Current limiting fuses: proposed NFPA 70-2017 240.67, arc modeling and an assessment based on IEEE 1584-2002

Abstract

When operating in current limiting mode, currentlimiting fuses can effectively mitigate the heat and pressure hazards associated with an arc event because they limit the current in the first ¼ cycle and interrupt current flow in less than 8.3 ms. The effectiveness of current limiting fuses is reinforced by their inclusion in NFPA 70E-2015: Note to Table 130.7(C)(15)(A)(b), Annex D.4.6, and Annex O.2.4(4). The incident energy levels associated with current limiting fuses can be determined from IEEE Standard 1584-2002, IEEE Guide for Performing Arc-Flash Hazard Calculations. IEEE 1584-2002 presents a method for directly determining incident energy without first determining arc current or arc duration. The fuse equations were derived from 600V arc tests involving a range of current limiting fuse sizes. This data is reviewed to assess the performance of current limiting fuses in reducing arc energy in accordance with the proposed NFPA 70-2017 Section 240.67. The IEEE 1584-2002 equations are used to assess the performance of these fuses in comparison with an arc energy reduction maintenance switch, and the combination of both technologies. The viability of predicting arc current and incident energy for installations involving current limiting fuses and implementing the proposed NEC Section 240.67 using general arc models is discussed.