Selectivity analysis in low voltage power distribution systems with fuses and circuit breakers

Interest in complete overcurrent device selectivity has increased due to the addition of selectivity requirements to articles 700, 701, and 708 of the National Electrical Code (NFPA70). Many users, both commercial and industrial, use fuses and circuit breakers simultaneously. Traditional Time-Current Curve (TCC) analysis is known to not fully communicate fuse selectivity; hence fuse manufacturers publish device ratio guidelines for selection of fuse type and sizes. Recent publications of selectivity tables by circuit manufacturers also demonstrate that traditional TCCs are often insufficient to fully communicate circuit breaker selectivity. Traditional TCCs can lead to incorrect conclusions regarding circuit breaker fuse selectivity, indicating more or less selectivity than may be possible. The authors will describe various methods for assessment of selectivity in systems using both fuses and circuit breakers together, with either device on the line side. The methods will demonstrate that selectivity above what TCCs demonstrate may be possible if devices are selected correctly, and that traditional TCC analysis, can also incorrectly demonstrate more selectivity than a more thorough analysis would predict. The methods lend themselves to analysis that a power system engineer can perform with published information or information that may be requested from manufacturers.