Traditional Time - Current Curves Are Not Enough, Adding $I^{2}{t}$ Considerations

Overcurrent device selectivity has always been desirable in power distribution systems. Recent changes in the U.S. National Electrical Code (NFPA 70-2005 and subsequent) have increased the demand for near perfect selectivity. Simultaneously, heightened sensitivity to the effect of slow clearing times on arc flash hazard has increased the need for fast and sensitive overcurrent protection, particularly at the larger upper tier overcurrent devices. When the selectivity solution is to oversize or to slow down protection at midlevel feeders above the branch circuit breakers (CBs), arc-flash-incident energy at higher levels in systems can reach unacceptable levels. Optimized protection and selectivity may be achieved by optimizing instantaneous settings and algorithms in upstream protectors. Such selectivity may be accomplished primarily by understanding the current-limiting behavior of the downstream protective device and understanding the thermal capacitance needed to melt upstream fuse elements or intentional damping added to upstream CB trip units. When CBs trip in subcycle times and clearing times are instantaneous, traditional time-current curves (TCCs) are insufficient to correctly demonstrate device operation. The authors will describe two graphical representations based on the I²t let-through of the downstream device and I²t requirement to trip the upstream device. The I²t representations more effectively describe protective device behavior in the instantaneous region below 0.010 s normally excluded in traditional TCCs. Furthermore, a technique, a theoretical basis, and a proposed test protocol that are suitable for a wide range of devices will be presented.