Safety evolution of residential microsystems

Utilities typically supply low-voltage, single-phase residential units using a radial layout, which constitutes a common distribution system for the dwelling units. In typical US and EU distribution systems, a faulted unit may affect the healthy ones within the same distribution system, due to the transfer of ground potentials, sometime dangerous for magnitude or persistence. Ground potentials appear during a fault over the building grounding system (BGS), via the PE, if present, and via the common extraneous-conductive-parts (EXCP). In Europe, TT system is imposed to residential units and the PE is not permitted to be connected to the utility neutral wire at the service entrance of the building. Some of the European utilities, in fact, cannot guarantee the continuity, as well as the absence of potentials, of the neutral conductor, which serves, indeed, many users. As a consequence, TT systems require an independent ground electrode and residual current devices as the protective measure against indirect contacts under ground-faults conditions. Such independent operation of residential units within TT-systems renders in practice impossible the coordination and the organization of a maintenance service, which might monitor and improve their safety: units may not have earth electrodes and/or residual current devices. The liberalization of the distribution of electricity, which would allow the choice of the TN-system in all the European countries, would be the radical solution to the problem. Alternatively, to safeguard the electrical safety of buildings, the authors propose an evolved model for the residential buildings, which calls for local transformers (LT) with taps, grounded at the mid-point of the secondary winding, within each house. This proposed arrangement contributes to increase the fault immunity of dwelling units, by preventing the transfer of ground potentials; the fault protection within this local TN system would be effectively performed by overcurrent devices, and touch voltages would be reduced, with evident benefit for the safety in special locations, as it is substantiated in the paper.