Labeling standards based on arc flash risk
A maintenance worker is about to open a non-fusible safety switch, which serves as a local motor disconnect, when heâ€™s confronted by a co-worker. â€śHey, whatâ€™s up?â€ť inquires the co-worker. â€śI need to check voltages, because the motor is running hot,â€ť answers the maintenance worker. Although the co-worker is not an electrician, he recalls hearing something about wearing personal protective equipment (PPE) when working on electrical equipment. â€śShouldnâ€™t you be wearing PPE?â€ť asks the co-worker. â€śDonâ€™t need PPE for this job,â€ť he replies. Not satisfied with this simple answer, the co-worker continues with the inquiry. â€śAre you sure?â€ť At this point, the maintenance worker becomes visibly irritated. â€śYou must be kidding! Iâ€™m not about to wear PPE for this measly job!â€ť My question to you is: Which one of these workers is correct?
Clearly, the maintenance worker is mistaken not to wear PPE for this task â€” and is in desperate need of training on electrical safety-related work practices. Yet, even if the right decision had been made, the absence of an NFPA 70E-compliant arc flash warning label on the safety switch leaves the selection of the appropriate levels of PPE to chance.
This is the first of a three-part series on the importance of arc flash labeling of electrical equipment in which practical examples shed light on some common misconceptions surrounding this issue.
The one-line diagram (click here to see Figure) depicts the electrical arrangement noted in the situation described above. The non-fusible safety switch is rated 60A and serves as the local disconnect for a 20-hp induction motor. If an arc flash event were to occur at the safety switch location, due to a fault during voltage testing, the maintenance worker must rely on the upstream fuses in the combination motor starter of the motor control center (MCC) to clear the fault.
Using IEEE 1584 as a reference document, we can calculate the incident energy (IE) in calories per square centimeter and the corresponding Hazard/Risk Category (HRC) level for variations in fuse rating and class, length of motor branch circuit conductors, and available bolted 3-phase short-circuit current at the MCC bus. These calculations were performed for four different combinations of fuse type and rating level, which are summarized in Tables 1 through 4. Letâ€™s start by using NEC upper-limit 50A Class RK5 fuses. The results of our calculations in Table 1 (click here to see Table 1) show the wide variability in IE and HRC with length of the motor branch circuit. For the second case, lower-rated 40A upstream fuses of the same class (RK5) are chosen that still permit start-up of the motor. Take note in Table 2 (click here to see Table 2) of the significant reductions in IE and HRC levels with this simple change in fuse ampere rating. For the third case, we return to using 50A upstream fuses. However, this time we choose Class RK1 fuses instead of RK5 (Table 3) (click here to see Table 3). Comparing the data in Tables 1 and 3, note the dramatic changes in IE and HRC levels with the change in fuse class. Finally, the fourth case involves lower-rated 40A upstream type RK1 fuses that still permit start-up of the motor. Comparing the results in Table 4 (click here to see Table 4) with those of Table 2, once again we see a significant reduction in IE levels.
What can we take away from this example? First, PPE is necessary for this â€śmeasly job,â€ť and an NFPA-70E arc flash warning label on the safety switch would certainly reinforce this point. Second, an NFPA 70E-compliant label, specifying available incident energy level at 18 in. from the piece of equipment or HRC rating, would inform the worker as to the appropriate level of PPE to wear for the task at hand. Finally, this example also illustrates the importance of replacing blown fuses with identical replacements, so as not to inadvertently change the level of arc flash risk from that on the arc flash warning label.Frank Mercede, PE., is vice-president and Joseph Mercede is president of Mercedes Electric Co., Inc., based in Lester, Pa. They can be reached at firstname.lastname@example.org and email@example.com.