Broken PEN conductor in the context of rural South African households.
Low voltage distribution systems often receive the least attention as faults affect fewer customers and the elemental value of the network is insignificant compared to other parts of the grand electrical system. Ironically, this component of the grid defines the safety of the customer which is dependent on the earthing practice employed. The three primary low voltage earthing practices used worldwide, Terre-Terre, Isolated-Terre and Terre-Neutral are characterised by differences in the low voltage source and service installations earthing. South Africa implements the Terre-Neutral-Combined-Separated earthing system in which the neutral and protective earth conductors are combined on the utility side but separated on the customer side. A broken protective-earth-neutral conductor on such system with unbalanced load creates a hazardous condition at single-phase service-installations which may lead to loss of life and damage to property. The risk to rural households is compounded due to the use of overhead reticulation networks and the high probability of unbalanced systems. Simulation and experimental studies observed that during an incidence of a broken PEN conductor, a typical rural household experiences close to phase-phase voltages at the single-phase installation and the exposed conductive parts are energized up to 191 V (r.m.s.) even when an appliance is off but still plugged into the mains socket. Touch potentials in excess of 35 V (r.m.s.) are considered dangerous. The scenario of a child standing on a muddy floor in contact with a stove chassis revealed, via simulations, that currents in the order of 368 mA flows through the kid undetected by the earth leakage device and the utility and household over-current protection systems. This current-flow may lead to ventricular fibrillation and eventually death as prolonged exposure to currents in excess of 6 mA is considered harmful. A multiple-earthed neutral conductor provides a pragmatic supply-side solution to lower touch potentials at households but is ineffective at mitigating out-of-limits phase-to-neutral voltages. A more complete solution would be to install a voltage operated device at the household distribution box to monitor the voltage between neutral and true-earth and disconnect live and neutral supply to the house when the voltage exceeds 35 V (r.m.s.). It is recommended that these proposals be explored for feasibility through future work and consideration be given for regulations to make the voltage sensing device in the distribution box mandatory.