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Arc Flash Calculations Clarified

Jan 24, 2011 4:44 PM, By Frank Mercede, P.E., and Joseph N. Mercede, Mercedes Electric Co.

Performing sample arc flash computations using the IEEE Std 1584 method

Electrical equipment is required to be field labeled by the NEC (NFPA 70) and OSHA. This requirement is meant to inform personnel working on energized equipment about the electrical safety risk of arc flash. OSHA recommends that employers consult consensus standards, such as NFPA 70E and IEEE Std 1584, to protect employees from such risk.

Generic labels, not supported with proper calculations by a qualified professional, are inadequate for this purpose, because they do not provide adequate information regarding incident energy levels, arc flash and shock protection boundaries, and personal protective equipment (PPE) requirements. However, there are many software packages on the market that will perform arc flash computations and generate compliant labels. As with most software programs, it’s best that the user of the program fully understands the concepts and technical issues involved in the calculations prior to its use.

The purpose of this article is to discuss some issues affecting the results of arc flash calculations under the IEEE Std 1584 method. The example in the Figure (click here to see Figure) involves a 150kVA, 3-phase (three single-phase units), pole-mounted transformer serving a 200A commercial service. Assume the arc flash event occurs on the line side of the main overcurrent protective device of the main service panelboard. For more details on these arc flash calculations, visit the Engineering Publications section at www.mercedeelectric.com.

Available 3-phase, short circuit MVA of utility supply

Arc flash calculations were performed for three different values of available 3-phase, short circuit MVA of the utility supply: 100MVA (base case), 10MVA, and 1MVA. The results of the calculations in Table 1 (click here to see Table 1) show increasing incident energy levels for decreasing available 3-phase, short circuit MVA of the utility supply. Consequently, the Hazard/Risk Category for the selection of PPE increases from Category 1 for 100MVA to Category 3 for 1MVA.

These results appear counterintuitive to our experience regarding the calculation of short circuit current for the selection of withstand and interrupting duties of switchgear; namely, a “stiff” supply (i.e., high available 3-phase, short circuit MVA) results in high withstand and interrupting duties. The reason for this apparent contradiction lies in the fact that the incident energy depends on both arc current and arc time.

Under the IEEE Std 1584 method, arc current is calculated from the bolted 3-phase, short circuit current with an empirically derived equation; and, as expected, the arc current in Table 1 decreases with decreasing available 3-phase, short circuit MVA of the utility supply. For this example, arc time is the total clearing time of the upstream 10A, class K (fast) fuse for the value of arc current at 12.47kV; and, as expected, the arc time in Table 1 increases with decreasing arc current. In fact, the arc time for the 1MVA case (a whopping 5.5 seconds) is the principal cause of the unexpectedly high incident energy and corresponding Hazard/Risk Category of this case. In practice, the 2-second rule, described in Annex B (Sec. B.1.2) of IEEE Std 1584-2002, may be applied to the 1MVA case to lower the calculated incident energy to a reasonable level. However, misapplication of the 2-second rule poses a liability risk in the event of an arc flash accident.

Fuse time-current characteristic

Arc flash calculations were also performed for two types of primary fuses for the system shown in the Figure: 10A, class K (fast) (base case) and 10A, class T (slow). As expected, the results in Table 2 (click here to see Table 2) show an increase in the incident energy level from 6.19 J/cm2 for the class K (fast) fuse to 16.9 J/cm2 for the class T (slow) fuse, necessitating an increase in the Hazard/Risk Category from Category 1 to Category 2, respectively.

Length of service drop

As a last step, arc flash calculations were performed for three different lengths of the service drop feeder for the system shown in the Figure: 25 ft (base case), 100 ft, and 500 ft. The results of the calculations in Table 3 (click here to see Table 3) show increasing incident energy for increasing length of the service drop feeder, although the Hazard/Risk Category to select PPE remains the same (i.e., Cat. 1) for all three cases. As discussed previously, the culprit for the increase in incident energy with increasing service drop length is arc time.

Findings

The results of these calculations illustrate some of the issues that affect the results of arc flash calculations using the IEEE Std 1584 method. Knowledge of incident energy and arc flash and shock protection boundaries is essential in the selection of PPE to protect employees from debilitating and life-threatening burns and other injuries of an arc flash accident. Such information must also be included on arc flash hazard labels applied to electrical equipment in the field.

Frank Mercede, P.E., is vice-president and Joseph Mercede is president of Mercedes Electric Co., Inc., based in Lester, Pa. They can be reached at fmercede@mercedeelectric.com and jmercede@mercedeelectric.com.

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