McClung has authored or co-authored 26 technical papers, many of which won awards. He received the IEEE Standards Medallion, the IEEE Charles Proteus Steinmetz Award, IEEE-PCIC Electrical Safety Excellence, IEEE Medal of Engineering Excellence, and IEEE-IAS Outstanding Achievement. He also received Union Carbide’s Chairman’s Award, has been elected a member of West Virginia University Academy of Electrical and Computer Engineers. He serves as the IAS Distinguished Lecturer for Electrical Safety Issues.

Vincent Saporita graduated with a BSEE from the University of Missouri-Rolla and with an MBA from Lindenwood College. He is a Professional Engineer in the State of Illinois. He has worked for Cooper Bussmann since graduation in various roles, and is now Vice-President, Technical Sales and Services. Standards experience includes membership on National Electrical Code panels 10 and 11, NFPA 70E (Standard for Electrical Safety Requirements for Employee Workplaces), and IEC TC32B (Low Voltage Fuses). Organizations include National Fire Protection Association, International Association of Electrical Inspectors, IEEE/IAS/I&CPS and PCIC, and National Electrical Manufacturers Association (Switch, Fuse, and Industrial Control Sections) Craig M. Wellman graduated from the University of Delaware in 1966 with a BSEE degree. He has been a project engineer and consultant for the DuPont Company in Wilmington Delaware since 1973. He is a senior member of IEEE, the secretary of the Codes and Regulations Subcommittee of the PCIC Standards Committee, Chair of the PCIC Arc Flash Calculations Working Group P1584, author of six past PCIC papers and presenter of two tutorials. He is a member of ISA, ISA 12.12, NFPA, the NEC Technical Correlating Committee, and the ACC Electrical Codes and Standards Task Group. He is a registered PE in Delaware.