Broken PEN conductor in the context of rural South African households.
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Date
2015
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Abstract
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.
Description
Master of Science in Electrical Engineering.
Keywords
Rural electrification -- South Africa., Low voltage integrated circuits -- South Africa., Electric power systems -- Protection -- South Africa., Theses -- Electrical engineering., Protective-earth-neutral (PEN)