Mersen’s Selectivity Analysis in Low Voltage Power Distribution Systems with Fuses & Circuit Breakers White Paper describes methods for assessment of selectivity in systems using fuses and circuit breakers together. The methods assessed lend themselves to analysis that a power system engineer can perform with published information or information that may be requested from manufacturers.

Excerpt:

In 2005 the NFPA added the following requirement to article 700.27, Emergency Systems, of NFPA 70-National Electrical Code (NEC) [1]: “Emergency system(s) overcurrent devices shall be selectively coordinated with all supply-side overcurrent protective devices.” The same requirement was added in the 2005 edition to article 701.18[2], Legally Mandated Standby Systems and in 2008 to the new article 708.54[3], Coordination. Though local and state jurisdictions are interpreting these requirements differently, many of the interpretations require that
substantial portions of the power distribution system provide complete selectivity up to calculated bolted fault values for both utility and emergency generations sources. Similar requirements have existed previously in NEC article 620.62[4].

Traditional power distribution system design often ignored selective performance to such high levels of fault current due to considerations of safety, equipment, or conductor protection and the real or perceived difficulty in achieving such high levels of selective behavior. However, the stricter interpretations of the new NEC requirements do not allow for these considerations. Also, in many industrial and critical commercial systems…>>Read More