Selectivity via the Peak-Current-Let-Through Method: What Traditional Time Current Curves Will Not Tell, but Should

Time-current-curves are the accepted industry standard for predicting overcurrent device operation and analyzing selective behavior under overload or fault conditions. The cement industry has utilized this interpretation of drawn curves for many years and this method has provided acceptable performance. However, it is not always a correct interpretation. In most cases this overlay interpretation is conservative, while in others it may lead to expectations of selectivity that exceed reality. 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 system designers. However, no standard nor single methodology exists for the creation of these tables. The writers shall present analytical methods, confirmed via testing, of device interaction analysis based on peak let-through current, suitable for predicting instantaneous selective behavior of protective devices more accurately than traditional time current curves may indicate. The method may be applied with published information or manufacturer´s internal test information.