Unprotected Faults of Electrical and Extension Cords in AC and DC Systems

Abstract

In electrical power systems, all the wiring exposed to mechanical damages and other insulation stresses, such as flexible cords, can be involved in overheating, arcing, and burning. Mechanical damages of the stranded bare conductors can locally degrade the effective sizing of the cross section and cause anomalous local conditions. The circuit protective devices (PDs) can be unfit to detect the faults of cords that remain so energized and available to electric shock and fire hazards. Some fire ignitions result from these types of special unprotected faults. To highlight the local incident energy in case of fault, this paper introduces the parameters steady and transient current densities that assist in analyzing these fault events. Efficient protection can be achieved by the integration of active and passive techniques: adopting arc-fault circuit interrupters or detection devices, recognizing arcing faults, and wiring circuits with a grounding protection conductor to involve the ground in every fault that, in ac systems, is rapidly protected by residual current PDs (or ground-fault PDs). At this aim, the use of special cables, namely, ground-fault-forced cables, is recommended, particularly for cords and extension cords also supplying Class II equipment. The dc system protection requires special care.