Ground-fault loop impedance calculations in single-phase systems

摘要

Protection against electric shock in low-voltage installations by automatic disconnection of the supply must be provided in compliance with applicable national or international standards. Such regulations (e.g. National Electrical Code in the U.S., IEC 60364, BS 7671 in the U.K., etc.) take into account the earthing system of the installation, such as TT, TN-C-S, or IT systems. 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 (e.g. from TT to TN, from TN to TT, from TN-C to TN-S, etc.), by means of single-phase transformers. This conversion can be promoted by either utilities or users. Other cases of conversion of grounding type, although not driven by safety concerns, are also examined, as they can be used to study the influence of neutral earthing on single-phase short-circuits. This paper seeks to clarify the circuits to be employed for the calculation of ground fault-loop impedances when the earthing type changes across single-phase transformers. The quantitative analysis of this topic is not present in current technical standards.