Ground-Fault Loop Impedance Calculations in Low-Voltage Single-Phase Systems

Protection against electric shock in low-voltage installations by the automatic disconnection of the supply must be provided in compliance with applicable national or international standards. Such regulations take into account different earthing types for installations. To realize the protection against indirect contact, the disconnection of the supply must occur within a permissible safe time. To this purpose, the determination of the actual ground-fault loop impedance (which may, or may not, comprise the earth) is essential. This impedance, in fact, must not exceed a maximum value so that a sufficient ground-fault current can circulate and operate protective devices within the safe time. In some instances, explained in this paper, safety might be achieved, or increased, by converting one earthing system into another, by means of single-phase transformers (SPTs). This conversion can be promoted by either utilities or users. This paper seeks to clarify the circuits to be employed for the calculation of ground-fault loop impedances when the earthing type changes across SPTs. The quantitative analysis of this topic is not currently present in technical standards.