Arc-Fault Protection of Branch Circuits, Cords, and Connected Equipment

Abstract

In electrical power systems, the fault frequently involves arcing and burning of wiring exposed to mechanical damage and other insulation stresses including wiring not fixed and connected by flexible cords and cables. IEC Standard 60364 ends the design of electric power systems at the outlets of branch circuits or at the fixed equipment. A complete design should include the connections of portable equipment and of extension cords (as requested by NFPA 70) that are exposed to arc faults and may cause fire and/or electric shock hazards. Since cords supplying Class II equipment are without a grounding protection conductor, the failure of the double insulation, caused by external damage, is unlikely to be easily detected as a ground fault. Protection must be provided to prevent the fault from extinguishing itself without being detected and remaining energized, thus presenting an electric shock hazard by direct contact with a live part, rendered accessible after local insulation failure. The authors highlight this worst case and suggest the protection achieved by wiring the circuits, particularly extension cords, with special power cables. Ground-fault forced cables (GFFCs) convert a line-to-line fault into a line-to-ground fault, that will be detected and protected by ordinary ground-fault protective devices. By adopting the GFFC type of cables internally to Class II equipment, the disconnecting protection could also be extended to equipment.