Protective Device Coordination Study — Part 2 of 3

Last month, we discussed the protective devices coordination procedure for a low-voltage motor and demonstrated how the ratings and settings of branch circuit and short circuit protective devices are chosen to provide adequate protection under overload and short circuit conditions. In this second installment, based on the 2008 NEC, it’s time to continue the procedure at upstream levels progressively closer to the service entrance of the facility, applying it to the phase overcurrent protection of a feeder serving a motor control center (MCC).

Figure 1 (click here to see Fig. 1) illustrates the one-line diagram of the example system under review. The protective device is a low-voltage power circuit breaker with a solid-state trip unit that is equipped with long-time, short-time, and instantaneous functions. The 50-hp induction motor is the largest horsepower motor of the MCC, and its protection was covered in Part 1. However, it’s important to remember that this presentation is incomplete — other crucial application-specific considerations are not addressed. In practice, the electrical designer works closely with the client and application engineers to select the electrical equipment best suited for the application.

Ratings

Assuming continuous duty of the motors and an application with standard operating conditions, the 600A rating of the MCC horizontal bus satisfies the requirements of Sec. 430.24 of the NEC. Furthermore, the ampacity of the feeder conductors satisfies 430.24, accounting for reductions in the conductor ampacity from Table 310.16 for more than three conductors in a raceway, via Table 310.15(B)(2)(a), and ambient temperature higher than 30°C, via the Correction Factors in Table 310.16. The 600A rating plug of the feeder breaker satisfies the requirements of Sec. 430.94; however, the long-time pickup of the trip unit must be set to a lower value to comply with 430.62(A). The 800A frame size and 800A current sensor rating are dictated by manufacturer selection constraints.

Protection and coordination considerations

Because MCCs are tested to withstand their short circuit current rating for only three cycles per UL 845, Motor Control Centers, the instantaneous function of the solid-state trip unit is necessary to ensure that the MCC does not endure a fault much longer than the short circuit duration time of three cycles. The symmetrical rms current capacity of the motor control center in Fig. 1 is 42,000A.

In Fig. 1, motor branch circuit short circuit protection is provided by current-limiting fuses. Because a fuse is capable of interrupting a fault in its current-limiting range in less than one-half cycle (i.e., 0.0083 sec), time-current characteristic curves that begin at 0.01 sec are not reliable in assessing the selectivity of the fuse with the upstream circuit breaker for high-level faults. The minimum instantaneous pickup setting for the upstream circuit breaker to ensure selective operation with the downstream fuse is determined from the fuse peak let-through characteristic at the expected maximum fault current.

Based on the paper by Marcelo Valdes in the March/April 2010 issue of “IEEE Transactions on Industry Applications,” Fig. 2 (click here to see Fig. 2) shows the manufacturer’s peak let-through current characteristics for the type of fuse used in this example. The uppermost diagonal line, labeled 2.3 × RMS, represents the prospective peak current available at the fuse for a 15% test power factor. The lower diagonal line, labeled square root of 2 × RMS, is the range of available instantaneous pickup settings for the circuit breaker, since its peak-sensing instantaneous trip is set to the nominal rms current setting × square root of 2. From Fig. 1, the asymmetrical fault current at the fuse of 15,000A corresponds to a symmetrical rms current of 10,000A. Referring to Fig. 2, for this value of symmetrical rms current, the peak let-through current is about 5,800A; and dividing the peak let-through current by square root of 2 yields a symmetrical rms current of about 4,100A. Setting the instantaneous pickup of the circuit breaker trip unit above this value ensures reliable selective operation of the fuse and circuit breaker for the maximum fault current.

Finally, the short-time delay settings should be insensitive to the MCC short-time loading requirement or maximum load amperes (MLA) for which the largest motor load is started with the remaining loads operating at full load. Referring to Fig. 1, MLA begins at 545A + 430A = 975A and decays to 363A + 430A = 793A by 0.1 sec.

Time-current coordination plot

The time-current plot of this example is shown in Fig. 3 (click here to see Fig. 3). The long-time pickup setting complies with 430.62(A). The instantaneous function of the trip unit prevents the motor control center from being subjected to high levels of fault current for a time beyond its rating of 3 cycles (i.e., 0.05 sec). The instantaneous pickup setting of 7 × rating plug or 4,200A ensures reliable selective operation of the fuse and circuit breaker. The short-time delay settings are insensitive to the MLA.

Now that’s we’ve discussed protective device coordination procedures for commercial and industrial power systems, it’s time to address phase overcurrent and ground-fault protection considerations for the primary-side fused switch, transformer, and secondary-side main breaker where the MCC feeder breaker resides in the final part of this series next month.

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 [email protected] and [email protected].