Method for Determining Selective Capability of Current-Limiting Overcurrent Devices Using Peak Let-Through Current—What Traditional Time–Current Curves Will Not Tell You

Time-current curves are the accepted industry standard for predicting overcurrent device operation and analyzing selective behavior under overload or fault conditions. A conservative interpretation of drawn curves has sufficed for many years and provided acceptable performance. However, recent emphasis on better selectivity while still trying to provide optimal protection increases the demand for more accurate selectivity predictions. Currently, manufacturers are publishing tables and other guidelines to facilitate the selection of optimally coordinated devices by systems designers. However, no standard or single methodology exists for the creation of these tables. The writers shall present three methods of device interaction analysis based on peak let-through current, suitable for predicting selective behavior of protective devices above what time current curves may indicate. The method may be applied with published information or manufacturer´s internal test information.