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 accepted industry standard for predicting overcurrent device operation and analyzing selective behavior under overload, or fault conditions. A conservative interpretation of curves has sufficed for many years and provided acceptable performance. However, recent emphasis on better selectivity while trying to provide optimal protection demands more accurate selectivity predictions. Currently, manufacturers are publishing tables and other guidelines to facilitate the selection of optimally coordinated devices. However, no standard or single method 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 methods may be applied with published information or manufacturer´s internal test information.