Traditional Time Current Curves (TCC) are not enough, adding I2T considerations

Overcurrent device selectivity has always been desirable in power distribution systems. Recent changes in the US 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 slow down protection at mid-level feeders above the branch circuit breakers 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 circuit breaker trip units. When circuit breakers trip in sub-cycle times and clearing times are instantaneous, traditional Time Current Curves (TCC) are insufficient to correctly demonstrate device operation. The authors will describe a two graphical representations based on the I
²
t let-through of the downstream device and I
²
t requirement to trip the upstream device. The I
²
t representations more effectively describe protective device behavior in the instantaneous region below 0.010 seconds normally excluded in traditional TCCs. Furthermore, a technique, theoretical basis and proposed test protocol that suitable for a wide range of devices will be presented.