The second in a three-part series on the importance of arc flash labeling of electrical equipment, this set of articles provides practical examples to shed light on some of the common misconceptions surrounding arc flash protection.
In Part 1, which ran in the September 2011 issue, we considered the low-level task of taking measurements within a non-fused safety switch that serves as a local motor disconnect. In this piece, we learned that the level of arc flash risk for this task cannot be taken lightly, because it depends significantly on the amperage and class of the upstream fuses in the motor starter compartment and the length of the branch circuit conductors running from the fuses to the safety switch.
In this installment, we focus on the example system outlined in Fig. 1 (click here to see Fig. 1). Work is to be done at the next upstream level device of the electrical system (i.e., in the combination motor starter bucket of a motor control center or MCC serving a low-voltage induction motor). Unfortunately, the door of the bucket is missing an NFPA 70E-compliant arc flash warning label. Therefore, the worker cannot be sure of the level of arc flash risk at this specific location. So how should one proceed when no warning label is present?
As shown in Fig. 1, the upstream protective device that should respond to an arcing fault in the motor starter bucket is a low-voltage power circuit breaker equipped with a solid-state trip unit. The effect of the long-time (LT), short-time (ST), and instantaneous (I) settings of the trip unit are shown on the time-current plot in Fig. 2 (click here to see Fig. 2). The degree of separation of the fuse and circuit breaker time-current characteristics in the “I” region is due to the “I” pick-up setting. This setting guarantees selective coordination of these protective devices below 0.1 sec for a fault on the load side of the fuses. It’s important to note that selective coordination can still be achieved at a lower “I” pick-up setting. For a discussion on this topic, see the article “Protective Device Coordination Study — Part 2 of 3,” which appeared in the March 2011 issue of EC&M, available online at http://ecmweb.com/design_engineering/protective-device-coordiation-study-part-2-20110301/.
Calculations were performed per IEEE 1584 to determine the incident energy level in Calories per square centimeter and corresponding Hazard/Risk Category (HRC) for variation in feeder length. The results of these calculations are summarized in the Table. As expected, the bolted 3-phase, short circuit current and corresponding arc current at the starter bucket decrease with increasing feeder length. IEEE 1584 recommends using the worst-case results of arc time and incident energy for 100% and 85% of the calculated arc current. These results are summarized in the Table (click here to see Table).
What can you take away from the results shown in the Table? First, the effect of feeder length on incident energy level is not straightforward. As you can see, this value encompasses a wide range of values and covers three different HRCs. Second, the “I” pick-up setting significantly affects the arc time when calculating incident energy, besides its effect on the coordination between the fuses and circuit breaker. Note in the Table that the arc time is either 0.01 sec or 0.19 sec, depending on whether the arc current is above or below the “I” pick-up setting, respectively. Thus, a key take away from this example is that you should not change trip unit settings on the fly without careful consideration of the consequences on protection and life safety requirements.
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: email@example.com and firstname.lastname@example.org.