The use of protective multiple earthing and earth-leakage circuit-breakers in rural areas (Second report)

Abstract

Fault resistance is an important factor in protection, and was not dealt with fully in the first Report Ref. F/T102. A more thorough study of this aspect of earthing has revealed very interesting results, the principal one of which is that, with a properly constructed protective multiple earthing system, the occasions when a dangerous shock may be obtained are few, and that in the majority of these cases the earth fault responsible for the liability to shock develops such a large amount of heat energy that the existence of a defective condition is likely to be revealed in some much more obvious way than by a dangerous potential on the metalwork. The maximum voltage which can occur between metalwork and earth is limited to the product of the fault current and the earth resistance, and the limiting value of the latter is 2 ohms with a P.M.E. system. Actually in the majority of cases it will be appreciably less than this. The effect of fault resistance is to reduce the current and consequently the risk of shock—at the same time, however, it may prolong the period for which the defective condition is maintained, but analysis shows that up to an appreciable value of fault resistance the energy dissipated in the fault itself is adequate either to burn it out or to draw attention to its existence, and at greater values of resistance the current is so reduced that the voltage drop across 2 ohms earth resistance is quite negligible from the shock aspect.The situation is very different from this with ordinary earthing, since there is no limitation on the earth resistance such as occurs with P.M.E. Consequently the voltage on the metalwork may have almost any value and the combined influence of the fault resistance and earth resistance may frequently be adequate to prevent the fuse blowing, and the voltage across the earth resistance may be dangerous. There are circumstances in which the fault will reveal itself by generation of heat as with P.M.E., but this does not apply generally.With earth-leakage circuit-breakers dangerous conditions tend to occur when the fault resistances are high rather than low, since at low values the trip coil is more likely to operate. If the resistance is of the order of thousands of ohms, insufficient current may flow to trip the circuit-breaker, but sufficient current may pass through the person making contact with the metalwork to constitute an unpleasant, though probably not a dangerous, shock. Low fault resistances have no influence on the operation of the earth-leakage circuit-breaker, since this may already have several hundreds of ohms in its circuit due to the impedance of the coil and the earth electrode. The non-operation of an earth-leakage circuit-breaker due to a high-resistance earth electrode does not necessarily mean that no shock can be experienced—the two facts are comparatively unrelated since the electrode may have a high resistance because it is small or in a dry place, whilst the person touching the metalwork may be in contact with soil of quite a different resistivity, or he may be standing on a conducting floor of considerable size, in which case the soil resistivity has only a small influence.Arising from the earlier Report, and further evidence which has been secured since its issue, certain alterations have been made in the recommendations. It is now proposed that distinction be no longer made between farms and ordinary installations as regards protection, and that no links for testing or other purposes be permitted in the neutral of a P.M.E. system since there is no significance in applying any test to a neutral which is earthed at possibly hundreds of places. Modifications have been made in the previous recommendations dealing with consumers' earth electrodes and the earthing of the distributor neutral, and the opportunity has been taken to make several other less important modifications.